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Low slope roof with a PERSIST / REMOTE house? Possibly using insulated metal panels instead of steel deck

jchwang | Posted in Green Building Techniques on

Toronto, Climate Zone 6.

Follow-on to my last question about insulated metal panels in a PERSIST wall system, I am wondering about roof. My understanding is that one of the differences between REMOTE and PERSIST is that REMOTE utilizes a vented attic.

For a low slope (2 degree) flat roof, what would be recommended? Do I adopt PERSIST wall structure (modified with insulated metal panels instead of exterior foam board) with the addition of a top layer membrane and sheathing?
(inside to outside)
i) GWB
ii) MemBrain
iii) 2×6 framing (5.5″)
iv) 5″ Roxul (mineral wool) for R16 (yes, am considering leaving 1/2″ gap for wiring / plumbing chase)
v) 1/2″ Plywood structural sheathing
vi) Tyvek Drainwrap
vii) 3″ Roxul Comfort CIS for R12 (supports fasteners, drainage, fire barrier)
viii) 4″ IMP for R28 with screws fastened through to framing
iv) TPO or some other membrane.

OR totally get away from the IMPs for the roof (worried about water penetrating caulking in this horizontal application).

I do not have a lot of room height-wise due to zoning, so likely going with unvented structure.

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  1. GBA Editor
    Martin Holladay | | #1

    You cannot use metal siding panels for roofing (especially for a low-slope roof). You need roofing.

    More information here: Insulating Low-Slope Residential Roofs.

  2. jchwang | | #2

    Sorry, I reread my question, and it wasn't phrased very well. Can I use the same principles for the PERSIST wall for the roof?

    It wouldn't be identical (instead of framing, there would be a roof truss or I-joist), but there is a commercial insulated metal panel product that the vendor says replaces the steel deck, insulation and top sheathing:

  3. GBA Editor
    Martin Holladay | | #3

    If you want to combine air-permeable insulation (like Roxul) with rigid foam for an unvented roof assembly, you have to make sure that your rigid foam layer meets the minimum R-value requirements for your climate zone. These minimum R-value requirements are part of many building codes. More details here: How to Build an Insulated Cathedral Ceiling.

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