R-value measures resistance to heat flow; higher is better. Fiberglass and mineral wool batts give roughly R-3 to R-4 per inch, blown cellulose about R-3.2 to R-3.8, open-cell spray foam about R-3.5 to R-3.6 and closed-cell spray foam about R-6 to R-7. Rigid foam board ranges from about R-3.6 (EPS) to R-6 (polyiso). The US Department of Energy recommends roughly R-30 to R-60 in attics depending on climate. Air sealing first makes any insulation work better.
The right insulation depends on where it goes, how much space there is and whether air sealing is part of the job.
R-value per inch (approximate)
| Type | R per inch | Common uses |
|---|---|---|
| Fiberglass batts | 2.9 to 3.8 | Walls, attics, floors |
| Mineral wool batts | 3.0 to 3.3 | Walls, fire and sound separation |
| Blown cellulose | 3.2 to 3.8 | Attics, dense-pack walls |
| Blown fiberglass | 2.2 to 2.7 | Attics |
| Open-cell spray foam | 3.5 to 3.6 | Walls, roof decks |
| Closed-cell spray foam | 6 to 7 | Rim joists, basements, tight cavities |
| EPS board | 3.6 to 4.2 | Below grade, exterior |
| XPS board | About 5 | Below grade, foundations |
| Polyiso board | 5.6 to 6 | Roofs, walls above grade |
Attic recommendations
The DOE recommends about R-30 to R-60 for attics depending on climate zone, with higher numbers in colder zones. Blown-in insulation over existing material is often the most cost-effective upgrade.
Air sealing first
Seal top plates, plumbing and wiring holes, recessed lights rated for contact, and the attic hatch before adding insulation. Air leaks undermine any R-value.
Moisture
Vapor retarders and their placement depend on climate. Basements need insulation that tolerates moisture, such as rigid foam against concrete. See basements and moisture.
Rebates
Federal, state and utility programs often offer rebates or credits for insulation and air sealing. See energy upgrades and pair insulation with efficient windows.
Browse insulation and weatherproofing.
Common questions
How much attic insulation do I need?
The DOE recommends about R-30 to R-60 for attics, depending on your climate zone. Many existing homes have far less.
Is spray foam worth it?
Closed-cell foam gives the highest R-value per inch and air seals, which helps in tight spaces like rim joists. It costs more and should be installed by trained crews.
Should I air seal before insulating?
Yes. Gaps around pipes, wires, top plates and attic hatches let air bypass insulation.
Fiberglass or mineral wool batts?
Both insulate similarly. Mineral wool is denser, more fire- and sound-resistant and holds its shape; fiberglass is cheaper.