As you suffer through a summer heat wave, your attic may be the last place on your mind. After all, there’s no way you’d want to climb up there when the stifling heat can reach as high as 150F. However, it’s important to not neglect your attic, either, as it acts as your home’s heat shield and therefore has a major impact on the temperature in the rest of the building.

As the summer sun blasts down on your roof, the entire structure absorbs the heat energy, which radiates into the attic space and then is conducted downward by the home’s framing. This heats up the actual living space —making you turn up your AC and increasing your electricity bill. If you can improve ventilation in your attic and reduce the amount of heat the space holds and pushes into your home, you’ll reduce the cooling load on your air conditioner and save money on your utility bills. When exploring the best attic ventilation practices, consider installing an attic ventilation fan to keep the hot air from settling in.

How Does an Attic Fan Work?

Attic ventilation fans help cool air your attic by pushing out the stifling hot air from inside the attic and bringing in cool air from outside. This prevents hot air from seeping into your home and driving up the temperature in the living space, which reduces the load on your air conditioner.

Most homes already have some passive attic venting built in. Cool air enters the attic through soffit vents in the eaves. Once inside, the air heats up and rises higher, ultimately exiting through venting at the roof’s gables, through ridge vents cut into the roof’s apex or other vent holes in the roof. As the air leaves, it creates negative pressure behind it, sucking in cool air from below into the soffits and creating a self-repeating ventilation process. Even if your roof already has ridge vents and plenty of ventilation built in, it’s a good idea to install gable fans or roof fans to help blow hot, humid air out of the attic.

Attic ventilation fans also tend to be quite energy-efficient in terms of their own operation. Gable fans fit into the gable vent and can be set to operate only within a preset temperature range. Many are solar powered and require no other wiring, so they don’t cause any additional charges on your electricity bill at all.

According to the Home Ventilating Institute (HVI), powered attic ventilators need to move a minimum of 700 cubic feet per minute (cfm) for 1,000 sq. ft of attic space (for example, 20’ x 50’) to be effective. Ideally, there should also be plenty of soffit intake space- their calculations recommend 336 net square inches of open soffit ventilation to supply 700 cfm. HVI recommends a ratio of 60 to 40 for soffit ventilation to gable or ridge ventilation.

The Downside of Attic Ventilation Fans

Attic ventilation fans, whether hard-wired or powered by their own solar panels, seem like a low-cost and effective way to help keep your house cool. However, the utility of attic ventilation fans is actually quite a controversial topic.

Good insulation reduces fan effectiveness. On one hand, the National Renewable Energy Laboratory said “Attic ventilation reduces attic temperature 10 to 25 degrees and slows the transfer of heat into the living space.” On the other, the Florida Solar Energy Center/University of Central Florida  found that “attics with nominal natural ventilation and R-19 ceiling insulation do not need powered vent fans.” As it turns out, because the insulation slows heat from moving down into the living space, homes with well-insulated attics don’t see a significant reduction in their cooling load when they add attic ventilation fans.

Fans can cause increased loss of conditioned air. Another downside is that homes that do not have air-sealed attics can lose some amount of their conditioned air from the suction of the attic ventilation fan, depending on how much soffit vent space is available. Your soffit ventilation actually needs to be considerably more than 336 net square inches to supply a 700 cfm fan system, or the fan is going to pull the air it needs through unsealed holes and gaps in the attic floor from the conditioned living space. Not only does this increase the load on your air conditioner even as you try to do the opposite, it can cause dangerous combustion problems with appliances like natural gas water heaters by creating backdrafts and unleashing poisonous carbon monoxide gas into the home.

Does My Home Need Attic Ventilation Fans?

It may be starting to sound like attic ventilation fans aren’t worth the hassle and could cause more problems than they solve. However, there are certain circumstances where they can prove useful.

Attic ventilation fans could be effective if your home has:

  • Attic insulation that is less than R-19.
  • An attic floor that is thoroughly air sealed.
  • Bountiful eave soffit ventilation space.
  • HVAC equipment in the attic that is well-insulated and sealed.

On the flip side, if your insulation levels are that low, you would probably see more bang for your buck by beefing up the insulation instead, which will have the double benefit of lowering your winter heating bills, too. The fans could play a useful role as a quick, temporary intervention to keep cooling costs down in the summer, but in the long run they seldom prove to be the most pragmatic or effective solution for lowering your energy bills year-round and improving energy efficiency.