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Concrete wall insulation – how do I insulate?

user-231615 | Posted in Energy Efficiency and Durability on

I am building a new house with 8 inch concrete walls above grade. I plan to use wood furring strips, foam board insulation and sheetrock in Lexington, Va. How thick should insulation be? Should I use a vapor barrier?

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  1. GBA Editor
    Martin Holladay | | #1

    According to the 2006 IECC, the minimum code requirement for wall insulation in your climate zone (Zone 4) is R-13. The code allows thinner (R-5) insulation for so-called "mass walls," including concrete walls, as long as the insulation is installed on the exterior of the concrete. (Unfortunately, there is little building-science justification for promoting thinner insulation for concrete walls in most climates. Once the thermometer drops below, say, 50 degrees for an extended period of time, the homeowner ends up with a poorly insulated wall with no thermal mass benefit.)

    Assuming you haven't drunk the thermal-mass Koolade, R-13 represents the bare minimum in your climate. If you're using polyiso foam, that means 2 inches. If you're using XPS, that means 3 inches of foam.

    But you're not building a code-minimum home, you're building a green home. My own recommendation is to double the R-value beyond minimum code requirements. That would mean at least 4 inches of polyiso or 5 inches of XPS.

    It's far better to install the foam on the exterior of the home, not the interior. If you try to install it on the interior, you have to pay attention to thermal bridging, which occurs where the floor meets the walls, where the ceiling meets the walls, and where interior partitions meet exterior walls. If these thermal bridges are not addressed, tremendous amounts of heat can escape, undermining the whole purpose of your wall insulation.

  2. GBA Editor
    Martin Holladay | | #2

    To answer your second question: you don't need a vapor retarder with this type of wall construction. For one thing, the polyiso or XPS both qualify as vapor retarders.

    Secondly, the 2006 IECC requirement for vapor retarders includes two exceptions covering your case. Climate zones 1 through 4 fall under one of these exceptions; no vapor retarders are required in these zones. Another exception covers "construction where moisture or its freezing will not damage the materials" -- an exception that covers concrete walls.

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