Insulation Calculator
R-value is thermal resistance; more inches, more R. Pick a target from the Department of Energy guidance for your climate zone and see the depth and bags it takes.
How this calculator works
- R to add = target − existing. Existing loose fill is roughly R-3 per inch if you do not know the type.
- Depth = R to add ÷ R per inch (about 2.5 for blown fiberglass, 3.5 for cellulose).
- Bags assume a typical bag covering about 40 sq ft (fiberglass) or 30 sq ft (cellulose) at R-19.
Insulation questions
What R-value does my attic need?
ENERGY STAR / DOE recommends R-30 to R-49 in the warmest US zones and R-49 to R-60 in the rest of the country for uninsulated attics.
Can I blow insulation over old batts?
Yes, if the old insulation is dry and free of mold or pests. Loose fill over batts is a common way to top up an attic.