Exterior polyiso or XPS for wall and roof assembly?
I have plans for construction of a 15 x15 foot addition to my home in climate zone 5. The walls are currently designed as 6-5/8 inch Murus OSB polyurethane SIPs with adhesive spray foam in the panel joints, All Weather Flashing tape on the inner seams and Grace Vycor adhesive on the outer seams. Tyvek Drain wrap is over the SIPs to act as a small air gap. The drain wrap is covered by 2 inches of foil faced polyiso with 1x4 furring added to support UNA-CLAD steel siding. The entire wall assembly is roughly R 53 or so.
The sloped roof is also 6-5/8 inch SIP again with adhesive spray foam in the panel joints, All Weather tape on inner seams but with Grace Vycor covering the entirety of the external SIPs as per a PERSIST design. I then have 4 inches of foil faced poly iso on top of the Grace Vycor adhesive with 2x4 furring over the 4 inch poly iso then 5/8 inch CDX plywood, #30 felt and UNA -CLAD standing seam metal roof. Roughly R 66 for assembly.
My architect is now telling me that she has reservations about having poly iso exposed to possible water in both wall and roof assembly and thinks that it would be wiser to use XPS for both assemblies as she believes XPS to be 10 times less absorptive of water compared to poly iso. She also is making a case that poly iso is less stable than XPS, degrades in R value, and shrinks more than XPS. She is also making the argument that the permeance of foil faced poly iso is so low that the walls will not be able to dry to the outside and that XPS would have higher permeance and therefore better for that reason as well.
As for the roof she also now has concerns that nailing of insulation and furring strips through the Grace Vycor adhesive will compromise the water proofing function of the Grace Vycor. She is now suggesting for the roof the usage of 4 inch wide metal Z clips to hold the insulation over the Grace Vycor and to mount the furring strips onto the Z clips so as to keep penetration through the water proof adhesive to a minimum.
My questions are the following: If her statements about the inferiority of poly iso to XPS are true with regards to moisture and durability issues why would one ever use poly iso as an exterior insulation? Not with standing her concerns, I have seen many pictures ,and read many articles on GBA , in which poly iso has been used as the exterior insulation with furring strips added on both wall and roof assemblies. Are her concerns about the poly iso legitimate and should I be using Z clips for the roof?
Posted Wed, 12/11/2013 - 16:46
Edited Thu, 12/12/2013 - 07:01
Other Questions in Green building techniques