How to best add insulation to sheltered exterior wall?
Having recently moved into a large, 1980s-built home in central Maine (read: cold) I’m in the process of improving air sealing and insulation. The home has a large attached garage with a non-insulated second floor, and an adjoining second floor room that was framed and insulated but never finished or heated. It’s not clear how or where the builder intended to access this space it from the finished second floor but I don’t need the space anyway.
These spaces combined create a wall that divides heated from non-heated areas along the full length of the second floor. The wall was 2×6 built with fiberglass batts, and OSB on the cold side. No siding or no windows because it doesn’t face the exterior, and there is no baseboard heat along the interior side.
I think this long wall is low hanging fruit for increased the R value, but I assume it was built with polyethylene sheeting on the warm side so I can’t cover the exterior with rigid foam. I’m debating adding a 2×4 or 2×6 wall on the cold side, but structurally it seems like overkill.
Other ideas?
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Replies
Nick,
If I were you, I would install a layer of rigid foam on the cold side of this wall, to interrupt thermal bridging through the studs, improve the wall's R-value, and reduce air leakage.
If you take this advice, you'll need to follow the rules outlined in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing. Since you are apparently in Climate Zone 6, you'll need to specify rigid foam with a minimum R-value of R-11.25.
Once the rigid foam is installed, you will want to protect it with a thermal barrier (1/2-inch drywall).
I wouldn't worry too much about the fact that your house has interior polyethylene.
Many energy experts have worried whether it's a good idea to install exterior foam on a house with interior polyethylene. Although it would be better if the poly weren't there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven't been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits are "not so risky as most people think. These homes will probably be fine."