Batt insulation is sold by square feet per bag, and coverage drops as R-value rises because the batts get thicker. Match the batt to the cavity depth first, then count bags.
R-value is limited by cavity depth
A 2×4 wall has a 3.5 inch cavity and holds R-13 or R-15. A 2×6 wall has 5.5 inches and holds R-19 to R-21.
Forcing a thicker batt into a shallower cavity compresses it, and compressed fibreglass loses much of its rating. The air spaces are what insulate, not the glass.
Fit matters more than the number
A batt cut short, bunched at the edges, or split around a wire performs far below its label. Gaps at the edge of a bay are effectively holes in the wall.
Cut batts slightly oversize and friction-fit them so they touch every side of the cavity. Split them around wiring rather than compressing over the top.
Attics want depth, not compression
Attic floors have no depth limit, so this is where higher R-values actually pay. R-38 to R-60 is typical depending on climate zone.
Do not compress attic batts by storing boxes on them or laying flooring directly over them. Build a platform on raised joists if you need storage.
Faced or unfaced
The kraft paper facing is a vapour retarder and goes towards the heated side of the wall. Two vapour retarders in one assembly trap moisture between them.
Use unfaced batts when adding a second layer over existing insulation in an attic, and lay them perpendicular to the joists.
Air sealing comes first and matters more
Insulation slows heat conduction. It does almost nothing about air moving through a gap, and in most older houses air leakage accounts for a larger share of the heat loss than the insulation level does.
The places that matter are the top plates of interior walls, plumbing and wiring penetrations, recessed lights, the attic hatch and the rim joist — all of them connecting conditioned space to the attic or the outside.
Caulk and foam before insulating. Insulation installed over unsealed penetrations lets air pass straight through it, and fibreglass filters that air, which is why it goes black in exactly those places.
The rim joist
The rim joist — the band of framing around the perimeter at each floor level — is one of the largest and most consistently missed heat losses in a house.
It is awkward, it is often full of wiring, and fibreglass batts stuffed into it perform badly because they do not seal the air path.
Rigid foam cut to fit and sealed at the edges, or closed cell spray foam, is what works there. It is a small area and a disproportionate share of the result.