Does one need to ventilate a small roof area when replacing old fiberglas insulation with closed cell polystyrene?
I have recently gutted a bathroom in my 53 year old house which had old fiberglas insulation in the shed roof and walls. The roof line is a low pitch (3/12)shed roof framed with existing fir 2×8 rafters, and ties into the main gable roof (7/12) at the eaves. I would like to insulate the walls and roof with closed cell polystyrene because it is a tub area extended out off the back wall of the house. The framed floor of the tub area is insulated with R-40 closed cell polystyrene, under 3/4 t&g plywood. Do I need to leave ventilation space above the polystyrene if I fill the rafter cavities with polystyrene, or do I install it tight to the underside of the existing roof sheathing with no ventilation. There is a vented soffit detail existing already on this shed extension. In order to reduce thermal bridging I was going to use 1 inch closed cell polystyrene or 1 inch reflective polyisocyanurate with any and all joints taped, installed perpendicular to the rafter faces with 1/2 inch Hardiboard for tile. I am limited to the depth of insulation that I can install on the rafter faces due to the fact that where the descending roof meets the wall there is a new low e argon awning window and the heights are critical at the juncture of the wall and roof. Thanks for the help Edward J. Palma
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Replies
So long as you truly seal the space and prevent bulk air filled with humidity from the bath from infiltrating into the area between the foam and the sheathing you should be fine.
The key is getting a good seal. Your idea of filling the 2x8 rafter space with chunks of foam and then running over it all with a sheet of foam under the backer board that is well detailed with tape should work fine. It may void your shingle warranty but I've never heard of anyone collecting on one of those anyway.
If your main 7/12 roof is a ventilated assembly and is getting it's air from the soffit vents on the 3/12 roof then you will need to be sure you don't cut off that supply.