In this Q&A post, a homeowner asks for help after a sewer backup flooded his basement. In response, “RastaCook” had to remove the flooring and the lower portion of the drywall, at which point he decided it was time to renovate the basement.

The house was built in 1982 and is located in Climate Zone 6A. On the interior side of the concrete foundation there is 2-in. EPS rigid foam on the upper 4 ft. of wall between light-gauge steel studs installed on the flat, a poly vapor/air barrier, and drywall.

One obstacle Rasta faces is framing new walls to add more insulation; the interior space is limited by stairs and other structures.

He’s consulted with contractors. Some have recommended covering the interior of the foundation with 2 in. of closed-cell-spray foam, but he’s afraid moisture will be driven toward the top of the wall and possibly rot the sills; off-gassing is another concern.

One contractor suggested replacing the EPS with 2 in. of XPS covering 5 ½ ft. of wall starting from the top. But he says the bottom should be left open to allow any accumulated moisture to dry.

Rasta doesn’t know what to do. He says he doesn’t mind cranking up the heat, but “my priority is to not do anything that could cause condensation, mold, and/or rotting issues.”

In spite of his worries, Rasta hasn’t seen much evidence of bulk water problems since he fixed the basement windows, re-graded around the foundation, and installed a dehumidifier.

So, what is the best insulation solution for this basement?

Keep the EPS, but what about the poly?

Bill Wichers wonders whether Rasta has a moisture problem, and recommends he test for this by taping a piece of poly over the concrete somewhere and watching for moisture…