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Community and Q&A

Air-Sealing a Cathedral Ceiling

John Warstler | Posted in General Questions on

I’m in zone 6 and would like some advice on details on a new construction cathedral ceiling. It’s a simple roof with no hips or valleys.   I’ve read numerous articles about T & G wood products being a terrible choice for cathedral ceiling due to warm air rising through to the underside of the cold roof sheathing and condensing.

I have 6/12 24″ deep parallel chord trusses and have installed a baffle system that creates a 2″ air space from eave to ridge using recycled 1 1/2″ polyiso.  I am very pleased with the rigidity of the foam and I’m confident I can make the underside of the polyiso baffle system air tight. I plan on filling the remainder of the truss space, 20″ with insulation.  I’m also debating on whether or not to add another layer of recycled polyiso on the underside of the truss.

So my question is:  With the air tight site built baffle system, do I have to be concerned as to how I treat the ceiling below? I realize best practice, if I wanted to use wood for the ceiling,  would be air tight drywall attached to the bottom chord of the truss, 1x or 2x furring strips and then wood on the underside attached to the furring. I’m not hell bent on a finished wood ceiling, but I feel it would be a nice touch. I’m also wondering whether I could attach the finished wood ceiling directly to the bottom chord of the truss.  I know I have ample insulation, but does inadequate air sealing on the underside reduce the effectiveness of the insulation above?

Thank you in advance.

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    John,

    Air-sealed baffles are a good secondary air-barrier, but not reliable enough to be treated as the primary one. You need a continuous air-barrier under the trusses.

    1. John Warstler | | #2

      Great to know, I’m not surprised. We’ll scrap the wood ceiling idea. Thank you very much.

      1. Expert Member
        Malcolm Taylor | | #7

        John,

        I wouldn't scrap the wood ceiling. Drywall, a smart vapour-retarder, or (depending on your climate) poly will all work as effective air-barriers above the wood.

  2. Stephane Fortier (zone 6A) | | #3

    I'm not an expert but I agree with Malcom, you should rely on a primary air-barrier. In my case, I used Polyiso under the trusses, fully air tight, with double furring in allow space for electric wires and LED recessed lights. I kept holes to a bare minimum: plumbing and radon vents, chimney and hatch, being very careful with air tightness around them.

  3. John Warstler | | #4

    Thanks for the reply and the pic. Did you use 1x or 2x? Initially I assumed it was 1x, but my eyes tell me it might be 2x. Since the wood ceiling is out and drywall is in, I will also do a system similar to yours. Lumber is creeping up again so I was going to price
    2 x 4’s running perpendicular to the trusses or 1x the way you used it. Thanks again.

  4. Stephane Fortier (zone 6A) | | #5

    They are 1x3. You are right, needed some 2x4 this week and they are back close to $9/piece ($CDN)... ridiculous. Last year they came close to $10 and went down o $3 in August, was not expecting another raise like this.

    For recessed light, I need about 1 inch of space for the box, so 2 perpendicular 1x3 gives me enough.

    Good luck with your project!

  5. John Warstler | | #6

    Thanks again.

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