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Elaborate on the closed-cell vs. open-cell unvented roof debate

michiganman18 | Posted in General Questions on

I have been reading many of the articles on this here, and why closed cell foam is better than open cell for thinner insulation numbers.

Mostly I agree. I can understand why closed cell foam performs better for only smaller and thin numbers of insulation, but why if there will be 5” of OC is moisture still a concern? The warm air will not reach the cold sheathing so would not 5” of OC foam be a viable option at keeping the roof deck dry?  This would address thermal bridging, it is an out building so code is not as pressing.

Background: I am building an outbuilding with an overhead door, in the topside of zone 5. The building will be conditioned year round, unvented roof. I am putting  one layer of 1inch EPS foam on the exterior walls, r15 mineral wool, in 2×4 wall studs.  The roof already is on or I would add exterior foam to the roof as well.  I consider this to be a good practice run, for building a better building in the future.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Michigan,
    Q. "Why if there will be 5 inches of open-cell is moisture still a concern? The warm air will not reach the cold sheathing, so would not 5 inches of OC foam be a viable option at keeping the roof deck dry?"

    A. Vapor diffusion is possible through some materials -- even through air barriers. Although 5 inches of open-cell spray foam is, indeed, an air barrier, the foam is still vapor-permeable (with a permeance of about 10 perms).

    For more information on this issue, see "All About Vapor Diffusion."

  2. Expert Member
    Dana Dorsett | | #2

    >"I am building an outbuilding with an overhead door, in the topside of zone 5. The building will be conditioned year round, unvented roof. I am putting one layer of 1inch EPS foam on the exterior walls, r15 mineral wool, in 2×4 wall studs. "

    Just 1" of EPS (R4) isn't sufficient dew point control for R15 at the cold edge of zone 5 ( if that's what's meant by "topside of zone 5", but 1.5" is. The IRC prescribes R5, with a presumptive R13 in the walls for zone 5 to be able to skip the interior side vapor retarder. To keep the same R-ratio at R15 (and thus the same average temp at the sheathing) takes R5.77.

  3. Jon_R | | #3

    WIth unfaced EPS, your wall exterior is probably more than 1 perm (wet), so best to use a Class III (vs lower) vapor retarder on the interior side. Code allows you to do that with vented cladding or 1.5" EPS foam (both is even better).

    Lower exterior perms should use Class II (see table 3 below):

    https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

  4. michiganman18 | | #4

    Thank you Martin, Dana, and Jon for your replies.

    Martin: Vapor diffusion, not condensation is the problem that seems to arrise with open foam. I think I understand now.

    Dana, I must have had an R value mixed up somewhere, as I read a sheet rigid foam product spec that was at an R value of 6 for an inch. Perhaps it was an XPS product. This is what I planned on installing on the exterior. Yes, topside meaning the cold side of zone 5.

    Jon: (or all) Such as latex paint as a vapor retarder? Cladding with be vinyl.

    https://www.energy.gov/sites/prod/files/2014/01/f6/4_3b_ba_innov_vaporretarderclassification_011713.pdf

    Thanks for all the help. Im getting closer.

    1. Expert Member
      BILL WICHERS | | #5

      R6 per inch is the usual spec for polyiso. Perhaps that is what you were thinking of?
      XPS is typically specced as R5 per inch. EPS is the lowest R per inch of the three common types of rigid foam insulation with a value as stated by Dana. XPS does gradually degrade over time and begin to approach EPS values eventually, but that’s not how it’s rated by the manufacturers.

      Bill

    2. Jon_R | | #8

      Yes, 3 coats of latex should get you below the 10 perms needed to be a Class III vapor retarder. But switch to Membrain if you are going to use XPS or a faced rigid foam.

  5. michiganman18 | | #6

    Thanks Bill, I do believe I was looking at a polyiso.

    I will have to go with two layers of EPS foam. Now only to find a decent way to insulate the overhead door beyond its 2.5inch layer of insulation.

    1. Expert Member
      Dana Dorsett | | #7

      In zone 5 with just an inch of polyiso most products would have to be derated to R5/inch from a dew point control perpective. Dow Thermax claims to have found the holy grail and does not need derating, and can even be up-rated at lower temperatures enough to use with R15 in zone 5, if that makes the construction any easier. At 25F mean temp through the foam (say 40F on the warm side, +10F on the cold side) it runs about R6.8/inch, and continues to climb in performance hitting R7/inch a when the mean temp through the foam hits 0F (say 30F on the warm side, -30F on the cold side, probably colder than the record cold outdoor temp in your area.) See the graph on page 1 of this bit o' marketing fluff.

      http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_098a/0901b8038098a7e2.pdf?filepath=styrofoam/pdfs/noreg/179-00379.pdf&fromPage=GetDoc

  6. MattJF | | #9

    $/R is generally better for 1.5" and 2" foams, while not requiring significantly more work to install. As stated above, you probably want 1.5"+ of exterior foam. With the correct amount of foam you can skip an interior vapor retarder because there is no cold surface for vapor to condense on.

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