Rigid Foam Board Calculator

Polyiso gives the most R per inch (about R-6) and suits above-grade walls and roofs. XPS (R-5) handles moisture best and is the usual pick below grade. EPS (R-3.9) is cheapest and fine under slabs.

Yes if the foam doubles as your air barrier. Tape every seam and seal the edges with canned foam — untaped board can lose most of its air-sealing benefit.

Polyiso has the highest R per inch and suits interior applications and above-deck roofing. XPS sits in the middle and handles moisture and ground contact well. EPS is the cheapest per R and is the usual choice below grade and under slabs. Depth available, moisture exposure and budget decide it more than the R rating alone.

If the board is doing any air-sealing work, yes, and that is usually the point of installing it. Untaped seams leave a continuous air path straight through the layer, which undoes much of the benefit of going continuous in the first place. Use tape rated for the board rather than general-purpose tape.

Inside a building, almost always. Rigid foam is combustible and codes generally require it to be covered by a thermal barrier — half-inch drywall is the common answer. This applies in basements and garages, which is exactly where people are most tempted to leave it exposed. Check your local requirement before finishing.