Roof insulation plan – low slope
I have a new question about our roof — I have edited my original question…
Climate zone 5
Low slope, unvented roof assembly
We are following what is recommended in Martin’s article “Insulating Low-Slope Residential Roofs” https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
—“You can install a more moderate thickness of closed-cell spray polyurethane foam on the underside of the roof sheathing, supplemented by a layer of air-permeable insulation below that.”
We need a minimum R-value of 41% foam for our climate zone. Our insulation contractor likes BIBS for the air-permeable insultation. With an available space for insulation of 11-7/8” (I-joist), I calculate minimum of 3.6” of closed-cell spray polyurethane foam (R6.5/in) with the balance (8.275”) of BIBS (about R4.1/inch). That would be a total of R57.3 – except for thermal bridging…
Sounds like thermal bridging is not addressed with this assembly. How bad is thermal bridging with 11-7/8” I-joists? Can you put a layer rigid foam between the I-joists and the roof deck to mitigate thermal bridging? I imagine that the rigid insulation would need to be bonded to the plywood to eliminate any air.
We are building a home in the northwest suburbs of Detroit in climate zone 5a. Ground snow load of 25 or 30 lb/ft2. It is a ranch home with a gable roof – basically no valleys or dormers. The ridge beam runs east-west. It’s a pretty flat roof – the north side is 1-1/2:12 and the south is 2.4:12.
Most of the roof is I-joists – 11-7/8” @16” OC with a cathedral ceiling. Some areas will be trussed on the north side with a flat 9′ ceiling.
Planning a standing seam metal roof – probably a darker color like dark bronze or something a few shades lighter.
I have attached the current details (for both the I-joist and truss areas). For the wall assembly, we have already made plans to add rigid insulation outside the sheathing to a minimum of R7.5 with a 1” airspace between the insulation and the brick– this change is not reflected in this detail. I have questions about that, too – I’ll ask in a later post.
The layers are noted on the details.
Is this a good plan?
The eaves are 4’ from the studs in most places – would you insulate the eaves on the south and west sides to avoid ice dams from cladding thermal plumes?
Should our strategy be to ventilate as it’s planned? If so, would we need a ridge vent or vapor diffusion port?
Alternatively, should this be a conditioned space?
Thanks in advance, Alan
GBA Detail Library
A collection of one thousand construction details organized by climate and house part