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What is the best rigid insulation to use when building the compact unvented roof assembly (2004 Building Science Corp)?

27brLutkxr | Posted in GBA Pro Help on

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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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure about Title 24 requirements, but the 2006 IRC requires at least R-30 ceiling insulation in Climate Zone 3. That means at least 5 inches of polyiso (which has an aged R-value of about R-6.5 per inch).

    Most green builders favor EPS or polyiso over XPS, because the blowing agents used for XPS have a higher global warming potential than the blowing agents used for EPS or polyiso.

    I'd be a little nervous sandwiching any plywood or OSB between two layers of Ice and Water Shield. One layer is plenty. Put it on the bottom layer of roof sheathing, and omit it from the top layer. What's wrong with ordinary roof underlayment like asphalt felt?

  2. 27brLutkxr | | #2

    Thanks for your reply Martin.

    The building is a workshop which is exempt from T-24 requirements. We are using this outbuilding as a method to refine construction details for the future house. The proposed house will have at least 5 inches of rigid insulation.

    The second layer of W.R. Grace "vycor" added to the metal roof underlayment was recommended by the roofing subcontractor as a means to mitigate the low pitch ( 3 in 12 ) and the extreme temperatures reached ( it gets to 114 deg. F in the summer on occasion). I will discuss with him the option of asphalt felt.

    From your reply to the original question I am assuming that polyiso rigid insulation still has an R value of 6.5 per inch in California. Polyiso is also the most competitively priced.

    I am concerned as well with the two layers of WR Grace with plywood between but if there is no way for the warm moist air within the living space to get to the cold surface of the roof underlayment what problems could arise?

  3. GBA Editor
    Martin Holladay | | #3

    If there is any imperfection in the top layer of Vycor, a roof leak could cause the top layer of roof sheathing to get wet.

    Or, here's another way to look at it: if the roof sheathing happens to be damp on the day you install it, it will hold that moisture content forever. So you had better hope that the weather is perfectly dry on the day you install the top layer of sheathing -- and that your contractor has stored the plywood perfectly, so that the plywood never got wet before installation.

  4. 27brLutkxr | | #4

    Thanks for your insight. It has been helpful.
    Is there any benefit to foil faced polyiso given in the compact unvented roof assembly there would be no air space?

  5. GBA Editor
    Martin Holladay | | #5


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