R-24 spray foam enough?
I’m in Zone 6 up here in Nova Scotia Canada. Got a old 150 year post and beam house that has some serious rot in the roof sheathing. The roof was insulated with fiberglass and no vapour barrier or proper ventilation, so it was mouldering in there awhile. Going to be laminating the rafters, ripping off the sheathing and putting on new sheathing and a metal roof. Thinking of doing an unvented roof assembly as there are dormers that would be heard to vent properly and I’m not convinced the baffles would adequately vent the sheathing. There is no attic, the upstairs right below the roof is our second floor and will be a living space, and so we would have to do a drop ceiling and make a mini attic to properly ventilate. Also losing 1-2 inches for vent space below the sheathing would cut into our living space. So i’m leaning away from ventilation but I am open to be convinced otherwise. To do an unvented with rigid foam on the outside of sheathing and batts on inside I would need an R25 on the outside. This is 5 inches of XPS rigid foam and seems a little over the top. My roofer wasnt gung ho on this idea. But is it the best route? Other option is spray foam. The code requires R 31 in sloped roofs here in Nova Scotia, that would be more than 5inches of spray foam, which I cannot afford. However the spray foam contractor says that 4 inches is fine. Is he telling the truth? If the code require R 31 why would R 24 of spray foam be ok? Also, according to spray foam contractors around here, 2 inches of spray foam is enough to seal the roof in as a proper vapour and air barrier. Is this true? If so I’m leaning towards 2 inches of spray foam to seal roof and then adding roxul batts below to reach my R31. Any insight into the potential consequences of just an R 24 in spray foam or choosing vented vs non vented, rigid foam exterior vs spray foam interior would be much appreciated! Thanks in advance.
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