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Replacing fiberglass with foam to save space

skidmorebay | Posted in General Questions on

I’m trying to make room to run a 3.5″ x 10″ HRV duct in my new-build cathedral ceiling cape, unfortunately as a bit of an afterthought. Unfortunately, I don’t have a convenient interior wall to use, so it needs to be installed in an exterior wall.
One way I can make room is to take one rafter bay and one stud bay and use RMax polyiso there instead of the fiberglass I will be using everywhere else. This way I can achieve approximately the same R Value in less space, freeing up room for the duct I wish to install.
My question is, from a vapor diffusion standpoint, in a house designed to dry to the outside in a cold, rainy climate, are there any concerns with using solid foam inside a stud or rafter bay? What I would have in the end will be somewhat like a unvented, “hot” roof – an assembly which has had such a poor track record here that builders refuse to build them.
thank you,

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, what is the depth of your rafters? It's hard to imagine that your Cape has rafters that are deep enough to permit the installation of a 3.5-inch-deep duct as well as adequate insulation.

    A few facts:

    1. In most of the U.S., the minimum code requirement for ceiling (or roof) insulation is R-49. In Climate Zones 2 and 3, the minimum is R-38.

    2. It's perfectly acceptable to design an unvented insulated roof assembly, but you have to get the details right (and you need to install at least as much insulation as the minimum level required by the building code). For more information on code-approved unvented roof assemblies, see How to Build an Insulated Cathedral Ceiling.

    3. The insulation approach you are contemplating is called "cut-and-cobble." This approach requires between 8 and 10 inches of rigid foam in most of the U.S. Even if your rafters are deep enough to insert 8 inches of rigid foam between your duct and the roof sheathing, I don't recommend this approach, because cut-and-cobble foam is associated with failures (mold and rot) in unvented roof assemblies. For more information, see Cut-and-Cobble Insulation.

    4. The bottom line: your rafter bays are almost certainly not deep enough to proceed with your plan. Don't do it. Instead, install an interior soffit or chase to hide the duct.

  2. GBA Editor
    Martin Holladay | | #2

    You didn't mention whether or not your rafter bays include ventilation baffles under the roof sheathing. If most of your rafter bays are insulated with fiberglass, I hope that you included a ventilation gap between the top of the insulation and the underside of the roof sheathing.

  3. skidmorebay | | #3

    Thanks for your reply. I'm sorry I didn't include all the details.I am in southeast Alaska, near Juneau. My roof assembly consists of 2x12 rafters. I plan on having a 2" airspace, R38 fiberglass in the remaining 9 ¼", and 1 ½" of RMax polyiso on the interior side, for a nominal R48. I'll have ¾" strapping over that, followed by ½" sheet rock.
    I could achieve something like the R48 with five layers of 1 ½" Rmax against the vent channel. This would take up 7 ½", leaving 3" of the 3 ½" I would need for the duct (to the interior side of the sheet rock.) This would leave a much smaller bump on my ceiling than running the duct against the finished ceiling.
    When the duct reaches the wall on it's way to the crawlspace, a similar approach would be used in my 2x6 walls with R21 fiberglass and 1 ½" of interior polyiso. That is, if this is an acceptable approach...

    A further question on cut-and-cobble: My outside rafter bays at each gable are partially taken up by the 2x4 lookouts that hold up the barge rafters. I'm curious if inserting 100% foam in these bays would get back some of the R value lost by the space the lookouts take up. I wouldn't want to take any risks with rot and mold, though, because this climate is already prone to these things.
    thanks again,

  4. GBA Editor
    Martin Holladay | | #4

    Thanks for the further details. You have more room available than I imagined. I'm glad that you are including a ventilation space.

    If the cut-and-cobble foam is installed on the interior side of your vent channel, the risk disappears. The vent channel makes this approach safe.

    I was confused by this sentence in your original question: "What I would have in the end will be somewhat like a unvented, 'hot' roof." So I guess I need one more clarification: will the rafter bay that has the duct be vented or unvented?

  5. GBA Editor
    Martin Holladay | | #5

    At the gable ends, it's probably a good idea to insert cut-and-cobble foam between the lookouts. After all, these rafter bays can't easily be vented.

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