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Community and Q&A

Insulating block walls

man of steele | Posted in GBA Pro Help on

I am currently remodeling an old gas station. The exterior walls are cinder block. The building is 26 feet by 48 feet with 12 foot high walls. One third of the building (26 feel by 16 feet) will be used for a meeting area and have two bathrooms. All of these walls have been framed up with dimensional lumber. The perimeter walls of this space have been insulated and a vapor barrier applied followed by drywall. The remainder of the building will be used as a heated storage area with no interior insulation. The exterior of the building is to have LP smart siding. In order to install the siding I will be applying 2x4s furring strips 16 inches on center. I would like to insulate the exterior of the building using 1 1/2 inch ridged foam placed between the furrng strips. I am considering using a slicker rain screen between the siding and the foam. I feel confident this will keep moisture issues under control for the siding. My question is will I have a moisture problem between the foam and block wall or will the interior block wall sweat? The project is in Wisconsin. our average rain fall is 30 inches per year high humidity. Snow fall average of 30 inches with temps that go into the negatives. What is the best solution for exterior block way insulation?

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  1. user-2310254 | | #1

    Man of Steele,

    This issue was explored in a somewhat similar threat:

    I would skip the cut and cobble type of approach and consider reusing recycled foam over the entire wall. You can attach your strapping over the foam and have a more resilient stack up.

  2. Expert Member
    Dana Dorsett | | #2

    It's better to make the foam layer continuous, taping the seams, and strap it to the walls with 1x4 furring through screwed to the CMU with masonry screws, hanging the SmartSide on the furring.. Otherwise the furring is a less than R2 thermal bridge through the foam layer. Even at 24"o.c. a 2x4 furring becomes a ~15% framing fraction, and that's not counting any window framing or top plate, etc.

    With a 3/4" rainscreen gap provided by the 1x4s you won't need any RainSlicker (tm) or similar product. Both the furring and the siding will dry easily into the gap.

    Foil faced polyiso would give the best performance, but take care to keep the cut bottom edge off the ground or pavement, since it can wick moisture. If you expect snow drift to saturate the bottom few inches, use EPS for the bottom foot or so, with polyiso the rest of the way up.

    LP SmartSide instructions call out 1.5" minimum nail penetration into the wood, which you obviously can't get with 1x furring. Many manufacturers with similar specs would allow 3/4" penetration if using ring-shanked nails. But it would be worth running that by the manufacturer first.

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