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Interior foam for thermal bridging?

Birdo | Posted in General Questions on

I’m building a wood shop. It’s a pretty conventional building. 2×6 with blown cellulose insulation. I’m thinking of putting 1/4” foam on the interior walls to help with thermal bridging. Is that enough insulation to help? Or maybe just a waste of time?

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  1. Jon_R | | #1

    I'd spend the time/money on better air sealing and blower door testing. You could put thicker strips of polyiso foam on the stud edges.

  2. Expert Member

    It may help to think of the studs as one form of insulation, and the cellulose as another.

    There really isn't such a thing as a thermal break. All the foam does is add its R value to whatever it covers.

    The studs have a value of about R-7. The cavity filled with cellulose about R-19.

    When you add the 1/4" foam, the studs are now about R-8. The cavity is now R-20.

  3. Aedi | | #3

    Malcolm is right that there really is no such thing as "enough" to help with thermal bridging. More is better, and 1/4" is not really worthwhile. If you are going that thin to save on foam cost, consider 1-2" foam cut into 1.5" wide strips to cover just the studs, and filling the rest with cellulose. It will be simple to attach drywall through the strips, and it will make a pretty significant difference in the wall assembly. See:

  4. Expert Member
    BILL WICHERS | | #4

    About the only 1/4" rigid foam insulation commonly available is the fanfold XPS stuff. That's R1 -- not much. Going to 1/2" polyiso gets you up to about R2.7-3 (depending on manufacturer), so about three times the insulation in twice the space. The fanfold stuff is sold at a bit of a premium per square foot too, so you might not even see much, if any, cost difference between the two.

    I'd go with at least 1/2" thick rigid foam, but more is better.


  5. Birdo | | #5

    Thanks for the ideas. Decisions, decisions. I appreciate the description of the studs in terms of their R value. That puts it into perspective.

  6. Expert Member
    Peter Yost | | #6

    We can't "stop" either heat or moisture flow; just slow it down. We think of materials that do a much better job slowing down those flows than wood, metal, or concrete as thermal breaks.

    Maybe we should call them thermal benders and not breaks...


    1. Expert Member
      MALCOLM TAYLOR | | #7

      I don't think I've ever heard of materials that reduce moisture flow referred to as thermal breaks. Isn't that conflating two issues?

    2. Expert Member
      BILL WICHERS | | #8

      I agree with Malcolm. Best to not confuse the two things.

      Also, it is possible to completely stop moisture migration with a hermetic seal. Heat can make it past even a vacuum barrier. Basically you can completely stop the movement of physical things like water (moisture/vapor), but not radiation (heat). You can only slow down the movement of heat.

      I think it’s appropriate to call something a “thermal break” when that something is significantly better at slowing the transfer of heat than whatever “other” material you are using. In engineering, usually a few orders of magnitude (10x, 100x, etc). Even electrical insulators are like this. Electrical insulators are rated in terms of dielectric stengths in volts per mil. In really small dimensions you get leakage currents, but in “regular” thicknesses those leakages currents are so low as to be nearly immeasurable.


  7. Birdo | | #9

    Ok, I've been looking at the Bonfiglioni 's methods. I have a slightly different plan. Will be using a combo of spray foam and blown cellulose in the stud cavities. I thought I'd attach foam strips to the studs, but skip the 1x since I'm using a wooden wall board.

    This would leave a space. I'm thinking a space could present some new issues. Or should I have the blown-in stuff brought out to the level of the foam strips? (Might cause some problems for the installers when they clean off the excess.)

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