My garage was originally a carport. To my surprise I discovered the ceiling's actually thin plywood, whereas the walls are drywall. I doubt it's wise (or legal) from a fire point of view. I was just going to put drywall directly over it, I don't care if it puckers or not, it's a garage. I did then think, with the wisdom brought on by winter (my bedroom floor, which is over the garage, has been getting awfully cold. I was thinking of putting battens up, then sheet foam insulation (I guess that goes inbetween) and drywall/sheetrock over it, but then thought maybe I should check what's up in there, if its's insulated at all, at the moment it wouldn't be hard to pull the ceiling off - and I want to get that sorted now, so I can start building shelving in there etc.

It also occurred to me that I could drill holes at intervals in the ceiling and put expanding foam up there, although would that creat issues if it covers wires? Wouldn't want any overheating, not to mention it'd probably stop me fishing wires through in the future - although for one, if it's already insulated, that'd be difficult anyway, and for another, I can always surface mount in conduit.

Only downside I can see is garage will lose some of its default heating, (the garage rarely drops below freezing thanks to the dryer outlet pipe running through garage and out wall (so not venting into garage, but heat comes from the pipe) plus leakage of heat from the house - but floor in master b/r will feel a little nicer.