Insulating a four-season roof/ceiling.
We are planning to turn our screened, roof-covered, screened porch (approx. 22’ x 14’) into a four-season room. I would love to hear people’s thoughts on our plans for the different areas of the project. Yet, here, I will address only my doubts about the insulation of the ceiling.
We are in zone 5.
ANY feedback, even if not to all of my questions, would be very much appreciated!
The existing ceiling has exposed beams. Above the beams, there are wooden boards, then a gap (where some electrical wires, for the porch’s/future room’s lights, run) plywood, ice-and-rain shield for the entire surface of the roof, and then asphalt shingles. The beams give us a depth of 5 1/2″, which we are thinking of filling with spray foam insulation. We are also planning to apply, in addition to the spray foam, foam board panels (polyiso), which will be screwed to the beams; 1″ foam panels would be a minimum, but I am hoping we can go for 2 inches of polyiso. (That depends on whether we have enough height, as the roof is slanted––very high on the house side, and then sloping down; as the floor will have to be raised, as per another question I had posted days ago, I fear there might be not enough height for adding a 2″ polyiso board.) As a ceiling, we would like to go for beaded board, rather than drywall––we like the look and, also, hope that the installation would be simpler.
I am thinking of three possible options, though I don’t know whether the third is even admissible.
1) 5 1/2″ of OPEN-CELL spray foam + 2″ faced polyiso, taped + furring running perpendicular to the beams + beaded board. R value should be 30.9, right?
Is it OK to use polyiso boards inside (healthwise I mean)?
Should a vapor barrier be added to this assembly and, if so, would it make sense to place it between the sprayfoam layer and the polyiso board? (I suspect I do not need an additional vapor barrier, but I might be wrong about that.)
2) Same as above, but with CLOSED-CELL spray foam. The total R value would be, I believe 48.5.
In this case, I am sure the spray foam would constitute a vapor barrier.
I know that, in zone five, ideally roofs should have a minimum of R-49; so, this solution would be great, if more expensive (and, also, MUCH LESS environmentally friendly). I do worry, however, on:
(a) it making ANY sense to go past 3 1/2″ of closed-cell spray foam, as I understand there is a maximum thickness for it;
(b) any health concerns with having closed-spray foam inside, possibly releasing gases within the living space;
(c) the possibility that (despite the ice-and-water shield) a roof leak could be produced, and go undetected for a long time because of the closed-cell foam.
3) Is it possible to use Zip-System Insulated R Sheathing INSIDE the house? If that were to be the case, then I could skip the furring, and just install the beaded board on the sheathing. Of course, if such a use of the Zip-System sheathing is allowed (if, for example, there are NO health concerns related to using it inside), it could be applied to the open-cell or the closed-cell option.
Any thoughts on pros and cons on the above options? Thanks!
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