I have a garage that I would like to insulate and heat but it has an exposed screened porch above it. So, if I insulate the ceiling of the garage do I need some sort of air space above insulation between the joists? Will moisture condense against the underside of the porch floor?
You are required by Code to ventilate the space above the insulation. If you have an effective continuous vapor barrier plus effective ventilation (cross-ventilation) it is unlikely condensation will occur. Perhaps you could install a removable access panel in the garage ceiling in order to check.
Yes an air space is required.
I think the best option is installing R10 ridged foam insulation sheets between all the trusses. Use tuck tape to intall these. Leaving spacing of 11/2" between them and the lumber onder the deck. Then you can install R20 batt insulation on the interior side of the ridged foam. This will take care of it and give you great r factor.
Venting flat roofs does not work effectively.
Best to foam the ceiling of the garage, which does not require venting.
The floor of the porch is solid, yes?
If not, make it solid with plywood over, then whatever finish you would like for the porch floor.
Search the TrustedPros directory and discover the best contractors in your area.Find your home service pro