About the Insulation Calculator
This insulation calculator works out how much insulation you need to bring an attic, wall or floor up to a target R-value. Enter the area, the R-value of any insulation already there and the R-value you want, choose a material, and it calculates the R-value to add, the depth that takes, and how many bags of blown-in insulation, packs of batts or board feet of spray foam to buy.
It helps homeowners planning a DIY attic top-up, anyone comparing cellulose, fiberglass and spray foam, and contractors sanity-checking a bid. ENERGY STAR (the EPA/DOE program) recommends R-30 for an uninsulated attic in climate zone 1, R-49 in zones 2–3 and R-60 in zones 4–8, so older homes with a few inches of settled insulation often benefit from adding a thick new layer.
R-values per inch and installed densities are typical averages, not product ratings. The bag count assumes about 1.5 lb/ft³ for cellulose and 0.5 lb/ft³ for loose-fill fiberglass; real products vary (loose fiberglass is often 0.5–1.0 lb/ft³, which can mean up to twice as many bags), and blown-in insulation settles over time. Enter your product’s density if you know it, and always compare the result with the coverage chart printed on the bag for your target R-value.
With the default inputs, the depth to add is 10.86 in. Change any value above to recalculate instantly.
How to use the insulation calculator
- 1Measure the attic floor, wall or crawlspace area in square feet.
- 2Estimate the existing R-value from the depth and type of old insulation.
- 3Choose your target R-value for your climate zone.
- 4Pick the insulation type and enter bag weight or pack coverage.
- 5Enter the price per unit to see the material cost.
Formula and method
R-values of layers in series add together, so the R-value you need to add is simply the target minus what is already there. Dividing by the material’s R-value per inch gives the thickness to install. Typical values are about R-3.5 per inch for cellulose, R-2.5 for loose fiberglass, R-3.2 for fiberglass batts, R-4 for mineral wool, R-3.7 for open-cell foam and R-6.5 for closed-cell foam.
For blown-in insulation the volume (area × depth ÷ 12) is multiplied by a typical installed density — about 1.5 lb/ft³ for cellulose and 0.5 lb/ft³ for loose fiberglass unless you enter your product’s density — and divided by the bag weight. These densities are rough averages, so the manufacturer’s coverage chart should be the final word. Batts are counted by area ÷ coverage per pack. Spray foam is sold by the board foot, which is one square foot one inch thick, so board feet = area × depth in inches.
- ΔR
- R-value to add
- Depth
- Thickness of new insulation (in)
- density
- Installed density of blown insulation (lb/ft³)
Worked examples
Top up a 1,000 sq ft attic from R-11 to R-49 with cellulose
Going from R-11 to R-49 adds R-38. At R-3.5 per inch that is 10.86 in of cellulose, or 904.8 cu ft. At 1.5 lb/ft³ it weighs about 1,357 lb, so you need 55 bags of 25 lb, about $825 at $15 per bag.
Blown fiberglass to R-60 in a cold climate
Adding R-41 with loose fiberglass at R-2.5 per inch requires 16.4 in over 1,200 sq ft, or 1,640 cu ft. At 0.5 lb/ft³ that is 820 lb, so 30 bags of 28 lb at $45 each cost $1,350.
Same R-60 top-up with a denser loose-fill product
If the bag chart implies about 0.8 lb/ft³ installed, the same 1,640 cu ft weighs 1,312 lb, or 46.9 → 47 bags of 28 lb costing $2,115 — about 57% more bags than the 30 the 0.5 lb/ft³ default gives.
Closed-cell spray foam in new walls to R-21
Closed-cell foam at about R-6.5 per inch reaches R-21 at 3.23 in thick. Over 500 sq ft that is about 1,615 board feet, costing roughly $2,019 at $1.25 per board foot.
Fiberglass batts to R-19 under a floor
R-19 of fiberglass at about R-3.2 per inch is roughly 6 in thick (R-19 batts are sold at 6¼ in). 800 sq ft ÷ 40 sq ft per pack is 20 packs, or $1,000 at $50 each.
Frequently asked questions
What R-value do I need in my attic?+
ENERGY STAR recommends R-30 for an uninsulated attic in climate zone 1 (south Florida, Hawaii and similar), R-49 in zones 2–3 and R-60 in zones 4–8, which cover most of the country. If you already have 3–4 inches of insulation, it recommends adding R-25 in zone 1, R-38 in zones 2–3 and R-49 in zones 4–8.
How many inches of insulation is R-49?+
It depends on the material: about 14 inches of cellulose (R-3.5 per inch), about 19–20 inches of loose-fill fiberglass (R-2.5 per inch) or about 15 inches of fiberglass batts.
Can I add new insulation on top of old insulation?+
Yes, as long as the old insulation is dry and free of mold or pests. R-values add, so blowing new loose-fill over existing batts is a common, cost-effective way to raise the total R-value. Do not put faced batts on top of existing insulation.
How many bags of blown-in insulation do I need?+
Multiply area by the depth needed to get volume, multiply by the installed density and divide by bag weight. The coverage chart on the bag for your target R-value is the most accurate check, since densities vary by product.
What is a board foot of spray foam?+
A board foot of spray foam is one square foot sprayed one inch thick. Contractors price spray foam per board foot, so 500 sq ft at 3 inches thick is 1,500 board feet.