GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Retrofitting walls with interior vapor barrier?

Calum_Wilde | Posted in General Questions on

My house is in climate zone 6a, has 2×6 exterior walls, sheathed with OSB, with batt insulation, and an interior vapor barrier. Is there any way to retrofit these walls with exterior insulation that won’t cause issues as they can’t dry to the inside?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    If you are conservative, the best answer is to install a continuous layer of mineral wool on the exterior side of your sheathing, followed by furring strips and new siding. Mineral wool is vapor-permeable, so this kind of wall can dry to the exterior.

    That said, installing exterior rigid foam to your wall isn't as dangerous as you think. Here are links to two articles that discuss the riskiness of walls with interior polyethylene and exterior rigid foam:

    The Exterior Rigid Foam is Too Thin!

    Rethinking the Rules on Minimum Foam Thickness

  2. Calum_Wilde | | #2

    Thank you!

  3. Expert Member
    Dana Dorsett | | #3

    The Canadian manufacturer IKO has vapor permeable polyiso products could also be used:

    At any of their standard thicknesses it's supposed to be greater than 1 US perm, and thus only a Class-III vapor retarder.

    According to their marketing fluff:

    "Available in 4' x 8' and 4' x 9' boards in 12 mm (0.5"), 16 mm (0.625"),
    18 mm (0.75"), 25 mm (1.0"), 38 mm (1.5"), and 50 mm (2.0") thicknesses. "

  4. Calum_Wilde | | #4

    Thanks Dana.

    Is the cold weather derating still required for polyiso?

  5. Yupster | | #5

    For what it's worth, in the Ontario climate, people have been building 2x6 with poly and 1" rigid exterior foam for a long time and no widespread failures so far. Our rain and wind loads are pretty low though. When you do have a bad flashing or leak it definitely doesn't dry out!

  6. Expert Member
    Dana Dorsett | | #6

    Calum: You'd have to ask the manufacturer. Dow can't be the ONLY manufacturer to have made headway on this. But even with derating it will outperform rigid rock wool inch-for-inch (and probably dollar-for-dollar too.) Rigid rock wool is good for about R4/inch. An inch of exterior polyiso would meet or beat that even on the coldest days of winter. See the derating/uprating curves on p.2 of this document:

    The R3.6/inch @ 75F mineral wool board rises to about R4/inch @ 0F mean temp through the layer, about an 11% improvement. R4/inch @ 75F rigid rockwool would likely have a similar rise, hitting R4.4 @ 0F. NS is cold enough, but it's not THAT cold. Only on the coldest hours of the coldest days would the mean temp through the insulating sheathing hit 0F, and not much below that in your stackup.

    The crummy R.5.2/inch @ 75F polyiso in DOW's derating curve is still R4.4 @ 0F, no worse than the inch of rock wool estimate.

  7. Calum_Wilde | | #7


    I've read dozens of your posts here and I learn something new every time. Thank you so much for your patience and for sharing your knowledge with others less educated in your field. I don't know where you find the time, but you're helping to make the world a better plac, thank you.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |