polystyrene on garage rafters - moisture problems?

[chithawk]

i was told this will cause moisture problems, but im not sure the "expert" understood what i was doing now that i think of it

i have a house built in 00' the garage has open rafters with storage we made just on some OSB panels across the rafter "floor"

i was planning on attaching 3/4 foam sheet to the cieling of the garage (bottom of the rafter storage, not the actual bottom of the roof) the overhang is vented also there is a vent near the peak.

will there be any moisture problems ? i havea feeling the person i was talking with misunderstood where i was placing the paneling

i will be heating the garage occasionally and i wanted to seprate the rafter area so im not heating it, i would sheetrock but it would be more time consuming and less R value

[chandler]

Any insulation you add should be between the garage ceiling and the decking you installed with the vapor barrier facing down. If I understand you correctly you want to add foam insulation under the ceiling? You will certainly have moisture problems. One more thought not mentioned, do you have a ceiling in the garage?

[chithawk]

also its not going to be super sealed, the foam sheets would be just to help keep the heat from my furnace where im working and not up in the rafters where its can escape easily

[hrjrkr]

That should not be a problem. I have seen garage ceiling insulated and sheet rocked. As long as it's vented above the ceiling, which you say its, I don't see why you would have a moisture problem.

[chandler]

Key to hrjrkr's comment is the order in which it is done...insulated then rocked. Your insulation should be above the rock. You never did answer if you already had a ceiling or not.

[chithawk]

no its just open rafters no ceiling

[chandler]

Then why not go ahead and install regular insulation and drywall. The polystyrene panels won't span the joists like drywall will and you will have done it correctly. Cut and install the insulation with the vapor barrier down and hold it in place with insulation placement wires, then install the drywall.