Q. A client wants to build a heated room over an existing deck. I’m suggesting that we insulate the deck floor by placing two layers of 2-inch "polyiso" foam board between the existing joists. Two layers should bring the floor to the required R-30 after discounting for heat loss through the joists. Will this work, and what should I do to protect the foam board from moisture, insects, fire, and so forth?
A.Corresponding editor Henri de Marne responds:

Assuming it wouldn’t create any floor height problems with door swings, I would lay the foam board directly over the top of the existing deck instead of fitting it between the joists (see illustration). Not only would this be a lot easier and faster, but it would also eliminate conductive heat loss through the joists. Plus it helps protect the insulation board from the various problems you mention.

Set the sole plates of the new walls directly on the deck, forming a perimeter for the installation of the insulation. By installing the insulation in two layers as you propose, you will eliminate any thermal short circuits at the wall plates, because the wall insulation between the studs will reach below the top of the rigid insulation (as long as you use only a single plate, that is). Stagger the second layer of foam board so that the joints don’t overlap. Use long screws to fasten the plywood subflooring to the deck through the insulation.

I don’t know where your building is located, but you are probably aware of the controversy about the outgassing of polyisocyanurate rigid insulation: It is said to reduce the R-value significantly over a few years. Extruded polystyrene (XPS) may be a better buy. I am not sure you should be overly concerned about the "required" R-30 for floor insulation. (Is this a local code requirement?) Rigid insulation of any type is so efficient at air sealing that the R rating is not the only consideration. I believe that the double layer of 2-inchthick XPS will give you excellent performance even though its official rating would only be around R-20.