Choosing Rigid Foam Insulation for Exterior Application
Re-thinking type of 3″ rigid foam for exterior of 2×6 framing in Climate Zone 6a (Maine).
– No XPS (blowing agents)
– Relatively easy for builders inexperienced with exterior foam (…fastener length aside…we’re working on that!)
I was looking at 3″ Type II EPS, but someone mentioned switching to 3″ GPS (graphite-infused) which is more expensive but has higher R-value. I’d written off polyiso since it loses R-value as temps decline but should I reconsider?
2 x 6 stud wall with dense-pack cellulose
7/6″ ZIP sheathing with tape
3″ exterior rigid foam
Painted pine clapboard
I don’t have an energy model going and would love a simple calculation to compare cost-effectiveness of these options.
Assumptions for cold temps (are these reasonable?):
Type II EPS @ R-4 per inch = R-12
Graphite-infused EPS @ R-4.75 per inch = R-14.25
Polyiso @ R-4.5 per inch in cold temps = R-13.5
Rest of wall @ R-23? (R-4 dense pack cellulose for 5.5″ = R-22, plus R-1 for ZIP 7/16″)
Roof will be flash-and-batt within 2×12 cavities with low-GWP foam + dense-pack cellulose.
Building is 734 sq ft with one monopitch shed roof and one flat roof, requiring 34-38 panels of rigid foam. I think my overall glazing ratio may be close to 20%; more on south; still finalizing windows.
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