Insulating and Venting a Cold-Climate Cathedral Ceiling
I’m building a high performance ICF home in Zone 5 with a single slope mono pitch (3:12) roof and a cathedral ceiling. The Roof layers are currently as follows:
Standing seam metal
5/8 Zip Roof Sheathing
We were planning to have an unvented assembly with 3″ of ccSPF applied directly to the underside of the sheathing (technically about R21, not counting the thermal bridging of course), followed by open cell or mineral wool to get us to R49 or R60 (we have very tall parallel chord trusses with plenty of space for insulation…and the downside that there will be more thermal bridging since the bays will never be fully insulated, although we are considering encapsulating them with a thin coating of ccSPF to help with that). We are planning to use a T&G wood ceiling attached to the bottom of the trusses with no drywall above it – we believe this should work because the air barrier is the 3″ of ccSPF on the underside of the roof, so we’re basically creating an unvented, conditioned “attic” in the 12″ or so space between the T&G ceiling and the SPF on the underside of the roof (please let me know if anybody disagrees and sees a problem with that design).
The roof will be light colored, fairly low slope and facing due north, in a high snow area in Zone 5. We will have 2′ overhangs at the ridge (high side) and 1′ overhangs at the eave (low side). While I believe that this assembly should technically work, I’m thinking that with such a simple roofline (no dormers, skylights, etc. – only roof penetrations will be 1 wood stove pipe and 2 PVC plumbing vents), it might make sense to include a ventilation channel below the sheathing to allow for some drying potential in case it were to ever get wet. With 3″ of ccSPF below I’m not too concerned about air or vapor leakage from the house, but eventually every roof leaks and I’m concerned that the sheathing would have virtually no potential for drying and the leak may go undiscovered for a long time increasing the risk for catastrophic failure. It might also help with ice damning (not a huge concern with standing seam) and if I ever need to repair or replace roof panels (and sheathing below), we wouldn’t have to tear out all of the insulation.
We’re considering adding vents in the soffits at the eave and ridge and building in ventilation channels (probably at least a 3″ space since we have plenty) all the way from the eave to the ridge using rigid foam or maybe fanfold stapled to the sides of the rafters and then keeping our plan of using a combination of 3″ of ccSPF applied directly to the back of the ventilation baffles, followed by air permeable insulation to get the rest of our R-Value.
I’m wondering if there are any downsides to adding this vent channel? It is a mixed-humid environment in summer, with high outdoor humidity levels which could unnecessarily introduce moist air to the underside of the sheathing in the vent channels. We also have a very tall detached garage very close to the house on the eave (low) side, which might block a lot of potential for wind exposure from that side which would limit the usefulness of the vents. I’m also concerned about truss uplift caused by basically putting all or part of the top chord in the ventilation channel outside of the building envelope while the rest of the truss members are inside the envelope. On a similar note, I’m concerned that the thermal bridging through the trusses might be made much worse by exposing the top chord and bottom of the roof sheathing to the exterior air in the vent channel rather than keeping them inside the conditioned envelope (at least inside the foam) so that the cold has to conduct from the air to the roof, through the ice and water shield, through the sheathing into the trusses).
I would greatly appreciate your thoughts on these options and proposed assemblies. Our main goal is to build the house in the most durable and energy efficient manner within the confines of our mono-pitch roof design.
Happy to provide drawings or answer any questions if I’ve omitted any details that we would help.
Thanks in advance!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part