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Fans in the Attic: Do They Help or Do They Hurt?

Homeowners in hot climates need to understand the difference between whole-house fans and powered attic ventilators

Posted on Oct 26 2012 by Martin Holladay

There’s a lot of confusion surrounding attic fans. Here at, we regularly receive e-mails from homeowners with questions about attic fans: What’s the purpose of the fan in my attic? How often should I run it? Do I need a bigger fan?

Before addressing these recurring questions, it’s important to define our terms. First, we need to distinguish between three different types of ventilation fans.

The most common kind of residential ventilation fan is one used to provide fresh air for building occupants. Examples of this type of fan include the fans in a heat-recovery ventilator (HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. ) or an energy-recovery ventilator (ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.), as well as some types of bathroom exhaust fans. (For more information on this type of ventilation fan, see Designing a Good Ventilation System.)

Whole-house fans are sometimes confused with ventilation fans that provide fresh air. Unlike a ventilation fan, a whole-house fan — an attic-mounted fan that exhausts air from a home at night — is designed to cool a house (that is, to lower the indoor temperature).

A powered attic ventilator has a different purpose: it is designed to lower the temperature of an attic by exhausting air from the attic and replacing attic air with outdoor air.

At the risk of oversimplifying, whole-house fans are good. Powered attic ventilators are bad.

Whole-house fans

Whole-house fans are used to cool a house at night, when the heat of the day has passed and the outdoor temperature has dropped enough to feel comfortable. When should you turn on a whole-house fan? The answer depends on your climate and your comfort range. The outdoor temperature should certainly be below 80°F — or, better yet, below 70°F.

The main advantage of using a whole-house fan instead of an air conditioner is to save energy. A whole-house fan usually draws between 200 and 700 watts — about 10% to 15% of the power drawn by a central air conditioner (2,000 to 5,000 watts). If evenings are cool enough, it’s fairly easy to lower the temperature of your home and your furniture with a whole-house fan — sometimes in less than an hour.

Whole-house fans are intended to be used in homes that are not air-conditioned. It makes no sense to introduce lots of (potentially humid) exterior air into a house at night if you intend to turn on an air conditioner the next day. If you're using your air conditioner, keep your windows closed, 24 hours a day, so that the air conditioner isn't faced with an increased latent loadCooling load that results when moisture in the air changes from a vapor to a liquid (condensation). Latent load puts additional demand on cooling systems in hot-humid climates. due to exterior humidity entering the house at night.

In most cases, a whole-house fan is mounted in the attic floor, above a rectangular grille in the ceiling of a central hallway. Once the outdoor temperature cools down — usually in the evening or early morning — the homeowner opens a few downstairs windows, closes the fireplace damper, and turns on the fan. (The wall switch that controls a whole-house fan should be properly labeled so that it isn’t accidentally turned on during the winter.)

The fan pulls air from the hallway and blows it into the attic. Since whole-house fans are relatively powerful — they are usually rated between 2,000 cfm and 6,000 cfm — they quickly exhaust the hot indoor air, allowing cooler outdoor air to enter through the downstairs windows. Once the house has cooled off, the fan can be turned off and the windows closed. Most people who have whole-house fans keep their windows closed from early morning until evening, so that the cool air inside the house doesn’t escape.

You need enough attic vents to let the air escape

Since a whole-house fan blows all of the hot air from the home into the attic, the fan won’t work effectively unless the attic has large openings to exhaust the hot air. Most old-fashioned whole-house fans require more attic venting than the minimum amount required by the building code — anything from a little more to about twice as much, depending on the size of the fan.

Here’s the rule of thumb: you need one square foot of net free vent area for every 750 cfm of fan capacity. The vent area can be made up of a combination of soffit vents, ridge vents, and gable vents. If the vent has insect screening, remember to make the opening 50% larger than the rule of thumb dictates. It’s better to have too much vent area than not enough.

Manufacturers of ridge vents and soffit vents provide information on the net free area of ventilation per linear foot of their products; for example, the Air Vent website lists different ridge vent products that provide between 9 and 18 square inches of net free area per linear foot of product.

How do you size your whole-house fan? The traditional recommendation is to choose a fan that can move between 15 and 20 air changes per hour (achACH stands for Air Changes per Hour. This is a metric of house air tightness. ACH is often expressed as ACH50, which is the air changes per hour when the house is depressurized to -50 pascals during a blower door test. The term ACHn or NACH refers to "natural" air changes per hour, meaning the rate of air leakage without blower door pressurization or depressurization. While many in the building science community detest this term and its use (because there is no such thing as "normal" or "natural" air leakage; that changes all the time with weather and other conditions), ACHn or NACH is used by many in the residential HVAC industry for their system sizing calculations.). If you’re aiming for 15 ach, that means you need to divide your home’s volume by 4 to obtain the cfm rating of your fan. If your ceiling height is between 8 and 9 feet, just multiply the floor area of your house by 3 to obtain the cfm rating of your fan.

Where does a whole-house fan make sense?

If you live in the right climate, whole-house fans are a great way to keep your house cool. In the U.S., they make more sense in the arid West than in the humid Southeast, since most homeowners don’t want to invite lots of humid air into their homes.

Whole-house fans make sense in areas with cool nights. If you live somewhere where the temperature stays in the 80s all night long, a whole-house fan won’t help you much.

However, even if you need to seal up your house and turn on your air conditioner during the hottest months of summer, a whole-house fan may be useful during the spring and fall seasons, when nights are cool but days remain hot.

A few caveats

Whole-house fans make sense in some, but not all, homes:

  • They don’t make sense for homes in neighborhoods where security concerns prevent homeowners from leaving their windows open.
  • They don’t make sense for homes with a furnace or water heater in the attic.
  • Because they depressurize a home, whole-house fans can cause atmospherically vented appliances located inside a home — for example, a gas-fired water heater — to backdraft. If the homeowner remembers to open plenty of windows before turning on the fan, backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney. probably won’t occur. But the best way to avoid backdrafting problems in a house with a whole-house fan is to make sure that the house doesn’t have any atmospherically vented combustion appliances.
  • Whole-house fans represent a big hole in your ceiling — a hole that is likely to leak a lot of heat during the winter unless it is properly sealed.

Finally, it should be noted that some homeowners complain that whole-house fans are noisy. However, newer models of whole-house fans — especially the Tamarack HV1000 — are quieter than traditional whole-house fans with higher cfm ratings.

Sealing up the big hole

There are two ways to address the “big hole in the ceiling” problem. One solution is to build an insulated box that fits on top of fan. The main disadvantage of this solution is that you have to climb up into the attic twice a year to install it and remove it.

One document posted online — — includes instructions for building a “box cover” for a whole-house fan. Unfortunately, the document suggests that it’s acceptable to build a cover insulated only to R-5. Clearly, that’s not enough insulation.

For a better approach, make a site-built cover as shown in the detail in GBA's CAD detail library. Or you can follow the advice given by Erik North in his blog on building a “coffin” for insulation pull-down attic stairs. (North advises building a box with an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. ranging between R-26 and R-49.)

The second solution to the “big hole in the ceiling” problem: buy a whole-house fan from Tamarack.

Tamarack fans

of Buzzards Bay, Massachusetts, makes the best whole-house fans available. Since Tamarack fans include motorized doors insulated to R-38 or R-50, you won’t have to climb up into your attic twice a year to wrestle with an insulated box if you install a Tamarack fan.

You can choose between two models of Tamarack whole-house fans. The is rated at 1,150 cfm and draws just 70 watts. It costs $579.

has two speeds (1,150 cfm and 1,600 cfm) and draws 230 watts at high speed. It costs $859.

Tamarack fans have lower cfm ratings than most other whole-house fans, but the low power ratings confer certain advantages. The fans are quieter; they use less electricity; and they are smaller than other fans, and therefore easier to air-seal and insulate when not in use. Moreover, since a Tamarack fan blows a smaller volume of air than the typical whole-house fan, it usually doesn’t require any extra vents in your attic. Most homes have enough soffit and ridge ventilation to accommodate a Tamarack fan. The HV1000 requires a minimum of 3 square feet of net free vent area, and the HV1600 requires a minimum of 5 square feet.

Of course, since these fans don’t move as much air as a fan rated at 4,000 cfm, you’ll have to run the fan for more hours to get the same cooling effect.

Powered attic ventilators

Now that we’re done talking about whole-house fans — the “good” kind of attic fan — it’s time to address powered attic ventilators — the “bad” kind of attic fan.

Powered attic ventilators are usually mounted on a sloped roof or the gable wall of an attic. Most powered attic ventilators are controlled by a thermostat so that they turn on when the attic gets hot.

The intent of a powered attic ventilator is to exhaust hot air from the attic. The installers of powered attic ventilators hope that the exhausted air will be replaced by outdoor air. They also hope that the outdoor air will be cooler than the exhausted air, and that the effect of operating the fan will be to lower the attic temperature.

The idea is to save energy by reducing the run time of your air conditioner. Installers evidently hope that a powered attic ventilator will save more energy that the electricity required to run the fan.

Well, it's an interesting theory...

Although the logic behind powered attic ventilators is compelling to many hot-climate homeowners, these devices can cause a host of problems. Here’s the basic problem: a powered attic ventilator will depressurize your attic, and it’s hard to predict where the makeup air will come from. Although the “smart arrows” in the sales brochures shows outdoor air entering the attic through the soffit vents, that’s not what usually happens.

In many homes, powered attic ventilators pull conditioned air out of the home and into the attic through ceiling cracks. The net result: powered attic ventilators increase rather than decrease cooling costs.

As the cool air is being sucked out of the house through the ceiling, hot exterior air enters the house through other cracks to replace the exhausted air. The net result: the air conditioner has work harder than ever as it struggles to cool all that entering outdoor air.

Several studies show that even in a house with a tight ceiling, a powered attic ventilator uses more electricity than it saves.

Flue gases get sucked backwards into the house

A more alarming problem: researchers in Florida and North Carolina have shown that powered attic ventilators can depressurize a house enough to cause water heaters to backdraft. Since backdrafting sometimes introduces carbon monoxide into a home, the phenomenon can be dangerous.

John Tooley of Natural Florida Retrofit and Bruce Davis of Alternative Energy Corporation’s Applied Building Science Center in North Carolina conducted a field study to investigate powered attic ventilator performance. According to , “As a result of this research, Davis said that he wouldn’t recommend the use of powered attic ventilators. … The potential for hazardous conditions is particularly high in homes with combustion gas appliances, because the ventilators can create negative pressures that cause backdrafting.”

One of the researchers working with Tooley and Davis was Arnie Katz, : “In most of the houses we’ve tested, the attic fans were drawing some of their air from the house, rather than from the outside. In other words, they are cooling the attic by drawing air-conditioned air out of your house and into the attic. Air conditioning the attic is not recommended by anyone I know as an effective strategy for reducing your bills. ... In one house we tested, we measured substantial levels of carbon monoxide (CO) in the daughter’s bedroom in the basement. The CO was coming from the water heater next to the bedroom, which was backdrafting. The daughter had been suffering from flu-like symptoms for some time. The backdrafting was caused by the powered attic vent fan.”

Like a little boy looking for a job

Researchers at the Florida Solar Energy Center (FSEC) have reached similar conclusions to those reached by Tooley, Davis, and Katz. In an FSEC publication called researcher Subrato Chandra wrote, “Data measured at FSEC and elsewhere show that attics with nominal natural ventilation and R-19 ceiling insulation do not need powered vent fans. Such fans cost more to operate than they save in reduced cooling costs, so they are not recommended.” Of course, if your ceiling insulation is deeper than R-19 — as it should be — there’s even less reason to worry about your attic temperatures.

William Rose, the renowned building scientist and attic-ventilation expert, was interviewed for an article on attic ventilation that appeared in the August 1997 issue of Energy Design Update: “‘Ventilation is like a little boy who goes around the house looking for a job,’ notes Bill Rose ... ‘He can do some things well, but can’t do anything really well.’ … Research suggests that the energy to run the fan for a powered attic ventilator can be higher than the savings in cooling energy. The biggest potential problem, says Rose, is that power venting can cause a negative pressure in the attic. ... He says, ‘One of the worst things that can happen is to draw quantities of indoor air into the attic, and powered equipment is more likely to do this.’ ”

What about solar-powered attic fans?

For some reason, proponents of powered attic ventilators just don’t want to give up. In hopes of answering critics who complain that these fans use more electricity than they save, the industry has developed powered attic ventilators equipped with small photovoltaic panels. They developers of these products proclaim: these fans don’t require any grid power!

Well, that doesn’t really address the problem of potential backdrafting, does it?

Researchers at FSEC looked into solar-powered attic ventilators, and noted that the devices could, in some circumstances, reduce the electricity used for air conditioning. In their report, however, the researchers concluded, “Based on the matching period analysis, estimation of annual space cooling savings are on the order of 460 kWh. These savings have a value of approximately $37 at current Florida energy prices. Given that the costs for the two units was approximately $600, or about $850 installed, the payback of the ventilators is not very favorable at over twenty years.”

My favorite quote on solar-powered attic fans comes from , who wrote, “In my opinion, powered attic ventilators are generally not a good idea, whether they’re powered by nuclear electricity, burning water buffalo dung, landfill-generated methane gas, or directly by the sun…. A solar-powered attic fan … is like smoking cigarettes made with vitamin C.”

What do I do if my attic is too hot?

A hot attic isn’t necessarily a problem. If you don’t have any ductwork or HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment up there, who cares how hot it gets? After all, you should have a thick layer of insulation on your attic floor to isolate your hot attic from your cool house.

If you do have ductwork or HVAC equipment in your attic, the designer and builder of your home made a major mistake. Solutions include:

If you believe that your house has a hot ceiling during the summer, the solution is not a powered attic ventilator. The solution is to seal any air leaks in your ceiling and to add more insulation to your attic floor.

Martin Holladay’s previous blog: “Rating Windows for Condensation Resistance.”

Tags: , , ,

Image Credits:

  1. U.S. DOE
  2. Tamarack Technologies
  3. GBA
  4. Pacific Gas & Electric

Oct 26, 2012 12:16 PM ET

Night Breeze doesn't need windows opened
by Kris Knutson

Another fantastic article Martin!

Davis Energy Group of Davis, CA developed the Night Breeze system which does not require window opening to cool the house. This would seem to mitigate the concerns about security - it also supports a filter for incoming air. I have never used one, but it seems like a good system.

The attic "coffin" you mention, which is used to air seal and insulate pull-down stairs, scuttle hole, or a whole house fan, is good in theory, but in practice, is cumbersome to install. Even though a manufactured cover costs more, it is far easier to install. It also requires no assembly.

Oct 26, 2012 1:05 PM ET

Edited Oct 26, 2012 1:08 PM ET.

Response to Kris Knutson
by Martin Holladay

The NightBreeze has been around for a while; I first wrote an article about the product 8 years ago, for an article that appeared in the September 2004 issue of Energy Design Update. In that article, I wrote:

"The NightBreeze includes a variable-speed air handler with an ECM blower, a hydronic heating coil, a large motorized damper to control the intake of exterior air, and a wall-mounted control unit. The thermostat-like control unit also regulates the operation of the air conditioner, if any. In addition to providing nighttime ventilation cooling, the NightBreeze system provides year-‘round whole-house ventilation.

"In order to pull in enough outside air to cool the house at night, the NightBreeze requires a large exterior intake grille — usually about 3’x3’ (assuming the grille has 50% free area). If the air handler is located in the attic, the air intake grille can be mounted in a gable, in a doghouse dormer, or at the top of a false chimney. The air-intake duct is connected to a damper box containing a large hinged motorized damper blade. Damper leakage is minimal — at 25 Pascals, it’s about 1.4% of total fan flow. According to David Springer, the president of Davis Energy Group, one of the main consulting groups that developed the NightBreeze, ‘It is possible to achieve 6 percent duct leakage in a system that includes the damper.’ The wall-mounted NightBreeze control replaces a conventional thermostat. ‘At my house, the control is set for a 5-degree delta T,’ says Springer. ‘The upstairs thermostat is set to 80°, so it’ll start ventilating when the outdoor temperature drops to 75°. By morning, the outdoor temperature might be in the fifties or low sixties, and the indoor temperature might be 68°.’

"The control unit uses historical monitored temperature data to predict next-day outdoor temperatures. When the control predicts relatively mild weather, the ventilating blower will run at a lower speed than when hotter weather is predicted. ...

"Like a whole-house fan, the NightBreeze system will not provide effective ventilation cooling in all climates. According to Springer, ventilation cooling works best in areas of the country with a mean daily temperature range of at least 30°F. According to the ASHRAE Fundamentals book, such areas include Idaho, Nevada, Utah, and Wyoming, as well as most of Arizona, California, Colorado, Montana, New Mexico, and Oregon. In most of the rest of the country, however, the mean daily temperature ranges are narrower than 30°F. An investment in ventilation cooling equipment makes more sense in a dry Western climate than in a humid Eastern climate. ‘I was convinced it would never work in Florida, but Danny Parker [from the Florida Solar Energy Center] convinced me otherwise,’ said Springer. ‘He showed me that even in Florida you can do some good with ventilation cooling, at least during a few weeks of dry weather in the fall. Whether the investment in the equipment can be justified is another question.’ ..."

Oct 26, 2012 10:07 PM ET

Good stuff!
by Curt Kinder

I'm constantly up against folks who sell or have bought powered attic ventilators for use in Florida. Most who have them "get it" when I explain that conditioned air is pulled up out of every ceiling penetration.

Oct 28, 2012 5:27 PM ET

whole house 'attic fan'
by greenhouse437

Grew up with one in the 60s in NY. worked pretty well and was an obvious choice before the family had central AC. Another thing to remember is to cover the large attic exhaust vent in the off season. In our case it was a louvered opening about 30in square that we covered with a plywood piece every September or the attic would get really cold.

Oct 31, 2012 3:41 AM ET

Edited Oct 31, 2012 5:57 AM ET.

by kelly Jones

Solar Attic Fans are easily managed and fixed and a great product for proper attic ventilation. [Brand name deleted by editor] solar attic fans are no. 1 by builders and are the highest attribute made in the USA!
[Website link deleted by editor]

Oct 31, 2012 5:58 AM ET

Response to Kelly Jones
by Martin Holladay

It's amazing that your urge to post spam is stronger than your willingness to read.

Oct 31, 2012 5:18 PM ET

Edited Oct 31, 2012 5:20 PM ET.

Attic Temperatures and Roof Lifetimes
by Lee Dodge

Standard asphalt shingle roofs is northern Vermont, where Martin Holladay lives, last 30 or more years, while roofs in south Texas last about 15 to 18 years. I think that the difference is due to the hotter attic temperatures in the south, which increase the shingle temperatures. The cost of a new roof for a modest size house is roughly $8000, or $500 per year for a roof that lasts 16 years. If an owner could get an increase in roof lifetime by 4 years by keeping the attic cooler, that would cover the cost of about 20,000 kWh at $0.10/kWh!

That 20,000 kWh would go a long way toward operating a pair of thermostatically controlled, powered attic ventilators (operated off the same thermostat) that had fans both blowing air into and out of the attic, through gable vents, with a balanced pressure so that air was not sucked through the ceiling of the house. It would also reduce heat gain into A/C ducts in the attic, a standard practice in older homes.

I get the idea that people that suggest a hot attic with a well-insulated ceiling have never crawled around an attic in south Texas on a summer day trying to string a TV cable or electrical wiring. It is not a matter of comfort, but rather, a matter of survival.

Oct 31, 2012 5:39 PM ET

Flat roofs & whole house fans
by kim shanahan


Since the desert southwest is prime territory for whole house fans and a common building style is flat roofs with either no attic space or very minimal, non-accessible attic space, what is an appropriate detail for installing a whole house fan? This is relevant as a remodeling question also.

Oct 31, 2012 5:55 PM ET

How important is insulating the fan cover?
by Derek Roff

While I agree that more insulation is generally better, I wonder how important it is to attain large R-values in the cover for a whole house fan installed in the floor of the attic/ceiling of the hallway. The article says of R-5, "Clearly, that’s not enough insulation." I wonder what the energy consumption difference would be, between an R-5 cover, and the R-26 advocated as the low end by Erik North. If we are talking about less than 10 sq ft of opening for the fan, then the difference in energy consumption between R-5 and R-26 for the cover will add up to how many pennies per year?

Obviously, it depends on the average ambient temperature, humidity, and the nature of the attic. Still, an example calculation for an average home in New Jersey, and another in Vermont, would be enlightening.

My guess is that way more attention should be given to creating a flawless air seal around the cover than to adding a few more Rs to the R-value. Of course, doing both is nice.

Nov 1, 2012 6:23 AM ET

Edited Jul 3, 2013 2:42 PM ET.

Response to Lee Dodge
by Martin Holladay

Your argument rests on three legs. You assert:

1. Asphalt shingle roofs last 30 or more years in Vermont, and only 15 to 18 years in Texas.

2. The best way to lower the temperature of asphalt shingles is with a powered attic ventilator.

3. Cooler asphalt shingles last longer than warmer asphalt shingles.

To determine whether your argument has any merit, we need to look at each of these three assertions in turn.

1. In fact it is quite rare for an asphalt shingle roof to last 30 years in Vermont. Your estimate of 15 to 18 years is much closer to the average here (as it appears to be in Texas). Many factors affect shingle longevity; shingle temperature is only one factor.

2. The best way to lower asphalt shingle temperatures is to choose white-colored shingles over black shingles. No other factor, including attic ventilation, plays as much of a role as do color and orientation. This fact holds true in Vermont as well as Texas. While the correlation between shingle temperature and longevity has not yet been definitively shown, it remains true that anyone concerned about elevated shingle temperatures should simply choose white shingles.

More information on this topic can be found here:

"In his recent book, Water in Buildings, Rose summarizes what he has learned from years of careful research: “Does ventilation significantly reduce shingle temperatures? The short answer is no, not significantly. Shingle color is a very strong determinant of shingle, sheathing, and attic temperature. ... In a vented cathedral ceiling, venting can cool shingles in the lower part of the roof; it cannot cool shingles high in the roof. ... As long as shingle manufacturers ignore the effects of shingle color as a determinant of temperature, they may be admonished for asserting so strongly the importance of venting to control temperature. ... In short, on the basis of currently available information, attic ventilation is only marginally beneficial to shingle durability. Attic ventilation does not deserve the attention it has received in relation to shingle durability.” If your clients prefer cool shingles to warm shingles, the amount of ventilation behind the sheathing is far less important than the shingles’ color; so advise them to choose white shingles."

"Our simulations and measurements have shown that asphalt shingles applied over vented roofs in hot-dry climates operate warmer than the same asphalt shingles applied over unvented roofs in hot-humid climates. To our knowledge, there have been no shingle warranty adjustments for Las Vegas versus Orlando, and that difference in location is more significant in regards to shingle temperature than vented versus unvented."

Insulated Rooflines and Shingle Temperatures:
"One study showed that shingle life is reduced by less than a year on homes in Miami with no attic ventilation."

3. On the third point, the jury is still out. However, if you really care about shingle temperatures, the best solution is clear: choose white shingles! Roof ventilation won't help much.

Nov 1, 2012 7:19 AM ET

Edited Feb 14, 2018 6:37 AM ET.

Response to Kim Shanahan
by Martin Holladay

Q. "Since the desert southwest is prime territory for whole house fans and a common building style is flat roofs with either no attic space or very minimal, non-accessible attic space, what is an appropriate detail for installing a whole house fan?"

A. Several manufacturers make whole-house fans for homes without attics. to see a website with many inexpensive models of whole-house exhaust fans for flat roofs.

A better option would be the -- a model with an automatic insulated damper that prevents heat flow and air flow when the unit is not in use.

Nov 1, 2012 7:38 AM ET

Response to Derek Roff
by Martin Holladay

Q. "I wonder what the energy consumption difference would be, between an R-5 cover, and the R-26 advocated as the low end by Erik North?"

A. Let's do the calculation. As I explained in my article, How to Perform a Heat-Loss Calculation — Part 2, "The heat loss formula for determining transmission losses through floors, roofs, and walls is Q = A • U • ΔT. In other words, the rate of heat flow through a building assembly (in Btu/h) is equal to the area of the assembly (in ft²) times the U-factor (in Btu/ft² • hr • F°) of the assembly times the ΔT (in F°)."

To perform the calculation you requested, we'll have to make some assumptions. You can adjust the assumptions to fit your own case and run your own calculations if you want.

Let's assume that the insulated box in the attic measures 40 inches by 40 inches, and it is 24 inches high. The exposed area of this box is therefore 26.6 square feet for the sides, and 11.1 square feet for the top. The total area is 37.7 square feet.

Let's assume that the attic temperature is 32°F, and the indoor temperature is 72°F. That means that the ΔT is 40 F°.

First we'll do the calculation assuming that the box has an R-value of R-5:
Q = A • U • ΔT = 37.7 • 0.2 • 40 = 302 Btu/h

Next we'll do the calculation assuming that the box has an R-value of R-26:
Q = A • U • ΔT = 37.7 • 0.038 • 40 = 58 Btu/h

The difference in the rate of heat flow is 244 Btu/h. Over 24 hours, that would be 5,856 Btu. Over 5 months, that would total 878,400 Btu, or 257 kWh.

If you were supplying heat to your house with electric resistance heat at a cost of $0.14/kWh, that would cost you $36 over the winter.

If your space heating costs were only 1/2 the cost of electric resistance heat -- a more likely scenario -- the difference in R-values would cost you $18 over the winter.

Cold climates will have a higher ΔT than I assumed, and warm climates will have a lower ΔT than I assumed. My calculation is only an example to show you how to figure this out for yourself.

Nov 1, 2012 10:51 AM ET

Thanks for the help with the calculations
by Derek Roff

Thank you, Martin, for showing me how to do the heat-loss calculation, and providing an example. It certainly may be a good, average example, but I'd like to take other information from your article, to suggest a significantly different example. Let me know if I have misunderstood or miscalculated.

Choosing the Tamarack HV1600, the larger of the two Tamarack whole house fans that you recommend, the product literature says that the rough opening in the ceiling should be 14 1/2” x 22 1/2". This gives an area of ~2.25 sq ft. If we keep everything else equal, this is 1/17th of the insulated area in your calculation, so the rough cost differential over a season drops from $18 to just over $1, for the cheaper heating source. Of course, that isn't entirely fair, since the needed insulating "coffin" in the attic would be smaller for this fan, and the fan comes with insulating doors. But let me finish explaining the difference I am contemplating in insulation location.

We could get R-5 out of an inch of a high-quality foam. Even adding the necessary cosmetic and structural elements, it would be reasonable to build this 1" plus thickness into a flat insulating cover mounted below the fan opening, on the ceiling, inside the living space. In that scenario, calculating the insulation area based on the rough opening seems reasonable to me. Higher insulation levels would require an insulation coffin in the attic, which leads to greater surface area and greater heat loss area. So I'm thinking that there is even less final energy loss/cost difference between an R-5 insulated cover in the living space and an R-26 coffin in the attic, if we consider the advantageous placement of the insulation possible with R-5.

We could make a similar comparison with your example 40"x40" fan. The inside-the-house flat R-5 cover that I am suggesting would have an area of 11 sq ft. This is less than a third of the surface area of the R-26 coffin in your example. Again, keeping other things equal to your example, this would give us a seasonal cost difference of roughly $6.

Considering lower materials cost, easier fabrication, and simpler attachment and removal, I'm thinking that I would be very happy paying an extra $1-$6 per year to use an R-5 flat cover on the ceiling of the hallway. Have I missed something critical?

Nov 1, 2012 12:07 PM ET

Response to Derek Roff
by Martin Holladay

Most whole-house fans require a rectangular opening that is at least 32" x 32", and often larger; some whole-house fans are 48 in. in diameter. A ceiling grille measuring 32"x32" or 36"x36" isn't unusual.

Of course, the size of the Tamarack fans is smaller, as is the required ceiling grille.

Your plan to install an interior cover should work fine. But as long as you're making a cover, why not use at least 2-inch-thick rigid foam instead of 1-inch-thick foam?

Nov 1, 2012 7:41 PM ET

Asphalt Shingles
by Tom Barrett

Asphalt shingles and other asphalt-based materials are the reason attics get so hot. I've recorded temperatures up to 180 deg F on back side of asphalt shingles in the California summer sunshine and attic temperatures of 160 deg F. White shingles are marginally cooler. The key is solar reflectivity and thermal emittance of the roofing material. Asphalt is a great material for absorbing solar radiation and has poor thermal emittance. Walk barefoot on an asphalt road and see how that works.

It is interesting that asphalt single warranties are based on having attic ventilation to code or the warranty is voided. I've measured attic temperatures with various types and amounts of attic venting and there is really little effect that can be detected with venting on attic temperature. Cooling ends up being localized around the vent openings, while the attic stays hot.

So the biggest cause hot attics is our roofing materials, not the lack of ventilation. Of course if you want to make lemonade out of that lemon, use the attic heat to preheat water and call your roof a solar collector.

Nov 2, 2012 8:53 AM ET

Response to Tom Barrett
by Martin Holladay

Thanks for your comments.

I agree with your conclusion that "there is really little effect that can be detected with venting on attic temperature."

Nov 2, 2012 9:33 AM ET

Thanks for the follow-up response
by Derek Roff

Thank you, Martin, for your further comments on my point. I picked one inch of foam in my example to match the R-5 mentioned in your article. However, I agree that choosing two inches of foam for a cover would be twice as good.

Nov 5, 2012 8:21 AM ET

Ridge vent versus powered attic ventilator
by Chris Grabowy

So one part of my roof/attic has a ridge vent but does not have a gable vent. And then the main part of my roof/attic has NO ridge vent but does have gable vents on both ends. I was thinking about adding some sort of powered attic ventilator. But I was wondering if I would just be better off installing a ridge vent along that main roof/attic. According to this article the powered option is not an option. So are the gable vents enough or should I install ridge vents? Also we have debated about putting in a whole house fan, since there are some weeks when its cool outside but hot inside the house with no wind outside to help. BTW, I live west of Philadelphia, PA. Anyway, great article!!

Nov 5, 2012 11:29 AM ET

Response to Chris Grabowy
by Martin Holladay

If your attic doesn't have any problems, I wouldn't worry about adding more attic ventilation.

What problem are you trying to solve?

Nov 5, 2012 12:08 PM ET

Edited Nov 5, 2012 12:23 PM ET.

Attic fan reduced my electricity bill
by j wing

I've been an energy engineer for over twenty years. My experience has taught me to think beyond articles like this one. In the real world, comfort, low first cost, and immediate gratification take precedence over theory and lab results. To wit, I have intalled two attic fans; both installations were resounding successes:

1) My 1960-vintage home in northern Alabama runs 5-tons of central air conditioning plus one small window A/C unit. In the summer, the upstairs bedrooms are unbearably hot, due to the ceiling radiating attic heat. The bedrooms can be filled with cool air, yet the hot ceiling makes them feel like being inside a broiler. This past summer,a major outdoor renovation forced me to disconnect the condensing units from the central A/C, leaving me with just the small window unit. So, I put in an attic fan to relieve the hot ceiling. The A/C is off during the day when I'm at work; but the fan runs. By the time I go to bed, the attic is cooler than the inside of my house, so it is drawing heat out. The small window unit is plenty to keep the house comfortably cool.

2) My mother-in-law stays in her hot home all day and was considering replacing her central A/C system with one of higher capacity. However, the cost was prohibitive, so I installed an attic fan. With a cooler attic, the A/C system had lower heat gain, therby reducing the supply air temperaure at the diffusers. Additionally, the hot ceiling effect was alleviated. MIL was able to turn her thermostat up a few degrees, thereby saving energy and increasing comfort.

It's very easy to write that better insulation, radiant barriers, and light-colored roofs are components of a superior solution. And they are; I can't argue otherwise. But in the real world, we look for practical solutions that people will actually implement. In the two cases, I've described, the most practical (by far) solution was contrary to the conventional wisdom.

Nov 5, 2012 12:45 PM ET

Edited Nov 5, 2012 12:47 PM ET.

Response to J Wing
by Martin Holladay

J Wing,
You have installed powered attic ventilators in two homes in recent years, so you obviously like them. According to your anecdotes, the electricity use decreased in both of these buildings.

Even if your anecdotes are accurate in all details, and even if you have no financial interest in promoting powered attic ventilators, two anecdotes aren't convincing. Needless to say, there are all kinds of reasons that electric bills fluctuate from one month to the next; one of the biggest reasons (when it comes to air conditioning) is variations in weather.

You have described two buildings with "hot ceilings." I have no idea why anyone with a hot ceiling would think it was easier to install powered attic ventilators than a layer of cellulose insulation. After all, the cellulose insulation doesn't require any electricity.

Here at GBA, we strive to advise readers of the simplest and best solutions to common building problems. I'd like to repeat my advice: if you have a hot ceiling, install more insulation on your attic floor. Don't install a powered attic ventilator, since these devices, on average, use more electricity than they save.

Nov 5, 2012 3:33 PM ET

by John Bigler

Excellent article. Thank you Martin.

Nov 5, 2012 6:11 PM ET

Well, to me two anecdotes are
by j wing

Well, to me two anecdotes are more compelling than one article, no matter how many times it's been re-written. I have been a professional energy engineer for 2 decades; I understand that there are many variables that account for enrgy use.

The most important variable, however, is the human beings involved and the particular situation they are dealing with. In both of my anecdotes, the human beings felt that the cost in money, time, work, and hassle would not have sufficient return in comfort. In the case of my house, it is impossible to lay down another layer of insulation.

On the other hand, each attic fan took about $100 and hour to install. As a curious professional, I was willing to risk the investment to see for myself what effects are. If the attic fans didn't do the trick, then I'd move on to the next solution and be out only $100. The results of my experiments, so far, differ from this article and all the other ones that say the same thing. I report my findings not to argue or to be off-handedly accused of having financial interest, but to advance our understanding and knowledge.

Building a new house? Definitely insulate the roof and add a radiant barrier. Is your fan-coil unit in a hot attic? Consider an attic fan, especially if you turn the A/C off during the day. Are you at home with the A/C on all day? Consider a different roof heat-gain management system, if you can afford a long-tem investment. Not staying in the house long enough to make the investment worthwhile? Consider a less-expensive solution that may have some drawbacks. Does your attic have insufficient ventilation openings that will cause an attic fan to draw through your house? Increase your ventilation area, or consider a different solution.

Energy engineering is way more complicated than telling people that there is but one solution to a problem.

Nov 5, 2012 6:21 PM ET

Response to J Wing
by Martin Holladay

Thanks for sharing your anecdotes. I certainly appreciate the information, as I appreciate all information provided by GBA readers. And I accept your assertion that you have no financial interest in promoting powered attic ventilators.

However, after having read the reports of researchers who have studied the issue and measured energy use in buildings with powered attic ventilators, I'm going to stick with the conclusions and advice of the researchers, even in the face of two anecdotes that buck the trend.

Nov 8, 2012 3:21 PM ET

What about a garage attic??
by Robert Williams

I'll be building a large garage with attic storage in NE Texas. I will use a radiant barrier decking, but does a powered attic ventilator make sense here ??

Nov 8, 2012 3:34 PM ET

Response to Robert Williams
by Martin Holladay

Running a fan always requires electricity. Of course, a fan cannot cool your garage unless the outdoor air temperature is lower than the indoor air temperature. If you have radiant barrier sheathing, the temperature difference between the two spaces won't be very great, and probably won't be enough to justify the use of the fan.

The other question: what are you storing in your attic? Does it matter what the temperature is? If you have an old lawnmower and some Christmas ornaments up there, it probably doesn't matter if the garage is 80 degrees or 110 degrees.

Feb 2, 2013 8:14 PM ET

Whole House Fans
by Kurt Shafer


Your research into attic fans is of great interest. It is valuable to have the data from those who have studied them recently.

Your comments about Tamarack are surprising. Tamarack is a fine company with leading edge insulation technology but you do your readers a disservice by inserting your opinion that they "make the best whole-house fans available".

I had hoped you would have done a little more research into other suppliers in order to offer a more unbiased view.

I would be very happy to assist you in a thorough market overview which I do regularly for the

Apr 30, 2013 9:50 AM ET

Like so many other "plug in"
by David Jones

Like so many other "plug in" solutions attic fans certainly have the potential to make things worse rather than better. Your article does a great job of outlining how this happens. As a general rule unless a homeowner has a highly qualified person to evaluate their house, I think its safe to say that attic fans should be avoided.

I do think that under CERTAIN circumstances an attic fan can be beneficial. The problem is that in most cases, there is no one available to accurately make that determination.

The primary reason attic fans are not helpful is that they can depresureize the attic and draw air out of the house. Many houses have significant leaks in the air barrier(attic floor) and it would be easy for the attic fan to draw a significant amount of air from the house. If a house has been air sealed by someone with an understanding of the air leakage, then it would seem that the depressurization would be minimal and the attic fan could then be beneficial.

The amount of air being drawn out of the house is a function of the pressure difference and size of the air leaks in the attic floor. In an attic with gable vents, soffit vents, and ridge venting, I suspect that the attic fan would not create a significant pressure difference and the air leakage from the house is probably small. (assuming even a rudimentary level of air sealing).

Lee Dodge suggests a "balanced" system where there is an incoming and exhaust fan. This seems like an approach that could also limit depressurization.

If air leakage is minimized, and depressurization is controlled with either a balanced fan or adequate openings for incoming air then it seems reasonable that the amount of depressurization would minimal and the attic fan could be beneficial.

Arnie Katz's study concluded that (an undefined amount of?) depressurization was enough to increase cooling costs. This assumes that there is a cooling system in place and operating. In a house not running an ac system any air being drawn from the house is being replaced by outdoor air. If the windows are open in the house there is no "cooling penality" to having some air exhausted from through the attic. if late in the afternoon the house temps rise above the outdoor air temps there is actually a cooling benefit. (we hope the amount of air being exhausted through the attic floor is minimal because the house is air sealed).

If the house without AC running was closing the windows during the day to keep the house cooler, then any air being exhausted to the attic would be replaced by warmer outdoor air, but that warming of the house is tempered by a cooler ceiling.

You propose the question "who cares how hot the attic is if there is enough insulation in the attic floor?" Insulation can only slow and not stop the energy transfer. Regardless of what the insulation level is, there will be some level of energy transfer from the warmer attic to the cooler house. So everything else being equal, any house would benefit from a cooler attic. In many houses it is impractical to increase the insulation in the attic. (At least when compared to the difficulty/cost of installing an attic fan. Of course the insulation is the better option if possible.).

My own experience points to anecdotes showing the attic fan is beneficial. I know of two houses using them and the owners are certain that they make a large difference in comfort. Both houses do not use AC. Both have a large amount of venting for incoming air.

Apr 30, 2013 10:11 AM ET

Edited Apr 30, 2013 10:13 AM ET.

Response to David Jones
by Martin Holladay

If you install a powered attic ventilator, there is one thing you can be sure of: your electricity bill will go up. That's because running a fan requires electricity. This increase in your electric bill is especially guaranteed in homes without AC (because there is no possibility that running the fan might decrease electricity devoted to AC). But research has shown that, even in a house with AC, powered attic ventilators increase homeowners' electricity bills -- they don't lower them.

You wrote, "Insulation can only slow and not stop the energy transfer. Regardless of what the insulation level is, there will be some level of energy transfer from the warmer attic to the cooler house. So everything else being equal, any house would benefit from a cooler attic."

Just because insulation slows down heat transfer rather than stopping it, doesn't mean that insulation isn't the best (and most cost-effective) way to prevent hot attics from making homeowners uncomfortable. Insulation is still the best approach. It's cheap and it doesn't require any electricity to run. If you are worried that your ceiling is still warm, the solution is simple: pile on a little more insulation.

You wrote, "Everything else being equal, any house would benefit from a cooler attic." That statement is only true during the summer. Believe it or not, many homeowners with powered attic ventilators leave them running all year long, because they forget to turn them off or the thermostat breaks. As a result, the fans increase the homeowners' energy bills during the winter.

From start to finish, powered attic ventilators are nothing but trouble. Homeowners should avoid them like the plague.

Apr 30, 2013 11:10 AM ET

Yes, of course more
by David Jones

Yes, of course more insulation with no moving parts and year round benefit is the better option. I should have made that clear.

Yes, as the insulation level increases the potential benefit from the fan decreases.

In some cases additional insulation on the attic floor is not practical. In those cases the options are to "sweat it out", cool the poorly insulated house with a large AC system, or try to limit the heat gain to reduce the temperature in the house. If a $100 attic fan (which consumes little electricity compared with an AC unit and contains far less embodied energy) can be part of a plan to reduce heat gain there may be a significant energy and environmental benefit compared to the AC option.

Can an attic fan offer some benefit? I think so under some very specific circumstances.

In a house with no ac running and the windows open there has to be some level of cooling benefit from lowering the attic temp with an attic fan. (You can certainly argue that the cost of electricity to power the fan outweighs the comfort benefit).

Yes, installing an attic fan raises the electric bill. If the attic fan increases the comfort level of the house, it may be worth it. If the attic fan keeps the house cool enough so that the owners leave the AC off, there is almost certainly an energy benefit.

Of course if the attic fan isn't working correctly and is running all year long that would be a significant problem. The same can be said for any mechanical equipment in the house that is not operating correctly.

Jul 17, 2013 11:06 PM ET

Unpowered vents?
by Jack Coats

Growing up in NE and West Texas, we used un-powered attic 'turbine' vents. Do those make sense these days? ... After moving to Houston roofers only wanted to remove the un-powered ones and put in powered ones, and I insisted they put them (and more) back in PLUS ridge and soffit vents. ... In the summers the vents spun day and night relentlessly. ... In North TX and OK (where wife grew up, and West Texas attic fans were great. In dry climates day to night temp difference was often 30 to 40F. So they worked great. In the Coastal plains and East Texas the humidity make the whole house fans not a reasonable solution. ... Sorry about the diatribe. The discussion was centered around powered fans and vents. How about the un-powered vents? They are inexpensive. Are they normally cost effective? (They are cheap and not hard to DIY install properly.)

Also, living in West Texas (think El Paso or Lubbock) 'swamp coolers' work great for all but about 2 days a year. Even running without water is good to act as a 'whole house fan' at night. My house in ELP had a roof top swamp cooler that was attached to the ducting in the house. (Yes, we insulated and sealed the vent (drained the water and cleaned/painted them inside in the fall. Checked them and replaced the filter pads in the spring when un-winterizing the systems.) When moving to ELP it wasn't initially natural to open the windows in the rooms you wanted to cool, and close them when the room is not in use.

Ahh, the differences from living in the NE and the SW (and the South) is really different on how we live with our environment. Saving energy and needing insulation is the same. Just the numbers and how to build reasonably changes.

Jul 18, 2013 5:02 AM ET

Response to Jack Coats
by Martin Holladay

Most attics are vented; for years, this was required by code. Recent code changes allow unvented roofs and unvented attics, but most attics in the U.S. are vented.

The usual way to vent an attic is with soffit and ridge vents. A ridge vent is basically a hole in your roof. Most experts don't see any advantages to turbine vents (or whirligig vents). When the wind isn't blowing, the devices are no different from any other hole in the roof; and if the wind is blowing, the devices can potentially depressurize the attic, sucking indoor air into the attic through ceiling cracks. That's undesirable.

Roofers hate them because they are a frequent source of roof leaks.

There is a basic fallacy to all attempts to move more air through an attic, since moving lots of air through your attic isn't the best way to keep your house cool. Ideally, you have sealed your attic floor (otherwise known as your ceiling) tightly against air leakage. Ideally, you have installed a thick layer of insulation on your attic floor. If you've done that, you don't have to worry about attic ventilation. Who cares what your attic temperature is?

It's a waste of effort to try to move more air through your attic. A better use of your efforts would be to perform air sealing work on the attic floor and to install thicker insulation. I'll repeat Bill Rose's words: "Ventilation is like a little boy who goes around the house looking for a job. ... He can do some things well, but can’t do anything really well."

Jul 22, 2013 11:22 PM ET

who cares how hot the attic is?
by David Jones

Martin, even if you have R-100 insulation and perfect air sealing there will always be an energy flow from a 150 degree attic to a 78 degree living space. There is just no way around that.

Regardless of the insulation level there is always a benefit (in the summer) to a cooler attic.

In a house with no ac running and the windows open there has to be some level of cooling benefit from lowering the attic temp with an attic ventilation fan -even with a less than perfectly air sealed attic floor. (Remember all make up air going to the air space is coming from the outdoors. Once the outdoor temperature is equal to or lower than the indoor air temp, air leakage in the attic floor isn't hurting anything at that moment.)

Your primary point is that an attic fan results in an energy penalty when it draws conditioned air through a poorly sealed envelope. That's a great point. To conclude from that point that ALL attic ventilation is ALLWAYS a bad thing surely can't be correct.

Jul 23, 2013 11:19 AM ET

Edited Jul 23, 2013 11:23 AM ET.

Response to David Jones
by Martin Holladay

You are correct, of course, when you note that insulation doesn't stop heat flow; it merely slows it. That said, if we examine the heat flow across your hypothetical R-100 attic insulation, we will find that the heat flow is so small that any heat flow -- even if the attic is at 160 degrees F -- into the conditioned space below will be so low that it doesn't represent much of an energy penalty. That's why the energy required to run an attic fan is wasted: the attic fan consumes much more electricity than any possible saving from reduced air conditioner run time.

So, yes: during the summer, there is always a benefit to a cooler attic. But it is senseless to run a counterproductive electrical appliance (the attic fan) if that appliance is just adding to your electrical load -- especially when the benefit is much smaller that the added cost from running the fan.

In your next case -- a hypothetical house without any AC -- does an attic fan make sense? Well, it's possible that it will lower the attic temperature a few degrees. But it will also add to your electric bill. Let's say it costs you $5 a month (or $35 a season) to operate. After 5 years, you've spent $175. I maintain that you would be much better off spending the $175 on air sealing work or added insulation rather than on running the fan.

Once you get your attic floor insulation up to the minimum code level (R-30 in Florida), it really doesn't matter very much what your attic temperature is (with the usual proviso: you shouldn't have any ducts or HVAC equipment in your attic).

Aug 3, 2013 11:55 PM ET

When HVAC equipment is in the attic
by Bob Barnum

Interesting article. I'm in the position where there is HVAC equipment installed in the attic of a South Florida home I just purchased. Moving the unit is not an option at this time.

While I am planning on insulating the attic properly, is there sense in me having an attic fan as well? There is currently a fan installed, but it's not operating so would need replacing.

Any advice would be greatly appreciated. Thanks!

Aug 4, 2013 5:43 AM ET

Edited Aug 4, 2013 5:45 AM ET.

Response to Bob Barnum
by Martin Holladay

You wrote, "I am planning on insulating the attic properly." That's good. If you are investing in new insulation, I strongly urge you to install spray polyurethane foam on the underside of your roof sheathing. If the work is done properly, this step will create a conditioned, unvented attic, bringing your HVAC equipment and ducts inside your home's thermal envelope.

For more information, see Creating a Conditioned Attic.

Aug 4, 2013 12:16 PM ET

Response to Martin Holladay
by Bob Barnum

Thanks for the advice Martin.

Am I correct in noting that a properly 'conditioned attic' would have zero vents in the roof?

Currently there is already a hole in the roof (for the attic fan). I imagine this hole would need to patched. Is that correct?

Aug 4, 2013 3:31 PM ET

Response to Bob Barnum
by Martin Holladay

Q. "Am I correct in noting that a properly 'conditioned attic' would have zero vents in the roof?"

A. Yes. That is correct.

Q. "Currently there is already a hole in the roof (for the attic fan). I imagine this hole would need to patched. Is that correct?"

A. Yes.

Aug 6, 2013 10:24 AM ET

green washed?
by Brite Niezek

I just had 2 attic ventilator fans installed in my 1979 home. When I called the attic fan company I'd intended to get 1 whole-house fan installed, however when the installer came out we realized that the upstairs of my home had a separate roof that wasn't connected to the lower level roof, so he said I would need an attic ventilator on the lower roof. The attic ventilator was much cheaper, so I opted to get two of these installed & see what happened. My home has no air conditioning & we routinely open all the windows in the evening & close them in morning. We have ceiling fans in 4 rooms that work wonders. I have very little insulation in my attic. It is only about 1 foot thick. This is something we intend to remedy prior to winter. I have a gas furnace & a gas hot water heater in the basement. I also have a gas pottery kiln. Another reason I opted for the attic ventilators is that I was hoping it wouldn't create a negative pressure situation that would disrupt my gas appliances. Currently I have the access panels to the 2 attics opened a few inches, hoping this will help draw hot air from my house into the attic so it can be ventilated. The ventilators are programmed to come on when the attic reaches 100 degrees & go off when it reaches 90. I can change this set-point, however I'd like to avoid having the ventilator run constantly. I was just looking for a solution to vent the night-time heat gain that piles up throughout the house between the hours 5pm & 9pm. If anyone has any comments to this, feel free to pick it apart. I realize I may have gotten green-washed & duped by the cheaper price! I'm just keeping my fingers crossed that my particular home & conditions might allow the attic ventilators to be sufficient.

Aug 7, 2013 7:57 AM ET

Response to Brite Niezek
by Martin Holladay

It sounds like you wanted to install a whole-house fan, but ended up with two powered attic ventilators instead.

I don't think that the fans you have make any sense, especially if they are controlled by attic temperature. Instead, you want a whole-house fan that is run when the outdoor temperatures are low -- not when the attic is hot -- and you want that fan to have a wall-mounted switch that you can control.

Ideally, your installer would have found a way to increase the vent openings in your attic so that a whole-house fan could be installed. If it is impossible to add vent openings to your attic, then your house is not a good candidate for a whole-house fan.

Again, I recommend that you disable the powered-attic ventilators.

Aug 7, 2013 5:19 PM ET

Response to Martin
by Brite Niezek

hi, thanks. well you've confirmed I definately made the wrong choice. Since I've already wasted $1050 for the two attic ventilators, do you think there's a way I could extend the wiring & install an indoor manual controller on the two ventilators & open the access hatch into the attic fully & just turn the ventilators on once it has cooled off outside? I guess it would be like having an really expensive roof window with a box fan in it. I'm inclined to call the company & see if they can un-install them & patch the roof back up, however I imagine that would cost another $700 or so.

Aug 8, 2013 6:31 AM ET

Response to Brite Niezek
by Martin Holladay

Q. "Do you think there's a way I could extend the wiring & install an indoor manual controller on the two ventilators & open the access hatch into the attic fully & just turn the ventilators on once it has cooled off outside?"

A. Yes, an electrician could perform the work you describe. If you do that, your fans will operate better than with the current controls. But it's too bad that you don't have a louvered opening in your ceiling instead of a hatch that needs to be opened and closed.

Aug 15, 2013 5:43 PM ET

Attic Duct Sweating
by Micah Morgan

My house was built around 1950, and has been added onto several times. It is a low slope ashpalt shingle roof with deteriorated batt attic insulation. There was at one time a whole house fan but at some point it was replaced with an air handler in the attic with hard duct. The insulation on the duct is in very bad shape. The roof has soffit vents, but no other form of venting. I live in Louisiana and this summer the attic has gotten to be over 140 degrees on a regular basis. This has caused my AC ducts to sweat causing water damage to the ceiling. I priced getting open foam insulation at the roof line, but that solution is price prohibitive to me at the moment. I am looking at installing an attic ventilator fan, radiant shielding, new ducts and blown in insulation to combat the problem. This will be significantly cheaper than insulating the attic line. This article strongly recommends against the attic fan, but the only solutions presented to problems such as mine are very expensive. If I can't insulate the attic line, and I shouldn't install attic ventilation then what is the alternative.

Aug 16, 2013 7:35 AM ET

Response to Micah Morgan
by Martin Holladay

Clearly, you need to address your immediate problem -- sweating ducts. But that doesn't mean that you need to rush out and buy a powered attic ventilator.

You admit that "The insulation on the ducts is in very bad shape." In order to prevent condensation from forming on your ducts, the most important step you can take is to fix your deteriorated ducts and deteriorated duct insulation. That should solve your immediate problem.

First, inspect the existing ducts. It sounds like you have galvanized ductwork. Strip off all of the deteriorated insulation and throw it away. Make sure that there are sheet-metal screws in each duct seam. Make sure that each duct seam is sealed with mastic. Then install new vinyl-backed duct insulation with taped seams. That should solve your problem.

If the existing ducts are in very bad shape and aren't worth saving, you could remove and discard all of the exiting attic ductwork are replace the ducts with new insulated flex duct.

Good luck.

Aug 29, 2013 1:43 PM ET

old vs new whole house fans
by Larisa Solonenko

Our fan is probably 20+ yo. It is noisy, but pretty powerful, and we try to remember to take advantage of the drop in temps at night. How do the old fans compare to the new ones in efficiency? Do the new fans come with an outside temp sensor to coordinate with the AC? Are they noticeably quieter? Is it worth replacing a functioning fan?

Aug 29, 2013 1:58 PM ET

Edited Aug 29, 2013 1:59 PM ET.

Response to Larisa Solonenko
by Martin Holladay

Q. "How do the old fans compare to the new ones in efficiency?"

A. I don't have any specific information on this topic, although there is a good chance that a newer fan -- especially one from Tamarack -- will be more efficient than the fan you have. However, the difference in efficiency probably isn't enough to justify the investment in a new fan.

Q. "Do the new fans come with an outside temp sensor to coordinate with the AC?"

A. Not that I know of.

Q. "Are they noticeably quieter?"

A. Yes -- especially if you buy one of the Tamarack models.

Q. "Is it worth replacing a functioning fan?"

A. Not if you think that the investment will have a payback in terms of efficiency. But if your fan is so noisy that it's driving you crazy, and you can afford a new Tamarack fan, go ahead and buy one.

Sep 8, 2013 4:36 PM ET

Whole House Fans for sealed attics?
by Mike Baldinelli

Newer construction calls for some attics to be completly sealed, no vents. The space is more friendly for air handlers located there. The question is if the house, including the attic, is very well insulated is there a application for an attic fan. (It seems to me there is) But a sealed attic has no vents to dump the fan air from the interior of the house. So I guess there would have to be a duct from the fan discharge through the roof. I guess it would have to be pretty big, too. Any thoughts.

PS. My home is in the Sacramento CA area.

Sep 9, 2013 8:59 AM ET

Response to Mike Baldinelli
by Martin Holladay

If your house has an unvented conditioned attic, then your attic is just another room in the house -- just like a bedroom in a finished attic.

Several manufacturers make whole-house fans designed to be installed in insulated roofs. Here is a link to some models that are not necessarily optimized for air sealing:

Here is a link to a Tamarack fan that is optimized for air sealing (and that provides high-R shutters controlled by a motorized operator):

Sep 9, 2013 1:54 PM ET

Attic fans and mold
by Hannah Mills-Fee

Hi, Martin,

Thank you for this informative, well-written article. My husband and I are closing on our first house next week (we live outside Chicago), and one of the red flags in our home inspection was the non-working attic fan. The inspector also found a bit of mold in the attic, which he deduced came from the non-working attic fan, his logic being that if the fan worked, there would be more air circulation and no mold would be able to grow. But after reading your article, I don't know that we need an attic fan because we should have a whole house fan. So that being said, how do we prevent more mold from appearing (and deal with the current mold) if we do not install the attic fan? Does the whole house fan allow for adequate air circulation even in the winter to prohibit mold growth?

Thank you.

Sep 9, 2013 2:43 PM ET

Edited Sep 9, 2013 2:45 PM ET.

Response to Hannah Mills-Fee
by Martin Holladay

Most attics are dry and mold-free. If you see mold in an attic (and you know that you don't have a roof leak), it's almost always a sign of a leaky ceiling. Warm, humid indoor air is leaking out of your house and into your attic through ceiling cracks.

In many homes, this type of mold is associated with a wet basement or a wet crawlspace.

If you know how to inspect an attic for air leaks, the first step is to spend a few days in your attic sealing air leaks. For more information on this task, see these articles:

Energy Upgrades for Beginners

How to Insulate and Air Seal an Attic Hatch

How to Air-Seal an Attic: Introduction

If there are signs that you have a damp basement, you'll need to address those issues. The first steps to addressing a damp basement are to make sure that you have roof gutters connected to conductor pipes that carry the roof water to a location far from your foundation, and to adjust the grade around your foundation as necessary so that the soil slopes away from the foundation in all directions.

If those steps don't work, you may need new footing drains.

Good luck.

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