I live in central Florida. My single garage is concrete block, with unfinished ceiling, just rafters and a ridge vent with new shingles on the roof. Is there an inexpensive insulating sheet that I can tack up to the cross members (ceiling joists) so I can run a 8,000 btu portable a/c just to cool the inside air from 90deg. to 75? I thought a 1/2 in foam board with foil side facing up to roof. Will that be ok? will it cause moisture to build up in what is then an attic space? Plain drywall has negligible R-value and is heavy to install.

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Really any insulation will do enough to make your air conditioner do the job. Obviously the more the better, but only to a point. I'd guess that anything more than about R-15 won't improve efficiency much given the small temperature differential. The key is to reduce overall conditioned space volume and prevent mixing with outdoor air. If you use foil-faced I might install it foil-up, since its purpose is to reflect radiant heat.

Your garage should have vents down low to allow a full circulation path. Soffit vents are typical. without that there's very little movement in the attic, and moisture buildup could be a concern.

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