What is the best rigid insulation to use when building the compact unvented roof assembly (2004 Building Science Corp)?
The project is located in northern Sonoma County, California. According to the California Energy Commission this is in California climate zone 2.
We are using a continuous layer of W. R. Grace “Ice and Water Shield” as the air barrier membrane. This also allows for a water poof protection for the exposed decking during course of construction.
We will apply another layer of W.R. Grace “Vycor” on top of the cdx plywood underlayment for the metal roof.
I was thinking about using 2 layers of 2″ rigid polyiso insulation and staggering and taping the seams of each layer. A EPS rigid insulation salesman told me that California has required changes to the way polyiso is made and therefore polyiso now only has a R value of 5/inch. I am a bit skeptical.
Also due to the fact that there is no air space I would not receive any R value benefit to getting foil facing on the panels. I would also prefer not to add 2x sleepers to the top of the rigid for an air space if it not necessary – the additional buildup would be detrimental to the design.
Thank you in advance.
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