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What happens if you turn an unvented roof assembly into a vented roof floor?

Onnie M | Posted in General Questions on

We’ve just purchased a 1935 ranch with a whole mess of gables going every which way. One of the gables is a vaulted cathedral ceiling over a large living room that appears to be unvented. I’m not sure how it’s insulated aside from poorly. Not sure of the rafter size either but my guess is 2×6 based on other house framing. Other insulation in the rest of the house appears to be fiberglass in a vented attic; I have yet to measure the depth. In 5-10 years we’d like to do an addition and frame a new roof that would have some second story space but would also effectively “frame in” the vaulted ceiling; we were planning to do just basic air sealing until then and not worry too much about upping the insulation until later. However, we just got back our first gas bill and it’s making me long to start working on that wonderful/terrible ceiling now, not later. We are climate zone 5 (Chicago burbs).

My understanding of a pretty good, low-cost assembly that would bump our cathedral ceiling’s R-value now would probably be a couple inches of closed cell with another material (such as mineral wool) below, maybe using rigid mineral wool panels between the studs and the ceiling drywall to further increase the R-value. But I’m wondering if trying to get to R-35 or so now using closed cell plus other insulators against the existing roof sheathing is going to limit my abilities to try and get to R-50+ later? Originally I was thinking when we build a new roofline I would just have the “roof over a roof” assembly treated like a vented attic, just piling cellulose along the old roofline till I got my desired R-value. Now I’m thinking I’d like to start moving on this ceiling now, and the best unvented ceiling assemblies seem to be closed cell closest to the roof deck. But would that limit my abilities to modify it later? My understanding is that the closed cell structurally bonds to the sheathing, so once I go closed cell, I can’t just remove what it’s sprayed against; I’ll have to work with what I got after that, unless I remove all that (expensive!) foam. But what if I might like to later do a “vented attic” by say putting rigid foam up along the old roofline to increase our R-value even further in the new vented space – is that asking for trouble if I’m sandwiching rafters and old roof sheathing in between two layers of foam (rigid top layer, then sheathing below, then closed cell below that)? Or could I still pile cellulose or mineral wool on top of the old roofline with the sheathing under, then closed cell underneath, instead of rigid foam? If the cathedral ceiling no longer needs to function as a roof and effectively becomes just a ceiling, are the drying/moisture permeability concerns different, especially as regards a potential closed-cell foam sandwich?

Thanks in advance. I’ve really enjoyed perusing this site!

Replies

  1. GBA Editor
    Martin Holladay | | #1

    Onnie,
    Q. "What if I might like to later do a 'vented attic' by say putting rigid foam up along the old roofline to increase our R-value even further in the new vented space?"

    A. You could do that.

    Q. "Is that asking for trouble if I’m sandwiching rafters and old roof sheathing in between two layers of foam (rigid top layer, then sheathing below, then closed cell below that)?"

    A. No, it's not asking for trouble.

    Q. "Could I still pile cellulose or mineral wool on top of the old roofline with the sheathing under, then closed cell underneath, instead of rigid foam?"

    A. Sure.

    Q. "If the cathedral ceiling no longer needs to function as a roof and effectively becomes just a ceiling, are the drying/moisture permeability concerns different, especially as regards a potential closed-cell foam sandwich?"

    A. Everything will be fine. What you don't want is a cold surface exposed to leaking interior air, and your plan won't have that.

  2. Onnie M | | #2

    Thank you Martin! I was 99% positive that either assembly would work, since the old roofline would no longer be directly exposed to the cold air and water, but I kept wondering if there was something I was missing since I'm not an engineer. Now to actually start sealing up the place...

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