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What are good alternatives for the air barrier when building a cathedral ceiling using tongue and groove instead of gypsum?

Alan Shuman | Posted in Energy Efficiency and Durability on

I am building a log home in North Georgia (Zone 4) with cathedral ceilings and tongue and groove on the inside. Build up from the inside out will be: Pine tongue and groove/ AIR BARRIER / I-Joist Rafters with R-30 fiberglass insulation between / OSB / Felt / Metal Roof. The structure will be vented with soffit vents and ridge vent and 1” air space over insulation. I read the article “HOW TO BUILD AN INSULATED CATHEDRAL CEILING” from this site and the suggested air barrier was gypsum board between tongue and groove and rafters. Is there a good alternative to gypsum? Poly? Typar? In Georgia I will need to pass blower door test. I also want to avoid condensation issues from air passing through the tongue and groove.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Alan,
    Experience shows that gypsum wallboard works for this application, and that polyethylene doesn't.

    Personally, I wouldn't depend on Typar or Tyvek. As you know, there will be lots of fasteners penetrating the air barrier. You might consider using one of the new European air-barriers-in-a-roll like Intello Plus -- but it's hard to beat gypsum wallboard for affordability and easy of installation. There hasn't been enough experience with Intello Plus in the U.S. to answer questions about its long-term performance or its performance when penetrated by lots of fasteners.

  2. Alan Shuman | | #2

    Thanks for the reply. Is there a an alternative way to seal the seams of the gypsum other than conventional mud/drywall paper tape?

  3. GBA Editor
    Martin Holladay | | #3

    Alan,
    Paper tape and mud are cheap and effective.

    All you need is one coat of mud and the paper tape. There is no need to do any sanding, finishing, or painting.

  4. Alan Shuman | | #4

    If I instead choose to install an unvented roof assembly using a combination of rigid foam on top of the osb roof sheathing (at the thickness suggested in 2009 IRC to achieve R-15 or more) and then use air-permeable fiberglass insulation under the sheathing between the rafters to meet my total desired R value, Can I then simply attach the tongue and groove to the bottom of the rafters and forgo an air barrier at this location? In this arrangement, there would likely be some air space above the fiberglass insulation but there would be no venting on soffit or ridge.

  5. GBA Editor
    Martin Holladay | | #5

    Alan,
    There are two problems with your suggested assembly:

    1. There should be no air gaps between the top of your fiberglass insulation and the roof sheathing. If you are designing an unvented roof assembly, choose the right thickness of fiberglass batt so that it fills the rafter bays completely, without a gap.

    2. You still want an air barrier under the fiberglass batts, to stop convection currents and air leakage through your roof assembly. Even if you don't install a ridge vent, you can easily get air leakage through unintended gaps near your ridge, and these air leaks can draw indoor air through your T&G boards.

  6. Randy Reich | | #6

    Martin,
    I am starting a project similar to Alans': T&G Knotty Pine walls & sloped ceiling / fiberglass insulation between 2X12 rafters / air gap / OSB / ashphalt shingle roof. I live in Michigan and experience very cold and very hot/humid temps. There are crawl spaces behind 4 ft knee walls on both sides, and a small "attic" space above a 7' wide flat area at the top of the room (11ft ceiling). There is just no way I am going to put up gypsum wallboard and mud - avoiding that hassle is one of the perks of T&G! My questions - I would be willing to put up 7/16" OSB instead of gypsum wallboard if it offered similar air-barrier properties. Assuming I taped the edges/corners, would that work as well as gypsum as an air barrier? What tape should I use? Do you think this would be better or worse than Intello Plus you mentioned (which would be slightly less expensive than OSB or gysum wallboard)? The project is the 3rd floor fo my house, so my goals are to minimize heat loss due to air leaks in winter and avoid any moisture/condensation problems.

  7. GBA Editor
    Martin Holladay | | #7

    Randy,
    You wrote, "There is just no way I am going to put up gypsum wallboard and mud - avoiding that hassle is one of the perks of T&G!"

    Your statement is a little scary. A T&G board ceiling will always be more complicated than a drywall ceiling -- unless you skip the air barrier, which would be a disastrous approach.

    Drywall and mud are inexpensive. The system goes up fast, and paper tape is cheap (and creates a good air barrier).

    OSB is much harder to tape than drywall. If you decide to use OSB, you'll have to invest in a top-quality tape. I recommend Siga Wigluv tape. For more information on taping OSB, see Backyard Tape Test.

  8. Don J. | | #8

    "Drywall and mud are inexpensive" that's true. and those are probably the only good characteristics. Once you do a drywall demo, you wish you never had it in your house in the first place.....
    I left a sheet of cement board outdoors for an entire year. After snow, rain, summer sun, and winter freeze it was like brand new and ready for installation. However, it's heavy, costs more, and no good for fire proofing.
    If you cathedral ceiling ever leaks you'll have to take it down and re-do it. Drywall also absorbs odors quickly, is an excellent base for mold, and accumulates moisture. The 'greenboards' may be 'moisture resistant' but swell up several inches when 'moist'.... you get what you pay for

    If any one knows a drywall vendor who makes drywall with cement board characteristics please let me know I will buy it right now, thank you for your recommendations. In particular a high fire rating and low weight would make it superior to cement board if such thing exists

  9. Stephen Martinson | | #9

    Don J.,
    Have you looked into magnesium oxide board?

  10. Expert Member
    Malcolm Taylor | | #10

    Alan, You might consider using taped or sealed OSB or plywood as your air barrier.
    You may also want to increase the air space between your insulation and roof sheathing. A 1" gap is hard to maintain with FG batts and easily compromised. Our code minimum is 3".

  11. Floris Keverling Buisman | | #11

    The INTELLO Plus membrane that Martin mentions is many times lighter and easier to put on the ceiling than either sheetrock or OSB. It is meant to be taped airtightly with TESCON Vana and as shown in the BLDGTYP post - and their first blowerdoor in that project easily beat the PH airtightness standard.

    They used a T&G finish on the ceiling. See this blogpost. You can use INTELLO in combination with dense pack cellulose or batts (fiberglass, mineral wool, cotton etc).

    The roof is best vented outside the plywood sheathing in your climate. This is easily done with a metal roof. Just roof vertical battens on top of a high quality underlayment (like the Pro Clima SOLITEX MENTO roof underlayment) as shown in this post. Replacing the vented OSB/asphalt shingles with your metal roofing.

    More info and all these Pro Clima membranes are available from http://www.foursevenfive.com, the company I established to bring these high performance material to the US market.

  12. Alan Shuman | | #12

    Martin,

    I am in zone 4 right on the border of zone 3 (my property line is the county line and the adjacent county is zone 3). If I use open cell foam between my 2x10 rafters, then cover with intello plus membrane and finally install my tongue and groove, is this a very good roof system? Would I be better off using gypsum instead of intello plus? I don't want to be dealing with mold or rot down the road but filling the voids with closed cell is cost prohibitive.

  13. GBA Editor
    Martin Holladay | | #13

    Alan,
    If you fill your rafter bays with open-cell spray foam, you are all set. Spray foam is an air barrier, so you won't need the Intello Plus. In your climate, open-cell spray foam does not need an interior vapor retarder.

    If you install 9.25 inches of open-cell foam, you'll have an insulation R-value of about R-34. That will meet the minimum code requirement in Climate Zone 3, but not Climate Zone 4 -- so be sure to drag your house to the south side of the county line before you call up the building inspector.

  14. Alan Shuman | | #14

    Good point. Perhaps I'll increase my rafters to 2x12. Alternatively, I could put a layer of polyiso below the 2x10 rafters and then install the tongue and groove. Does tongue and groove meet code as the thermal barrier for fire protection or will I still need gypsum for thermal barrier?

  15. GBA Editor
    Martin Holladay | | #15

    Alan,
    Q. "Does tongue and groove meet code as the thermal barrier for fire protection or will I still need gypsum for thermal barrier?"

    A. Good question. The easiest way to satisfy the thermal barrier requirement is to cover the spray foam with 1/2-inch drywall. In the older versions of the IRC, Section R314.4 (Thermal barrier) had the following language: "Unless otherwise noted in Section R314.5, foam plastic shall be separated from the interior of a building by an approved thermal barrier of minimum 0.5 inch (12.7 mm) gypsum wallboard or an approved finish material equivalent to a thermal barrier material that will limit the average temperature rise of the unexposed surface to no more than 250°F (121°C) after 15 minutes of fire exposure complying with the ASTM E 119 standard time temperature curve." I'm not sure whether the language has been changed in newer versions of the code, but I think the requirement is substantially unchanged.

    According to one comment posted on a web forum, "In the 70s the Society of the Plastics Industry in cooperation with the foam plastics industry conducted testing of various products as thermal barriers over foam. 3/4" tongue and groove wood decking was one of the materials tested that passed the thermal barrier tests. However, it is not specifically listed in the national codes and I am not aware of recent tests on the product." I have no way of verifying whether this statement is accurate, or whether it would be easy or difficult to convince your local code official that the statement is true.

  16. Alan Shuman | | #16

    Martin,

    Researching this further, the minimum required spray foam for Ga in an unvented rood assembly is r-19 if trade-offs are used. If I spray a layer of 5" open cell open cell between the 2x10 rafters to archieve the r-19 minimum, can I then install the tongue and groove directly to the bottom of the rafters? This would leave an air space of 4 inches between the t&g and spray foam but I assume this is ok (ie wouldn't require an additional air barrier between rafters and t&g.). If instead I fill the 4" space below the spray foam with fiberglass insulation, then I have to install a second air barrier barrier such as gypsum before installing the t&g. Am I correct on these two options? Why is the second air barrier required if I use fiberglass to fill the 4" space but if I leave it empty, a second air barrier isn't required?

  17. Expert Member
    Dana Dorsett | | #17

    R-19 center-cavity is a bit skimpy on total R, and I'd be surprise if it actally meets code in GA, though I'm sure there are plenty of foam installers selling the line that "It's all you really need." GA code is based on IRC 2009, which calls out R38, but most jurisdictions allow R30 for "compact roof", where the insulation is at the roof deck:

    http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_11_sec002.htm

    http://energycode.pnl.gov/EnergyCodeReqs/?state=Georgia

    If you did 2.5" of closed cell (R15, code required for dew point control: http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_8_par093.htm ) you could then fill the rest of the cavity with fiber insulation (cellulose recommended) blown in mesh and you'd be at the full R38 or better.

    If you had timber rafters rather than I-joist rafters you could use rock wool batts under the R15 closed cell and skip the interior side air-barrier, but there's no way to get a good enough fit with batts in I-joists, therefore blown is it. It's possible to do reasonable DIY cellulose blown in mesh- if you can't find the purpose-made mesh, use landscaping fabric stretched tight. With the R15 on the exterior the benefits of dense packing are much reduced, and whatever density you end up will be good enough, as long as it's blown reasonably tight. You'll have to roll the cellulose-filled mesh flat to the rafter edge plane when it's time to put up the t & g, but that's not a big deal. With blown fiberglass you'd need to put up at least a layer of paper or permeable housewrap as an air barrier to keep the friable fiber from ending up as microscopic aerosol particle pollution in your house (a serious irritant for asthmatics), but with cellulose you'd be fine without it.

    Alternatively you skip the closed cell & fiber, spray two 4-4.5" lifts of open cell and be done with it, which may be cheaper than R15 closed cell + fiber blown in mesh.

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