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Fill soffit with open cell spray foam?

2screws | Posted in General Questions on

Thank you for helping me understand things!
I’m planning a new slab house build in CZ3 with OC spray foam conditioned unvented attic with shingle roof and planning a dehumidifier along with HVAC in attic.

I requested nonperforated soffit planks for the enclosed soffit. Spray foam sub said then he’d have to fill soffits with the same open cell spray foam he’d use under roof deck.

I know it will increase cost, but is it the right thing to do?
Chris

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Replies

  1. insaneirish | | #1

    From your other posts, we know you're looking to do an unvented roof assembly with vapor permeable insulation and a vapor diffusion port.

    However, I don't think we know what you plan to be your air control layer. The soffit doesn't need to be filled with foam, but if you have an unvented roof assembly, there does need to be an air control layer.

  2. 2screws | | #2

    Roof layers: fiberglass shingles, felt, OSB sheathing, OC spray foam.
    Drop the plan of a vapor diffusion port.

    1. insaneirish | | #3

      Unvented or vented roof?

  3. 2screws | | #4

    unvented, conditioned, dehumidifier

    1. insaneirish | | #6

      Total R-value of roof?

  4. lesprice | | #5

    Good Luck. I am also building a home on a slab (in CZ4 East TN). After much research, I have decided to go with OC foam (9,5 in for 2x10 rafters) and plan to have dehumidifier to avoid the moisture diffusion to the roof deck problem. I discussed the CC+OC option, but the insulation contractors around here won't do it because of the "can't see a leak" issue. I have specified no overhang on the gables and only 6 in at the eaves, so there will be only a small soffit cavity, which will be filled with foam

  5. insaneirish | | #7

    > I am also building a home on a slab (in CZ4 East TN). After much research, I have decided to go with OC foam (9,5 in for 2x10 rafters)

    9 1/2" of open cell foam does not meet code minimum insulation requirements in climate zone 4. I suggest re-evaluating this decision in a brand new house.

    > I discussed the CC+OC option, but the insulation contractors around here won't do it because of the "can't see a leak" issue.

    This is an erroneous argument. From Building Science Corp. (https://www.buildingscience.com/documents/insights/bsi083-mea-culpa-roofs): "There has been much discussion about open-cell being better under a roof deck than closed-cell because if there is a leak the leak either dries faster or you can see it sooner. Not true. There is no compelling evidence one way or the other. None."

    1. lesprice | | #10

      Thanks Patrick. re R value - yes I am aware that 9-10 inch oc does not meet the R49 code. What I am told is that that thickness of foam (~R38) in a conditioned attic is considered as effective as R49 in the attic floor, which is the basis for the code value.

      Yes, I am aware of Lstiburek's (and many others) view, but I got the same answer from 3 different foaming contractors. I suspect they just want to give a simple answer to customers and don't want to drive them toward a cc foam approach which would be so expensive that they would abandon the unvented attic idea.

      1. insaneirish | | #14

        > What I am told is that that thickness of foam (~R38) in a conditioned attic is considered as effective as R49 in the attic floor, which is the basis for the code value.

        Never heard this claim before, nor can I think of any basis for it. In fact, one could argue *the opposite* is true; that it's more important to have higher R value on a sloped roof than a ceiling. The reason for this is that, thinking of a simple gable roof over a flat ceiling, the roof has more surface area than the ceiling. The total amount of energy transferred over a temperature gradient is dependent on the surface area of the interface between the two zones, so a sloped roof of a given R-value will have more heat loss/gain than a ceiling insulated to that same R-value.

        1. insaneirish | | #15

          > Never heard this claim before, nor can I think of any basis for it.

          I realized what they're using to claim this, and it's mentioned here: https://www.greenbuildingadvisor.com/article/three-code-approved-tricks-for-reducing-insulation-thickness

          However, don't confuse code approved as being "as effective". Your energy bills will be higher than they would be with R-49.

          1. lesprice | | #16

            Actually, I think the answer is simply that in the county where I am building, they only require compliance with the 2012 IRC, which is 38

  6. Expert Member
    Rick Evans | | #8

    2screws,

    You Wrote:

    "I requested nonperforated soffit planks for the enclosed soffit. Spray foam sub said then he’d have to fill soffits with the same open cell spray foam he’d use under roof deck."

    I feel like I am missing something here... The only reason that I can think of to insulate the overhang is to help guard against ice dams. I doubt that this is an issue where you live. Why insulate the soffit at all? And what does it have to do with you adding soffit boards?

  7. 2screws | | #9

    Yes Rick, I didn't think leaving the soffits empty and using nonperforated soffit boards would be a problem. Spray foam sub claims that will risk roof deck rotting over the soffit sections.

    There's only snow/ice in our area once every 2 or 3 years. And I hadn't come across any articles reporting that risk. That's why I posted the question. I suppose more insulation is better and would protect top plate areas better, but I'm just trying to learn more from the forum.
    Chris

  8. jberks | | #11

    Hi Chris,

    I do unvented flat roofs, and last one had about a 1' overhang. I added insulation in the soffit only against the blocking in between the rafters to maintain the exterior insulation. It was 2" of ridged rockwood. But I did CC spray foam from the inside as part of the airsealing strategy.

    So I don't see a point to filling the entire soffit cavity with foam. and I assume you don't have exterior insulation.

  9. 2screws | | #12

    Correct Jamie, no exterior insulation.

  10. 2screws | | #13

    Here's the reasoning of the oc spray foam contractor: Solid soffits are fine, but it’s going to mean extra foam to cover the entire roof deck including that which overhangs the outside of the building. If you don’t have an air barrier on it that roof deck will rot much faster in that area.

    Any insights?

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