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I have a stand alone garage that is 24X 36. It is uninsulated. It has housewrap over the outside of the studs, and metal bands nailed in diagonals for rigidity. There is no sheathing but the hardboard siding. A cheap construction but it has held up for 21 years and still is square and looks good.

I would like to insulate the entire garage using fiberglass insulation, and since I use the area above joists for storage, I would prefer not to hang a ceiling with insulation above it. I would rather insulate walls and under the roof decking. I have no soffit vents, ridge vents or any kind of vents in the garage.

I know I am not supposed to allow the insulation to touch the decking, but I am wondering even if I did insulate without touching the decking that in the absence of soffit vents and a ridge vent I would have a terrific heat buildup during the summer causing the deterioration of the roof shingles. Or am I worrying about nothing?
 

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Question #1. Where are you located?
Q. #2. Do you intend to heat the garage some way?

There are specific code rules that you can follow that would allow you a non-vented attic space such as using a foam directly under the deck of the roof and fill the rest with inexpensive fiberglass insulation. Is that interesting to you?

Andy.
 

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I live in southern Illinois where we can be below zero in the winter to 100+ in the summer.

I would heat the garage in the winter, but only when I would be working out there.

Spray on foam is an option, although I know I that isn't a DIY project. I am hoping to do the work myself.
 

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You can surely insulate the walls with no problems, then finish them off with drywall or whatever, if you wish.

As for the ceiling, you can have insulation up there between the ceiling joists, and still store stuff on top of the ceiling joists. For example, if you have 2x8 ceiling joists, you can have 7 1/4" of insulation up there. If you finish the ceiling with sheetrock or some other material, you could blow in cellulose above it.

You would need some way to access the area, like one of those pull-down staircases which one sees from time to time.

You would want to vent the attic first.

7 1/4" of cellulose is not much, but it will certainly help if you are planning on heating up the space while you are working there during the day.

I have a garage workshop which is insulated to about these same standards (r-13 in the walls and something minimum in the attic) and it has only been down to 36 F in there so far, and we have had lows down to about zero.

My workshop is about 16x20 and I have a furnace which I believe is 75,000 btu. I have no plenum on the furnace and just a filter in front of the hole where there should be a filter rack.

I can turn that furnace on when I open the garage and it will go from say 40 degrees to 60 in about as much time as I can bring my lunch bucket from my car.
 

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if you insulate the ceiling and install pull-down stairs, that access hatch, and everything else, should be well air sealed first. try to find some roxul (or cellulose, or cotton) insulation for the walls, too; fg is the worst. if you want to have a cathedral ceiling, greenbuildingadvisor.com has good info on "how to insulate a cathedral ceiling". if i were going to store stuff up above the lid, i'd not want to be walking in insulation all the time.
 
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