There’s a lot of confusion surrounding attic fans. Here at GBA, 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) or an energy-recovery ventilator (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 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 load 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 (ach). 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, backdrafting 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-value 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.
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 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:
- Moving your ductwork and HVAC equipment to the interior of your home;
- Sealing leaky duct seams and adding insulation on top of your ductwork;
- Moving the insulation from your attic floor to the sloped roof assembly, creating an unvented conditioned attic.
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.”