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So I recently had a new 30x24 detached garage built and intend to leave it mostly unfinished. I live in Iowa and we have pretty cold winters here, which will be coming soon, and I'm looking for a low-cost way to provide enough insulation to allow me to heat my garage when I want to do some work in it during the winter. I have a 5,000 W electric garage heater installed, but it has no chance in keeping up with the winter as the garage is now. The garage was installed with no roof vents, but all of the soffits are vented.

I've already installed fiberglass insulation on all of the walls and my concern is now the ceiling. I may drywall the ceiling at some point in the future, but definitely not this year. My questions is the best low-cost and fast way to provide at least a reasonable amount of insulation in the ceiling area. Can I insulate the soffits to try to reduce the amount of air/draft that comes in through the soffits, at least for the cold months? I figure this would be a cheap and easy solution, but I'm not sure if it would solve the problem or is a good idea. Alternatively, would it provide much of a benefit to install vapor barrier on the ceiling and then leave the soffit vents open (so the roof area would be vented).

Again, I'm not looking for a super insulated garage, I just need something that I can work in and heat on occasion. Right now every time the wind blows half of my heat is lost from the air coming in through the soffit vents.

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It may very well be feasible to temporarily insulate the soffit area between the rafters above the top plate. But if you do this it would be very important to remove this insulation when you eventually close in the ceiling. –  Michael Karas Nov 17 '13 at 23:22
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2 Answers

If the roof/ceiling area has no insulation, even closing the vents off will be little help in keeping in heat. The heat will be quickly lost through the roof sheathing and roofing. Installing just a vapor barrier is probably equally effective, but you gain a bit by needing to heat less air volume. But it's still not much insulation, so heat will still be quickly lost.

If you were to install just R-11 insulation over the vapor barrier, you would be much much better off. You can always add more later. Another approach would be to install 1/2" XPS foam panels as a ceiling. If you seal all the edges, it will serve as a vapor barrier when you finally insulate for real. A drywall ceiling can be applied directly to the XPS when time comes to properly finish off the garage. Just be sure to use longer screws.

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Definitely close off the ceiling rather than plugging the vents, IMHO.

The "fanfold" insulation [typically 1/4" X 4ft X 50 ft - R1 possibly some thicker versions available, on a quick look around - thicker would of course be higher R-value, as well] intended for use under siding (and sometimes roofing) might be another option, in larger sheets (less joints to seal.) More than just vapor barrier...and it might support more serious insulation (just not too much) above it.

Fanfold

I presume that you recognize and accept that until you do drywall over it, this (or vapor barrier, or XPS sheets, etc...even exposed fiberglass, to an extent) is a fire hazard. In a similar situation, I'm putting my effort towards getting the drywall on the ceiling so I can insulate the heck out of the ceiling - a 5000W heater does not go very far, otherwise. Specifically, 5KW * 3413 btu/kWh = 17065 btu/hr (input) - .vs. 720 sqft (30x24) of R1 (1 Hr x sq-ft x degree F/BTU) ceiling would only net you 23.7 degrees (F) (17065/720) above outside ambient if that were the ONLY place heat was lost (floor, walls, doors all infinitely insulating, and zero air exchange - obviously an incorrect assumption - just illustrating the point.)

If the walls are 8 ft high, amounting to 864 sq ft, plus the 720 sq ft of ceiling, and we assume that all are insulated to R11 (optimistic for many garage doors and all regular doors & windows - but low for most modern walls and ceilings) the same 5KW could heat to 118 F above outside ambient (but I'm still ignoring the floor and air exchange...)

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