Replies

1. 
   [email protected] | Feb 22, 2024 10:27pm | #1

   No this does not make sense. Two layers of zip and xps and closed cell foam is too many layers of things that do not diffuse vapor.
   Personally probably put zip panels on my roof rafters, xps foam on top of the zip, then I’d screw 2x4 purlins to attach the metal roof to.
   if you go with foam go with open cell and fill the rest of the cavity with batt insulation

1. 
   southern_water | Feb 23, 2024 09:07am | #2

   Thanks Primitive. I understand the vapor concern; that's what I've been trying to wrap my head around. The ZIP documentation says it is not a vapor barrier- just air/water. So this seems to indicate the XPS foam and the Closed Cell foam are the true vapor barriers.

   Thinking through this for my climate zone (3A-Hot/Humind) the greatest vapor concern will be in the warm-humid months with warm-moist air trying to push its way into the unvented attic. The risk then would seem to be water/moisture/vapor penetrating the first layer of zip, the XPS, the second layer of zip, and then condensing against the back side of the CC foam. Is that how you see it?

   We've all seen Matt Risinger's house with the assembly I mentioned only with mineral wool instead of foam. I've also done a bit more reading on the Building Science website and found one article by Lstuberik that seemed to indicate this assembly would work (https://buildingscience.com/documents/insights/bsi083-mea-culpa-roofs) but he does mention that should water get into the space it would be bad and this was the only information I found directly addressing my planned assembly.

   Regarding the vented roof deck, I have gone around and around about that. As I'm planning metal shingles I think I would need a second nail base regardless which would go on top of the furring strips/purlins and then I'll have to deal with the venting somehow our the fascia...more thinking on this one.

   Thanks for the input and the additional homework.

1. 
   southern_water | Feb 25, 2024 10:31pm | #5

   I found this in some saved browser tabs (forgot I had it). It is the energy.gov guide on insulting roofs. This guide specifies Open Cell foam under the roof.
   https://basc.pnnl.gov/resource-guides/above-deck-rigid-foam-insulation-existing-roofs#edit-group-scope

2. 
   firedudec56 | Feb 23, 2024 01:03pm | #3

   if you go with the metal roof, be sure all the components are rated for the higher temperatures the metal roof will have from the sun (not saying that as well as I'd like)
   bottom line, the metal roof will probably be hotter than conventional asphalt shingles and anything under it will need to be able to take the "heat"
   good luck

1. 
   southern_water | Feb 25, 2024 10:21pm | #4

   Thanks. Understand. The Zip OSB will be fine underneath.
   And I did find this informative article about XPS foam that includes a discussion on dealing with heat on the roof: https://www.roofingcontractor.com/articles/86305-xps-design-considerations-for-extruded-polystyrene-roof-insulation

3. 
   Donaldgallo | Feb 26, 2024 06:59am | #6

   Your proposed roof assembly for a hot and humid climate (Climate Zone 3A) with a sealed attic is well-considered. The metal shingle roof, ZIP sheathing, XPS foam board, ZIP sheathing on roof rafters, and closed-cell spray foam collectively create a robust thermal and moisture barrier. The optional addition of mineral wool or fiberglass batts enhances insulation. Ensure meticulous installation, paying attention to sealing, moisture control, and ventilation. Consulting with a local building professional is advisable to tailor the design to specific climate nuances and ensure compliance with local building codes.