underlayment
I am planning to build this summer in Northern NY, zone 6, and will construct our vented roof assembly by installing rigid foam insulation above the first layer of roof sheathing. R-30 continuous R-19 cavity. The roof sheathing will be taped prior to placing the rigid insulation. We plan to use a permeable underlayment over this sheathing. Is permeable underlayment correct to use at this location? We will then place 6 inches of rigid foam insulation and 2×4 for a ventilation channel and extension of the eaves. A second layer of sheathing will then be placed over the 2×4. The plan is to use an impermeable underlayment at this location and then to place a metal roof on this. Is our selection of underlayment correct at these locations? Thank you in advance for your advice and all the information available on GBA.
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You don't really need any underlayment on the first layer of sheathing. But if you do use it, permeable underlayment would be correct. You can use either permeable or impermeable underlayment below the metal roof. It won't make much difference either way. You might get somewhat less condensation on the underside of the metal roof with impermeable underlayment. You should still pay attention to air sealing the interior ceiling plane. You should also use at least a Class III vapor retarder (drywall and latex paint can be OK) on the interior ceilings.
Thank you Peter. Would we air seal the ceiling plane using a membrane just prior to placing the drywall?
My thinking for this type of assembly is to place the vapor barrier at the roof sheathing. Everything above the roof sheathing than is either permeable or semi-permeable.
So in your case it would be
-drywall
-batts
-taped roof deck
-non permeable underlayment
-semi permeable rigid insulation ( unfaced EPS or fiber faced polyiso)
-permeable underlayment
-horizontal 2x4 or 1x4 on flat
-metal roof
This would allow the assembly to dry in both direction (limited through the foam but not zero). If it makes the assembly more robust or not is hard to quantify but since the cost is about the same, it doesn't hurt.
Most metal roof panels can be installed onto horizontal strapping. With the current cost of plywood, I would skip the 2nd deck. The ribs and striations on most panels allow for some airflow even with horizontal strapping providing a bit of ventilation.