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Adding rigid foam over existing insulation

vkurKcnMdF | Posted in Energy Efficiency and Durability on

I like in northwestern North Carolina (zone 7) in a Cape Cod style house. The upper rooms are pretty uncomfortable in the summer. You can feel the heat radiating into the room I presume because it basically has “attic” on three sides that is unconditioned space. There are seven foot-high knee walls on two of the three interior walls in each bedroom with a bath in the middle. The knee walls have fiber glass batt R-19 which sits just proud of the 2X4 stud bay. My thinking is that if I apply some or a couple of inches of rigid foam on top of the wall and leaving the existing batts it would help the batt perform better and reduce the amount of heat being transmitted through the studs, which is really obvious to the touch. If this is sound thinking should I use a Poly Iso board with a foil barrier or just some Dow blue Styrofoam. Then there is the question would this cause any moisture problems even if i detailed wall penetrations such as receptacles and switches with tape and spray foam? What would be the level of foam I would need to lower the condensation point so it wouldn’t be in the wall?

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  1. GpJzCv5bV7 | | #1

    Your main concerns are obvious. Reflect the summer heat out and and not create a situation that contributes to
    condensation. I would not use foam board as it is costly, difficult to install and is not a permeable product.
    I would add furring strips to the existing studs as you would have to do to add foam board and then wrap your attic living space in LOW-E attic blanket. It is well perforated and provide's breathing should condensation be present. Is an air barrier and a thermal barrier , but most importantly, a great reflector of that summer heat gain.
    An inexpensive and easy to use product.

  2. GBA Editor
    Martin Holladay | | #2

    Your suggested solution is a good one. Kneewalls always need an air barrier on the back side, and rigid foam is preferred for this purpose. Foil-faced polyiso is best -- Thermax is permitted to be exposed by most building inspectors, while XPS (blue styrofoam) may require a thermal barrier like gypsum wallboard.

    The polyiso should be installed in an airtight manner, using caulk, canned spray foam, or high-quality tape to seal the perimeter of each sheet. In your climate, you don't have to worry about winter condensation in your walls. I would use 2 inches of foam, simply for the R-value, but 1 inch will work.

  3. wjrobinson | | #3

    Garth said foil. Martin said foil faced foam.

    No one even mentioned the ceiling.


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