Ensuring drying on flash and batt unvented standing seam roof
I am currently building a 2600 sq ft home – with beyond code insulation, ducted mini splits, HRV for ventilation, and high efficiency windows – and the framing is nearly complete.
This is located on the line between Marine 4 and 5 (Clark & Skamania County line in SW Washington State). We are located at 1000 ft and see everything from 5 degrees to 100 degrees, about 60 inches of rain, and as much as several feet of snow (county design snow load is 70 psf roof).
From the start we planned a unvented cathedral roof at 10/12 pitch, using flash and bat, 2×12 rafters (stick framed room, all ceilings in the house are vaulted), and a standing seam metal roof. Thanks to many articles I’ve read here, and the community here, I felt I had a good understanding of what was required to do this (spray foam under the sheeting, then batts, then well sealed drywall with no can lights) and why it was best (the roof is a simple gable but has skylights and dormers in it).
Now as we get close to putting the roof on, with quotes from roofers in hand and a few selected based on their experience with standing seam – I realize that one thing I did not fully understand is ensuring the sheeting can dry in at least one direction.
Our roof stackup is currently planned as:
1. Standing seam metal (ASC Metal Skyline 16 in a medium gray color “Old Zinc Gray”)
2. Synthetic Underlayment (Titanium 50 is what the roofer wants to use)
3. 5/8 Plywood (already in place)
4. 3″ of closed cell spray foam (exceeding the R value requirements for flash an batt in Marine 4 since we are next to zone 5)
5. Remainder of 2×12 rafter bay filled with blown in fiberglass or cellulose
6. Drywall ceiling with all seams taped.
I’ve seen several suggestions here and elsewhere to ensure the plywood sheeting can dry, however some don’t seem ideal for our construction:
1. Rigid foam on the exterior – both cost and aesthetics prevent this from being a good option. The metal roofing requires a solid substrate so we’d have to put down more plywood over the foam. The resulting 4-5″ sandwich would also look rather strange because the roof already has 2×10 fascia and exposed tails – I think a 2×16 fascia would look too strange.
2. A vent channel under the metal combined with #30 felt or breathable underlayment – again this would require another layer of plywood which would be expensive, and would make the deck look thicker. If #1 and this are the only options I guess we would pick this.
3. I’ve seen some mention of using Cedar Breather between the underlayment and the roofing – this seems doable and affordable but I’m not sure how effective – has anyone done this? what are the opinions of whether this provides enough drying in a stackup like we want to use?
4. I’ve seen some mention of using #30 felt or a breathable underlayment and just putting the metal over it, but it isn’t clear to me if this has any benefit – I don’t think it does since the metal cannot breathe. Would this work? Can it dry to the inside with 3″ of closed cell spray foam?
Any help understanding this fully and how best to build this roof without having a thicker roof deck would be most appreciated!
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