Zone 6 roof replacement: vent or unvented?
Preparing to replace my 1972 colonial roof – 8/12 pitch, Northeast Zone 6. Currently the roof creates large ice dams on both the east and west sides which eventually cause leaks. I am questioning if I should leave the roof vented or create an unvented/conditioned space in the attic by insulating the roof deck.
Current Installation: 2”x6” trusses and 2”x4” walls. Attic has 6” fiberglass insulation with 6” blown in on top. Soffit and ridge vents. No defined vent channel. There is no question why the ice dams are forming – lack of insulation and poor installation.
Below are the options I have been considering.
Vented Roof: Install synthetic underlayment on existing roof sheathing, install 1 or 2” of polyiso above original sheathing, new sheathing, ice/water shield and synthetic underlayment, new shingles. Correct the eave vent/insulation and install 1” polyiso 1” below the original roof sheathing (inside the attic) to create a defined vent space for the roof. The eave work would be done from the outside by removing the first course of original sheathing. Once roof work is complete, increase depth of blown in insulation.
Unvented Roof: Install synthetic underlayment on existing roof sheathing, 6” polyiso above original sheathing, new sheathing, ice/water shield and synthetic underlayment, new shingles. Install 3” polyiso below the roof deck. Insulate gable ends. Remove existing fiberglass and blown in insulation in attic.
Despite all the info on ice dams and roofs on GBA I still find myself questioning which I should do. If either?!
Thanks for the help in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part