R-Value by Zone

The R-value your attic should have depends on where the house is, and the answer is set by the IECC climate zone map rather than by opinion.

Zone 2 wants R-30 to R-49. Zone 5 wants R-49. Zones 6 and 7 want R-49 to R-60. Those are the code minimums for new construction — an existing house is not required to meet them, but they are the right target because they were chosen where the economics stop making sense.

Local code beats the map

The IECC is a model code. Your jurisdiction may have adopted a stricter version, an older version, or amended it. Check locally before ordering, particularly if the work is permitted.

R-value on the bag is not R-value in the wall

A batt rated R-19 in a 2×6 wall gives a wall that performs closer to R-13, because roughly a quarter of a framed wall is wood and wood conducts about three times as readily as insulation. Attics are much better in this respect — the framing fraction is smaller and the insulation lies over it rather than between it — but the principle holds anywhere framing interrupts the layer.

Compressed insulation is not the R-value on the bag either

Stuffing an R-19 batt into a 2×4 cavity does not give R-19. Loose fill that has been walked on, or wetted and dried, performs below its label. If an existing attic looks flattened, treat its rating as optimistic.

Where the last R-value is worth having

At the top of the range the returns are thin — see the payback numbers. The exception is a house with a cooling load: attic insulation works in both directions, and in hot climates the summer benefit often justifies more than the heating figures alone would.