Why they don’t work and get a cool attic!
With the summer season approaching quickly, now is a good time to discuss an uncomfortable topic. Hot Attics And Why Traditional Exhaust Fans Don’t Work To Cool Them Down!
A very hot problem
Last week we had warm weather for the first time this year. A customer called and said the second floor was extremely hot and asked us to install a larger, more powerful roof fan. The customer had decided that a new fan was needed to replace a solar system they had installed a few years ago. The solar fan “didn’t seem to be working as it should.” Compared to the rest of the house, the rooms on the second floor were extremely hot and could not be inhabited in the afternoons and evenings.
Typical attic construction
Upon inspection, I found the villain in the attic exactly as the customer described. A single solar powered roof fan on the roof. It worked.
There were no other plumbing or HVAC equipment in the attic. The insulation consisted of a 6 to 8 inch layer of insulation on top of the ceiling, plus random pieces of R-19 fiberglass mat thrown at an angle on the blown material but not completely covering the room.
The 2 × 6 rafters with 4 “jump jackets and ½” plywood scissors were all fully visible. 6 “x 24” have all 8 ‘vents installed around the perimeter and the vents were clear and not blocked by the insulation. The temperature of the roof cladding in the attic was 147 degrees. The loft, which was 45 by 24 inches (about 4500 cubic feet), 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 throw their money away because it doesn’t work.
Imagine lying in the sun on a Cancun beach. Everything is fine for about 5 minutes, then you feel hot. You get a large fan and set it up so that air blows over you. Ah, feels better for about 5 minutes. Soon the air will even feel hot, so swap out the fan for an air conditioner and let that cold air blow you. Feels great now so you can stay outside in the sun until you notice your skin is burned red and blistered and crispy.
What happened? Radiant heat! This is why you can still get a terrible sunburn even on a cold day. Radiant heat flows through cold air without any effect. What does this have to do with your house?
The roof structure of your house is like your skin. Solar radiation hits the roof surface and heats the entire roof mass (roof, roofing paper, nails, cladding, rafters). Soon the roof mass is so hot that it emits heat itself (like the sun). This radiant heat travels through the attic and hits the material on the surface of the ceiling structure (insulation, wooden beams, drywall, piping, etc.). Very quickly, the entire mass of the ceiling structure becomes a huge radiant heater. If your ceiling is not airtight and extremely well insulated, all of that heat will radiate into your home.
Equipment and lines suffer badly in a hot attic
A hot attic is bad enough. However, if you have equipment, plumbing, or piping there that operates at elevated temperatures, you can expect a stressful and likely shortened lifespan for these items.
If you have plumbing, stoves, air conditioners, water heaters, water pipes, etc. in your attic, here are some things to consider:
- Move the plumbing and equipment into the crawl space or inside your home.
- Carefully seal the seams and add reflective insulation around your plumbing
- Convert your attic into an air-conditioned space. To do this, move a radiation barrier and insulation to the roof slope. The spray foam insulation makes this possible. However, you need to create and follow logs to avoid trapped moisture and ventilation issues with the existing roof frame.
The point here is that if you have a hot attic space it will shorten the life of any equipment in there.
A retrofit example that works
- Seal air leaks on the ceiling. This prevents air from reaching the attic from the living space. This also prevents excessive moisture from migrating in and out of your attic.
- Install extra insulation to bring the total to over R-30 anywhere – even above the access door. Cover the tops of wooden ceiling joists at least 3 inches.
- Make sure the eaves openings are open and clear.
- Install radiation protection film over the insulation. You can get Radiant Barrier Film from many places online.
Removing or circulating attic air does not prevent radiation from transmitting heat. During the day, the air introduced is immediately heated by the surrounding structure. At night, after the sun’s source of radiation has stopped, the air brought in from the outside cools down in the attic, but this happens very slowly. As soon as the sun rises in the morning, the radiant heating process starts again.
Attic fans can actually cause more problems than they solve.
- If there aren’t enough reveal, gable, or ridge vents, a powerful roof fan can pull air from your house through the ceiling if it’s not perfectly sealed.
- Powerful roof fans can actually retract draft stoves or water heaters by pulling combustion gases from their burners into the house.
- Good ventilation of the attic is great at preventing moisture and condensation, but not enough to keep it cool in summer.
If your house has a hot attic and ceiling in the summer, the solution is not an electric attic fan. To get rid of a hot attic, you need to have a radiation barrier between the attic space and the hot roof structure. This is best done when the house is under construction. Once built, it can be difficult or impossible to retrofit without major work on the structure. The good news is that heat alone isn’t very harmful if you don’t have plumbing or appliances in your attic. It’s just uncomfortable.
What you need to do is stop the heat from being radiated back into the house. This requires a fully sealed ceiling, a very thick layer of insulation, and radiation barriers (layers of reflective film) over the insulation to block radiation and isolate your hot attic from your cool house.
Our client ultimately decided to create an air-conditioned room in their attic with foam insulation applied to the pitched roof combined with a radiation barrier. This resulted in a completely cool attic, drastically reduced electricity costs and easy-to-maintain temperatures in the rooms on the second floor. The bottom line is that this solution provided an absolutely comfortable second floor. I will publish further details on this option later on request.
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Disclaimer of liability
The views expressed above are the author’s own.