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Community and Q&A

Spray foam retrofit from the exterior?

user-5367238 | Posted in Green Building Techniques on

I’m re-siding a house built in the 60’s with minimal fiberglass insulation in the walls. Many of the rooms are quite cold. I am considering two options below: (Note: removing the interior finish, or even blowing in from the inside is not an option)
1) removing the siding, sheathing, and insulation, then applying open cell spray foam in the stud cavities from the outside, then sheathing with R6 zipwall.
2) just removing the siding, applying the R6 zipwall

Obviously option 1 is more work because of removing the sheathing, but I get a higher R value. However, I’m concerned about not having a vapor barrier on the inside of the wall. I know I will have to deal with the windows if I remove the sheathing, but that is part of a more complicated discussion which is better not discussed for clarity.

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  1. Expert Member
    Dana Dorsett | | #1

    It's usually possible to dense pack cellulose in over the pre-existing batts with 1-3 holes per stud bay, which is both cheaper & easier than stripping the sheathing and spraying open cell foam.

    Before putting up the ZIP-R it's important to know if there is an interior side vapor barrier such as polyethylene sheeting, or foil-facers on the interior side of the batts. If there is a true vapor barrier on the interior, putting up the ZIP-R could create a moisture trap, due to the sub-1-perm vapor permeance of the ZIP-R:

    Local climate is another important factor to consider- where are you?

  2. user-5367238 | | #2

    Located in Lincoln MA, outside of Boston. My concern with drilling from the outside is that it never seems to get effectively past the wiring, stud pockets, random blocking etc. When I have demolished blown in walls I often see voids. Also, I assume the fiberglass was foil faced, as that's what it is in the attic. Maybe one more reason to remove the sheathing (so I can avoid the moisture trap)?

  3. Expert Member
    Dana Dorsett | | #3

    Drill some test holes to verify the facers. It's sometimes possible to yank them out without pulling the sheathing or wallboard, other times not. But yes, that could be a deciding factor.

    When drilling from the exterior with multiple blowing holes you can find the blockages and fill the potential voids. Even if you can't get the hose fully past the batts, a hole every 3-4 feet will usually get good coverage.

    R6.6 ZIP-R is marginal from a dew-point control perspective in US climate zone 5, especially with temperature derating of the polyiso for a cool climate. On the exterior side of the assembly that R6 attributable to polyiso will be more like R3 whenever it's below 20F outside. The binned hourly average January temp in Lincoln MA is about 25F, so it'll probably make it, but not by a huge margin.

    To meet current MA code-min performance with 2x4 framing you'd need R5 on the exterior. 3" of closed cell foam in the cavities would meet the letter of code, but under-perform an air tight R13 + R5 c.i. assembly by quite a bit. A cellulose or open cell foam full fill with R6 ZIP-R would make it, despite the slim dew point margin. If you pull it apart and air-sealed the gypsum to the framing before damp-spraying cellulose it will perform as-well or better than open cell foam from both a moisture & thermal point of view.

    BTW: Not particularly relevant, but I drive through Lincoln on my daily commute.

  4. user-5367238 | | #4

    thanks for the comprehensive answer! I think I will open up a section somewhere to verify conditions, but I suspect that I will do the foam and R6 zip, since the client has already agreed to that. Do you have experience with using zipR6 as sheathing? I spoke to Huber (zip's parent) who said it is code approved as sheathing, but I'm curious how stiff it feels. On the other hand, the foam will provide stiffness as well. Are you an energy consultant? Builder? Architect?

  5. GBA Editor
    Martin Holladay | | #5

    You will obviously get a higher R-value, and less air leakage, if you fill the studs with open-cell spray foam than if you just leave the stud bays filled with the existing minimal fiberglass. But you know that.

    I'm somewhat confused by your statement, "I'm concerned about not having a vapor barrier on the inside of the wall." Are you saying that it is important to you to have an interior vapor barrier, or that it is important to you not to have an interior vapor barrier?

    I assume that you are aware that open-cell spray foam is vapor-permeable, so installing open-cell spray foam will not provide an interior vapor barrier. I assume that you also know that in your climate, you don't need an interior vapor barrier. (The important thing is limiting air flow. There is no particular reason to worry about vapor diffusion, although in your climate a wall without an interior vapor barrier usually performs better than a wall with one.)

    For more information, see Do I Need a Vapor Retarder?

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