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Community and Q&A

Why do basement walls need to dry to the interior?

Richard Baumgarten | Posted in Energy Efficiency and Durability on

Hello all,
I’ve always read that basement wall insulation should be semi permeable to water vapor, allowing the wall assembly to dry to the interior,… but I don’t recall ever reading why.
If one was to apply a foil faced foam board, or a closed cell spray foam directly against the concrete wall, what’s the danger?


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  1. User avatar GBA Editor
    Martin Holladay | | #1

    I have puzzled through the same question, and I have concluded that there is no reason for a foundation wall to dry to the interior, in spite of what you sometimes read.

    Walls insulated on the interior with closed-cell spray foam perform very well -- and they certainly don't dry to the interior.

  2. Kohta Ueno | | #2

    A semi-permeable basement insulation system has a slight benefit, in that moisture will not accumulate as much at the insulation-to-masonry interface. As a result, the relative humidities there will not rise as high. This is the reasoning, for instance, behind having 1" of XPS and a stud frame wall (see Figure 10 in BSD 103 Understanding Basement: This makes it slightly "safer" if there are accidental connections from the interface to interior space-i.e., less chance of it being a mold farm.

    That being said, completely impermeable insulation materials, such as foil-faced polyisocyanurate, can work pretty well. However, you have to be well aware that the space behind the foam is likely to be at 100% RH much of the year--and has potentials for IAQ problems. Overall, you need to seal it off from the interior in a pretty bulletproof way. Think of it like when they shut the hatch of the flooding torpedo room in submarine movies--the stuff back there is in bad shape, so it has to be cut off from the interior.

    Finally, closed cell spray foam does allow *some* degree of drying--at least more than foil-faced insulation. ASHRAE Fundamentals lists it at 1.6 to 2.2 perm-inch (perms in a one-inch thickness)--you need to divide by the number of inches, so a 2" thickness would be 0.8 to 1.1 perms. Closed cell foam also has the advantage that there shouldn't be an air space created behind the insulation, reducing the chance of "bad stuff" leaking inside.

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