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Community and Q&A

Building vent chutes in a cathedral ceiling

AatFrNweUd | Posted in Energy Efficiency and Durability on

Could someone build and / or be wise to build vent chutes for a cold roof in a cathedral ceiling out of strips of 1″ foam ( butted up against the sides of rafters and the underside of the roof sheathing) and then attach a sheet of 1″ or 1 and 1/2′ foam under the strips forming a chute? Then fill the remaining cavity with fiberglass?

Yes, yes, yes people don’t like fiberglass. I personally think spray foam is not cost-effective. The purpose of my proposal is to 1. create the vent space, 2. gain a bit on R-value over FG alone.

The question primarily revolves around the possibility of the foam trapping moisture in the FG. I would likely apply poly to the underside of the rafters.

I apologize in advance if I offend any spray foam lovers. Each is entitled to their own cost benefit analysis.

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  1. GBA Editor
    Martin Holladay | | #1

    Your method is a good one, and many other builders have done it.

    Just be sure to do a good job of air-sealing at the perimeter of each piece of rigid foam. You can cut the pieces a little narrow, leaving deliberate gaps at the edges, and foam them in place with canned foam. The seams between adjacent pieces of foam can be taped with housewrap tape.

    I would skip the poly under the rafters; kraft facing is all you need for a vapor retarder. Be sure you install your drywall in an airtight manner -- no can lights!

  2. Riversong | | #2

    Do NOT use a double vapor barrier. You can use XPS, rather than foil-faced polyiso, as the vent chutes and limit it to 1" thickness to allow some drying outward. Rather than using foam spacers, I would use wooden strips - easier to attach to the rafters and you can use roofing nails to hold the foam board in place until the edges are foamed.

    Do NOT use a poly vapor barrier - they cause more problems than they solve. Use air-tight drywall (as Martin suggests). And if you must use batts, use cotton (recycled bluejeans with borate). Though a bit more expensive and a bit of a challenge to cut, it has most of the advantages of blown cellulose without the disadvantages of fiberglass.

  3. AatFrNweUd | | #3

    Thank you for your replies. Clearly I would not use the foil backed foam. It makes sense to use the most breathable foam possible as I was concerned that my idea could trap moisture.

    It is interesting that some say don't use the poly, but use air-tight drywall. To use air-tight drywall is ineffect a vapor barrier, so what is the harm of using the poly under the drywall. I suppose you say moisture can be trapped between the poly and the drywall and rot the drywall paper. But that is a bit doubtful since if the mositure can go through the drywall it can just as easily dry back teh way it came.

    Is there no way to use can lights in a cathedral ceiling? Light is pretty hard to get without some.

    One more thought or question. Would there be any problem with doing the system first described here for shoots, and on top of it run stripping perpendicular to teh rafters on the underside, filling the space between straps with more foam. The thinking is to break thermal bridging and to gain more R value. Of course with all this foam and labor spray foam might start to make more sense. I would ahve to run some numbers, but again would like to know if some many changes in insulation types could start causing moisture problems.

    Thanks again everyone.

  4. GBA Editor
    Martin Holladay | | #4

    Airtight drywall is not a vapor barrier, it is an air barrier. Drywall is vapor-permeable.

  5. Riversong | | #5

    "Is there no way to use can lights in a cathedral ceiling?"

    Not unless you want moisture problems and unnecessary heat loss. Use surface ceiling fixtures with polypans around the electrical boxes, seal all penetrations with canned foam and caulk the polypan flanges before hanging drywall.

    "Would there be any problem with doing the system first described here for shoots, and on top of it run stripping perpendicular to teh rafters on the underside, filling the space between straps with more foam?"

    If you want a thermal break under the rafters, then install XPS directly to the rafters, tape all seams and use can foam at the perimeter edges. You can either install drywall through the foam with longer screws or apply strapping horizontally over the foam and attach drywall to that.

  6. GBA Editor
    Martin Holladay | | #6

    You asked, "Is there no way to use can lights in a cathedral ceiling?"
    The answer is, yes -- if you put all of your insulation on top of your roof sheathing. Install R-40 to R-60 foam on top of the roof sheathing, followed by another layer of plywood, with or without venting above the foam. Then you can install as many can lights as you want under the lowest level of roof sheathing.

    There's nothing wrong with track lighting, by the way -- especially since can lights make so many of us cringe. You don't want guests in your house to be cringing.

  7. Riversong | | #7

    The problem with putting the insulation outside of the roof sheathing, when that is intended also as the air barrier layer, is continuity of air barrier on all six sides of the "box" we call a house. An air barrier is useless without continuity.

  8. GBA Editor
    Martin Holladay | | #8

    You're right, of course, that air barrier continuity is important. If your air barrier is above the roof sheathing, the wall air barrier needs to be extended from the wall top plates to the roof sheathing. The easiest way to do this is with spray polyurethane foam (a couple of Handi-Packs).

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