Insulation
Wall Insulation Calculator
Insulation for wall cavities, where the stud depth limits how much R-value will physically fit.
Result
Wall cavities cap what is achievable: a 2x4 stud bay is 3.5 inches deep, which holds about R-13 of fibreglass or R-21 of closed cell foam. Reaching higher means continuous exterior insulation, not a denser batt squeezed into the same space. Required R-values are set by climate zone and by local energy code, and they differ for attics, walls and floors. The figures offered here are common code minimums rather than recommendations, and a minimum is a floor, not a target.
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About this calculator
What R-value fits in a 2x4 wall?
About R-13 with standard batts, R-15 with high-density ones, or around R-21 with closed cell spray foam. A 2x6 wall takes roughly R-19 to R-21 in batts.
Does compressing a batt work?
No, it reduces the R-value. A R-19 batt squashed into a 3.5 inch cavity performs worse than a R-13 batt that fits properly.
The formula
thickness = (target R - existing R) / R-value per inch
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