Insulation calculator for attics, walls and floors
This insulation calculator tells you how much insulation you need to reach a target R-value. Enter the area of the attic, walls or floor, the R-value you want and what is already there, then choose the material: batts, blown-in cellulose or fiberglass, open- or closed-cell spray foam, or rigid foam board. It works out the thickness, the number of batts or bags, the board feet of spray foam and the number of boards, with a cost if you enter a price.
Homeowners, contractors and energy auditors in the US, Canada and Australia use it as an insulation calculator for the attic or a quick spray foam calculator by square feet. Pick a US climate zone to fill in the ENERGY STAR recommendation, or switch to metric to work in square metres, millimetres and RSI.
The calculator runs in your browser. Its R-per-inch and density figures are typical published values; the manufacturer's bag chart and label are what inspectors check, so use them for the final order.
How to use the Insulation Calculator (Batts, Blown-In & Spray Foam)
- Choose US or metric units and where the insulation goes: attic floor, walls, a floor over a crawlspace or somewhere else.
- Enter the length and width (or wall height) and any openings or areas you will skip.
- Pick your climate zone to fill in the ENERGY STAR level, or type a target R-value (or RSI). Enter the R-value already in place.
- Choose the insulation. Adjust the R-value per inch, density and bag weight — or type the bags per 1,000 ft² from the bag chart — for blown-in; batt R-value and spacing for batts; kit yield for spray foam.
- Add a waste allowance and a price, then read the depth, quantity and cost and copy the summary.
Insulation Calculator (Batts, Blown-In & Spray Foam) features
Five ways to insulate
Batts and rolls, blown cellulose, blown fiberglass, open- and closed-cell spray foam, and EPS, XPS or polyiso board.
ENERGY STAR levels
Attic and floor recommendations for US zones 1–8, with the add-on level used when insulation already exists.
Blown-in bags two ways
From depth × settled density ÷ bag weight, or straight from the bags-per-1,000 ft² figure on the bag chart.
Spray foam board feet
Area × thickness in board feet, and the number of kits or sets at your yield.
Wall cavity check
Warns when the thickness needed will not fit a 2 × 4 or 2 × 6 cavity.
R and RSI
Switch between US R-values and metric RSI, as used on Canadian and Australian products.
How the insulation is calculated
R-values add in layers, so the R-value to add is the target minus what is there. Thickness = R-value to add ÷ R-value per inch. For loose-fill, volume = area × settled thickness and weight = volume × settled density; bags = weight ÷ bag weight. Spray foam is sold by the board foot — one square foot one inch thick — so board feet = area in ft² × thickness in inches.
Worked example: a 40 × 30 ft attic (1,200 ft²) with no insulation, target R-49, blown cellulose at R-3.5 per inch: 49 ÷ 3.5 = 14 in settled. With 15% settling it should be blown to about 16.5 in. The settled volume is 1,200 × 14 ÷ 12 = 1,400 ft³; at 1.75 lb/ft³ that is 2,450 lb, or 98 bags of 25 lb.
The same attic in closed-cell foam at R-6.5 per inch would need 7.5 in — 9,000 board feet — which is why foam is usually reserved for walls, rim joists and roof decks.
ENERGY STAR recommended R-values (retrofit)
Based on the 2021 International Energy Conservation Code, as published by ENERGY STAR (checked 19 October 2026). For walls ENERGY STAR recommends blowing insulation into empty cavities and adding R-5 to R-15 insulated sheathing when siding is replaced.
| Zone | Attic, uninsulated | Attic with 3–4 in already | Floor |
|---|---|---|---|
| 1 | R-30 | add R-25 | R-13 |
| 2 | R-49 | add R-38 | R-13 |
| 3 | R-49 | add R-38 | R-19 |
| 4A/4B | R-60 | add R-49 | R-19 |
| 4C/5/6 | R-60 | add R-49 | R-30 |
| 7/8 | R-60 | add R-49 | R-38 |
Typical R-value per inch
- Blown cellulose R-3.2 to 3.8 per inch settled, at 1.5–2.0 lb/ft³, settling 10–20%; blown fiberglass R-2.2 to 2.7 at 0.5–1.0 lb/ft³, settling 2–4% (PNNL Building America Solution Center).
- Open-cell spray foam about R-3.6 per inch (R-12.6 at 3½ in); closed-cell about R-6.5; EPS board R-3.8 to 4.4; XPS R-5; polyiso R-6.5 (US DOE Guide to Home Insulation).
- Batts are sold by R-value: R-13 and R-15 fill a 2 × 4 wall, R-19 and R-21 a 2 × 6, and R-30 or R-38 go in attics. A 15 in batt fills a 16 in on-centre bay; a 23 in batt a 24 in bay.
- Planning the framing too? The board foot calculator counts studs and plates. Sizing cooling for the insulated room is in the AC tonnage calculator.
Insulation Calculator (Batts, Blown-In & Spray Foam) FAQ
How do I use an insulation calculator for an attic?
Measure the attic floor, choose your climate zone or type a target R-value, enter the R-value already there and pick blown-in or batts. The calculator shows the depth to blow or the layers of batts and the number of bags or batts.
How deep should attic insulation be for R-49?
About 14 in of settled cellulose (R-3.5 per inch) or 18–22 in of blown fiberglass (R-2.2 to 2.7 per inch). Blow it slightly deeper to allow for settling.
How do I calculate spray foam board feet?
Multiply the area in square feet by the thickness in inches. 280 ft² of wall at 2 in of closed-cell foam is 560 board feet, about one 600 board-foot kit.
What R-value do I need for walls?
A 2 × 4 wall holds R-13 to R-15 in the cavity and a 2 × 6 wall R-19 to R-21. In colder zones ENERGY STAR suggests adding R-5 to R-15 of insulated sheathing outside the studs when residing.
How do I convert R-value to RSI?
Multiply the US R-value by 0.1761. R-49 is RSI 8.6, and RSI 6.0 is R-34. Australian and Canadian product labels often show the metric value.
Why is the bag count different from the bag label?
Bag charts are based on the maker's tested density, which varies by product and installation. If you enter the bags per 1,000 ft² from the chart, the calculator uses it instead of the density estimate.