Insulation is rated by R-value, its resistance to heat flow; higher numbers insulate better, and layers add together. How much you need depends on where you live and which part of the house you are insulating. This calculator looks up the recommended level for your climate zone, subtracts what is already there, and converts the difference into inches and into the bags, batts, board feet or foam sheets you would buy.
How to use the insulation calculator
- Choose your climate zone.
- Pick where you are insulating: attic floor, floor over unheated space, exterior wall or basement wall.
- Enter the area to insulate.
- For attics and floors, enter the depth and type of existing insulation.
- For walls, choose 2×4 or 2×6 framing.
- Choose the new insulation. You can override the R-value per inch, the target R-value, and for blown products the installed density and bag weight.
Insulation formulas
For fiberglass batts the calculator picks the combination of one or two standard batts (R-11 to R-38) that reaches the target with the least excess. Spray foam is measured in board feet, one square foot one inch thick.
Worked example: topping up an attic in Zone 5
Target for Zones 4C–6: R-60. Existing: 3½ in of loose fiberglass × R-2.5 per inch ≈ R-8.8.
R to add: 60 − 8.8 = R-51.3. Blown cellulose at R-3.6 per inch: 51.3 ÷ 3.6 = 14.2 in.
Over 1,000 ft²: 1,000 × 14.24 ÷ 12 × 1.3 lb/ft³ × 1.05 = 1,619 lb → 65 bags of 25 lb.
With batts instead: R-38 plus R-15 (R-53), laid in two layers.
Recommended levels by climate zone
| Zone | Attic (total) | Already 3–4 in? Add | Floor over unheated space |
|---|---|---|---|
| 1 | R-30 | R-25 | R-13 |
| 2 | R-49 | R-38 | R-13 |
| 3 | R-49 | R-38 | R-19 |
| 4A and 4B | R-60 | R-49 | R-19 |
| 4C, 5 and 6 | R-60 | R-49 | R-30 |
| 7 and 8 | R-60 | R-49 | R-38 |
These retrofit recommendations come from the U.S. Department of Energy and ENERGY STAR and are based on the 2021 International Energy Conservation Code. For new wood-framed walls, the same code calls for R-13 in Zones 1–2, R-20 (or R-13 plus R-5 continuous insulation) in Zone 3, and R-20 plus R-5 continuous (or R-13 plus R-10) in Zones 4–8. Basement walls are R-5 to R-15 continuous, or R-13 to R-19 in a framed cavity, from Zone 3 north.
R-value per inch of common insulation
| Material | Typical R per inch |
|---|---|
| Fiberglass batts | 2.9–3.8 |
| Mineral wool batts | 3.8–4.3 |
| Blown fiberglass (attic) | 2.2–2.9 |
| Blown cellulose | 3.2–3.8 |
| Open-cell spray foam | 3.5–3.8 |
| Closed-cell spray foam | 6.0–7.0 |
| EPS foam board | 3.6–4.2 |
| XPS foam board | 5.0 |
| Polyiso foam board | 5.6–6.5 |
Before you add insulation
- Air-seal first. Gaps around top plates, plumbing and wiring holes, recessed lights and attic hatches leak more heat than thin insulation does.
- Keep ventilation open. Install baffles at the eaves so blown insulation does not block soffit vents.
- Keep clearances around chimneys, flues and recessed lights that are not rated for insulation contact.
- Mark the depth. Rulers stapled to trusses help you blow to an even depth, and blowing slightly deeper allows for settling.
Better insulation lowers the size of heating and cooling equipment a room needs; the BTU calculator shows how insulation level changes a room’s heat loss. If you are opening walls, the drywall calculator estimates the board to close them up.
Frequently asked questions
What R-value should my attic have?
DOE and ENERGY STAR guidance for existing homes, based on the 2021 IECC, recommends about R-30 in Zone 1, R-49 in Zones 2 and 3, and R-60 in Zones 4 through 8. Most of the country therefore aims for R-49 to R-60 in the attic.
How many inches of insulation is R-60?
It depends on the material. At typical values, R-60 takes about 17 inches of blown cellulose (R-3.6 per inch), 24 inches of blown fiberglass (R-2.5 per inch) or about 19 inches of fiberglass batts. Check the coverage chart on the bag for the minimum settled thickness of your product.
How do I find my climate zone?
The IECC climate zone map assigns every US county to a zone from 1 (hottest) to 8 (coldest), with marine (C) and dry (B) variants. Zones follow county lines, so look up your county rather than relying on the state; mountain areas can be two or three zones colder than nearby valleys.
Can I add new insulation on top of old insulation?
Usually, yes, as long as the old insulation is dry and free of mold or pests. Do not add a second vapor retarder: use unfaced batts or loose fill on top of existing insulation. Wet, matted or contaminated insulation should be removed, and vermiculite that may contain asbestos should be tested before it is disturbed.
Is blown-in or batt insulation better for an attic?
Blown-in insulation fills around joists, wiring and irregular spaces with fewer gaps, which is why it is the usual choice for topping up an attic floor. Batts are easy to handle and work well in open joist bays and walls. Either performs to its rated R-value only if it is installed without gaps or compression.