Air space under roof insulation?
I just read an article by Fred Lugano that suggested installing rigid foam with a slight air gap even though ventilation isn’t necessary with air tight insulation methods. I’m thinking of rigid foam insulation, cut to fit between rafters, and sealed in with spray foam. My thinking is that this should be air tight and a vapor barrier, and not require ventilation or an air gap. I don’t have soffits or roof ventilation, so I have no access to fresh air, so the air space doesn’t make sense to me. He claimed it was just a minimum amount of space for moisture to accumulate if it did get in, and then would be dissipated through the roof sheathing slowly. Wouldn’t that also happen if the rigid foam was up against the roof deck? BTW, my roof has 1×4 planks nailed across the rafters, with plywood over that, then shingles. So the inside underneath surface has gaps between the planks.
Replies
ccampbell, your approach can work, but depending on what climate zone you're in you might have problems. If the foam is too thin, the inner face could become a condensing surface with the ability to dry to the inside only.
If the product is foil-faced polyisocyanurate (e.g., "Thermax") leaving an air space allows it to function as a radiant barrier.
Thanks for the reply. I was thinking of using 2 2" layers, plus a 1" layer of R5 foam board, for R25 in a 2x6 space. I would put them directly against the roof deck, foamed in place - no ventilation.
I'd be cautious about using
I'd be cautious about using extruded polystyrene as it may not be code approved for how you describe it. The other posters suggestion to use polyiso is probably abetter approach and 'more normal' for this type of application. Raw styrene is always a concern. It's a fire code issue.
Otherwise your approach is probably fine ... again depending on your climate ... and I might add ... the condition and functionality of your exhaust/ventilation systems in the house. Properly ventilated, you can get away with a lot in a house in terms of a vapor retarder/system.
I'm assuming you have a vaulted ceiling that you are working with (i.e. not an open attic).
I'm actually looking at
I'm actually looking at converting an attic to living space, so yes, it would be a vaulted ceiling. The Foamular I was thinking of using would be covered with drywall, so why would that be a problem?
I'm in Kansas City. I think that's zone 3.
As long as it's covered with
As long as it's covered with drywall you should be okay. I've found the easiest way to get a good seal it to cut the sheets (on a table saw) about 1/2" narrower than the framing cavity, tacking them into place with nails, then foaming the edges with canned foam. I don't know what your code minimum R-values are but if R-25 is not enough you can get a little extra R-value with polyisocyanurate, but XPS is a little easier to work with and costs less. The only other thing you might consider is adding a full layer of 1" foam under your drywall to eliminate thermal bridging.