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Yes, usually you will need some kind of vapor barrier on the exterior walls when the framing is attached to concrete and masonry surfaces.

While adding insulation can be beneficial, there is too much of a good thing. Too much insulation can seal your house to the point where it can't release moisture that enters it—essentially trapping water vapor inside. Warm, humid climates are extra susceptible to mold from moisture issues, but homes in colder climates can experience condensation from an air-tight home when freezing temperatures thaw.

Polyisocyanurate foam, or polyiso for short, has the highest R-value per square inch: 6.5 to 6.8. Polyiso is a rigid foam that comes with a reflective foil on both sides to serve as a radiant barrier. Given its high R-value, rigid foam is usually meant for foundations and basement walls.

We recommend that you cut batts of fiberglass insulation with a sharp utility knife. However, in a pinch, you can use tools such as a box cutter or even a pair of scissors. No matter what cutting tool you use, just make sure it’s sharp so you get a clean, straight cut. Always cut along the back (foil) side of the fiberglass, rather than the “fuzzy” side of the material.