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.
Jason M. Hoerter, P.E.
Sr. Product Manager
Copyright © 2012 NCFI Polyurethanes