Insulation Services Case Studies: Why Attic Fans Don't Always Work
Challenge
Many homes are either built with or add on an attic fan to help "cool" a hot attic. While they may help to vent hot air when present, they don't work to eliminate the cause of the hot air. Additionally, when not in use they can operate as a space for uncontrolled air to move in and out of an attic. With the summer season upon us, now is a good time to review an uncomfortable topic. Hot attics, and why traditional attic fans don’t work well enough to cool them off!
Anatomy of attic heat
Imagine yourself laying in the sun on a beach in Florida. 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. It feels better for about 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 burned to a crisp.
The problem is UV rays. It’s why you can still get a terrible sunburn even on a cold or cloudy day. Radiant heat zips right through cold air without any affect and cooks your skin. So, what does that have to do with your house?
The roof structure of your home is like your skin. Radiation from the sun heats the surface of the roof. From there, the entire mass of the roof (roofing, roof paper, nails, sheathing, rafters) warms up through conduction. Soon the roof’s structural 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 also becomes a giant heat radiator. This heat moves back up towards the roof again and will continue to radiate heat well after the sun goes down. If your ceiling is not sealed airtight and extremely well-insulated, much of this heat is going to radiate downward into your home too.
Air in the attic that comes in contact with the surfaces of the hot framing, will become heated through conduction. This creates a kind of hot air sandwich formed between the roof and ceiling structures. The warming of all these surfaces is why the attic interior gets so much hotter than the exterior temperature. All that an attic fan will do is push some of that air outside. It will not eliminate the heat source.
Solution
We offer a multitude of solutions to help combat attic heat (and cold during the winter) to create a better conditioned space and help reduce your energy costs. a radiant barrier, spray foam insulation, panel insulation and various forms or air sealing can reduce the heat and eliminate any need for an attic fan which uses electricity when running.
