Good question and thanks for telling us where your project is located. According to the 2012 International Residential Code (2009 version too) section R806.5, you must install at least an R-10 of air impermeable insulation such as open or closed-cell spray polyurethane foam under your roof deck in climate zone 4C (Portland) to help control condensation. If you prefer to use a combination of closed-cell and open-cell, I would suggest you install at least an R-10 of closed-cell before adding the open-cell. That's about 1.5 inches of closed-cell depending on who's closed-cell product you choose. That section of the code also has recommendations for all US climate zones when you want to combine different types of insulation under your roof deck.

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Regards,

Jason M. Hoerter, P.E.
Sr. Product Manager

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