More Q&A Spotlight
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…
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5 Comments
"Steel studs are designed to provide the maximum possible conductive energy transfer across a wall using the minimum amount of material—a thin web with cleverly designed heat transfer fins (flanges) on both sides to efficiently absorb heat on one side and reject it on the other. It is pointless to insulate the cavity to fight this efficiency of heat transfer."
The best description of steel studs I've read!
One more suggestion: DO NOT use polyisocyanurate foam board in basements. It absorbs water like a sponge - which kills the R value and encourage rot and mold the wall assembly. EPS and XPS, or closed cell spray foam, are the only options.
It can absorb liquid water, which is why it's a good idea to keep it a couple of inches above the slab. The foil-faced type does not readily absorb water vapor so as long as the foundation walls are reasonably water-tight, there is no problem with using polyiso on walls. I use it on all of the basements I design, either on its own or in conjunction with a framed wall insulated with something fluffy. EPS is far more risky if the interior is to be finished, in my opinion, because it lets so much water vapor pass through.
Sorry Mike, but you highlighted why polyiso foam board should NOT be used for basements or exterior applications - it absorbs water. As you noted, "as long as the foundation walls are reasonably water tight". If any moisture accumulates / condenses on the concrete walls, it can be absorbed by the polyiso foam board. If the foil facing is placed against the concrete, it will corrode over time - allowing moisture to be absorbed by the polyiso foam board. If the foil facing is on the room side of the wall, you have created a classic 'diaper wall' with increasingly soggy polyiso foam board next to the concrete wall.
Plus, I have found through experience that condensation will accumulate on the foil face of polyiso foam board if the humidity rises on a cool morning. I returned to a job site one morning, where foil faced polyiso foam board was stacked in the basement - and there was a puddle under it. I was very surprised and started looking for a leaking pipe. There was none. Then I started to think about the dew point that morning, and how the foil - very well insulated on one side - could chill to the dew point, leading to condensation. I also found this problem inside an exterior wall of a home, where moisture had repeatedly condensed in the foil surface - leading to rot in the framing.
Kohta doesn't mess around! A great, very thorough explanation.
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