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Community and Q&A

Materials for vented cathedral ceiling

Chris Barnes | Posted in Green Building Techniques on

I am designing a house in a 4C climate. Currently the roof design includes using engineered wood 16″ deep I-beams (i.e., TJIs or BCIs). A portion of the house has cathedral ceilings, and the remainder is an insulated attic space for ducting, wiring, plumbing, etc (from a structure and insulation point of view, the entire house might as well have cathedral ceilings). My current plan is to use the top plate of the I-beam to mount an Accuvent or plywood-like material to hold in insulation. This would leave a 1.125″ gap between the roof sheating and the insulation barrier, then insulate with blown-in cellulose below that. The bottom of the I-beam would have OSB nailed to it to form an air barrier. Drywall would be attached on furring strips, which are attached to the ODB. The I-beams are 24″ OC.

In the “how to insulate a cathedral ceiling” article, it indicates that using think plywood is an option on the bottom of top plate of the I-beam. It seems to me that this would be the least trouble once installed as the accuvent appears to be pretty flimsy and may not hold up to dense packing. Also, the I-beam flanges are not the same width as typical 24″ OC trusses, so it could making fitting the accuvents awkward.

Why is it that it would be acceptable to use plywood in this case when I cannot simply make an unvented assembly? In other words, it seems that using plywood to hold in the insulation is equivalent to making an unvented assembly in regard to the potential for developing condensation.

(On a related/side note, the Oregon state building code requires R-20 impermiable insulation on unvented cathedral ceilings, even though IRC states that only R-10 is necessary for the 4C climate. I assume this is because Eastern Oregon is in climate zone 5. This is pretty lame as it makes unvented cathedral ceilings pretty expensive when it’s not really needed….)

Thanks!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Chris,
    Q. "Why is it that it would be acceptable to use plywood [as a ventilation baffle] in this case when I cannot simply make an unvented assembly?"

    A. If you have a ventilation gap, there should be air moving through that ventilation channel. The moving air helps dry the damp plywood. If you have plywood roof sheathing and plywood ventilation baffles, the moving ventilation air can dry both, since the air is passing between the two layers of plywood.

    If you pack air-permeable insulation tight to the underside of the roof sheathing (an illegal unvented assembly), you allow warm, moist interior air to migrate through the fluffy insulation, and the moisture in that air can contact the cold roof sheathing, resulting in condensation or ice. And in this case, you'll have no moving air to dry out the roof sheathing. That's bad.

  2. Chris Barnes | | #2

    Martin,

    Thanks for the response. I had one more question with this assembly. I was planning on the air barrier being the bottom of the I-beam/truss (i.e., towards the interior of the house). In another post back in ~2011, I saw that you posted in another Q&A that the air barrier should be at the top of the insulation, just under the vent. Is this still the case? Are there going to be any issues is the air barrier underneath the insulation, rather than on top of the insulation?

    (The insulation will be dense-packed cellulose)

    Thanks again,

    Chris

  3. GBA Editor
    Martin Holladay | | #3

    Chris,
    This is one type of assembly where two air barriers are better than one. The interior air barrier prevents air from entering the roof assembly, where it can find its way through unintended cracks and wreak havoc. And the air barrier above the insulation reduces the energy penalty associated with wind-washing (a function of the air moving through the ventilation channel).

    For more information on this issue, see One Air Barrier or Two?

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