Over roof insulation in an old farmhouse
Time to replace the roof on a 1930’s farm house in zone 6A. The two story 1,300 sq house gable roof has most of the second story attic as living space in a large dormer on one side. Roof construction is 2×4 24″oc on the 9/12 pitched roof and 2×6 24″oc on the shed dormer. There is currently only a little bit of fiberglass insulation in the roof ad despite being an energy hog this structure is in pretty good shape for something so scantily built. I’m thinking of adding insulation on top of the existing sheathing to help with the energy situation. Of course this is often done during deep energy retrofits but I’m not going that deep, all I want to do is address the roof at this time. My idea is to strip the old shingle and paper. Cut off the eaves, put a high perm air barrier down, add 6 inches of rockwool comfortboard 80, add strapping running the height of the roof from the soffit to the ridge to act as a vent. Build new eaves, install sheathing and new roof and ridge vent. The existing gable vent would be blocked off and additional comfortbat added in the small area where the peak and gable vent is (only about 18″ from the ceiling). I know this is nowhere near current insulation code requirements but its what I can fit without making the house look ridiculous.
A few things I need confirmation of my thoughts on.
My idea is to keep this vapor open as there is no interior air/vapor barrier other than painted plaster and I don’t want to trap vapor in the roof assembly.
Does the comfortboard need protection in the from the top in the vent?
Structurally there should be a concern about roof load but there are currently three layers of shingle and the existing structure has handled the snow load for 90 years. The only structural issues are around the now abandoned chimney that has had some leaks over the years. That chimney will be removed during this process.
Are there any other concerns I should have with this retrofit?
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