Insulating a Cathedral Ceiling
My gable roof has suffered some wind damage and needs to be replaced. I have a 1920s-built house in upstate NY (zone 5a) with a poorly finished attic. The attic has a cathedral ceiling with fiberglass batts between the 6″ rafters. The sheetrock took on some water after the roof damage, so it’ll need to get ripped out and replaced. If practical, I’d like to do what’s required now to someday convert the attic to a zoned living space.
I had a spray foam contractor come out who recommended 5″ of closed cell spray foam under the roof deck, (I’ll also probably do enough demolition behind my knee-walls to expose the top plates and enable him to foam down to them as well). But that much foam would get me to only ≈R30, and I’m told that code enforcement in my locality is likely to insist on R49 before signing off on a bedroom or bathroom in my attic.
I’ve identified three potential options, but all of them have significant down-sides:
Option 1: Nail 3.5″ of rigid polyiso below the rafters, which will rob me of 3.5″ of scarce headroom.
Option 2: Add ≈2.5″ of depth to my rafters with furring strips and increase the depth of spray foam. This solution also robs headroom but a smidgeon less, and hanging sheetrock might be easier without the rigid foam.
Option 3: Use nailbase insulation. As I understand it this would preclude me from using closed cell foam, which would prevent my roof deck (OSB on top of skip sheathing) from drying. If I use open-cell foam between the rafters, my interior R value drops to R21 and I need R28 above the roof deck, or ~6″ of EPS foam. This solution wouldn’t rob me of headroom but it’s likely to be expensive, might complicate my roof deck’s ability to dry, and I’ll also have to worry about the details the insulation will require (e.g. fascia boards) and the effect it will have on my semi-historic house’s appearance.
Questions:
Are there any alternatives I’m missing? Any factors I’m not considering, or am overthinking? Any best practices or materials I should be requesting of my contractors?
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Replies
They probably have a secondary roof requirement for “low profile” roofs. But the easiest option would be to call and see what they require. You will probably end up just filling the bays with fluffy insulation and then adding exterior foam to get to your dew point.
The town architect was in today. I asked about carve-outs for different ceiling types, but she said that there isn't a requirement for increasing the insulation from the current level, even for upgrading to zoned habitable space. So Thanks for the push to follow up, and not rely on hearsay from a contractor.
The R-value of closed-cell sprayed foam is R-6.5-7 per inch. 5.5" of ccSPF is R36-38. If you add perpendicular 2x2 strips, the additional 1.5" will give you an extra R10, for a total R46-R48. I would call that a good option if you don't mind loosing the 1.5" headroom.
If you need to reroof in the near future, option 3 is a good option to add 5"-6" rigid foam R35-40 min on top of the roof decking and R10 bellow the roof decking with any kind of permeable insulation. Reclaimed polyiso is a good inexpensive option, if you can find it.
2x’s perpendicular to the rafters makes a lot of sense; thank you. I was assuming 6.5/inch R value, but I’ll have to see what the local code overlords will accept.
I do need a new roof right now, which is what’s prompting this whole project. I’m really just trying to weigh the pros and cons of both approaches, and as much as I love the idea of nailbase insulation and the extra headroom the likely cost, contractor familiarity (most haven’t even been willing to give me a quote for it), rot potential, and fascia details are all giving me pause.
Since you need the roof right now, I think Armando's suggestion is probably the best. I'd like to add in that it may be a good time to ask your roofing contractor how they feel about making a small modification to their plans to follow the fortified roof guidelines. Since it's a cathedral roof, it'll help air seal and waterproof the entire deck layer for not a huge increase in costs.
Since you live in NY there may be financial incentives to help with the additional costs.
https://fortifiedhome.org/roof/
The perpindicular 2xs would also reduce thermal bridging since you'd only have small areas of wood where the pieces cross instead of the entire linear edge of the rafter. This would make your insulation inside act more like continuous insulation, and probably make up that extra R1 or so in Armando's numbers and get you up to code minimum. I would use 2x3s though, 2x2s are too easy to split.
I would recommend having your spray foam contractor install the first layer of closed cell spray foam, then finish off with a trimmed overfill of open cell. This would ensure your entire rafter bays get filled with no voids in the middle in the way you usually get with underfilled closed cell spray foam.
It's not really an issue to have exterior rigid foam on a roof. You do limit drying, but you don't get a lot of drying through a typical roof anyway with the layered shingles. I have roll roofing (basically one huge shingle) on my cathedral ceiling that has spray foam underneat, so I have spray foam on the inside and what amounts to a thin sheet of asphaltic coating on the outside. No drying, but also no issues. This is a fairly common roof assembly, just be sure to get the details at the edges right so that bulk water doesn't get under anything.
Bill
Thanks Bill. My energy consultant estimated just an extra 1kBTU/hour benefit from exterior insulation. Now that my town architect has confirmed that rafter bays filled with closed cell spray foam will suffice for code purposes, I'm finding it impossible to justify the expense of nailbase insulation. The foam will also help me to thoroughly air seal the leakiest parts of my house: my soffits and ridge. The biggest negative will be that I won't be able to beef up the insulation above the roof deck in the future if I ever want to, but with the heat gain/loss difference being so minimal, I think my home improvement dollars can be better spent elsewhere for the foreseeable future.