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We're working on a project with a limited budget (who isn't?) and the contractor is suggestiong that as a cost saving measure, we use a thin layer of closed cell spray foam insulation along the underside of the roof plywood followed by a thicker layer of open cell. The thought being this will provide the air and vapor seal we need, yet the open cell exposed to the interior will be less expensive. Is this logic flawed or can this be done without casuing potential condensation problems? The project is in the Portland, OR area.
 
Answer
SprayFoam.com

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.

Feel free to click on the "Contact the FAQ Administrator" link to ask me a follow-up question.

Regards,

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

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