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Attic ceiling project

Chris Haddox | Posted in General Questions on

OK…read lots, know lots, but have a tough situation: 1924 unvented hot/cold attic, shallow rafters (4″), plank roof deck, asphalt shingles/tar paper and wishing to finish the attic space for office/play area. Previous owner installed very thin layer of foil faced batt against roof deck at least 15 years ago–no thermal impact, but more importantly there has been no moisture issues for the past 43 years (28 him, 15 me). I really don’t want to go closed cell spray. Instead, I am figuring scabbing the rafters to get 6″ depth, then faced glass batts in the cavities against roof deck and rigid/taped seams over that for thermal bridging, then T&G pine for finished ceiling. I know T&G is not anywhere close to air/moisture barrier, but I’ve zero moisture issues in the past with the tiny amount of foil faced batt that has been there for at least the past 15 years, so just don’t think I am getting much moisture laden air up there in the first place…my rationale for not doing drywall. I know I won’t hit a code requirement for thermal, but this is not a permanent living space–just looking to add some comfort level for when the space is being used. Crazy? Better ideas? Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    Bad idea.

    If you want to use fiberglass batts, you have to have a ventilation channel between the top of the insulation and the underside of the roof sheathing.

    The fact that the existing assembly has no moisture problems now may simply be due to the fact that the existing batts are so thin and poorly installed that they are ineffective. Thicker batts could easily lead to moisture problems.

    To read about all the different ways you can do this correctly, see this article: How to Build an Insulated Cathedral Ceiling.

  2. Chris Haddox | | #2

    Thanks, Martin. I was thinking of baffles (channel) between the batts/deck, but as I have no soffit/ridge vents, how would the system benefit?

  3. GBA Editor
    Martin Holladay | | #3

    You have two choices:
    (a) Install soffit vents and a ridge vent so that you can create a vented roof assembly, or
    (b) Create an unvented roof assembly with spray foam or rigid foam.

  4. Chris Haddox | | #4

    with rigid, cobble into the bays, or just leave them open and go with rigid sheets to the rafter bottoms....then drywall over that, i suppose. thanks again, martin

  5. GBA Editor
    Martin Holladay | | #5

    No. If you want to use rigid foam for an unvented roof assembly, the rigid foam has to be installed on top of the roof sheathing. This method will require you to install new roofing.

    If you want to do all the work from the interior, you will need to use spray polyurethane foam.

    All of this is explained in the article I linked to: How to Build an Insulated Cathedral Ceiling.

  6. Chris Haddox | | #6

    Thanks, Martin....foolishly hoping for something new, but you comments snapped me back to reality! Appreciate your time.

  7. Chris Haddox | | #7

    Martin, I went with open cell foam insulation against my attic roof deck. The job was challenging as the house is 1920s with unvented hip rafter roof (6" depth) with 3 dormers and a short ridge. There are several short jacks interspersed in between the common rafters, as well. Regardless, we were able to seal the entire perimeter over the wall top plate and get good coverage on the roof deck. The job was well done and once I address the old windows, I should have a very nicely sealed/insulated space. My question now centers on vapor retarders and thermal bridging via the exposed bottom of the rafters. IECC Zone 5 appears to warrant a Class II retarder. As I am finishing the ceiling with T&G pine, I am considering a thin application of fanfold insulation...stapled/ cover the spray foam, then the T& address vapor and thermal bridging through the exposed rafter bottom. Thoughts? Overkill? Thanks!

  8. GBA Editor
    Martin Holladay | | #8

    You definitely need an interior vapor retarder. Fan-fold foam is one option, but it's only a good idea if you are sure you know the vapor permeance of the product you install. If it is thin EPS with no facings, it could be quite vapor-permeable -- and in this case, you're probably looking for a product with a rating of 1 perm or less.

    One option is a smart vapor retarder like MemBrain. Another option is 1 inch of XPS; the only disadvantage of XPS is the fact that it has a blowing agent with a high global warming potential.

  9. William Howell | | #9

    Martin's answer in Dec 2011...

    Don't bother to include ventilation. Instead, fill the 3.5 inch rafter bays with 3.5 inches of polysio (one 2-inch layer, and one 1.5-inch layer). Seal all gaps with canned spray foam. Then install a continuous layer of 2-inch polyiso under the rafters to stop thermal bridging.
    Using several layers of foil-faced foam is fine -- there is no problem with making a sandwich like this, except that it is slow, fussy work.

    I am considering doing this in my Victorian in Salt Lake City for similar reasons and would like to avoid spray foam if I can. Has this recommendation changed?

  10. GBA Editor
    Martin Holladay | | #10

    A few years ago, I hadn't heard any reports of failures of unvented roof assemblies insulated with the cut-and-cobble method. Since then, I have heard of several such failures.

    I no longer recommend the cut-and-cobble approach for cathedral ceilings unless the assembly includes a ventilation channel between the uppermost layer of insulation and the roof sheathing.

    For more information on reports of cut-and-cobble failures, see Cut-and-Cobble Insulation.

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