Insulation Calculator
Pick where you are insulating and your US climate zone, enter the area and any insulation already there, and get the R-value to add plus the number of batt packages or blown-in bags to buy.
1. Area & climate
2. Existing insulation
Vermiculite can contain asbestos. Don't disturb it – have it tested first.
3. New insulation
Estimate only. Coverage varies by brand – use the figure printed on the package or the bag chart.
How to use the Insulation Calculator
- Choose the area type and your climate zone; the recommended target R-value fills in automatically.
- Enter the length and width (or height for walls) and subtract windows or hatches.
- Describe the existing insulation by material and depth, or type its R-value.
- Choose batts or blown-in and check the package coverage or bag details against the label.
- Press Calculate insulation to see packages or bags, depth to add and the final R-value.
Recommended R-values by climate zone
Total R-values recommended by the US Department of Energy (ENERGY STAR) for existing wood-framed houses, and wall cavity values from the 2021 IECC:
| Zone | Example cities | Attic | Floor | Wood-frame wall |
|---|---|---|---|---|
| 1 | Miami, Honolulu | R-30 to R-49 | R-13 | R-13 |
| 2 | Houston, Phoenix | R-30 to R-60 | R-13 | R-13 |
| 3 | Atlanta, Dallas, Los Angeles | R-30 to R-60 | R-19 to R-25 | R-20 or R-13 + R-5 ci |
| 4 | New York, Washington DC, Seattle | R-38 to R-60 | R-25 to R-30 | R-20 + R-5 ci |
| 5 | Chicago, Denver, Boston | R-49 to R-60 | R-25 to R-30 | R-20 + R-5 ci |
| 6 | Minneapolis, Milwaukee | R-49 to R-60 | R-25 to R-30 | R-20 + R-5 ci |
| 7–8 | Duluth, Fargo, Fairbanks | R-49 to R-60 | R-25 to R-30 | R-20 + R-5 ci |
"ci" means continuous insulation, such as rigid foam sheathing over the studs. Check your county on the DOE climate zone map if you are unsure of your zone.
How the insulation calculation works
- R to add = target R − existing R (existing R = depth × R per inch)
- Batts: layers = ⌈R to add ÷ batt R⌉; packages = ⌈area × layers × (1 + waste) ÷ coverage per package⌉
- Blown-in: depth = R to add ÷ R per inch; volume = area × depth ÷ 12; bags = ⌈volume × density × (1 + waste) ÷ bag weight⌉
Worked example: a 40 × 30 ft attic (1,200 sq ft) in zone 5 with 3½ in of old fiberglass batts (R-10.85). Target R-49, so add R-38.15. With R-30 batts that is 2 layers; at 40 sq ft per package plus 10% waste you need ⌈1,200 × 2 × 1.1 ÷ 40⌉ = 66 packages. With cellulose at R-3.5 per inch you need 10.9 in, 1,090 cu ft, about 1,799 lb with waste, or 72 bags of 25 lb.
R-value per inch of common insulation
| Material | R per inch | Inches for R-38 | Inches for R-49 |
|---|---|---|---|
| Loose-fill fiberglass | 2.2–2.7 | 15.2 (at 2.5) | 19.6 |
| Fiberglass batts | 2.9–3.8 | 12.3 (at 3.1) | 15.8 |
| Loose-fill cellulose | 3.2–3.8 | 10.9 (at 3.5) | 14.0 |
| Mineral wool batts | 3.0–3.3 | 11.5 (at 3.3) | 14.8 |
| Open-cell spray foam | 3.5–3.7 | 10.6 (at 3.6) | 13.6 |
| Closed-cell spray foam | 6.0–7.0 | 5.8 (at 6.5) | 7.5 |
Loose fill settles over time, so manufacturers' bag charts list both the installed and the settled thickness. Keep insulation away from recessed lights not rated IC and leave soffit vents clear with baffles.
Frequently asked questions
What R-value do I need in my attic?
The US Department of Energy recommends R-30 to R-49 in zone 1, R-30 to R-60 in zones 2 and 3, R-38 to R-60 in zone 4 and R-49 to R-60 in zones 5 to 8. That is roughly 10 to 20 inches of loose fill or batts.
How do I calculate how much insulation I need?
Subtract the R-value you already have from the target to get the R-value to add. For batts, divide it by the batt's R-value and round up to get the layers, then divide area × layers by the coverage per package. For blown-in, divide the R-value by the R per inch to get the depth, then work out volume, weight and bags.
How many bags of blown-in insulation do I need for 1,000 sq ft?
To add R-38 of cellulose at R-3.5 per inch you need about 10.9 in, or 905 cubic feet. At 1.5 lb per cubic foot that is about 1,357 lb, or 55 bags of 25 lb, before waste. Always check the bag chart for your product.
What is the R-value of my existing insulation?
Measure its depth and multiply by the R per inch: fiberglass batts about R-3.1, loose fiberglass R-2.5, cellulose R-3.5 and rock wool R-3.0 per inch. 3½ in of old fiberglass batts is about R-11.
Can I put new insulation on top of old insulation?
Yes, as long as the old insulation is dry and free of mold. Use unfaced batts or loose fill on top so you don't create a second vapor barrier, and don't disturb vermiculite, which may contain asbestos.
What is RSI?
RSI is the metric R-value (m²·K/W). Multiply the US R-value by 0.1761 to get RSI, so R-49 equals RSI 8.6.