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Spray foam hits a point of diminishing return. Unlike fiberglass, because spray foam is a air barrier as well as a thermal barrier its performance levels can not be measured in R-value alone. With simply 1″ of spray foam you will gain over 70% of your insulating value, 2″ will give you 84% and 3″ 90%. At 3″ you will hit a point of diminishing return.
