Potential Ice Damming on New Roof
I’ve worked with an architect to design a pretty good house in Zone 7 (60psf Snow Load Region).
We’re using 16″ i-joists for the roof system with the intent of nailing plywood onto the top flange to create a 1.5″ vent chute. Then, dense packing the vaulted section of the roof with 14″ of dense pack cellulose. Drawing attached. There will actually also be an interior Intello air barrier and a service cavity on the ceiling.
In working with an energy consultant on mechanical design, heat load calcs and “redlining” the building for performance improvement opportunities, they expressed concern over the roof system. The r-value was not high enough (wanted “whole wall” R50 with less thermal bridging) and they wanted to see a 2″ vent chute.
They’re concerned that I will see ice damming on the eaves and that for the level of effort, there are higher performing roof assemblies.
1. Should I be concerned about ice damming with this assembly?
2. Are there other durability and performance concerns with the assembly?
3. Are there other roof assemblies that you would suggest using instead?
The energy consultants would like me to change to either a lumber or i-joist rafter with plywood and 6″ of rigid foam on the roof deck, then doing 2x rainscreen strapping and a second layer of roof sheathing with underlayment and roofing on top of that. I would have to apply the overhangs as outriggers using the rainscreen strapping.
Would the external insulation on the roof be THAT much better that it is worth what seems would be some additional effort and hassle?
CAN I do less external foam on the roof, perhaps 3-4″ to make it easier or do I then introduce concerns around condensation in the roof cavity?
Thank you for your feedback.
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