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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 GBAGreenBuildingAdvisor.com, 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

51.
Sep 27, 2013 11:27 AM ET

Powered Attic Ventilators
by Richard James

Martin,
My excursion into PAV's has yielded some important side benefits, not the least of which was discovery of this site. First things first. I live in low humidity inland CA and I needed to deal with a very hot attached garage. My workshop is out there, along with a freezer, washer dryer etc. I did not want to cut a hole in the ceiling for an exhaust vent as this opens up code issues (fire dampers) that complicate the design. There are code complaint exhaust fans available w/dampers but displacement of the air has to be addressed, especially since I have a gas WH in the garage, as most CA homes do. On my home this meant cutting inlet vents in the wall or garage door. Wall option is expensive, and of course ugly. I did not want to cut a hole in my garage fire envelope in any case, code or not. Additionally, drawing 100 degree air into a space does not cool it. Ergo, buy a high efficiency (40 watts, 15 cents a day) gable mounted PAV to reduce the radiant broiler effect on the ceiling of my garage. I have generous soffit vents (but no ridge vents) in this space. As it happens, there is a 6 sq foot cutout in the OSB boundary between my garage and house attic so the garage exhaust drew a bit of the hot air via intermittent soffit vents. Not sure if this cutout is code but the house passed inspection. My findings:
1. As you suggest, lay persons are jumping into PAV's without doing proper research and the on line vendors are not helping with this problem. They want to sell you a product, first and foremost. This applies to whole house fans as well. Creating a vacuum in your home can cause serious problems. For instance, filtering dirty air through the edges of your nice carpet and wall outlets. Air quality, controlling the inlet source, is a huge concern. These fans should be supplied and installed by licensed contractors. If you are required to open windows to create displacement you are adding a human element that is not acceptable. Position switches on windows or automatic inlet dampers, something is needed.
2. The high efficiency PAV I purchased from a reputable CA company worked for me. My garage was much cooler this summer and my home AC bills dropped, with caveats. Part (maybe all) of the cost savings were due to sealing air leaks (thanks GBA!), weather variables, and behavioral changes. Will never know. I plan on upgrading my R38 fiberglass batt insulation and attic venting (soffits and ridge vents) but that is $$$ with a long payoff. My heating and cooling bills in my small house are manageable for now.

Bottom line, for my unique circumstances the PAV works, a bridge to the next step. I think I will still need it at times to reduce garage temperatures regardless of insulation and passive ventilation improvements.

Engineering question: How much pressure is developed in a hot attic? Regardless of ventilation design I would guess that X relative temperature would lead to Y pressure. I run my PAV when it is hot only and I get a sense that all I am doing is neutralizing the pressure, and reducing reverse stack effect. Your input will be appreciated.


52.
Sep 27, 2013 11:51 AM ET

Edited Sep 27, 2013 11:54 AM ET.

Response to Richard James
by Martin Holladay

Richard,
Your account raises several questions. I assume that your garage is not conditioned. Yet (if I understand correctly) you have R-38 insulation on your garage ceiling.

If you do, indeed, have R-38 insulation on the garage ceiling, the temperature of your garage attic should be irrelevant. There certainly shouldn't be enough heat flow downwards through the R-38 ceiling to cause any problems in your garage -- certainly not enough to merit the installation of a powered attic ventilator.

There are many possible explanations for this puzzle. One possible explanation is that your R-38 insulation is sloppily installed. Another possible explanation is that you are really battling heat sources inside your garage, including your water heater, your freezer, and your clothes dryer -- and that your powered attic ventilator is a placebo rather than an appliance that succeeds in altering the temperature of your garage.

You asked about the pressures exerted by the stack effect. They are relatively low -- usually on the order of 1.5 to 4 pascals.


53.
Sep 28, 2013 11:29 AM ET

Martin, I took another look.
by Richard James

Martin,
I took another look. The plans called for R38 in all ceilings, R19 in walls and crawlspace. The attic over living space is indeed R38, 10-12 inches thick and well placed. Garage batts look thinner, perhaps R19 and not as well placed. Some day I'll make a compromise. Move the R38 batts to the garage attic. Seal the living space attic floor and implement passive ventilation improvements as Joe Lstiburek suggests, then blow in cellulose. The passive ventilation improvements are the biggest expense and it makes sense to go all in at once. No point in the insulation alone, once cellulose is in it is a pain in the butt working around it. The air leaks that I have addressed, and still working on, are already paying dividends. Regarding the heat source, no question it is the ceiling. Before the fan I could put my hand on the ceiling and feel the heat. Since installing the PAV no heat to touch and the garage is more comfortable. Other factors have not changed. All other comments are well noted and thanks for your support.


54.
Sep 29, 2013 4:11 AM ET

Response to Richard James
by Martin Holladay

Richard,
OK. During the summer you can put your hand on your ceiling and feel the heat. You investigate, and discover R-19 insulation that is "not well placed."

The normal solution to the problem you describe is to install adequate insulation -- and to make sure that it is installed well. If, instead, you choose to install a fan and pay for the electricity to run it... well... it's your choice, but that decision doesn't make any sense to me.


55.
Feb 17, 2014 6:33 PM ET

Whole House Fan Installation
by Fan Installation

Newer construction calls for some attics to be completely 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.


56.
Feb 17, 2014 7:32 PM ET

Response to Fan Installation
by Martin Holladay

As I noted in previous responses to questions on this issue, you can buy a whole-house fan that is designed to be installed in an insulated roof. Here, once again, are some links:


57.
Apr 28, 2014 12:35 PM ET

Bought a house with a whole house fan
by Penny Dorneman

I bought a ranch house last year in Central PA. The previous owners had installed a whole house fan, but never insulated the attic beyond 6" batts between the joists, and nothing under part of the attic. After our first winter's oil bill, it's pretty clear that insulation is more critical than the fan -- particularly for our humid location. My contractor wants to use blown-in insulation. That seems a bad move for using the fan. What are my options?


58.
Apr 28, 2014 1:42 PM ET

Response to Penny Dorneman
by Martin Holladay

Penny,
Blown-in-place insulation like cellulose is compatible with a whole-house fan. However, you will probably need to build an insulation dam to surround your fan so that the insulation doesn't interfere with fan operation.


59.
Jun 29, 2014 9:31 PM ET

Do you have actual references?
by Green Asalep

You state: "Several studies show that even in a house with a tight ceiling, a powered attic ventilator uses more electricity than it saves." What studies? Where? Real or imaginary? Your article comes across as biased. In what year were those studies written? In what location? Electricity costs vary greatly over time and location.
And what's what the "tight ceiling" claim over and over again? Does everyone you know live in a dilapidated log cabin? A properly maintained home doesn't have cracks in the ceiling.


60.
Jun 30, 2014 5:51 AM ET

Edited Jun 30, 2014 5:53 AM ET.

Response to Green Asalep
by Martin Holladay

Green,
Q. "What studies? Where? Real or imaginary?"

A. My article provides several links to studies by researchers, including researchers at the Florida Solar Energy Center (FSEC) and Advanced Energy in Raleigh, North Carolina. Of all of the links in the article, I would start with this one from FSEC: .

That paper notes, "Data measured at FSEC and elsewhere show that attics with nominal natural ventilation and [at least] 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."

Q. "And what's what the 'tight ceiling' claim over and over again? Does everyone you know live in a dilapidated log cabin? A properly maintained home doesn't have cracks in the ceiling."

A. In fact, energy raters and home performance contractors who do blower-door testing know that it is very rare to find a house with a tight ceiling. Most U.S. homes have lots of ceiling leaks: at the attic access hatch, at the plumbing vent pipe, at wiring penetrations, at duct penetrations, around the bathroom exhaust fan housing, and along the cracks between partition top plates and partition drywall. It's sad, but it's true.


61.
Sep 11, 2014 5:26 PM ET

Whole House Ventilation
by J Freeman

I recently moved into a three-year-old manufactured home with a switch labelled "whole house ventilation." I am not sure what this is for. I have emailed the manufacturer of the home asking for any information they can give me. I'm not sure whether this term refers to what you call whole house fans or something else.

I live in Arkansas; our summers are hot and humid and our winters not too cold. I'm hoping to learn whether I should be using this ventilation system in conjunction with my air conditioner, or if it is only useful for spring and fall when I can open my windows, or whether it should never be switched on in my climate. Can you advise me?

Thanks.


62.
Sep 12, 2014 6:35 AM ET

Response to J Freeman
by Martin Holladay

J. Freeman,
It's hard to know what a switch labeled "whole house ventilation" controls. Options include:

1. A whole-house fan designed to lower indoor temperatures at night when the air conditioner is off.

2. An HRV or ERV that provides fresh ventilation air to the indoors in all seasons.

3. An exhaust fan installed as an exhaust-only ventilation system.

One way to try to figure it out is to turn on the switch on a quiet day and listen for a motor coming on.

If that doesn't work, climb up in your attic and look for a whole-house fan. Look in your basement or mechanical room for an HRV or an ERV.

If all else fails, hire an electrician to come to your house and figure out what the switch is hooked up to.


63.
Jan 12, 2013 12:06 PM ET

Edited Jan 12, 2013 12:08 PM ET.

Insulation vs. lowering attic temps
by David Johnson

It is not an accurate statement to say that if you have good insulation, you don't care what the attic temperature is. Heat conduction from the attic to the house is proportional to the temperature difference divided by the R-value. So, for example, going from R-30 to R-38 insulation is the equivalent of lowering the attic temperature by 15 deg F (assuming starting point of 150 F attic temp and 78 F interior temp). I am not making conclusions as to the relative difficulty of either of those approaches, but a quick look says that to insulate 2000 ft^2 of attic space to R-38 vs. R-30 has a marginal cost of about $375. So, if you can lower the attic temperature by 15 F for under $375 (with no recurring energy penalty per a powered attic ventilation fan), that would be the more advisable route.

I am not arguing in favor of powered attic ventilation fans, but I just favor some clarity with regards to the general topic of attic ventilation (passive or wind/solar driven) vs. insulation.

Should also note that the user should do his own calculations based on his climate. Since for example, insulation is helpful year round whereas the attic ventilation is only favorable during air conditioning season. So, the trade-off varies depending on your climate of course.


64.
Apr 19, 2013 5:15 AM ET

Any advice on cooling a house in Kauai?
by Greg Calarco

Martin,

We moved to Kauai about a year ago. Most of the year the temperature was fine, but for about 3 months in summer, the house temperature became almost unbearable. Our house is about 2000 square feet. It is one story with an attic space. The attic, apparently like most on the island, doesn't have any insulation. We have no central AC, but do have 3 window AC units that can help to cool those individual rooms. Also, like many of the homes here, our house isn't built directly on a foundation. It is raised up about 4 feet on stilts. I don't know if that matters, but it seems it might, with the air being able to circulate under the house as well.

During those hottest 3 months last year our house heated up quickly in the mornings and by early afternoon it was often hotter in the house than it was outside on the decks. It would stay extremely hot inside until almost midnight, despite us having (screened) doors and windows open and ceiling fans going. We tried completely shutting the house up during the day, as well as keeping the house completely open during the day. Neither seemed to help. The only way be reasonably comfortable was to be in one the of rooms with the AC and have it running. Even at 10 or 11 at night, when it was comfortable outside, turning the AC off in those rooms meant that they would quickly heat up and become very uncomfortable.

Afterwards we were told by a friend that the heat problem was most likely caused by heat coming from our attic and that we should get more attic vents and potentially an attic fan. After reading your article it looks like we shouldn't get the powered attic ventilator that our friend was talking about. But, in a humid climate it sounds like the whole house fan wouldn't work that well either. What are your suggestions for the best way to keep the house from turning into a sauna this summer?

Thanks,

Greg


65.
Apr 19, 2013 5:47 AM ET

Response to Greg Calarco
by Martin Holladay

Greg,
The first step would probably consist of installing some attic insulation. That will reduce the transfer of heat from your hot roofing to the interior of your home.

If you think you might be able to get away without any air conditioning, you might try installing a radiant barrier on the interior side of your rafters. However, if you expect that you will need to continue running your air conditioners, insulation makes more sense than a radiant barrier.

If you have any east-facing or west-facing windows that get direct sunshine, you should check whether these windows have low-solar-gain glazing -- that is, glazing with a low SHGC. If they don't, you might consider replacing the windows that get the most sun with new windows with low-SHGC glazing, or installing exterior shading devices.

Finally, it's worth pointing out that there are many climates where three months of air conditioning is totally normal.


66.
May 20, 2013 4:36 PM ET

Attic Ventilation Fan
by Kaye Kittrell

All I know is when ours worked, it kept the second floor a tad cooler. We have no AC, heater is in attic, in old, renovated ('81) house. The second story is over back half the house and the original front half of the attic butts up against our clothes closets and the master bedroom. So it gets HOT up there, even here not far from the ocean in L.A. It's 90° in my clothes closet in hottest part of summer. Without spending a fortune I was just going to replace the ventilation fan in the attic. Do you have a recommendation for fans? The heating svc company wanted to put one in but seemed like a lot of money. Can an electrician do this job?f Thanks so much!


67.
May 20, 2013 5:32 PM ET

Response to Kaye Kittrell #66
by Dana Dorsett

Verifying that the attic insulation is still there would be an important first step. If it's just 6" of blown fiberglass or R19 batts (common in 1981), an overblow of 3-6" of cellulose would improve the performance considerably, since low-density fiberglass is fairly translucent to infra-red (radiating heat) coming down from the hot roof deck.

If the partition wall between the attic and bedroom/closet is not currently insulated, installing R13 batts in the cavities then an inch of foil-faced polyiso on the attic side will do quite a bit toward reducing the unwanted heat gained through the wall from the attic side.

If that isn't cutting it, painting the underside of the roof deck with a low-E paint or installing perforated aluminized fabric radiant barrier on the underside of the rafters in the attic will knock the attic temps back a bit without using power or driving air infiltration.

For a ventilation approach, a self contained solar attic ventilator can purge heat from the attic without the cost of wiring it up and powering it from the grid will do some good:


68.
Jun 22, 2013 11:51 PM ET

Edited Jun 23, 2013 12:21 AM ET.

Cooling attic from subarea
by Gary Seifer

I live in a relatively benign climate north of San Francisco, moderate humidity but summer days often exceed 90 and, rarely, 100 degrees. I created 2 additions and extensively remodeled our now 1,800 square foot single story rancher in 2010 and 2007. I am a general contractor and all systems were upgraded to modern and very energy efficient units.

Note: The bottom 2 paragraphs describe the essence of the problem

As part of this construction I installed a small, 350 cfm, exhaust fan in the sidewall of a skylight well which was both unobtrusive and the high point of the house. It worked ok, but slowly, so last year I installed a 3,200 cfm house fan in the same location. The evenings and mornings here can cool down to the 50's even if the days are in the 90's, which is one of reasons grapes grow so well here. The whole house fan worked great in rapidly reducing the interior temperature of the house so that we rarely require the a/c to kick on. There was one large flaw with the fan though, while the large fan duct was insulated and it had a damper on the end that exhausted to the attic, the heat build up in the attic caused heat to flow back into the house and would noticeably warm the house during the day. I thought I had made a large mistake, but I mostly fixed that problem by wrapping the duct with more insulation and by modifying the damping system creating new damper flaps insulated with foam sheet.

One mistake in the remodel was installing the furnace and duct work in the attic and heat is transferred from the attic to the ducts and then to the house. I want to address this issue so I am exploring solutions. Today was low 80's and a thermometer inserted one of the ducts registered 95. I have not tested attic temperature. Attic has a light colored roof and is well ventilated with extensive eave venting, eyebrow vents, gable end vents and two turbine vents. Even with this, we do not often use the a/c, the ceiling is insulated with R-30 between the joists with an additional R-13 or R-19 run across the ceiling joists giving a total of about R-45.

I have wrapped the ducts with additional insulation which helped a bit, but I want to cool the attic by 10-25 degrees if possible. Since the discussion here recommends against an attic venting fan, I have another idea for consideration. There is a dead spot in the corner of a closet where I could create a 100 square inch chase connecting the subarea with the attic and install a 500-800 cfm in line fan. This would draw cool air from the well vented subarea and pull it into the attic hopefully displacing the hot attic air and thereby cooling the attic without creating negative pressure. I would probably distribute this air with over size ducts so this cool air can reach most of the attic space. Most of these fans require an 8-10" duct.

What do you think, I have not seen this solution discussed here? One thing that comes to mind is fire damping since I will be creating a chimney. Everything is easily accessible including power, I would control the fan with a switch and/or a thermostatically controlled switch in the attic.


69.
Jun 23, 2013 5:04 AM ET

Response to Gary Seifer
by Martin Holladay

Gary,
What is a "well-vented sub-area"? Is it a crawl space? Basement? Garage? Shed? Outdoor shady spot under your grape arbor?


70.
Jun 23, 2013 8:40 AM ET

Crawl space, typical !8"
by Gary Seifer

Crawl space, typical !8" clearance from joists to grade, no moisture problems, a lot of foundation vents.


71.
Jun 23, 2013 12:49 PM ET

Edited Jun 23, 2013 12:55 PM ET.

Response to Gary Seifer
by Martin Holladay

Gary,
I'm not in favor of any attempt to use fans to try to change the temperature of the attic -- whether the fans are used to depressurize the attic (as typical powered attic ventilators do) or to pressurize the attic (as you propose).

Either approach can cause unexpected side effects -- in the case of your proposal to pressurize the attic, you might end up forcing attic air (or crawl space air) into the home through ceiling cracks.

Moreover, if you use a powerful fan for your proposed pressurization, the fan will quickly remove all of the cool air from the crawl space, and the fan will be pressurizing the attic with hot air drawn from unknown sources, but probably from outdoors.

The best way to limit heat transfer across the thermal boundary (the attic floor, in this case) is to make sure that the thermal boundary is air sealed, and that the insulation is very thick.

There are many reasons to doubt that your proposed fan installation will change the temperature of the attic enough to save any of the energy required to operate your air conditioner. Moreover, it is certain that operating the new fan you want to install will increase your electricity bill, because fans use electricity.


72.
Jun 23, 2013 2:21 PM ET

Thanks for the response, i am
by Gary Seifer

Thanks for the response, i am still interested in cooling the attic and I might try it after monitoring attic temperatures before and after to see if it makes much of a difference. If I do it, I figure a few mornings and $200-300 since I can do it myself. Not a massive loss if I abandon it. A Tjernlund 6" fan is rated at about 530 cfm and draws 125 watts, an 8" pushes 670 cfm and draws 210 watts so I am not too concerned about electrical operating costs it if increases the comfort of the house and reduces the a/c load.

Again, I am most concerned by the ducts in attic space warming up and transferring heat to interior. If not for them, we would rarely need a/c. Yes, a mistake to install the HVAC system in the attic, but given job restraints I would probably do it again but wrap the ducts with more installation at the install. Right now I closed the registers to restrict hot air from entering the system and I shut off the system to prevent an accidental firing of the forced air system, but I realize this is not a long term solution because of the risk of the system turning on with the registers closed.

Per your warnings I will better seal ceiling penetrations, smoke detectors, electrical boxes etc. to reduce chances of positive pressure pushing air into the house. Wiring and plumbing penetrations of the framing were generally not caulked, but since there are two layers of insulation across the the attic, I hope that will be enough to restrict penetration of the ceiling envelope and that cooler air forced into the bottom of the attic will force warmer air to be pushed through the higher eyebrow and gable end vents rather than through the any penetrations in the framing, the path of least resistance.

If the attic is 115-140 degrees and I push 80-100 degree exterior air into the attic, I will monitor to see how much of a difference it makes. Now all I have to do is satisfy the WAF, the Wife Acceptance Factor. If I decide to continue, I will inform you of the results, it should be fairly simple project. All this and trying to massively conserve water and reuse water where possible. Ah, life in California.


73.
Jun 23, 2013 5:57 PM ET

Other Solutions
by Gary Seifer

The mind begins to reel as other solutions begin to present themselves. On the theme of creating positive pressure in the attic and displacing hot attic air with cooler exterior air, ventilation fans designed for foundation ventilation could possibly used in place of 22.5 inch eave vents to push in exterior air and thereby push out attic air through high gable vents and eyebrow vents.

The problem is not the hot attic but the ducts in the attic. So I am beginning to search for motorized registers that would open if the thermostat starts the a/c or furnace. With all the home automation occurring, a plethora of products are beginning to emerge, but so far what I have found are registers controlled in individual rooms by temperature and humidity. Since I would need only 5 or 6 of these, that seems like an optimal solution if it can be easily retrofitted using a low voltage system and I can find something relatively affordable and controlled by the thermostat rather than these products designed to zone a system. There are even some Kickstarter ventures that have products on the verge of release.


74.
Jun 24, 2013 4:04 AM ET

Response to Gary Seifer
by Martin Holladay

Gary,
I'm not sure where you are thinking of installing these "motorized registers." It sounds as if you intend to install them in the supply ducts that are located in your unconditioned attic, so that your air conditioner can send cold air to your attic. If that's what you are thinking, it's nuts.

That would be equivalent to trying to air condition the outdoors. Talk about wasting energy!

There are standard solutions to the problem you describe (ductwork in unconditioned attics), and the solutions don't involve motorized registers. Here are links to two articles you might want to read:

Keeping Ducts Indoors

Creating a Conditioned Attic


75.
Jun 24, 2013 12:47 PM ET

Sorry for confusion
by Gary Seifer

I am considering, if such a product is available, installing motorized registers on each duct where it enters the room so they are normally closed, but open when the air conditioning system starts. When closed, they would prevent warm air in the ducts from entering the room. but open to allow a/c air to cool the room. Right now the registers are closed manually and I will reopen when I next turn on the a/c at the thermostat, but that is not a long term solution. Yesterday the temperature in ducts reached 100 degrees when the exterior temperature was 88, so I realized the problem was worse than I thought. I noticed the difference in room comfort once I closed the registers. All this being said, the house is quite efficient and we rarely use the a/c, I am just trying to increase the performance of the house at the margin. Conditioning the attic or rerouting the duct work is completely impractical or way too costly in this instance.

I have seen motorized registers that operate as part of a zoning system, I need to find some, that will open as the a/c system starts. Duct dampers that can be installed at the boot in the ceiling could also work. All these seem to operate on 24 volts. The advent of smart and automated homes are increasing the availability of these products, I just need to find the right system and products, I only need 6 of these smart registers or duct dampers. Thanks for your help, my wife thinks I am creating a problem where none exists.


76.
Jun 24, 2013 1:34 PM ET

Response to Gary Seifer
by Martin Holladay

Gary,
Thanks for clarifying your intent. While your plan makes more sense than my earlier interpretation, it's still not as good as the two conventional solutions to your problem (either creating an unvented conditioned attic, or increasing the thickness of the duct insulation).


77.
Jun 24, 2013 4:09 PM ET

Increasing duct insulation is
by Gary Seifer

Increasing duct insulation is part of the plan, I have already wrapped about half of the duct work, which means I have done the easily accessible part. For the benefit achieved, creating an unvented and conditioned attic is way too costly and complex. I have lived with the current flawed system for 8 years and it is not that bad, mostly because of our moderate climate where temps rarely exceed 90. Trying to create increased energy and water efficiency at the margin.


78.
Jul 3, 2013 11:01 PM ET

Whole House fan with attic fans
by David Miller

Hi Martin,

Thanks you for the excellent article and thoughtful responses to questions. Here's one for you. I live in Southern California where it gets warm during the day and nice and cool at night. We have a 2-story 2600 sq ft home, and installed the EXCELLENT Tamarack Ghost along with two attic fans, thermostat controlled, one blowing air out of the the attic, and one blowing air in (against the advice of my electrician, who wanted both blowing air out). My thought was that to move air through the attic without drawing air up from the home, we wanted to create cross-ventilation through the attic. After reading your article, I'm wondering how the Tamarack would work if I had NOT installed the attic fans. How would all of that warm air ventilate out of the attic? I picture the Tamarack blowing hot air up into the attic, which joins the cross-ventilation and carries it out of our attic. We do have very low electric bills, despite the attic fans running from noon to midnight on hot days (thermostat-controlled) and the Tamarack running all night as well. But the house is incredibly comfortable. I also have a well insulated attic floor. Thoughts on my Tamarack + an in- and an -out attic fan solution? Thank you.


79.
Jul 4, 2013 5:58 AM ET

Response to David Miller
by Martin Holladay

David,
If (a) your house is incredibly comfortable, and (b) your electric bills are low, and (c) you have a well insulated attic floor, then it doesn't sound like you have a problem.

I think there is no need to run the fans you have installed to exchange air in your attic. You aren't living up there -- why waste electricity to move air around in your attic? Just use the whole-house fan when nighttime temperatures are cool and daytime temperatures are hot -- that's all you need.


80.
Jul 11, 2013 12:10 PM ET

Open flow on both gables
by Brian Chaszar

Greetings Martin --

I recently bought an electric attic ventilation fan in order to cool my attic space. I'm in Montana, do not have AC, but feel that it is incredibly hard to cool the interior of my house after the outdoor temperature cool down in the evening. My thoughts on this were that there is a heat load sitting on my ceiling (in the attic) keeping everything hot. So, the theory behind the attic fan was to be able to turn it on as the outdoor temps cool in order to clear the hot air out of the attic space. This fan would be installed on one end of the house, while the opposite end already has an open, screened vent -- thus air would be drawn in from one end and expelled out the other. With the screened vent opposite the fan, it seems I wouldn't get all that much draw from the interior of the house, and if I did, it would be cool outside air anyway. Blowing in additional insulation is not an option at the moment financially. Does this sound like an appropriate and effective use of an attic fan, or will I still be wasting my time and money? Thank you!


81.
Jul 12, 2013 5:47 AM ET

Edited Jul 12, 2013 5:49 AM ET.

Response to Brian Chaszar
by Martin Holladay

Brian,
Once more, I will repeat one of the basic themes of this article: If your ceiling is hot during the summer, then your insulation layer is too thin. Fix the insulation layer.

Here is a corollary: The money you spend on installing and operating powered attic ventilation fans will never be recovered in energy savings. It's money down the drain.

If you were thinking about spending $300 on powered attic ventilators, and another $30 per year in electricity to run them, take the $330 you would have spent during the first year and use it to buy $330 of insulation.


82.
Aug 15, 2013 10:28 PM ET

Powered attic fan with no AC?
by K Costa

I can't find a definitive answer as to whether or not powered attic fans are a good idea if you don't have AC or a whole house fan. Are they worth it? Will they cool the home? I've found two sites that recommend them if you have no AC and others saying there's almost always a bad idea. I read a little about back-drafting and other things of the sort. I also had a thought about adding an intake valve in the attic sheet rock access panel (have no idea if that's good idea or not) so the powered attic fans can draw from the house (like a mini whole house fan). Any advice is much appreciated! Thanks.


83.
Aug 16, 2013 4:20 AM ET

Response to K Costa
by Martin Holladay

K.,
Q. "I can't find a definitive answer as to whether or not powered attic fans are a good idea if you don't have AC or a whole house fan. Are they worth it?"

A. No, they are not worth it. If you have installed enough insulation on your attic floor, a hot attic won't make your house hot. If your insulation is thin, invest in insulation, not a powered attic ventilator.

Q. "Will they cool the home?"

A. See the answer to your first question. If you want to cool your home at night, when the outdoor air is cool, then you need a whole-house fan, not a powered attic ventilator.

Q. "I read a little about back-drafting."

A. Good. Then you know that a powered attic ventilator can pull dangerous fumes from a water heater into your house.

Q. "I also had a thought about adding an intake valve in the attic sheetrock access panel."

A. That would increase the rate of backdrafting, not decease it, so it is a bad idea. I certainly hope that your "sheetrock access panel" has insulation and weatherstripping. If it doesn't -- fix the panel.

Q. "Any advice is much appreciated."

A. I suggest that you read the article on this page and follow its advice.


84.
Aug 19, 2013 10:08 AM ET

Tamarack fan recommendation
by Tyler Davis

Thanks for the article. I agree with the theory and most of the recommendations. I am interested in replacing my effective, but noisy, whole house fan of 1970s vintage.

I spent some time researching the Tamarack HV1000 after the author's recommendation. The rule of thumb in the article (CFM/3) suggests that at 1000 CFM, the HV1000 is only adequate to cool 333 square feet of building space. This means it can only cool 1-2 rooms, not a whole house. If the response is to "just run it longer" than what is the purpose of the rule of thumb, and where does the energy cost savings come from when running 3x longer than a traditional 3000 CFM fan?

I looked on Amazon and some other review sites and there are a lot of negative reviews of the HV1000. Complaints about the noise level, inadequate airflow, cheap plastic components, malfunctioning remote controls, etc.

I am curious if the author has personally installed or used the Tamarack products, and if he has an affiliate relationship with that company?


85.
Aug 19, 2013 12:28 PM ET

Response to Tyler Davis
by Martin Holladay

Tyler,
Designers of whole-house fans have to balance competing goals when optimizing the specs. Large fans that move a lot of air have an obvious advantage -- they change the air in the house at a fast rate -- but several disadvantages: they tend to be loud, they use a lot of energy, and they are so large that it is hard to design a motorized insulated weatherstripped door to seal them.

The Tamarack isn't for everybody. If you want a big fan, you can buy one.

The Tamarack fans move fewer cfm but use less energy and seal tightly. Take your pick.

Tamarack fans have been around for years, and have a good reputation. It is certainly possible that the manufacturer has changed the specs on these fans or cheapened the way they are built, so (like you) I welcome reviews from GBA readers.

To answer your questions: (a) No, I have never installed a Tamarack fan, and (b) I have no affiliation or financial relationship with the manufacturer (and neither does GBA).

If you post your questions on the Q&A page, more people will see your question, and you are more likely to get answers from people who have installed a Tamarack fan. Here is the link:
http://lakesideca.info/qa


86.
Sep 7, 2013 9:35 AM ET

whole house fan
by Scott Widdows

I live in central Florida and have a 5000 sqft roof over conditioned and non conditioned parts of the house. My attic temperatures get extremely high over our 6 month summer. In addition, my garage is so hot that I cannot comfortably work in there. I was thinking of adding buying a floor fan for the garage but thought of an idea. What if I could kill 2 birds with one stone and installed a whole house fan in the garage ceiling to draw in air (albeit warm air, but air movement nonetheless) to the garage and at the same time push the same 90 degree air into the attic to replace the 130 air that is up there. So instead creating a negative pressure with attic ventilators I am creating positive pressure. I have plenty of soffit, ridge and gable venting to allow the air to escape under the pressure. I know there will also be the potential of the attic air pushing its way into the house through cracks as well. If the air infiltration is minimal, it would seem that a 90 to 100 degree attic would be easier on the the AC than 130 degrees. And, yes, unfortunately the AC equipment and ducts are in the attic.
Thanks you in advance for your comments.


87.
Sep 8, 2013 4:30 AM ET

Response to Scott Widdows
by Martin Holladay

Scott,
If you have ductwork in a hot attic, the best solution is to transform your vented unconditioned attic into an unvented conditioned attic by installing insulation along the roofline.

If you don't want to do that, the next best solution is to seal the seams of your ductwork to make your ducts airtight, and then to add more duct insulation around your ducts. This approach can be supplemented by adding more insulation on your attic floor.

I don't recommend the use of fans to depressurize or pressurize your attic. This approach usually increases rather than decreases your energy bills.

If you like to hang out in your garage, install an air conditioner in your garage, or install a fan that blows air on you to help you feel cool when you hang out there.


88.
Nov 18, 2013 5:04 PM ET

Attic ventilation-Useful or not-icicles on gutter
by Kay Alldone

I have a metal roof, I am getting icicles on my gutters. My understanding is that it is possible that my attic is too hot (I have adequate insulation since I added some prior to new roof and new gutter install. I never had a problem with icicles before the new roof and gutters. So, will installing additional venting to attic solve the icicle problem? I currently have a ridge vent so, I am mystified.


89.
Nov 18, 2013 10:59 PM ET

Icicles
by Charlie Sullivan

Kay,

Check out the excellent articles on ice dams, perhaps starting with http://lakesideca.info/articles/dept/musings/ice-dam-basics

It's likely you have air leaks from the house into the attic; finding and sealing those could be a big benefit. You might also need soffit vents if you don't have them, so that air can flow in there and then out at the ridge vent.


90.
Nov 19, 2013 4:15 AM ET

Response to Kay Alldone
by Martin Holladay

Kay,
Q. "Will installing additional venting to attic solve the icicle problem?"

A. No. In addition the the article that Charlie suggested, you may want to read this one: Prevent Ice Dams With Air Sealing and Insulation.

Q. "I currently have a ridge vent so, I am mystified."

A. The type of problem you are describing is almost never related to attic venting -- except in the sense that a ridge vent can actually make your problem worse. For more information on this issue, see All About Attic Venting.


91.
Jan 6, 2016 6:05 PM ET

attic fan but with no attic
by CARLOS ALVARADO

Dear Advisor. We live in a very hot and humid tropical area. Our house has no attic, just ceiling space and roof. Ceiling is sealed, and it is made of gypsum. If there are any leaks would be the lighting installations.
We are considering installing a gable solar fan following ideas researched in web sites from Australia and the US. Roof has vent intake openings, but don't have top exhausts. So we cam to the conclusion that rooms get hot due to radiation that goes down from roof to ceiling. How this can be a bad idea, considering climate situation is a little different from where most of these commentators come from? We still believe this is a good idea.


92.
Jan 7, 2016 6:36 AM ET

Edited Jan 7, 2016 6:40 AM ET.

Response to Carlos Alvarado
by Martin Holladay

Carlos,
I'm going to try to repeat what you wrote to see if I understand correctly.

You have a house with a gable roof. The ceiling is sloped -- it follows the roofline. Your roof assembly has no insulation; that means that the rafter bays are empty. There are soffit vents but no ridge vents. You are planning to install a wall-mounted fan indoors, on a wall, on the gable side of your house. Is that correct?

It's possible that I misunderstood, and the "gable solar fan" will not really be installed in the gable. Maybe you are talking about a roof-mounted fan that is solar powered -- a fan that is designed to pull air from your rafter bays. Is that it?


93.
Apr 16, 2016 7:47 AM ET

Options for non air conditioned homes
by Cathy Shin

Hi Martin - find your whole site very interesting (had visited reading about passive house and solar water/voltaic before) and almost thought my question would be answered in another comment above since the guy was also in Kauai but it was not.
Anyway, I also have a house in Kauai but no AC. Lived there for years comfortably without AC and also lived in Philadelphia, SF, NYC and upstate NY without AC (i like being warm) but some folks who go to the house in Kauai wish there was AC from about 5-9 PM mostly in July and August. Inside temps probably 80-90 degrees at that time, outside temps 70-80. Is this a good situation for a whole house fan? The house has tons of windows, many louvered (so leaks/ventilation everywhere) and no attic insulation. (No heating or AC in most houses there built by people not transplanted from the Mainland) Or would adding attic insulation be a better idea?
I just always used ceiling fans and kept windows open but that does not appear to be sufficient for those accustomed to AC. Electric is about 40 cents/kWh in Kauai so I started looking at solar attic fans because the whole house fans seem to consume so much energy and ended up here (very few non-manufacturer sponsored opinions out there on this topic) There is almost always a decent breeze so maybe one of those non powered turbine type fans?
Essentially, it is a climate where the outside temps are always comfortable but the house does build up heat during the day and needs some cooling. Your thoughts?


94.
Apr 16, 2016 8:52 AM ET

Response to Cathy Shin
by Martin Holladay

Cathy,
I'm not experienced enough with your climate to know whether a whole-house fan would work, but I suspect that it would work better than attic insulation. Some of the daytime temperature rise comes from internal loads (refrigerator, television, lights), and some from solar gain through windows, and attic insulation won't reduce those heat sources.


95.
Apr 19, 2016 11:11 PM ET

Attic bedroom with no AC but a gable fan
by Lesley Matson

We are approaching our first summer in Portland OR and it's already hot in our attic bedroom/office on these warm spring days. We're self-employed and working upstairs all day. We have no AC, and the only insulation is in the crawl spaces at the sides (since the room is half ceiling, which is directly underneath the roof). We do have blackout shades and close those during the day, which makes it dreary. The previous owners installed a gable fan at one end of the crawlspace. Fan has on/off switch in our office, not a thermostat. Is this thing of any use to us at all? Should we turn it on when it's hot out? Once it cools down outside in the evening? Or just get rid of it? We are not considering central AC, because the main level of our house stays pretty comfortable. But maybe we need a large window unit for just the upstairs? Thank you!


96.
Apr 20, 2016 4:45 AM ET

Edited Apr 20, 2016 4:46 AM ET.

Response to Lesley Matson
by Martin Holladay

Lesley,
Since the fan is already installed, it sounds like it's easy enough to turn on the switch and see what happens. If the fan lowers the indoor temperature, why not use it?

Plenty of people with the problem you describe end up installing a window-mounted air conditioner. That works too.

If your roof is uninsulated, the long-term solution is to install insulation with an adequate R-value in your roof assembly. This might mean installing rigid foam insulation above your roof sheathing the next time you need new roofing.


97.
Apr 20, 2016 1:04 PM ET

Thanks and follow-up question
by Lesley Matson

Thanks, Martin. When we have the fan on should we open the door between the bedroom and the crawl space where the fan is located? And what about the door at the bottom of stairs between the bedroom and the main level? Thanks again!


98.
Apr 20, 2016 1:12 PM ET

Response to Lesley Matson
by Martin Holladay

Lesley,
It's your house -- you can operate the fans any way you want. Experiment! See what works best.


99.
Jul 14, 2016 2:52 PM ET

Depressurization
by Dan Puccio

I live in FL, and I'm considering a fan to exhaust hot air from the attic. I have an area of wall where I can install it, so I won't need any holes in the roof.

It seems like the primary argument against using an attic fan is that it could cause a pressure difference, and cause cooler air to be sucked into the attic from the living space. I'm curious how this would be the case, as there are vented soffits completely around the perimeter of the house. Unless someone installed multiple fans or a fan which can really push a great volume of air, wouldn't the soffits allow for completely unrestricted airflow? (Surely, MUCH more air can be drawn through the soffits than a single fan could ever hope to draw.) If the airflow is completely unrestricted, how can there be a pressure differential?

In my case, I have the HVAC ducts and also the air handler in the attic space. It's so hot that it's impossible to work there in the summer.

If I don't depressurize the attic and I don't have leaks, what is the downside?


100.
Jul 14, 2016 3:58 PM ET

Response to Dan Puccio
by Martin Holladay

Dan,
Researchers have confirmed that powered attic ventilators depressurize the home under the attic ceiling. This conclusion isn't speculation; it's based on pressure measurements.

In theory, if your ceiling is perfectly airtight, you could limit or eliminate depressurization problems. But very, very few ceilings are airtight. They are generally filled with (invisible) holes.

You are basing your hopes on the "smart arrow" theory -- namely, that the air entering your attic will follow the "smart arrows" shown in diagrams produced by soffit vent manufacturers. Alas, air doesn't pay any attention to the smart arrows in illustrations. You are likely to increase your energy bills significantly if you install an attic fan, because your air conditioner will be working much harder (due to the conditioned air being pulled through ceiling cracks).

If you want a cooler attic, you need to install insulation along the roof slope. For more information, see Creating a Conditioned Attic.


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