I do not believe it makes sense to insulate a garage in a hot climate like the desert southwest. This assumes the homeowner does NOT plan to condition the space. I get many customers asking me to do it and I try to persuade them not to. I am wanting some opinions on this as I have a hard time explaining why. The jist of my argument is that the garage will still get as hot or hotter than it is outside very quickly. It will warm up a little bit slower in the morning, but not much. Just think of how in the middle of summer in a home, if you have no AC on, it is quickly hot as it is outside (albeit a bit slower rate than uninsulated)
My second reason is that you have a huge thermal mass of hot metal pulling in after an hour of rush hour traffic in 110 degree weather, which is released into the garage and kept in place by the insulation.
I can see a radiant barrier helping in the attic and the garage door though since attic temps are driven by radiation.
Oh, another thing...the reason it makes sense in a cold climate is both conduction, radiation and convection is heat transfer from hot to cold object/air. So in hot climate the heat transfers from the garage to the house - it will not gain "Cold" from the house to keep the temp down in the garage. In a cold climate some heat transfer happens from the warmed house to the cold garage, which is then maintained by insulation.
Here's my take form way up north. Do everything you can to keep the heat from getting in, ie, shading, radiant barrier (RB) shingles, RB coated sheathing, low SHGC windows, even exterior shutters, and anything else, before you have to deal with it on the inside. If the outside efforts are reasonably successful, then perhaps insulation and drywall on the inside would be beneficial. But as you said, if it is going to reach full temp somewhere through the day, then all of that insulation will simply slow the cooling. The next option would be to ventilate it like an attic, with exhaust up high and intake way down low, perhaps the north side. If there is an cooler air to be had, bring it inside. I have used power vents and one of their big benefits is a rapid recovery once the outside temps go down. Perhaps a power vent in combination with the finished/insulated garage would make everyone happy.
For your part, you need to measure what they really want. If it is a finished looking garage, then add the venting and other suggestions along with the optional power vent to do the best you can to keep it cool. If it is a cool garage and looks are of no concern, then outside first, and RB under the roof deck and vent the whole space. I might even be tempted to test some of the RB paint on the inside of a wall to see if I could believe their advertising :).
I may possibly be the person or product Brad refers to in the RB paint category. Ours is a thermal barrier, effective against all 3 modes of heat transfer.
Metal or wood, the interior OR exterior can be painted and the difference in heat transfer is tremendous. You can also use it as an interior paint for the walls and ceiling of the garage or house.
I can ramble on about what we have done but you see alot of what we have done at our site at
We have more than one energy calcs done that show us dropping the BTU's by 50% or more.
A steel storage container manufacturer recently proved that our coating on the exterior surfaces of a container is MUCH more effective at insulating their modified / conditioned units than their very expensive foam they normally use.
More discussion on garage doors here.
I do not live in the perma-hot climes, but it seems to me that a non-insulated steel garage door will radiate heat into the garage. If a homeowner is going to invest in insulating the garage, the door should be carefully considered. I just wrote a detailed response in the link that Evan Mills refers to above.
I think the original premise of this thread has gotten pushed aside. I am talking hot climate only. I am talking people wanting to insulate their attic floor only, and they are not conditioning their attic in any way. I see people ask for it all the time and I see other insulators doing it for people all the time.My take is that it is entirely a waste of money.
Blown or batt insulation ONLY slow conduction. This means in the hot summer, your garage is going to get just as hot with or without insulation in the attic. The only difference is your garage may be a bit cooler for a longer period of time in the morning, but by the same token that insulation is going to keep the heat IN the garage for longer at the end of the day...time shift only, no overall energy reduction or change in temp of the garage.(other than timeshifted differences)
Just spotted this old file / statement that says what our coating did on the inside of his metal garage door and what it did for him. He is in Orangevale Calif, near Sacramento.
He sent this as an email to me.
I think your first sentence of this post clarifies what you intended but did not say in the OP - insulating only the ceiling.
I think the garage door is the first choice. Second would be the ceiling and third the walls. If this is an attached garage, and the owner keeps the temp in the house set to below the outdoor temp, then you are completely missing the heat loss/gain from that attached wall or walls. If you can keep that attic temps out of the garage during the day, I think it would be beneficial. When the homeowner opens the door to drive a car in a large portion of the air is displaced, but the surface temperatures take longer to change. Leaving the door open to let the car cool and ventilate before closing the door would probably be a good strategy, too.
In my house here in Alabama, my garage/shop has a room above, 1 car door, and about 30 ft of its 110 ft perimeter in shared with the house. The house is insulated and sealed from the garage, that garage walls and door are insulated. The garage has no hvac, but on most days is not more than 10 deg different that indoor temps and at the coldest only 20. This is also the primary route for entering and exiting the house, so the door opens and closing ventilation is going into this space not directly out doors. I don't park my car in it - as it is workshop and I have 5 kids so perhaps higher than normal traffic.
Do I think it was wasted effort / money? No. Will all circumstances have similar results? No. My neighbor parks their car in the garage and leaves the door open all day - only closing it at night. That keeps the heat gain down during the day... but then security would be an issue for some.