Help with Exterior Insulation in Climate Zone 5
I’m in the process of adding exterior polyiso insulation to my small 24’x32′ garage-apartment in Strafford County, New Hampshire (Zone 5) with the first six panels of insulation on. In scouting GBA for the answer to a different question, I either missed or failed to read an important part about the lower performance of polyiso in cold climates and that builders should derate the R-value of polyiso when determining how much exterior insulation to add. I missed that the two inch standard for Climate Zone 5 applies to EPS. I purchased two inch polyiso, thinking that I was meeting the dew point requirements for Climate Zone 5. The house is framed with 2×6 framing and ZIP wall sheathing. There will be a rain screen between the insulation and Hemlock siding.
My questions are:
1.) Should I hit full stop and switch to EPS or XPS, or move to a 2.5 inch polyiso? No matter what, it looks like 2 inches of polyiso isn’t going to cut it?
2.) Or, does the fact that I’m installing mechanical ventilation (ERV) help with controlling any potential issues with the lower performance of the polyiso in the colder months?
3.) Is there anything else that can be done to offset the risk of installing only two inches of polyiso versus bumping up to 2.5 inches of polyiso?
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