How to Use This Tool
Enter the area, the cavity you are filling, and the batt you were planning to buy. The tool reports what you would actually get rather than what the label says.
R-value is trapped air, so thickness is not optional
Fibreglass and mineral wool work by holding still air in place. Compress the batt and the air goes, and so does most of the extra R-value. An R-19 batt is 6.25 inches thick; forced into a 3.5 inch stud bay it performs at about R-10.6.
That is a 44 per cent loss, on a product that costs more than the R-13 batt designed for the space — and it lands below that R-13. To exceed R-13 in a 2x4 wall the options are a high-density R-15 batt, or continuous foam on the outside — never a thicker batt squeezed in.
R per inch, from each batt's own rating and thickness R-13 3.50 in 3.71 per inch R-15 3.50 in 4.29 R-19 6.25 in 3.04 R-21 5.50 in 3.82 Compressed into a 3.5 in stud bay R-19 3.5 x 3.04 = R-10.6 44% below the label R-21 3.5 x 3.82 = R-13.4 36% below R-13 fits = R-13.0 nothing lost
The studs are a fifth of your wall
Wood is about R-1.25 per inch, so a 3.5 inch stud is roughly R-4.4 while the bay beside it is R-13. On a 16 inch layout, studs, plates, headers and corners come to about 23 per cent of wall area — 18 per cent at 24 inch spacing.
Averaging those two paths by U-value gives the effective R-value of the assembly. A nominal R-13 wall works out around R-8.9, which is why the label number and the performance number are never the same. This is called thermal bridging.
Continuous exterior foam is worth more per inch
Insulation between studs cannot fix the studs. Insulation over them covers everything, including the plates and headers where framing is densest. An inch of exterior foam at R-5 adds R-5 to the whole wall, not to 77 per cent of it, which is why it moves the effective figure more than the same R-value added inside.
Buy by the bag, and expect to cut
Batts are sold in bags with a stated coverage, commonly around 49 sq ft for a 2x4 wall bag. Every cavity that is not a clean 16 or 24 inches needs a cut piece, and cut pieces should be trimmed slightly oversize and eased in — not folded, which creates the same compression problem locally.
Gaps matter more than the last R-point
A batt with a 5 per cent gap around wires and boxes loses far more performance than the difference between R-13 and R-15. Splitting the batt around cables, rather than stuffing it behind them, is worth more than any upgrade on this page.
