Meeting commercial energy codes for slate roof insulation
I am detailing a commercial slate roof and I am having a hard time finding information on how best to insulate a slate roof while meeting commercial energy codes. The roof insulation must exceed 2012 IECC R value by 10% to meet a local green code provision. The project is a classroom building in the
4A climate zone.
The original roof detail has continuous insulation entirely above the roof deck with 5” poly iso insulation to achieve R value of 28. I have read Martin Holladay’s article “How to Install Rigid Foam on Top of Roof Sheathing” https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing. Do the assemblies discussed in the article apply to slate roofs or should the insulation be below the roof sheathing? Does the slate roof need to be ventilated or installed on battens if the insulation is above the roof sheathing?
Also, the roofing contractor has substituted the specified 2 layers 30lb asphalt impregnated roofing felt with SBS modified bitumen underlayment. NRCA mentions the modified bitumen underlayment can act as a vapor retarder that may cause problems with ventilation and moisture. The project does not have an eave condition so I do not think we have to worry about ice dams which is what the product seems to be for. Is there any good reason to stay with the SBS modified bitumen underlayment?
Slate roof assembly detail is as follows:
Underlayment (see question above)
5/8” CDX plywood sheathing (recommended by NRCA for plywood)
5” poly iso insulation – 28 ci R value
5/8” plywood sheathing
Light gage metal trusses 24” o.c.
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