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Q&A Spotlight

Rotting Walls in a SIP House

A project to add a deck uncovers decay. Now what?

An unwelcome surprise: What began as a project to rebuild an exterior deck has mushroomed into extensive repairs to structural insulated panels. Photo credit: B3.

A GBA reader named B3retta89tu (let’s just call him B3) has made an unpleasant discovery about his 1998 home in northern Ohio. As he began replacing a deck on the north side of the house, B3 found rot in the structural insulated panels (SIPs) that make up the exterior walls.

SIPs are a sandwich of oriented strand board and rigid foam insulation. Repairing a bad spot isn’t as straightforward as it would be with a stick-framed wall because the assembly is, in effect, a single component.

“About a month ago we started replacing a deck on our north wall and in the process of replacing the ledger drip cap we found rot underneath a lower window behind the housewrap,” B3 writes in this recent Q&A post. “Upon further inspection we determined the rot extended above the window to a second-floor window but no farther.”

The same issue was found under every window in the house. Housewrap was installed over the nailing flange of the window, followed by J-channel. There was no sign of any flashing tape. There was evidence of repair around a door, B3 said, adding, “To make the repairs on this door would have required removing the siding along the window, making them aware of the problem and thus the shoddy repair attempt.”

This is what the homeowner discovered beneath the siding around a window.

B3 has been in touch with Insulspan, the SIP manufacturer, which recommends repairing the damaged areas with stick framing or attempting to piece in new OSB.

“At this point I’m leaning toward a stick frame replacement and filling with foam panel,” B3 writes. “The other issue we found was that the interior seams were not taped and there is noticeable discoloration of the OSB on the exterior-side seams,…

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  1. synergytodd | | #1

    I disagree with advising the customer to skip the foam insulation and use a vapor-open insulation such as cellulose, especially in the Southern United States. Use open cell foam and it as vapor-open insulation.

    1. Expert Member
      Michael Maines | | #2

      I don't understand your comment. I do understand that in extremely humid climates, cellulose insulation may not be the best choice, though I know it can work with proper detailing. You seem to be recommending open-cell foam, which is also vapor-permeable, so is your concern with cellulose specifically or with the vapor permeance? Or are you simply a foam-for-every-situation advocate?

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