Insulation batts for a 15 × 9 ft 2×4 wall at 24″ on centre — mineral wool

What to buy

135 ft² of wall, plus 10% for cuts around boxes, windows and bracing — 149 ft² of mineral wool batts, or about 2 rolls.

What the bag says, and what the wall does

The bag says R-15. The wall performs at about R-9.8.

Wood conducts roughly three times as readily as insulation, and about 22% of a framed wall is wood once you count doubled studs at corners and intersections, headers over openings, and the top and bottom plates. That is 35% of the rated value gone before a single batt is installed badly.

This is calculated the way ASHRAE does it — averaging conductance across the two paths. Averaging R-values instead gives a friendlier number that overstates real wall performance by several points, and it is the version most calculators quietly use.

The fix is on the outside

Continuous exterior insulation is the only thing that addresses this, because it is the only thing that interrupts every thermal bridge at once. An inch of rigid foam outside the sheathing adds more to the effective wall R-value than upgrading the cavity batt does.

Install decides the rest

Split batts around wiring instead of crushing them behind. Cut around electrical boxes rather than over them. Make sure the batt touches the drywall — a gap on the warm side lets air loop behind the insulation and short-circuits it completely, and it is invisible once the wall is closed.