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Fri, Feb. 28

Hot Attics, Part I: Most people like hot cars, but not hot garages

Happy July Fourth weekend! With the unusually hot weather recently I’ve had three emails regarding hot garages. Each had different questions and concerns. Instead of repeating the questions, I will just summarize my recommendations regarding a hot garage.

I know some of you will find this hard to believe, but home inspectors do not know everything about everything. But fortunately I know some experts I can ask when I see something I’ve never seen before, or want to make sure I’m answering a question correctly. I would like to thank Mike Uniacke, a nationally known insulation expert and owner of Advantage Home Performance in Prescott, for taking the time to talk to me about some of my advice below. I did not let Mike review this column, out of consideration to those of you that like to correct me after every column, so any mistakes are mine.

A quick review: heat is usually transferred by three methods. Convection - hot air rises, this is how baseboard heaters heat a room. Conduction - some materials are great conductors, like metal, and some materials are terrible conductors, like insulation. Radiation - radiant heat will warm up anything it touches. Standing in the sun is a lot hotter than standing in the shade - this is radiant heat.

I don’t spend as much time in the garage as I used to, but probably more than some people, and I have a full size fridge in the garage. I hate to think how hard that fridge is working in a 90-degree garage. Usually the biggest heat gain in a garage is through the overhead doors. Uninsulated metal doors are great conductors. If the outside of the door is 140 degrees, the inside is probably 139.8 degrees. This heat than radiates into the garage, heating everything in the garage. We had insulated overhead doors installed last year, and it helped a lot. There are overhead door insulation kits available for uninsulated doors. Some are pretty good, but none work as well or look as nice as a manufactured insulated door.

Now we have the other walls. In attached garages, one or two walls are common with the interior, so there is no heat gain there unless you are running your furnace in July, in which case you have more problems that I can address here. One exterior wall is usually mostly overhead doors, which we have discussed. This leaves one or two exterior walls. These can also be a source of heat gain, since they are often not insulated. Insulating finished walls (meaning they have exterior cladding and interior drywall) is not easy, and is not a DIY project for most people. If you have stucco exterior walls, the stucco will provide some insulation. But if you have east- or west-facing walls, especially with wood siding, you may want to consult with an insulation company.

So that leaves the ceiling/attic. Well, also the floor, but there’s usually not much heat gain from the floor, which is good because there’s not much we could do about it. Most garages in our area do not have any attic insulation. There are some exceptions. In some custom-built or very high quality homes I find insulation in the garage attic, and also in some older homes with the laundry facilities in the garage. Usually the garage attic insulation is less than the home attic, often about half. For example, the home attic may have @ R-30 insulation, and the garage attic would have R-15 to R-20.

There is a lot of heat gain in garage ceilings with uninsulated attics. On a recent 100-degree day I checked the bottom of the roof sheathing (“plywood” under the roof shingles) with my infrared thermometer, and it was 160 degrees. The sheathing is radiating that 160 degrees into the attic. Remember that radiant heat will heat up anything it touches, so the drywall over the garage ceiling is getting very hot. That heat is “conducted” through the drywall, where it is “radiated” into the garage.

So attic insulation in a garage is effective at reducing the garage temperature in the summer. But you don’t need as much as insulation as in the home. The attic insulation is more to reduce the radiant heat gain from the attic than to reduce heat loss in the winter. Of course, the insulation will help with heat loss too, but most garages are not heated anyway. And over-insulating a garage can actually trap heat in a garage in the summer, e.g. from pulling in hot (temperature wise) cars.

As always, batt insulation (on a “roll”) is the least effective because there are always gaps on the sides and ends of the insulation, and it does not cover the framing. Blown in insulation is much better because it covers everything. Having loose fiberglass or cellulose insulation blown is not that much more expensive than installing batt insulation, and is a much better value because of the much better coverage.

Since heat gain in garages is mostly from radiant heat, I have to mention radiant barriers. I see these in attics sometimes. These are basically aluminum foil rolled out in the attic, intended to reflect the radiant heat back up to the sheathing and keep it off the interior/garage ceilings. The concept is much better than the execution. I have seen this installed on the bottom of the top chords (over your head) and on top of the insulation (under your feet). In both cases, the barrier is not very smooth, and is very dusty/dirty. This lessens the “reflective” quality. And I have not noticed attics with radiant barriers over my head being significantly cooler. My first thought when I see this in an attic is there was a good salesman in the neighborhood.

If I did not answer all your questions, don’t sweat it. Next time will be Hot Attics Part II.

Randy West owns Professional Building Consultants in Prescott. He is state-certified and has performed more than 7,000 home inspections in the Prescott area. West serves on the Home Inspector Rules and Standards Committee for the Arizona Board of Technical Registration. Contact him at or visit

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