What type of insulation to use?
We have just started a remodel on an old farmhouse in the Zone 6 region. The old 1 1/2 story farmhouse used 2X4 roof and wall framing with ship lap on both sides of the walls and on the top of the roof. The walls have blown in cellulose and the attic space has blown in fiberglass. The new part will be 2X6 construction and we are matching the roof lines but using 2X8’s for the rafters with the ceiling leveling off and 2X8 tie rafters. Our plan all along has been to put 1″ foam board (EPS) on the entire exterior of the walls with Tyvek Drain wrap under the foam and regular Tyvek on the outside of the foam. We were going to batt the walls with 6″ fiberglass and spray foam the ceiling of the new part (7 1/4 inches of closed cell). But we were talking with someone this week that doesn’t think the drain wrap is performing the way it should so we started looking for alternatives, came across your website, and now have many more questions with a project that is currently going on. We are using vinyl siding and sheet steel on the roof. After reading many different articles I have developed some ideas but our situation is always a little different than the articles I have found so I am looking for input to help clear my thinking. Thanks for any help.
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John,
If you want to install rigid foam on the exterior of your 2x6 walls in your climate zone, the rigid foam needs to have a minimum R-value of R-11.25. If you plan to use EPS, that means that you will need a minimum of 3 inches of EPS, not 1 inch.
Here is a link to an article that explains the reasoning behind this code requirement: Calculating the Minimum Thickness of Rigid Foam Sheathing.