Spray foam under roofs & moisture
I am planning to get my spray foamer in soon for apecidoc areas that need air sealing and higher R value. But I started to think twice about the spots under some of the roofs.
So my main roof has 6″ of polyiso above the deck, so I will have 9.5″ of Rockwool insulation in the joist bays to let the deck breath donward to the interior.
However, the rest of the roofs are very small decks, where there isn’t insulation on top, just the polyurethane roof membrane on top of the plywood decking. Thus I want to use 2lb spray foam to get the highest R value possible for both efficiency and hvac/comfort for the occupied spaces below.
My worry is if I sprayfoam the underside, within the 9.5″ joist bays, based on discussions here, it would create a rot sandwhich for the decking. But then I’m wonder how severe the moisture /rot would actually be, given that they’re relatively small areas.
These roofs are 112sqft, 38sqft & 18sqft.
Does anyone have any experience to offer? Should I not worry, or is there an alternative solution I’m not thinking of?
Any advice is always appreciated.