Discounting the R-value of fiberglass batts
I do energy assessments for our utility customers (Zone 6A). I have a question about a customer looking at an insulation retrofit. Her home was built in 1979 with 2×4 exterior walls. Using a probe, I was able to determine that the exterior walls have fiberglass batts. Thermal imaging did not reveal any major gaps or voids.
I know a perfect installation should achieve an R value of around 11-12 (assuming a 3.5 inch thick batt), but we all know that it is highly unlikely to be perfect. Can anyone give me a good number to use to discount for imperfect insulation? R10? R7?
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