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Rigid foam on top of studs

tkaj | Posted in Energy Efficiency and Durability on

I’m currently in zone 5 redoing a brick ranch home. The wall from the outside to inside is brick, plywood, black sheeting, R-11 and drywall. All of the drywall is down and wall cavities are exposed. The customer wants to use R-15 Roxul insulation and add rigid insulation on top of the studs with taped seams and drywall over that. I know some people do that but not sure which type of rigid foam insulation to use in order to prevent moisture inside the wall. I think we would like to use 1″ rigid foam to make sure we can attach drywall correctly.

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  1. GBA Editor
    Martin Holladay | | #1

    It would be helpful to know (a) whether there is a WRB (for example, asphalt felt) between the brick veneer and the plywood, and (b) what you mean by "black sheeting." Do you mean asphalt felt? Black polyethylene? Asphalt-impregnated fiberboard?

    My usual answer would be, "any type of rigid foam will work," but the assembly you describe may be at risk of inward solar vapor drive problems -- unless there is a layer (like asphalt felt) to slow down inward diffusion. If inward diffusion is a concern, then EPS without any facers would make more sense than a vapor-impermeable rigid foam like foil-faced polyiso.

    For more information, see these two articles:

    Walls With Interior Rigid Foam

    When Sunshine Drives Moisture Into Walls

  2. tkaj | | #2

    Black sheeting meaning something similar in texture like Tverek wrap. It's a material like texture. It's a house from 1965 so not sure exactly what was used back then.

  3. GBA Editor
    Martin Holladay | | #3

    There weren't any plastic housewraps on the market in 1965. Can you post a photo?

    Does this material feel like a polyethylene garbage bag? Or is the texture different?

  4. tkaj | | #4

    It has a fabric like texture.

  5. GBA Editor
    Martin Holladay | | #5

    Do any GBA readers want to speculate on what type of membrane is being described?

  6. Expert Member
    Michael Maines | | #6

    Sounds most likely to be asphalt felt. Hard to believe a builder wouldn't be familiar with that material but I guess that's where things are these days. TKAJ, can you gauge how thick the material is? Can you tear or cut off a small piece? If it's tearable and similar in thickness to a cereal box then it's probably asphalt felt. If it's thicker, usually 1/2", it would likely be asphalt-impregnated fiberboard. Both have fabric-like surface textures. I can't think of any other materials that would have commonly been used in that application in 1965.

  7. Expert Member
    Dana Dorsett | | #7

    Like Michael, I suspect TKAJ is talking about black SHEATHING, not "...sheeting...". It's common to see brick clad houses built with asphalted fiberboard sheathing. It's usually 1/2" thick, but often 3/4" thick, but usually on the exterior side of the plywood when used in conjunction with plywood.

    Take a hole saw, make a 1.5-2" hole in the fiberboard to see if there is an air gap between the plywood & brick, and measure both the depth of the gap and the thickness of the plywood + fiberboard or felt.

    If there is an air gap of 1" or greater it probably doesn't need rigid foam or interior side vapor retarders other than latex paint on wallboard. For moisture control, but if the gap is thinner the assembly will be more susceptible to exterior moisture drives from the brick.

    How deep are the roof overhangs (both eaves & rake)?

    Local climate matters, and 5A is a huge zone. Got a ZIP code?

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