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Can rigid insulation be safely used in the center of a double stud wall?

user-6233959 | Posted in Green Building Techniques on

Joe Lstiburek’s details for double stud walls show a layer of OSB/plywood in the center of the wall for an air and vapor barrier. Could a wall be assembled with R-13 batts in 2×4 framed walls, separated by a layer of 2″-3″ rigid insulation? It seems like the first condensing surfaces of the rigid insulation would safely remain below the dewpoint, and foil faced insulation like polyiso would be a very effective air and vapor barrier. This would be built in climate zone 4.

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  1. itserich | | #1

    This addresses that question nicely:

    The diagram at the top of the article shows Joe's ideal double-stud wall design. (For more details, see High R-Value Wall Assembly: Double Stud Wall Construction on the BSC website.) It's got two layers of framing, which is why it's called a double-stud wall or double-wall construction, but there's an important difference between his wall and the one in the BSC study. He specifies a layer of OSB or plywood on the outside of the inside wall when using an air permeable insulation like cellulose or fiberglass.

    That extra sheathing serves as the primary air control layer as well as the vapor control layer. That method works in a cold climate, like the climate zone 5 where the BSC study took place. It also works in other climates. "Plywood or OSB work in every climate zone," Joe said. "Both of them act as a rigid, smart vapor retarder and air control layer."

    Some builders in cold climates put a layer of 6 mil polyethylene inside a double-stud wall instead of the OSB or plywood that Lstiburek prefers. I asked Joe for his thoughts on that technique: "In terms of the physics, it's great. In terms of practicality, it's a very fragile way of doing things. That's why I like sheet goods. We did this in the early '80s and it was difficult to build."

  2. GBA Editor
    Martin Holladay | | #3

    You can design a safe wall as you propose. Here is a conceptual way to approach the design of this type of wall: first, consider only the inner stud wall and the rigid foam layer. In your climate zone (Zone 4), according to the principles explained in "Calculating the Minimum Thickness of Foam Sheathing," the rigid foam layer for such a 2x4 wall would need a minimum R-value of R-2.5 if we are talking about Marine Zone 4 -- otherwise, the foam layer could be any thickness. So your proposed rigid foam layer (which sounds like it will be rated at about R-8 to R-12) is plenty thick enough.

    Once you have verified that these two layers work together -- the insulated inner stud wall and the rigid foam layer on the exterior side of the inner wall -- then any vapor-permeable insulation that you install on the exterior side of the rigid foam works just fine.

  3. user-6233959 | | #4

    I feel like Gomer Pyle, "Thank you, thank you, thank you!" I realize that my idea is not one of the more common or probably even preferred ways of insulating a wall. I just have an unusual situation, and all of your answers and the links have been a great help. I am very much obliged.

  4. charlie_sullivan | | #5

    I would say Martin's design criteria are actually overly conservative--I would count the added exterior insulation with the foam R-value in checking whether it meets the minimum. In any case, it's fine in your zone.

  5. GBA Editor
    Martin Holladay | | #6

    "I would say Martin's design criteria are actually overly conservative." You're right.

  6. Matt_Michaud | | #7

    This could be a great solution (taped iso air barrier in a double stud) for our project as well. However, it isn't the smart vapor retarder that Joe spec'ed. Instead, it seems comparable to interior 6-mil which is often discouraged. Martin, when do you consider interior vapor barrier acceptable?

  7. GBA Editor
    Martin Holladay | | #8

    Although you asked just one question -- "When do you consider an interior vapor barrier acceptable?" -- I think you are also pondering a second question: "When do you consider it acceptable to have a vapor barrier in the center of a thick wall?"

    I'll tackle the first of these two questions first. There are lots of times when an interior vapor barrier are acceptable. Here are some examples:

    1. If we're talking about an ICF wall.

    2. If we're talking about a wood-framed wall in northern Canada or Alaska.

    3. If we're talking about a cold-climate wall that we know, absolutely, will never include air conditioning.

    4. If the insulation in question is 100% rigid foam, as with the PERSIST approach.

    You get the idea -- the answer is, "It depends."

    Here's my answer to the second question -- "When do you consider it acceptable to have a vapor barrier in the center of a thick wall?"

    The answer is, "When calculations show that the interior side of the vapor barrier will stay above the dew point during the winter."

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