Risky wall assembly design with stucco/stone over 2.5″ XPS outsulation
Building in Zone 5, NE US want to adhere to the concept of outsulation and break up thermal bridging of 2×6 stick framing (sheathing is 5/8″ plywood) and add 12.5 R-value to the wall. Final exterior product planned to be a combination of stucco and stone veneer (lick and stick).
Had enthusiastically wanted to use 2.5″ XPS, taped seams, over Tyvek applied directly over plywood followed by a layer of asphalt #15 felt over the Tyvek, followed by the taped XPS. The details to support the weight of stone veneer is the issue. The areas to get stucco (EIFS) would be fine atop the XPS, would apply a weep panel atop the XPS under the EIFS to vent the stucco layer but the weight of the stone veneer is an issue.
Was taking a belt and suspenders approach by starting with an angle iron acting as a brick ledge. Not that stone veneer is meant to transduce its weight to the brick ledge, but figured it can’t hurt to combat creep. Then to support the stone veneer weight was thinking first insert a layer to vent the stone and then secure furring strips AND the overlying metal lathe directly to the 2×6 studs with the appropriate caliber fasteners and frequency installation, according to the weight of the exterior stone.
Finally proceed over the metal lathe with a scratch coat and stone.
I have consulted with the structural engineer we used on other parts of the structure who obviously warned me to proceed with extreme caution and referred me to my architect citing wall assembly is not his area of expertise (albeit he was very helpful). My architect and the contractors I am working with all seem stuck on why I would wrap the entire house in 2.5″ of XPS. The architect has suggested I get another engineer involved. Is it me or does nobody want to go on record to design a 2.5″ XPS outsulated wall with exterior stone veneer? (for obvious reasons right?)
My engineer did suggest cutting the XPS layer to 1.5″ to help mitigate the structural risk. Given that thickness would suffice in Zone 5 to keep my plywood sheathing above the dew-point (from my understanding) than I am ready to say uncle and decrease it to 1.5″.
Any good suggestions/advice is appreciated and I offer my regards in advance.
btw: in the event of 1.5″ layer of foil wrapped poly-iso, given its low perm, would it obviate the need for a vent layer below the stone and/or stucco limiting drying to exterior only?
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