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Attic Exhaust Fans: Why they don’t work and what to do instead!

With the summer season fast approaching now is a good time to review a HOT topic, attic exhaust fans!

Last week in Palo Alto we had some hot weather for the first time this year. One of our HPS clients called to say that their attic was extremely hot and requested we install a larger a/c powered fan. They decided a new fan was needed replace a solar unit they had installed several years ago, that “didn’t seem to be working”. The second-floor rooms under the attic area were hot compared to the rest of the house.

On inspection, I found a single, roof mounted, solar powered attic exhaust fan. It was working. There was no other ducting or HVAC equipment in the attic. Insulation was a 6”-8” layer of blown-in fiberglass on the top of the ceiling, plus random pieces of R-19 fiberglass batts tossed askew atop the blown-in material, but not fully covering the space. The 2×6 roof rafters with 4” skip sheathing, and ½” plywood shear was all exposed to view. 6”x24” eave vents were installed around the perimeter every 8’ and were not blocked by the insulation. The temperature of the roof sheathing inside the attic was 147 degrees. The 45’x24’ (approx. 4500 cubic foot) attic space was very hot.

I explained to the client that the idea of attic fans to remove hot air from the attic area might seem logical. But it doesn’t work.

Here’s why attic fans don’t work to cool your attic

Imagine yourself laying in the sun on a beach in Cancun. All is good for about 5 minutes then you start to feel hot. You go and get a big fan and set it up to blow air over you. Ah, feels better for about another 5 minutes. Soon even the blowing air feels hot, so you exchange the fan for an air conditioner and let that blow cold air on you. Feels great now so you stay out in the sun till you notice that your skin is red and blistered and burned to a crisp.

What happened? Radiant heat! It’s why you can still get a terrible sunburn even on a cold day. Radiant heat moves right through cold air without any affect. So, what does that have to do with your house?

image of radiant heat and house
Radiant heat transfer

On your home, the roof is like your skin. Radiation from the sun hits the roof surface and warms the entire mass of the roof structure (roofing, roof paper, nails, sheathing, rafters). Soon the roof structure is so hot that it will radiate heat on its own. This radiant heat passes down through the attic space and hits the material on the surface of the ceiling structure (insulation, wood joists, drywall, ducting etc.). Very quickly the entire mass of the ceiling structure is a giant heat radiator and if your ceiling is not sealed airtight and well-insulated, all this heat is going to find a way into your home.

The point of this discussion is that if you have an attic space, it is going to get hot. And attempting to cool it off by pulling air through it doesn’t work*.

Here’s what works: 1. Seal off any air leaks at the ceiling. This will also prevent excessive moisture from migrating in and out of your attic. 2. Install additional insulation to bring the total to over R-30 everywhere-even over the access hatch. Cover the tops of any wood joists by at least 3 inches. 3. Make sure the eave vents are unobstructed. 4. Install radiant barrier foil over the top of the insulation as described in this VIDEO.

For a number of reasons, I do not recommend attic fans no matter how they are powered. They can actually create more problems than they solve.

  • If there are not enough soffit, gable or ridge vents, a powerful attic exhaust fan can pull the air from your home through the ceiling if it is not perfectly sealed off.
  • Strong attic fans can actually back draft furnaces or water heaters by pulling combustion gases out of their burners and into the home.
  • Maintain good attic ventilation for preventing moisture and condensation, not cooling.

Solutions for a hot attic

Unless you have ductwork or HVAC equipment in your attic, heat is not a terrible problem. You definitely must have a completely sealed ceiling and a very thick layer of insulation and radiant barriers to isolate your hot attic from your cool house. If you do have ductwork or HVAC equipment in your best attic solutions would be to:

  1. Move the ducting and HVAC equipment to the interior of your home
  2. Seal leaky duct seams and adding insulation on top of your ductwork
  3. Move the insulation from your attic floor to the sloped roof assembly, thus creating a “conditioned” space in your attic.

Bottom line: If you your house has a hot attic and 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 and a radiant barrier to the attic area of your ceiling.

*Removing or circulating attic air does not stop the radiation from transferring heat. During the day, any cooler outside air brought in will be heated up immediately by the surrounding structure. At night, the sun’s radiation source stops so any cooler air brought in from outside then, will eventually cool down the attic structure, but that will happen very slowly. As soon as the sun rises in the morning, the heating process will start again.

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