Insulating Basement Walls with 2 in. XPS Foam Board and Tape

How do you insulate a basement wall without trapping moisture behind it? In this walkthrough, Josh of The Excellent Laborer uses 2 in. thick Owens Corning FOAMULAR NGX XPS rigid foam board, rated R-10, applied straight to a block or poured concrete wall in a basement that is about to be finished.

Why foam instead of fiberglass

Rigid foam is stiff rather than flexible like fiberglass or rock wool, so it can be applied directly to the wall. Fiberglass in a basement holds moisture, and that trapped moisture can turn into mold and mildew. Behind drywall it would grow unseen. Because the foam sits against the wall, it doesn't have to be cut to fit between studs, which speeds up installation. It also pushes the finished wall out 2 in.

The sheets are 4 ft wide and 8 ft tall, with a compression strength of 25 PSI. Josh says the block wall alone is probably only R-1 or R-2, and the drywall and stud framing add perhaps another one or two. Thinner board in 1 in. and 1 1/2 in. exists, but his crew picks 2 in. as a good happy medium for their region. The same board can go on the exterior before backfilling, where the joints don't need taping.

Prep and cutting

First the wall is scraped of loose mortar and debris with a wonder bar, since adhesive won't hold to a dirty surface. The board is cut on a sheet of plywood across two sawhorses. The ceiling is only about 82 in., so each sheet is trimmed to fit with room above the footer: he measures 80 3/4 in., marks it with a utility knife, runs the knife along a T-square, deepens the cut, and snaps the board. For rip cuts a straight 2×4 serves as the straight edge, and he keeps the factory edge against the factory edge.

Adhesive and nails

Because of the volume of work, he uses foam adhesive from a gun with a canister that screws on, and a twist adjustment regulates the flow. It was about 35-40° in the basement, and he says the foam still comes out fine where a caulk-style adhesive in a cold gun would be more difficult. He puts three strips on the back of the board, thick enough to contact the wall despite irregularities, starts flush with the edge of the wall, and presses the sheet in. A nail in each top corner into the sole plate keeps it from falling while the adhesive sets; with 2 in. foam he uses 3 in. nails. Walls will later be framed against the board.

Pipes, bands and vents

Where the water supply line and the well pump power line come through the block, he measures 7 1/4 in. from the installed board to the pipe center and about 12 in. up, then cuts a 2 in. hole with a hole saw and fishes the board over the pipe. He dry fits first so the adhesive doesn't get wasted, then seals around the pipe with spray foam. The exposed band board at the staircase is covered with foam as well. The board was placed over vents high on the block wall; those areas will be cut out and filled in so outside air doesn't enter the conditioned space.

Taping and covering

Every joint is sealed with Owens Corning Easy Sealr tape, made for this foam board, which helps improve its vapor barrier qualities. It is laid over the joint, rolled with a roller and cut to length. Indoors he starts at the top and works down; on an exterior installation he would start at the bottom and go up so the overlap sheds water.

The foam does not meet the code fire rating if left exposed, so it must be covered with gypsum board, and he says inspections will likely require it.