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Smart vapor retarder over open-cell SPF on underside of roof in Zone 4 (Washington, DC)?

Elizabeth Hilder | Posted in Energy Efficiency and Durability on

I am having open-cell foam installed under several attic roof areas (collar attic, knee wall.areas, etc.) I have seen lots of statements about the need for a vapor retarder in Zones 5 and higher, as well as the general advice in any zone to add a supply and return (this would be complicated to install this, because it is not simply one big attic space and in any event our heat is from radiators, not forced air ducts).

My plan is to put humidity monitors in the relevant areas and see whether the levels indicate we need to do something to prevent the winter moisture concerns I have read about on this site.

Here are my questions:

1. If moisture concerns arise, it is ok to address them by installing the MemBrain vapor retarder over the open cell spray foam, even in zone 4? (no drywall is being installed over the foam; just the ignition barrier paint)

2. The spray foam will completely cover the rafters (to reduce thermal bridging? Is it feasible to install such a vapor barrier in that situation using tape?

Many thanks!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Elizabeth,
    There may be a fire safety concern when the MemBrain is left exposed. I would check with CertainTeed (the distributor of MemBrain) and your local building inspector before proceeding with your plan.

    My usual advice is that closed-cell spray foam is associated with fewer problems than open-cell spray foam when the insulation is applied to the underside of roof sheathing.

  2. D Dorsett | | #2

    From the short-sheet spec ( http://www.certainteed.com/resources/30-28-080.pdf ):

    -------------------

    Fire Resistance
    – ASTM E84
    Surface burning characteristics
    Max. Flame Spread Index: 20
    Max. Smoke Developed
    Index: 55

    -------------------

    Whether that's good enough to meet local codes in that type of installation is an open question.

    While some attics in zone 4 would have a moisture cycling issue, most don't. From a total vapor permeance point of view, most open cell foam would be close to class-II vapor permeance at R49, ~2 perms @ R38, so the risk to the roof deck is not super high, but lower permeance would be better. See Table 3 in this document:

    http://buildingscience.com/file/5809/download?token=18Y6NJQ8

    The zone 4A city example modeled with WUFI (Kansas City) showed higher than ideal roof deck moisture on the north facing pitches when only R38 half pound foam at the roof deck.

    But if the installer can give it a 1" shot of closed cell as the first layer of encapsulation with the rest being open cell foam the roof deck would be quite safe. Then it's only a matter of waiting to see whether there would be excessive moisture cycling in/out of the open cell foam without actively ventilating the attic spaces.

  3. David Meiland | | #3

    Choosing open cell over closed in this situation seems like an unnecessary risk to me. If I thought I had to put in monitors and make a contingency plan to deal with a possible moisture problem down the road, I'd probably just look for another approach to begin with. And, if there's not ductwork in the attics, why insulate the roof at all, as opposed to the floor?

  4. Expert Member
    Dana Dorsett | | #4

    David: I understood "...because it is not simply one big attic space..." to mean that there are partition walls and possibly even some built-out conditioned rooms in the attic, ergo insulating at the floor isn't much of an option.

    The risks of open cell foam in that climate are perhaps a bit exaggerated, despite the fact that there are existence proofs of homes with problems. The 1" closed cell against the roof deck with the rest being open cell fully protects the roof deck. Using all 1lb foam instead of half-pound foam would be sufficiently vapor tight to be very low risk too.

  5. David Meiland | | #5

    If the attic is divided up by intervening areas of cathedral ceiling, then there will need to be multiple strategies, with insulated sidewall in the attic, insulated rafters in the cathedral areas, and IMO insulated attic floor where possible. I've been in hundreds of attics and I know that many are not easy to insulate well, but I still don't see the appeal of open cell. In fairness, the 2-3 insulators that work in my market offer closed cell, and that's what I use when I need to insulate the roof deck. Minimum here (4 marine) is 3" open cell if the rest is vapor permeable (usually cellulose if I'm doing it).

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