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Community and Q&A

Painted concrete block

user-5469404 | Posted in Energy Efficiency and Durability on

I have an existing detached concrete block garage that is painted on the outside.  I want to use spray foam or rigid foam on the interior but am concerned that the wall will not be able to dry.  Is this a concern?  Exterior insulation is not an option due to the existing wall being only 2′ from the property line.

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  1. Expert Member
    BILL WICHERS | | #1

    There is no need for a concrete block wall to be able to “dry”. Concrete doesn’t care if it’s wet, and is happy to be wet forever (look at all the dams and drainage channels that are made of concrete, for example). You’ll be fine insulating that wall with either spray foam or rigid foam, although rigid foam is probably going to be both cheaper and easier to install well.


    1. user-5469404 | | #2

      No concern about the sill plate staying damp?

  2. user-5469404 | | #3

    Also... Polyisocyanurate foam okay for this application? Also if I cut and cobble the rafters with layers of Polyisocyanurate to create a continuous insulation layer between walls and ceiling, is that my best bet for cost effective retrofitting? I have access to used Polyisocyanurate for cheap.

  3. Expert Member
    BILL WICHERS | | #4

    Polyiso is fine as long as it's cut so that the bottom edge is not sitting in any puddles of water. If you want the wall to be able to breath a little, use polyiso with a kraft or fiberglass mat facer instead of the metal foil facer.

    I'm a little confused about your concern for the sill plate. I'd assumed you have a garage with above-grade block walls? I would not expect water to wick all the way up a full-height above grade block wall where it would get up into rafters and anything else set on top. If you're concerned about that, add a capillary break between the top of the block and whatever wood pieces are set up there. I like to use thin (1/32") strips of HDPE sheet, which is slick and stiff so it's relatively easy to slide into narrow gaps if you jack things up a bit for installation.

    You could also use EPS on these walls to allow a little bit of drying ability if you're really concerned. If the entire wall is above grade though, you probably don't have anything to worry about. Basement walls are different.


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