Recommended R-values by climate zone
Targets for insulation added to an existing home, following ENERGY STAR and Department of Energy guidance.
| Climate zone | Attic | Floor | 2×4 wall | 2×6 wall |
|---|---|---|---|---|
| Hot · IECC 1–2 | R-30 to R-49 | R-13 | R-13 | R-19 |
| Warm · IECC 3 | R-49 | R-19 | R-13 to R-15 | R-21 |
| Mixed · IECC 4 | R-49 to R-60 | R-25 | R-15 | R-21 |
| Cold · IECC 5 | R-60 | R-30 | R-15 | R-21 |
| Very cold · IECC 6–7 | R-60 | R-30 | R-15 | R-21 |
Walls are limited by the depth of the framing. To go beyond these values, add continuous rigid foam on the outside when the siding is replaced.
R-value per inch by material
| Material | R per inch | Best for |
|---|---|---|
| Blown-in fiberglass | 2.5 | Open attics; does not settle much |
| Blown-in cellulose | 3.5 | Attics and dense-packing closed walls |
| Fiberglass batts | 3.1 | Open walls, floors, unfloored attics |
| Mineral wool batts | 4.1 | Walls and floors; fire and sound resistant |
| Open-cell spray foam | 3.7 | Air sealing and filling irregular cavities |
| Closed-cell spray foam | 6.5 | Rim joists, crawlspace walls, tight spaces |
How many bags for an attic?
Bags needed to bring a 1,000 sq ft attic with about 4 inches of old fiberglass (R-10) up to each target, calculated by volume.
| Target | Blown fiberglass | Blown cellulose |
|---|---|---|
| R-30 | 16 bags | 30 bags |
| R-38 | 22 bags | 42 bags |
| R-49 | 31 bags | 59 bags |
| R-60 | 39 bags | 75 bags |
Bag coverage varies by brand: always check the coverage chart printed on the bag for your target R-value and the minimum settled depth.
Before you add insulation
- Air-seal first. Seal gaps around plumbing stacks, wiring holes, top plates and recessed lights. Insulation slows heat but does not stop air.
- Keep soffit vents clear. Install baffles at the eaves so insulation does not block attic ventilation.
- Mind recessed lights and chimneys. Older non-IC-rated fixtures and flues need clearance or a fire-rated cover.
- Check for old vermiculite. Gray-brown, pebbly insulation in older homes may contain asbestos. Have it tested before disturbing it.
Frequently asked questions
How much attic insulation do I need?
R-49 to R-60 in most of the U.S., which is about 20 to 24 inches of blown fiberglass or 14 to 17 inches of cellulose. Hot southern climates can stop at R-30 to R-49.
How many inches is R-38?
About 15 inches of blown fiberglass, 11 inches of cellulose or 12 inches of fiberglass batts.
Can I add new insulation on top of old?
Yes, as long as the old insulation is dry and not moldy, and you are not covering vermiculite. Lay unfaced batts or blow loose fill directly over it.
How much does attic insulation cost?
Blown-in insulation usually costs about $1 to $3 per square foot installed, depending on the R-value added. Doing it yourself with a rented blower costs mostly the bags, often a few hundred dollars for a typical attic.
Is cellulose or fiberglass better?
Cellulose has a higher R-value per inch and blocks air movement better; fiberglass is lighter, does not absorb moisture and settles less. Both work well in an open attic.
Results are estimates. Coverage, settled depth and prices vary by product and region; follow the manufacturer's coverage chart and local code.