What is an R-value?

You meet the number in four places: printed on the face of every batt, stamped on blown-insulation bags, written on the coverage card a contractor staples to a rafter after the job, and on window labels, where the industry uses U-factor, the inverse, instead. To find out what you already have, climb into the attic and push a tape measure straight down through the insulation in three or four spots, away from the hatch. Then convert with the rough per-inch values: fiberglass batt R-3.1–3.4, blown fiberglass R-2.2–2.7, blown cellulose R-3.2–3.8, rigid XPS foam board R-5, and closed-cell spray foam R-6–7. Eight inches of old blown fiberglass, a common finding, is about R-19.

R-value describes resistance to conducted heat and nothing else, which is where most insulation projects go wrong. It says nothing about air leakage, so a wall rated R-19 with gaps at the top plate performs closer to R-13; fiberglass filters air rather than stopping it. Air-sealing the attic plane first (top plates, wiring penetrations, the chimney chase, recessed cans, the hatch) costs $300–800 and returns more per dollar than the insulation above it. Compression is the second thief: an R-19 batt stuffed into a 3.5-inch cavity delivers about R-13, and plywood storage decking does the same to attic fill. Water is the third, because wet insulation loses most of its R until it dries, the real story behind a frosty or wet attic in winter. Too little R shows up as ice dams, a second floor the HVAC never satisfies, and ceiling joists poking above the insulation.

Insulation does not wear out, but it settles, gets shoved aside by anyone who crawls up there, and gets ruined by roof leaks, so look once a year using the attic insulation inspection guide. Blown cellulose settles 10–20 percent in its first few years, so a fresh R-38 job measures lower later. Topping up costs $1–2 per square foot installed, one of the highest-return projects in the house; skimping on attic insulation shows the shortfall per winter, and how long attic insulation lasts covers when material needs replacing. One note on incentives: the federal 25C energy-efficiency tax credit expired on December 31, 2025, so price the job on its merits and check your utility for a rebate.