A new 2x6 wall in Chicago — climate zone 5A — needs R-20 in the cavity plus R-5 of continuous insulation under the 2021 IECC. R-21 fiberglass batts and 1 in polyiso board do it, for an assembly U-factor of 0.042 against the 0.045 maximum. For 1,200 square feet of wall that is 33 bags of batts and 40 sheets of foam. The calculator above finds the requirement for any climate zone and part of the house, the thickness and quantity of the product you choose, and checks the wall for condensation.
How the calculator works
Pick a city or a climate zone and the part of the house: attic, wood-frame or mass wall, floor, basement, crawl space or slab edge. Choose one of the ways the code allows — all in the cavity, cavity plus continuous insulation (ci), or all continuous — and the products. The result is the thickness, the bags, boards or spray foam kits, and for walls the U-factor and the moisture check.
Requirements by climate zone
| Zone | Attic | Wood-frame wall | Floor |
|---|---|---|---|
| 0–1 | R-30 | R-13 or R-10 ci | R-13 |
| 2 | R-49 | R-13 or R-10 ci | R-13 |
| 3 | R-49 | R-20, or R-13 + 5 ci, or R-15 ci | R-19 |
| 4 (not Marine) | R-60 | R-20 + 5 ci, or R-13 + 10 ci, or R-15 ci | R-19 |
| 5 and Marine 4 | R-60 | R-20 + 5 ci, or R-13 + 10 ci, or R-15 ci | R-30 |
| 6 | R-60 | R-20 + 5 ci, or R-13 + 10 ci, or R-20 ci | R-30 |
| 7–8 | R-60 | R-20 + 5 ci, or R-13 + 10 ci, or R-20 ci | R-38 |
From 2021 IECC Table R402.1.3. An attic needing R-60 is satisfied by R-49 where the insulation keeps its full height over the wall plates at the eaves; R-38 likewise for R-49 (R402.2.1). Your state may have adopted a different edition or amendments.
Thickness
Thickness is the R-value divided by the product's R per inch: about 3.7 for fiberglass batts, 4.2 for mineral wool, 3.5 for blown cellulose, 2.5 for blown fiberglass, 3.7 for open-cell and 6.5 for closed-cell spray foam, 4 for EPS, 5 for XPS and 6 for polyiso board. R-60 in an attic is about 17 in of cellulose or 24 in of blown fiberglass.
Studs and the U-factor
Studs carry heat past the cavity insulation: in a wall framed at 16 in about a quarter of the area is wood. The calculator combines the cavity path and the stud path (the parallel-path method) into an assembly U-factor and compares it with the code maximum in Table R402.1.2 — U-0.045 in zones 4–8.
Condensation
In winter the temperature falls through each layer in proportion to its R-value. Where a surface inside the wall is colder than the dew point of the indoor air, moisture can condense — in a frame wall, on the inside face of the sheathing. Exterior continuous insulation keeps the sheathing warmer; a vapor retarder on the warm side keeps the moisture out (IRC R702.7). The drawing shows the temperature line and the dew point for the conditions you enter.
Common mistakes
- Compressing batts. An R-21 batt squeezed into a 2x4 wall loses R-value.
- Nominal R as wall R. The studs lower the assembly by a fifth or more.
- Blocking soffit vents. Keep baffles at the eaves before blowing the attic.
For the framing, use the Wall Framing Calculator; for the drywall that covers it, the Drywall Calculator.