Insulation calculator

US guidance for most climate zones: R-13 to R-21 in 2×4/2×6 walls and R-38 to R-49 in the attic. Attic insulation is the cheapest R you can buy — going from R-19 to R-38 up there usually beats any wall upgrade per dollar.

No — compressing a batt squeezes out the air that does the insulating and cuts its R-value below the label. Use a product made for the cavity depth, or a board insulation with a higher R per inch like XPS.

On the warm-in-winter side: inside the insulation in cold climates, taped at every overlap and penetration. On the cold side use a breathable membrane, never a second vapor barrier — moisture trapped between two barriers rots the assembly.

Add about 5% for straight stud bays and 10% when framing is irregular or you are cutting around pipes and boxes. Batts should be cut slightly oversize — about 1/4 in wider than the bay — for a friction fit without gaps.