Correcting a thermal bridge
I’m building a house in zone 6a, Central NH. My plan is to wrap the house with 2 layers of eps totaling 4.5 inches with the joints staggered. The framing is 2×4 and no interior vapor barrier will be used. I was planning to add 3.5″ of fiberglass to the cavities.I’m using DuPont drainwrap over OSB with the joints and stud lines taped with zip tape. The foam will be furred vertically with 3/4″ boards to ventilate behind the siding.
The roof will receive the same 4.5″ of foam and is 5/8″ Advantech with Tamko Moisture Guard between the foam and the Advantech. The roof will be re-sheathed with plywood and papered with metal roofing over. The rafter bays will be filled with fiberglass batts with no vapor barrier.
The foam I’m using has a permeance of 4.95.
My question regards the overhangs which are to be installed directly to the first rafter through sheathing and wrapped on both sides with foam but will contact the upper layer of roof sheathing. I think I’m creating a thermal bridge and may be in danger of moisture problems at the top of the wall.
An architect friend has suggested flashing the interior of the house with spray foam assuring me that there will be no dew point issues. I’m not too concerned about the wall as I believe the Drainwrap (36-50 perms depending on test method) and ventilation behind the siding will allow the wall to dry outward if it is necessary but I’m concerned about the roof as the Tamko product isn’t vapor permeable (.001 perms).
Naturally the house will have an HRV installed.
Sorry for the lengthy post and thanks in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part