Existing R-value: R-— | Est. annual savings: — | Payback: —
- Around the attic hatch, plumbing stacks, and wiring penetrations
- Top plates of interior and exterior walls
- Recessed light housings (use only IC-rated, or box them out)
- Chimney and flue gaps (use fire-rated sealant)
Shopping List
- Insulation type—
- Additional R-value—
- Added depth—
- Bags / batts (with waste)—
- Estimated material cost—
R-Value per Inch by Material
| Material | R per inch | Depth for R-49 |
|---|---|---|
| Fiberglass batt | 3.2 | ~15.3 in |
| Blown-in fiberglass | 2.5 | ~19.6 in |
| Blown-in cellulose | 3.5 | ~14.0 in |
| Spray foam (open cell) | 3.7 | ~13.2 in |
| Mineral wool | 3.3 | ~14.8 in |
Recommended Attic R-Value by Climate Zone
| Zone | Recommended attic R-value |
|---|---|
| 1 – 2 (hot) | R30 – R49 |
| 3 (warm) | R30 – R49 |
| 4 (moderate) | R38 – R60 |
| 5 – 6 (cold) | R49 – R60 |
| 7 – 8 (very cold) | R49 – R60 |
How Much Attic Insulation Do I Need?
Attic insulation is measured in R-value, the resistance to heat flow. To find how much you need, compare your target R-value to what you already have. Estimate your existing R-value from the depth and type of insulation already in the attic, subtract it from the target for your climate, and the difference is the additional R-value to add. Convert that to inches using the R-per-inch of your chosen material, then to bags or batts using the coverage on the package. The calculator above does all of this and adds a material cost and rough payback.
Recommended Attic R-Value by Climate Zone
The colder your climate, the more attic insulation pays off. The U.S. Department of Energy recommends roughly R30 to R49 for hot southern zones and R49 to R60 for cold northern zones, with most homes targeting R49 to R60 in the attic. Selecting your climate zone above sets a sensible target you can override. If you only know your ZIP code, the ZIP field picks an approximate zone, but verify it against the climate-zone chart since boundaries vary within regions.
Blown-In vs Batt Insulation
Batts are pre-cut rolls of fiberglass or mineral wool that you lay between and over the joists; they are cheap and DIY-friendly but leave gaps around obstructions. Blown-in cellulose or fiberglass is loose fill blown from a machine, which fills irregular spaces and gets into corners batts miss, giving better coverage in a cluttered attic. Cellulose has a higher R-per-inch than blown fiberglass and resists air movement well. For most retrofit attics, blown-in over the existing insulation is the fastest way to hit a high R-value.
Frequently Asked Questions
How deep should attic insulation be? To reach R-49, expect about 14 inches of cellulose or roughly 16 to 20 inches of fiberglass, depending on type.
Can I add new insulation over old? Yes. Blown-in or unfaced batts go right over existing insulation, as long as you air-seal first and keep soffit vents clear.
Should I air seal before insulating? Always. Sealing leaks first is what makes the added insulation actually perform.
Does more insulation always pay off? There are diminishing returns past the recommended R-value, so target your zone rather than overshooting.