The house was built with bedrooms directly over the unheated garage, and the ceiling of the garage has been sheetrocked with no insulation in between the garage ceiling and those rooms above. I acquired a free bunch of 4' x 8' sheets of 4" thick rigid styrofoam insulation and am thinking of attaching this insulation to the sheetrocked ceiling. Here I'm showing a chunk of the insulation along with the type of screw and washer I'm considering screw through the insulation and sheetrock into the joists above. Of course this screw is way too long but I'm just using it as an example because I don't currently have the appropriate length screw (which I suppose should be 5 inches, which would thread an inch into the joists which I suppose should be fine). Any comments/advice as to whether this approach with this type of fastener is a reasonable way to go here, or maybe a different fastener works better, some special type designed for this type of job?

 

Any commercial roofing supply will have these.
screws for EPDM roof - Bing Images

 

Actually you'll need screws at least 5 1/2" long...don't forget the sheetrock which is probably 5/8". Don't think the epdm screws in the link will work.

I'd be thinking some sort of adhesive, like Locktite Powergrab. Clear Power Grab All-Purpose Construction Adhesive Pressure Pack from Loctite Adhesives

 

Yeah I don't think those epdm screws in the link would be quite right either. But maybe if I could get some like this ASAP 3S - OMG Roofing Products in the 6-inch length that could work good. Trouble is they don't sell any locally and there are no big roofing supply outfits in my community. Would probably need to get em online from someplace but I have no idea where. I dunno about using that construction adhesive. These 4-inch thick panels are fairly heavy and seems to me might be rather difficult even with two guys to try to hold them up on the ceiling and press and hold firmly long enough for the adhesive to take hold.

 

What about using fender washers to fit whatever size screw you decide to use? I suppose you could also cut your own out of some scrap paneling.

 

I like those ideas fine. The problem seems to be getting long enough screws (6 inch). The two local hardware stores here have only up to 4 inch long, or can order 5 inch they say.

 

How will you cover the insulation after it is installed on the ceiling?

 

Well it's the foil face kind of rigid stytrofoam insulation, 4 inches thick. Was planning on probably filling in the seams with spray foam insulation or taping the seams. Didn't have a plan to cover the insulation but since you asked I assume the implication is it's no good to leave it uncovered.

 

Just wasn't sure what level of finish you were wanting.