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I am buying a home with a detached garage that the original owner insulated. The garage is probably 25 years old. At the time of construction, 1" expanded foam was nailed directly to the rafters to insulate the ceiling. Over time, the foam has shrunk (and deteriorated to some degree) leaving 1/2 - 1" gaps around all the seams and is not sufficient any longer. The attic is vented properly.

I would like to re-insulate the ceiling with standard blown-in fiberglass or cellulose. I would also like to sheetrock the ceiling to provide a better finish.

Can I install the sheetrock over the existing styrofoam and then blow in the insulation in the attic?

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Yes you can sheet rock with longer screws. However the screws will basically have no hold until they hit the wood. Your previous insulation will continue to shrivel. Your ceiling will sag (glue and screw your drywall to ceilings). And you will be like - "why didn't I prep this for a couple hours so I don't have to redo it." Nothing worse than completing a job and knowing that something you did could (probably will) go wrong.

Also I doubt your insulation is even/flat and I think this would make hanging it flat very hard.

I am all about saving time but if you are in the house for more than a year or two I think you are wasting time.

Also why are you insulating the roof of a detached garage?

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The bit on the unevenness and continued shriveling is good insight. Furthermore, you're right on the tear out - probably 10 minutes to rip down the foam and 110 minutes to pull all the nails. :) –  Derek Bromenshenkel May 14 '13 at 5:02
    
Regarding "Also why are you insulating the roof of a detached garage?" I live in MN and is common to heat a garage to melt off vehicles and use it as a shop for snow removal equipment and snowmobiles. –  Derek Bromenshenkel May 14 '13 at 5:04
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Probably not. Without pictures I imagine the drywall screws would not have enough grip into the foam to hold them to the ceiling during an earthquake or even a door closing. Sight unseen of course, I would pull the foam down before I installed drywall.

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I was thinking screws an inch longer than normal to drive through the foam into the wood. Wouldn't that work? –  Derek Bromenshenkel May 5 '13 at 3:41
    
I don't think that would work well. Even if you were able to get the screws to hold in the wood, you'll have a hard time getting the rock level and flat. Also eventually space between the rock and wood will shift and work loose. However, you may be able to install (2x4 or 1x2) bats that run perpendicular to the ceiling rafters (depending on the thickness of the foam). The bats would give you a good flat surface to screw and you would only need to cut a small strip of the foam away. –  Matt Johnson May 5 '13 at 4:12
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