Why attic fans don’t work and what to do instead!
With the summer season fast approaching, now is a good time to review an uncomfortable topic. Hot attics, and why traditional exhaust fans don’t work to cool them off!
A very hot problem
Last week we had some warm weather for the first time this year. One of our clients called to say that their second floor was extremely hot and requested we install a larger and more powerful attic fan. The client had decided a new fan was needed to replace a solar unit they had installed several years ago. The solar fan “didn’t seem to be working as it should.” Compared to the rest of the house, the second-floor rooms were extremely hot and impossible to live in during the afternoon and evening.
On inspection, I found the villain in the attic exactly as the customer described. 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 fully exposed to view. 6”x24” eave vents were installed around the perimeter every 8’ and vents were clear and unblocked 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.
Radiant heat: The reason fans don’t work to cool your attic
I explained to our client that the idea of attic fans to remove hot air from the attic area might seem logical. But they would be throwing their money away because it doesn’t work.
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 zips right through cold air without any affect. So, what does that have to do with your house?
The roof structure of your home is like your skin. Radiation from the sun hits the roof surface and warms the entire mass of the roof (roofing, roof paper, nails, sheathing, rafters). Soon the roof mass is so hot that it will radiate heat on its own (like the sun). 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 extremely well-insulated, all this heat is going to radiate into your home.
Equipment and ducting suffer greatly in a hot attic
A hot attic space is bad enough. But if you have equipment, ducting or piping up there, working in elevated temperatures, you can expect a stressful and likely shortened life for those items.
If you have ductwork, furnaces, air conditioners, water heaters, water piping etc. in your attic, it would be a good consideration to:
- Move the ducting and equipment to the crawlspace or the interior of your home.
- Meticulously seal seams and add reflective insulation around your ductwork
- Convert your attic to a conditioned space. Do this by moving a radiant barrier and insulation to the sloped roof assembly. Spray foam insulation makes this possible but you will need to create and follow protocols to avoid trapped moisture and ventilation issues with the existing roof framing.
The point here is that if you have a hot attic space, it is going to shorten the life of any equipment residing there.
A retrofit example that works
1. Seal off any air leaks at the ceiling. This prevents air movement from living space to attic. 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 ceiling joists by at least 3 inches.
3. Make sure the eave vents are open and unobstructed.
Removing or circulating attic air does not stop radiation from transferring heat. During the day, any air brought in will be heated up immediately by the surrounding structure. At night, after the sun’s radiation source halts, any 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 radiant heating process will start again.
Attic fans 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.
- Good attic ventilation is excellent for preventing moisture and condensation but it is not enough for cooling in the summertime.
If you your house has a hot attic and ceiling during the summer, the solution is not a powered attic ventilator. To eliminate a hot attic you must have a radiant barrier between the attic space and the hot roof structure. This is best done when the home is under construction. Once constructed it may be difficult or impossible to retrofit this without major work to the structure. The good news is that unless you have ductwork or HVAC equipment in your attic, heat is not a terrible problem.
What you must do is prevent that heat from in turn radiating down into the home. This requires a completely sealed ceiling, a very thick layer of insulation and radiant barriers (reflective foil layers) above the insulation to block the radiation and isolate your hot attic from your cool house.
Our client eventually opted to create a conditioned space in their attic using a foam insulation applied to the pitched roof, combined with a radiant barrier. This created a completely cool attic, dramatically reduced power bills and easy to maintain temperatures in the second floor rooms. Bottom line, this solution delivered a totally comfortable second floor. I will publish more details on this option later.
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