How Much Attic Insulation Do I Need? (R-Value Chart)
Part of Interior & Finishes
Quick answer
The U.S. Department of Energy recommends R-30 to R-49 for most attics, rising to R-49–R-60 in the coldest climate zones (6–8) and as low as R-30 in the warm South (zones 1–2). In blown fiberglass that's roughly 12–20 inches deep; in cellulose, about 8.5–14 inches. If you already have some insulation, only add the difference up to your target R-value.
Attic insulation is rated by R-value — its resistance to heat flow. More R-value means a warmer house in winter and a cooler one in summer, up to the point of diminishing returns set by your climate.
Recommended attic R-value by climate zone
| Climate zone | Example areas | Recommended attic R-value |
|---|---|---|
| 1–2 (hot) | FL, south TX, HI | R-30 to R-49 |
| 3 (warm) | GA, AZ, southern CA | R-30 to R-49 |
| 4–5 (mixed/cold) | VA, MO, NY, OH | R-49 to R-60 |
| 6–8 (cold) | MN, ME, mountain west, AK | R-49 to R-60 |
What that R-value looks like in inches
| Material | R per inch | Depth for R-49 |
|---|---|---|
| Blown fiberglass | ~2.5 | ~19.6 in |
| Blown cellulose | ~3.5 | ~14 in |
| Fiberglass batts | ~3.1 | ~16 in |
| Open-cell spray foam | ~3.7 | ~13 in |
| Closed-cell spray foam | ~6.5 | ~7.5 in |
Run the insulation calculator with your attic's square footage and target R-value to get the exact depth and the number of bags or batts to buy. If you're adding over existing insulation, set the existing R-value so it only sizes the top-up.
Topping up existing insulation
- Measure the current depth and multiply by the material's R-per-inch to estimate the existing R-value.
- Subtract that from your target — that's the R-value (and depth) to add.
- Add unfaced insulation over old insulation; never lay a second vapor barrier (faced batts) on top.
- Keep insulation off recessed lights unless they're IC-rated, and maintain soffit-to-ridge airflow with baffles.
FAQs
Is more attic insulation always better?
Up to a point. Going from nothing to R-30 makes a big difference; going from R-49 to R-60 saves much less. Match your climate zone's recommendation — beyond it, the payback period gets very long.
Can I put new insulation over old?
Yes. Blowing loose-fill or laying unfaced batts over existing attic insulation is normal and effective. The one rule: don't add a second vapor barrier, so use unfaced material on top.