Does My Home Need Attic Ventilation Fans?

By Vernon Trollinger, May 30, 2017, Energy Efficiency, Green, Home Improvement, News, Save Money

While most homeowners love their homes, not every home is perfect. There are one or two things the homeowner would like to change, such as an outdated bathtub for a new walk-in shower or updating the look of their kitchen. And then there’s occasional “uh-oh” of the unexpected repair. We’re going to identify several projects and some of the ins and outs of what you can expect.

Does My Home Need Attic Ventilation Fans?

Even though it’s been a very warm spring, there’s still lots of time to think about keeping down your home cooling costs. One of the big ones is your home’s heat shield —your attic. As the summer sun blasts down on your roof, the entire structure of your roof absorbs that heat energy, which will begin radiating and emitting heat into the attic space. It gets insufferably hot—up to 150°F. You home’s framing will conduct that heat downward where it will begin to heat up the living space —adding to your air conditioning load and increasing your electricity bill. Of course, if you can reduce the amount of heat in your attic, you’ll also reduce your cooling load and save money.

Does My Home Need Attic Ventilation Fans? | Bounce Energy Blog

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 is heated and rises higher in the attic, ultimately exiting through venting at the gables (located at the ends of the house roof) through ridge vents cut into the apex of the roof or through vent holes cut in the roof. As it leaves, it creates negative pressure behind it, sucking in cool air from below into the soffits and the process repeats. Yet, some homeowners believe that even if their roof already has ridge vents and plenty of ventilation built in, it’s a good idea install gable fans or roof fans to help blow hot, humid air out of their attic.

Gable fans fit into the gable vent of your home and can be set to operate only within a preset temperature range. Many are solar powered and require no additional wiring. There are also solar powered roof vents that require you to cut a hole in your roof to install them. Both types of fans are rated to move air in cubic feet per minute (cfm). According to the Home Ventilating Institute (HVI), powered attic ventilators need to move a minimum of 700 cfm for 1,000 sq. ft of attic space (for example, 20’ x 50’). Ideally, there should also be plenty of soffit intake space. The ratio of soffit ventilation to gable or ridge ventilation HVI recommends is 60/40. Their calculations recommend 336 net square inches of open soffit ventilation to supply 700 cfm.

Sounds good, right? So, does your home need an attic ventilation fan?

Well, it’s complicated.

Does My Home Need Attic Ventilation Fans? | Bounce Energy Blog

Attic ventilation fans — no matter if they’re hard-wired or powered by their own solar panels seems to be a good idea. They’re relatively low cost on Amazon. However, over the past 30 years, there’s been a lot of research and debate into the effectiveness of using attic ventilation fans. For example, in 2001, NREL said “Attic ventilation reduces attic temperature 10 to 25 degrees and slows the transfer of heat into the living space.” Yet, it was well established by the Florida Solar Energy Center/University of Central Florida in 1985 that “attics with nominal natural ventilation and R-19 ceiling insulation do not need powered vent fans.” Why? 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 further reduction in their cooling load when they add attic ventilation. Homes with less insulation do see a benefit. However, those same homes are stuck with higher heating costs in winter.

Another point of contention is that homes that do not have air-sealed attics will lose some amount of their conditioned air up into the attic due to the suction of the attic ventilation fan. The amount depends on how much soffit vent space is available. So, if there isn’t considerably more than 336 net square inches to supply a 700 cfm fan system, it’s going to pull the air it needs through unsealed holes and gaps in the attic floor from the conditioned living space. This can cause dangerous combustion problems with appliances like natural gas water heaters where suction by the attic ventilation creates backdrafting and unleashes poisonous carbon monoxide gas into the home.

In short, having an attic ventilation fan has the potential to cause more problems than not having one. That said, they may work best for the following circumstances:

  • Attic insulation is less than R-19.
  • Attic floor has been thoroughly air sealed.
  • There is bountiful eave soffit ventilation space.
  • HVAC equipment in the attic is well-insulated and sealed.

Because the effectiveness of attic ventilation fans can easily be negated by attic air sealing and installing more insulation to the attic (which also lowers winter heating bills, fans do not) installing an attic ventilation fan (in my view) is something I would use as a temporary measure if I was air sealing and insulating my attic piecemeal on weekends by myself. Naturally, not everyone is keen on that. Attic ventilation fans may be a pragmatic solution for someone looking for an easy fix. All the same, as with most easy fixes, you keep paying for the consequences of their conveniences over and over again.

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A native of Wyomissing Hills, PA, Vernon Trollinger studied writing and film at the University of Iowa, later earning his MA in writing there as well. Following a decade of digging in CRM archaeology, he now writes about green energy technology, home energy efficiency, DIY projects, the natural gas industry, and the electrical grid.

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