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How do you install rigid insulation to a foundation wall?

WALT DE WALT | Posted in Energy Efficiency and Durability on

Do I need to seal or prepare the interior foundation walls in any way to do this? Where can I get sheets of 4′ x 8′ sheets of rigic insulation . . . or does it come in other size configurations? I live in New England. What thickness(es) should I be considering?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "How do you install rigid insulation to a foundation wall?"

    A. Here is an article that explains the steps: How to Insulate a Basement Wall.

    Q. "Do I need to seal or prepare the interior foundation walls in any way to do this?"

    A. No, as long as the concrete is dry and as long as the basement shows no signs of dampness.

    Q. "Where can I get sheets of 4' x 8' sheets of rigid insulation?"

    A. Most lumberyards or building supply stores carry it. Some smaller outlets may only have 2 ft. by 8 ft. sheets.

    Q. "Does it come in other size configurations?"

    A. The two most common sizes are 2 ft. by 8 ft. and 4 ft. by 8 ft.

    Q. "What thickness(es) should I be considering?"

    A. The building code requires a minimum of R-15 insulation for basement walls in New England. That means at least 3 inches of XPS, 4 inches of EPS, or 2 1/2 inches of polyiso.

  2. Expert Member
    Dana Dorsett | | #2

    Fiber faced 3" -3.5" roofing iso works great for basement apps, and reclaimed goods often available for cheap from companies specializing in that stuff. In my neck of New England (central MA) there are multiple vendors, Insulation Depot in Framingham, MA being the biggest (, but others often advertise through craigslist. eg:

    Roofing iso typically comes in 4' x 8' sheets, but 4' x 4's are also fairly common.

    When using iso on a basement wall where the slab doesn't have a ground vapor barrier it's important to keep the cut edge off the slab, either with an air gap, or an inch or so of EPS/XPS, since iso can and will wick moisture slowly over time. If the basement has a history of water coming over the slab due to high water table, etc., stop the iso a few inches above the high-tide mark, inserting EPS for that section instead.

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