Insulation batts for a 20 × 8 ft 2×8 wall at 24″ on centre — fibreglass

fibreglass batts

176ft² to buy

160 ft² of wall plus 10% — R-25 in 2×8 framing at 24″ o.c.

Wall area20 ft × 8 ft	160ft²
Batts to buyincludes 10% for cuts	176ft²
Nominal R-valuefibreglass in a 7.25″ cavity	R-25
Effective wall R-valueafter 22% framing at R-9.1	R-18
Lost to thermal bridgingthe gap between the bag and the wall	28%
Studs24″ on centre over 20 ft	11studs
Rolls (88 ft² each)if buying rolls rather than pre-cut batts	2rolls
Cavity depth2×8 framing	7.25″

Wall length: 20 ft
Wall height: 8 ft
Framing: 2×8
Stud spacing: 24 in
Material: fibreglass

What to buy

160 ft² of wall, plus 10% for cuts around boxes, windows and bracing — 176 ft² of fibreglass batts, or about 2 rolls.

What the bag says, and what the wall does

The bag says R-25. The wall performs at about R-18.

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 28% 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.

Notes

The bag says R-25; the wall performs at about **R-18**. Wood conducts about three times as readily as insulation, and roughly 22% of a framed wall is wood once you count doubled studs at corners and intersections, headers and the top and bottom plates. That is 28% of the rated value gone before anything is installed badly.
This is calculated the way ASHRAE does it — averaging conductance across the two paths, not averaging R-values. Averaging R-values gives a higher, friendlier number that overstates real wall performance by several points.
Continuous exterior insulation is the fix, and it is the only one. An inch of rigid foam outside the sheathing breaks every thermal bridge at once and adds more to the effective wall R than upgrading the cavity batt does.
Fibreglass only reaches its rating when it fills the cavity loosely and completely. Compressing an R-19 batt into a 2×4 bay does not give you R-19, and cutting it short around wiring leaves voids that carry far more heat than their area suggests.
Install matters as much as the number. Split batts around wiring rather than crushing them behind, cut around boxes rather than over them, and make sure the batt touches the drywall — a gap on the warm side lets air loop behind the insulation and short-circuits it entirely.

Nearby sizes

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