How does an attic exhaust fan work?

Attic exhaust fans help cool 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, ridge vents cut into the roof’s apex or other vent holes in the ceiling. 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 built-in ventilation, installing gable fans or roof fans is an excellent idea to help blow hot, humid air out of the attic.

Attic exhaust fans also tend to be quite energy-efficient in their 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 additional charges on your electricity bill.

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 to installing an attic fan

Attic exhaust fans, whether hard-wired or powered by solar panels, seem like a low-cost and effective way to help keep your house cool. However, the utility of attic exhaust fans is 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.” Conversely, 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 into the living space, homes with well-insulated attics don’t see a significant reduction in their cooling load when they add attic exhaust fans.

Fans can cause increased loss of air conditioning. 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 exhaust fan, depending on how much soffit vent space is available. Your soffit ventilation needs to be considerably more than 336 net square inches to supply a 700 cfm fan system, or the fan will 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, but it can also cause dangerous combustion problems with appliances like natural gas water heaters by creating backdrafts and unleashing poisonous carbon monoxide gas into the home.

Are attic fans worth it?

It may start 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 helpful.

Attic exhaust 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 helpful role as a quick, temporary intervention to keep cooling costs down in the summer. Still, in the long run, they seldom prove to be the most pragmatic or practical solution for lowering your energy bills year-round and improving energy efficiency.