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Unvented cathedral ceiling and tongue and groove?

TNCave | Posted in General Questions on

Hello all,

We are breaking ground on our home in January and are currently getting the planned details worked out with our builder. We are in zone 4, and in a land with no codes.

Our house plan is spec’d for a ventilated cathedral ceiling, but our builder prefers unvented with closed cell foam. We are on a pretty tight budget, but attempting to build the best version of a pretty good house that we can. With closed cell in the roof, can we apply a tongue and groove ceiling without a backing or air barrier?  Or is there some reason to regulate air/vapor flow at the ceiling plane?

Thanks in advance for being an amazing resource.

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Replies

  1. user-723121 | | #1

    Why the spray foam and why an unvented roof assembly? I can tell you a T&G wood ceiling by itself leaks a lot of air. A blower door test along with a thermal scan will confirm this.

  2. Expert Member
    AKOS TOTH | | #2

    If your builder is paying for the cost of the closed cell, than there is nothing wrong with the plan. Since the SPF is both air and vapor barrier, you can install T&G over the SPF without any issues.

    For me SPF is a last resort type of solution, use it only when there is no other option. With a new build, it is always better to design it out of any assembly. The cost is high and not very environmentally friendly.

    If you are paying for the insulation costs, then a vented assembly with fluffy insulation is WAY cheaper (assuming a simple roof shape with soffit and ridge venting). You WILL need an air barrier before the T&G goes up. A layer of drywall is usually the easiest, one of the membrane type air barrier (ie Intello plus) would also work.

    One area to watch is near the ridge. If you are looking to leave part of the beam exposed, it is a hard to joint to air seal properly and can cause a lot of issues.

  3. TNCave | | #3

    We do have a very simple roof with no dormers etc. Construction is cost plus, so I’m paying for everything. Because we are in an area with no building codes, local builders are pretty unfamiliar with air sealing and general modern building science techniques. We are planning blower door testing and using Zip sheathing on the walls.

    I am not a builder, but the air sealing details of site built ventilation channels in an airtight envelope seems like there would be a lot of opportunity for mistakes or sloppiness, leading to poor performance. Especially in my environment, rural TN, where almost everything i’m asking for is new to our builder. And yes, we have interviewed a lot of builders. We ran quite a few off as soon as they heard “blower door test.” I’m not a fan of closed cell foam, but it is something our builder is familiar with, and it might be insurance against future moisture problems compared to a first-time attempt at a vented cathedral ceiling.

    A cutaway of our plans is attached and I’m open to alternative recommendations, but we are leaning toward CCF and I do want to be certain about the T&G at the ceiling plane.

    Thanks again.

    Cory

  4. TNCave | | #4

    In addition, we have, so far, been unable to find a local insulation installer who does dense pack. That also limits our ceiling insulation options.

    Cory

  5. DavidfromPNW | | #5

    I've read about t&g ceilings and see them on HGTV a lot when people are building their own home. I guess I must not be in the know, so maybe someone can inform me. Our house has 5/8" firecode drywall everywhere and we have two separate fire alarm systems that run independent of each other with one being monitored by a UL listed 3rd party with direct link to our Fire Department.

    Isn't t&g without drywall behind it a huge risk in the event of fire?

    1. TNCave | | #7

      We are only considering T&G on the cathedral ceiling, not anywhere with a floor above it.

  6. Expert Member
    Dana Dorsett | | #6

    With simple roof lines it's cheaper to do an unvented roof with exterior-side roofing polyiso and all-fiber in the rafter cavities. That can be made cheaper still if using reclaimed roofing foam at a fraction of the price of virgin stock foam. To duck under code-max U0.026 takes 6" of exterior foam board in 2 or 3 layers, held down by a 5/8" plywood nailer deck through screwed to the structural roof deck with 7.5-8" pancake head timber screws. The truss cavities could be left empty. An all-polyiso solution would then need a 1x8 facia board on the roof edges, which presents an aesthetic issue for some roof designs.

    From the bigger foam reclaimers I can usually get 3" polyiso in near-perfect condition for $15-25 per 4x8 sheet, which is pretty cheap performance compared to R49 closed cell foam under the roof deck, even when factoring in the additional nailer deck, screws, and fascia board costs. Nationwide Foam (https://www.nationwidefoam.com/) has regional depots within reasonable shipping distance of most zone 4 locations, but there is likely to be other sources near you. Repurposed Materials Inc (https://www.repurposedmaterialsinc.com/insulation/ ) has fewer locations, but also regularly gets this type of material in: https://www.repurposedmaterialsinc.com/polyiso-insulation/

    There are others. Running this search on the local craigslist every week or two will ferret out some of the more local or smaller players:

    https://jerseyshore.craigslist.org/search/sss?query=rigid+insulation

    https://cincinnati.craigslist.org/search/sss?query=rigid+insulation

    https://kansascity.craigslist.org/search/sss?query=rigid+insulation

    https://salem.craigslist.org/search/sss?query=rigid+insulation

    Alternatively, just 3" of exterior roofing polyiso board (R17) is sufficient dew point control for zone 4A or 4B (2" would do it for zone 4C) , for more than R35 of cavity fill. Then the remaing R32 (out of R49) could be 9" of damp sprayed cellulose (with water-activated adhesive, not dense packed) or open cell foam on the underside of the roof deck gets you there on an R-value basis. With a 5/8" nailer deck it only takes a 1x4 fascia board to cover the roof edges (or even a 4" commercial building type drip edge and no fascia board.)

    For fastener specs and fastening patterns use a nailbase polyiso panel vendor's recommendations as a guide, eg:

    https://www.hunterpanels.com/docman-categories/product-documents/hpanels/speciality-products/1388-h-shield-nb-application-guide/file

    With spray foam on the underside of the roof deck the ceiling needs to be a timed thermal barrier against ignition, but you may be able to get away with intumescent paint on the surface of the foam. With cellulose on the underside of the roof deck that is not an issue. The thermal mass of 11" of cellulose is also substantially higher than foam (any density), which offers a shoulder season performance benefit, as well as a summertime cooling benefit, since the peak ceiling temperature will lag the peak solar gains by several hours.

    1. TNCave | | #8

      Dana, thanks very much for this detailed response. This definitely gives me a lot to consider.

      Regarding splitting the insulation between exterior and interior, I was under the impression that damp spray cellulose wasn’t an option for cathedral ceilings.

      Thanks again,

      Cory

    2. TNCave | | #9

      Dana, is this cutaway from the detail library similar to what you described for the “split method”: https://www.greenbuildingadvisor.com/cad/detail/metal-roof-unvented-cathedral-exterior-4-1-2-rigid-insulation

      Except with 3” of exterior foam?

      Thanks again,

      Cory

      1. Expert Member
        Dana Dorsett | | #12

        Yes, that's pretty much it, except for the foam thickness, and the fact that you have trusses, not common rafters. If it were rafters it would be easy to make it work with high density batts on the interior, but with trusses the fit is harder to make perfect without using sprayed/blown insulation.

        Some damp spray products are alleged to have sufficient adhesive strength to apply directly to ceilings without other support, which should be cost-competitive with open cell foam. (I haven't used it myself.) The more common method is to blow cellulose behind mesh, which is more time-consuming & expensive- often more expensive than an open-cell foam solution for the interior side R.

        With more detail on the truss dimensions and spacing there may be other options to consider for the interior-R. I-joist type trusses 24" o.c. with 1.5" deep flanges and half-inch webbing can be fairly easy to get right using batts designed for steel framing.

        Have a ZIP code?

        1. TNCave | | #13

          Sorry for the delay. I missed this response. Thanks again for the expert assist. We’re located in 38564.

          Cory

  7. TNCave | | #10

    I just realized that we do have a shed-out porch roof on one side that would complicate exterior sheathing insulation. Plan photo attached.

  8. Expert Member
    AKOS TOTH | | #11

    There is no need to dense pack to go with a vented roof.

    Since you have no code, you can go with R30 batts in between the rafters. With truss roof construction you get very little thermal bridging, with R30 batts with drywall+T&G you end up with roughly an R32 whole roof, which is pretty good in warmer climates.

    If you want more R value, you can go with two layers of R23 batts which would get you up to an R44 roof.

    Go with high density batts designed for metal framing as they are wider. These are standard items at any commercial drywall house. You can trim the batts around the webs of truss framing. Important for the batts to snug up against each other when finished without a gap.

    Air tight drywall is not that hard. The only difference is putting a bead of caulk around the perimeter of roof sections, the rest is muded as normal. If you are putting T&G over the drywall, the drywall only needs to get taped, it doesn't need to be finish sanded. As belts+suspenders approach, you can put a bead of flex sealant between each drywall sheet before taping.

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