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We are installing 1×6 T&G Pine on our ceiling

carwood | Posted in General Questions on

We are installing 1×6 T&G Pine on our ceiling. Have recently heard we should install drywall layer first. Is this correct?

New construction house with 2/12 pitch roof on a gulf island about 80 miles north of Vancouver, BC. Climate very similar to Seattle, Wash. The roof costruction is 24 on center, 9 inch trusses (TJIs), with 2×4 horizontal strapping, 1/2″ plywood, ‘Nova Seal’ waterproof membrane and a standing seam metal roof. Insulation is two layers of Roxul, one R22, second of R14 for total of R36. No vapour barrier being installed on walls either, just 1/2 ” drywall. Do we really need a layer of drywall as well as the T&G Pine?

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  1. carwood | | #1

    Against our builder's advice, we are not planning to install poly vapour barrier on ceiling or walls. Hope we are doing the right thing.

  2. GBA Editor
    Martin Holladay | | #2

    You definitely need an air barrier on the interior side of your Roxul insulation, and T&G pine boards are not an air barrier. So, yes -- you need the gypsum wallboard, with taped joints, on the interior side of your TJI trusses.

  3. GBA Editor
    Martin Holladay | | #3

    I just realized that your roof has a 2/12 slope. That means that you probably want to read this article: Insulating Low-Slope Residential Roofs.

  4. Expert Member
    Dana Dorsett | | #4

    You will definitely need some sort of air-barrier between the t & g pine and the rest on both the roof and the wall assemblies.

    Unless measures were taken to fully vent the 1.5" air space between the 2x4 purlins to the exterior in a manner compliant with R806.1 of IRC 2012 (see: ) using poly would be a BAD idea. But you can use a "smart" vapor retarder such as Certainteed MemBrain or Intello Plus, detailed as an air barrier. You could also use half-inch OSB or plywood and seal all of the seams with acoustic sealant if you preferred that to gypsum. If you use the gypsum option, painting it with "vapor barrier latex" (about 0.5 perms) would be the right thing to do. OSB & plywood are already somewhat vapor-retardent, and would be sufficiently vapor retardent to work in that climate as long as there is at least SOME ventilation going on under the roof deck.

    Hopefully the exterior siding has at least some amount of rainscreen-gap? If yes, it would be OK to use poly detailed as an air barrier if you don't want to detail the dryall as an air barrier. But long as the gypsum is air tight, standard latex paint would still have sufficient vapor retardency to protect the sheathing from interior side moisture drives. Without the rainscreen gap it's advisable to use vapor barrier latex primer on the drywall to get the vapor retardency down to under 1 perms (class-II vapor retardnecy), yet vapor open enough to dry toward the interior. See:

    For comparing IRC prescriptions to your location, your climate is comparable to the cool edge of US climate zone marine 4 (also called zone 4C).

  5. carwood | | #5

    Thank you Martin and Dana. Very helpful thoughts. We do have ventilation in the roof around the perimeter between the joists, soffits not installed yet, but will be vented aluminum. Walls have rain screen lathes, both vertical and horizontal on top of tyvek and OSB. Siding will be fir board&batten.

    So, since we do have roof venting, as well as vented end flashing and vented soffits, and are not wanting to put drywall on the ceiling, it sounds like we could put plywood, sealed with acoustic sealant. However, I believe we only have about 1 inch space between the celing Roxul insulation and the roof sheathing. Should we add a vented 'dog house' to each roof (4 roofs in total)? The TJIs are actually 9 1/2" on two roofs and 11 7/8" on the other two roofs (these 2 roofs have R40 batts).
    As for the walls, we will install gypsum, some with latex, but were wondering about using 'American Clay' on some walls.

  6. carwood | | #6

    I should add, we are planning on installing gypsum painted with latex on the two ceilings with the
    11 7/8" TJI roof costruction.
    If we include the 2x4 strapping in the calculation on the two ceilings with the 9 1/2" TJIs, it looks like we have between 1 5/8" and 2" space between the top of the Roxul and the roof sheathing,. From Martin's article, if it appears we should increase this space to 6", we would need to remove the R14 layer of Roxul. Reluctant to decrease the insulation unless necessary!
    Our climate is marine zone 4 but not as cold in winter or as hot in summer as Wash. and Oregon.

  7. Expert Member
    Dana Dorsett | | #7

    Being on an inner-waters island I would assume your climate to track the temps of the San Juans or Whidbey Island in WA pretty closely, but perhaps a degree or two cooler overall.

    American Clay walls are going to be extremely vapor permeable- you may want to put poly or MemBrain between the clay and the wall cavities. What were you intending do use as the substrate?

    Low angle roofs don't normally convect the roof deck ventilation very well, but I would expect pretty consistent wind-washing to drive it at an island location. If there's a weather station on the island (or a nearby island) you can probably find weather datasets to verify that on

  8. GBA Editor
    Martin Holladay | | #8

    Since you don't have sufficient air space above your insulation in your low-slope (nearly flat) roof assembly, I suggest that you build an unvented roof assembly.

    Again, I urge you to check the details in my article, Insulating Low-Slope Residential Roofs.

    You can't simply remove some Roxul insulation to create a deeper air space above the insulation, since you still need to meet minimum R-value requirements to be code-compliant.

    It sounds like you are far advanced in your construction, and you may have painted yourself into a corner. If that's the case, it's an object lesson to others who may be reading this thread: it's always a good idea to nail down your insulation details before construction begins. If you hope to figure these details out at the last minute, it may be too late.

  9. carwood | | #9

    Thank you for your input Dana and Martin. Martin, we had considered the insulation and roofing construction before and throughout the construction process. Unfortunately, it now turns out our builder and others in the industry where we live were not well informed about building science. We have tried to educate ourselves on the topic, but a lot of it is confusing. We are also finding conflicting numbers about the space required between the insulation and sheathing in a vented ceiling. We had understood we had sufficient venting given we are venting on all four sides of each roof.

    We have read your article Martin, and considering all relevant variables, we now wonder if focusing
    on making the interior ceiling airtight with taping, sealing and vapour barrier latex, and installing a vented cupola on each roof will be sufficient to prevent moisture build up in our roof.
    We also have an unusual situation on our island in that we do not have any building codes and no inspections except for electrical.
    Finally, living near the water, about 500 ft. up, we have pretty consistent wind to drive the roof deck ventilation.

  10. Expert Member
    MALCOLM TAYLOR | | #10

    Carolyn, Although you do not have permits or inspections are you sure you are not subject to the BC Building Code? The code mandates a 2 1/2" (63mm) air space between the insulation and the roof sheathing. Probably a good idea whether or not it applies to you.

  11. GBA Editor
    Martin Holladay | | #11

    I don't have enough experience to predict whether your suggested solution will work. For your plan to work, it is vital that you do an extremely careful job of air sealing at the ceiling plane. This work would need to be verified with a blower door.

  12. Sal_123 | | #12

    I find its not uncommon to garner information from good building science sources, such as this site or and when you refer to this information or relay the same in conversation to the contractor, craftsman, mechanic in the field they have little to any idea what your talking about. Much of this "theory" seems to be lost on the day-to-day contractor doing the work in the field. In my area if you stray from the "accepted" standards, your suddenly in unchartered waters getting looks of confusion and skepticism.

  13. davidmeiland | | #13

    Carolyn, I'm not sure if I'm understanding the details of your roof correctly, but it sounds like you have rafters with purlins running across the tops. If that's the case, it seems like airflow under the sheathing is going to be inhibited by the purlins. Where exactly is the venting and what will be the driving force? I am south of you in the San Juans, where we have windy conditions quite often, and I think that helps marginal roof venting situations work, but I agree that your ceiling air-sealing has to be really well done. Personally, I would install 1/2" CDX plywood under the rafters, tape it with SIGA tape, seal it around the edges to the top plates with sealant, and then nail the T&G to that. Make sure the subs aren't drilling lots of big holes in the plywood, because you have to seal them.

  14. davidmeiland | | #14

    Another question... it the metal roofing you are going to use approved by the manufacturer for 2:12 pitch?

  15. carwood | | #15

    Thank you everyone for your many helpful comments. You hit the proverbial nail on the head Sal with your observation. I have even had flack here for wanting 24 on center framing! Not one person in the construction business here thinks we should eliminate the interior poly vapour barrier. It's confusing and frustrating.
    As for the roof construction David, our rafters are "iLevel Trus Joists, TJI Joists" with 2"X 4" cross strapping (purlins), then 1/2" plywood, then "Nova Seal" membrane (polypropylene fabric, that will "not support mold growth"). The 26 gage standing seam metal roof is from Cascadia Metals. I have checked the warranty and there is no mention of it excluding approval for a 2 :12 pitch. The roofing metal supplier (and quotes from two other suppliers) was well aware of our roof being 2:12 pitch.

    For venting, the theory is that airflow (wind) will travel through the vented openings, entering at the end of the trusses (tops and bottom ends of roofs) under the sheathing. Like you, we have some breezes here quite often. I agree, the airflow may be inhibited by the purlins. This is a concern.
    To increase the airflow, our latest solution is to attach 2"X 2"s to the bottom of each Rafter and drop the insulation down by the extra 2". All our ceilings are 9 ft or more. We should then have about 4" space between the top of the insulation and the bottom of the roof sheathing. We will then install the plywood, and make sure air sealing is well done.
    Just did a quick search for 'cdx plywood' from our area - none found.

  16. davidmeiland | | #16

    No CDX in Canada?? Never fear, it's the typical construction grade plywood used for sheathing--one 'C' face, one 'D' face, and eXterior glue. Heck, for skinning a ceiling you could even use OSB. It might be a bit less permeable and work in your favor.

  17. carwood | | #17

    Thanks David. I don't like the off gassing with OSB, so think we'll go with plywood. Thanks again for your suggestions.

  18. Expert Member
    MALCOLM TAYLOR | | #18

    Carolyn, Here in Canada the equivalent of CDX is just called Exterior Grade Plywood.

  19. carwood | | #19

    Thanks Malcolm!

  20. everettg | | #20

    Martin et al, I believe that when I rebuilt the roof on my 1771 Connecticut saltbox I followed the strategies for an unvented cathedral ceiling. But now as I begin the finish work I want to check for a final time. The house, before work began, had 5x5 beams on 4 foot centers, covered with horzontal planks, plywood, then asphalt shingles. I stripped it down to plywood, cut off all overhangs, and covered the entire house exterior with Ice & Water shield as an air barrier.

    Next came 2 layers of 3 inch polyiso foam, staggered, taped joints, etc..which were secured with VERTICAL 2X4 sleepers, screwed into the original oak beams/planks on 24" centers. Then plywood, another layer of water barrier (the black woven stuff - sorry I forgot the name), and Timberline shingles. Walls are the same only 2 layers of 1 inch foam, 3/4" furring, then clapboard.
    We thus have a continuous airspace from sill to ridge, with insect screen at the sill.

    The entire house is now within the thermal envelope, including attic, and I will use drywall or plaster between the attic beams, but in the one story "EL" (now the kitchen) the cathedral ceiling was going to be covered by bead board between the 4 foot on center roof beams. In this case do I also need to add a drywall vapor barrier? I thought that I was OK without it given the Ice&Water shield and foil faced Polyiso foam?

    Since I'm furring down the cathedral ceiling by 1.5" for wiring, etc, I'd like to add more insulation - any suggestions? I believe I cannot add more foam, and looking for 1.5" thick fiberglass is not easy - unless something like insulated fiberglass ductboard would work..

    Finally - The walls have also been studded with 2X4s to allow for electrics, plumbing, etc. I plan on adding R-15 fiberglass before the drywall/plaster goes on- should it be unfaced or paper faced?

    You've all been a great help so far in this project.. Stay tuned - I next need to tackle the leaky(i.e. wet) stone foundation walls and need guidance there..with a concrete floor which seems only 1/2" thick the option for inside remediation versus digging up the outside needs serious consideration..


  21. GBA Editor
    Martin Holladay | | #21

    Q. "The cathedral ceiling was going to be covered by bead board between the 4 foot on center roof beams. In this case do I also need to add a drywall vapor barrier?"

    A. No. Your Ice & Water Shield is a vapor barrier already.

    Q. "Since I'm furring down the cathedral ceiling by 1.5 inch for wiring, etc, I'd like to add more insulation - any suggestions?"

    A. I suggest that you leave this service cavity uninsulated. If you really want to insulate it, you can buy R-11 or R-13 fiberglass batts and split them like a layer cake to yield two batts that are about 1 1/2 inch thick if you want.

    Q. "The walls have also been studded with 2X4s to allow for electrics, plumbing, etc. I plan on adding R-15 fiberglass before the drywall/plaster goes on. Should it be unfaced or paper faced?"

    A. It doesn't really matter very much; the kraft paper won't cause any problems. But I would probably choose unfaced.

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