Insulation Calculator

This insulation calculator will help you determine the R-value of a selected material at a given thickness or the total R-Value of an entire barrier or wall made of different layers of materials at their respective thicknesses.

Thickness needed15.2 into add R-38 at R-2.5 per inch

R-value addedR-38

Finished R-valueR-38

Material volume1,266.67 cu ft46.914 cu yd

In metric386.1 mmRSI 6.692 total

Bags of blown insulation1,152roughly, at about 1.1 cu ft settled per bag

Standard batt depthsneeds more depth than a single joist baythe depth this would fit

Heat loss at the target1,578.9 BTU/hat a 60 °F temperature difference

Heat loss nowuninsulated

The formula

thickness = R-value wanted × the material’s R per inch

R per inch is what varies

All these materials do the same job; they differ in how much depth it takes. Closed-cell spray foam reaches R-38 in under six inches where blown fibreglass needs over fifteen — which matters enormously in a rafter bay and not at all in an open attic.

Two things the R-value does not capture. Compressing a batt into a shallower cavity loses more R than the lost thickness suggests, so an R-19 batt squeezed into a 2×4 wall performs worse than an R-13 made for it. And thermal bridging through the studs can cut a wall’s whole-assembly R by a quarter, whatever the cavity holds.

Air sealing first, insulation second. Insulation slows conduction but does nothing about a draught.