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Closed-cell foam, unvented ceiling, and vapor barrier

user-5254480 | Posted in General Questions on

This is a follow up to a previous question:

Roof is 2:12 slope with metal standing seam over 5/8″ decking. The joists ends have been blocked, sealed and wrapped.

I now have 2″ of poly-iso, above roof decking, which according to this chart ( gives us averaged R value of 11.4.

The overall target R value for the roof assembly is R49.

We are using closed cell foam for some other areas in the house and am considering using it to address the underside insulation needs of the roof. I also just a bit worried about any water build up in cellulose.

At a target total of R49 and with 11.4R on top – that leaves approx. 37.6R to fill. This would create 23% of total top side R. I could add another 6R below and still be above the 20% ratio.

If I target 37.6R below and use CC foam (6R per “), I’d need 6.3″ of foam. My joists are 12” deep.

– Should I add the additional bottom side foam to get closer to the 20% ratio? I’d rather stay closer to 22-23% to be safe.
– If I use CC foam do I still need a vapor barrier between the sheet rock and the joist bays?
– Is having a 5.5″ gap between the foam and sheet rock an issue? Should I look into alternate methods – to fill the entire bay – like Dana describes in the previous thread – to reduce convection? We are not using any cans in the ceiling.


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  1. GBA Editor
    Martin Holladay | | #1

    In this question, you forgot to tell us your climate zone. But I clicked the link you provided to your earlier question, and learned that your house is in Climate Zone 4C (Seattle area).

    Q. "Should I add the additional bottom side foam to get closer to the 20% ratio? I'd rather stay closer to 22-23% to be safe."

    A. It's your house. I can't think of any reason why you shouldn't choose the option you prefer.

    Q. "If I use closed-cell spray foam, do I still need a vapor barrier between the sheetrock and the joist bays?"

    A. No. I'd like to make two points to add to my answer: You wrote "still," but there isn't -- there never was -- any requirement for an interior vapor barrier in this location. (Your code probably requires a vapor retarder, but that's different from a vapor barrier.) In any case, 6 inches of closed-cell spray foam is already a vapor barrier. For more information on this topic, see Do I Need a Vapor Retarder?

    Q. "Is having a 5.5-inch gap between the foam and sheetrock an issue?"

    A. No. You can even put recessed can fixtures in there if you want (assuming they fit), because this space is on the interior side of your air barrier, and is entirely inside of your home's thermal barrier.

    If you will indulge me, I'm going to invent a question you didn't ask me.

    Q. "Is it a good idea to sandwich the roof sheathing between vapor-impermeable roofing and vapor-impermeable spray foam?"

    A. It's not great -- I would have used cellulose under the roof sheathing instead of spray foam, because it is more environmentally friendly and more vapor-permeable -- but people do it all the time. Just make absolutely certain that the roof sheathing is quite dry on the day that the spray foam is installed, because it won't dry out after it's been sprayed.

  2. Dana1 | | #2

    With a target total of R49 and R11.4 above the roof deck you are already at 23%, above the code prescribed 20%, and would not need additional closed cell foam to be moisture safe at the roof deck.

    You could install R38 fiber in the rafters and be done with it. (Cellulose is good, and would provide additional moisture protection due to it's wicking and moisture buffering capacity.)

    If you want higher than R50, add 2" of ccSPF on the under side of the roof deck, and 2" of polyiso on the rafter edges only, and install R38 batts in the cavity. That puts it at about R62 center-cavity, with R24 outside the first condensing surface, a ratio of 39%.

    With 5.5" of thermally bridging rafters the framing loss is twice that of a full cavity fill. With R12 polyiso rafter edge strips the framing loss is half that of a full cavity fill. The R62 cavity fill with R12 rafter edge strips will substantially outperform 6" of closed cell foam with a 5.5" deep empty cavity, at a lower overall cost, and a much lower overall environmental hit. (2" of closed cell foam instead of 6".) At 2" closed cell foam the vapor permeance is still only around 0.5-0.6 perms, which is a sufficient drying path, and it can be installed in one pass instead of three, with a cooling/curing period between lifts.

  3. user-5254480 | | #3

    Thank you for your thorough answers. I am putting together a plan.

  4. user-5254480 | | #4

    One more follow up on this thread.

    Recapping this is a 2:12 pitched roof over a living/dining room and kitchen. The high point is on the living side, with low point on kitchen side. For insulation we are moving forward with 2"CC + batts.

    The issue we are hitting is how to treat the existing ceiling area above the kitchen. This area has a flat ceiling so there is about a 7x15 area of sofit space with 3.5' on high side down to about 8" on low side. There are existing cans, however none of them stick up into the joist bays. I realize that this all in the same inside conditioned space, I was just worrying that since it has no ability to breathe or vent that it might create some heat or moisture issues.

    - Should I add some vents to help this area breathe?
    - Should I remove the cans and use low profile pucks and try to seal the space up more tightly?
    - Should I do nothing?


  5. GBA Editor
    Martin Holladay | | #5

    If you have polyiso above the roof sheathing and closed-cell spray foam under the roof sheathing, it's clear that this is an unvented roof assembly.

    Q. "Should I add some vents to help this area breathe?"

    A. No. Your attic is now indoors. Don't introduce any outdoor air into this space.

    Q. "Should I remove the cans and use low profile pucks and try to seal the space up more tightly?"

    A. The answer to this question depends on whether the recessed can lights intrude into space that is insulated. If the recessed cans are below the insulation layer, they are doing no harm. If the recessed cans are taking up room that should be filled with insulation, then you should removed the recessed cans.

  6. user-5254480 | | #6

    Correct - this is an unvented roof assembly. I should have been more specific about the venting. I was referring to some internal venting that let the sofit breathe back into the internal space - not venting to the outside. My concern is that this "attic"/sofit space will get hot and trap vapor since it has no venting as the top side of it is the unvented assembly and the bottom side is sheet rock. Thoughts?

  7. Expert Member
    Dana Dorsett | | #7

    Vapor isn't trapped by heat, it's released by heat (higher temps = higher vapor pressure.) Water vapor diffuses fairly freely through unpainted sheet rock, more slowly through latex painted sheet rock, but it's still sufficient for drying an air tight sealed off space toward the room air, and not a moisture trap.

    But soffit area needs an air-barrier at the plane of the exterior wall to STOP air from flowing into & out of the conditioned part of the attic, otherwise it's a thermal bypass channel. That barrier should also be insulated to at least the same level as the exterior walls.

  8. GBA Editor
    Martin Holladay | | #8

    Dana is right. Properly detailed, your attic is inside your home. Nothing is trapped -- it's just ordinary indoor air.

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