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Unvented roof assembly

Jekyll975 | Posted in General Questions on

Hello everyone, 

I plan to build an unvented, low sloped roof (12%), in climate zone 6. 

I’ve read that in an unvented assembly, rigid foam must be in direct contact with the roof deck, and can be used in combination with fluffy insulation. 

I’ve considered several techniques that i’ve found on this website and others. 

From what i’ve understood, the following assembly (proposed by a contractor I will work with) is not permitted, but I don’t understand why, if I pay good attention to air tightness on the underside of the rigid foam. This is the most cost effective and easy way i’ve found to build my roof, and  also a very high r-value (R-75), so i seriously consider it. 

Can someone tell me if it is risky, and why ? 

I thank you in advance.

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