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2021 IECC basement wall requirements
| Climate zone | Basement wall R-value |
|---|---|
| 1 | Not required |
| 2 | Not required |
| 3 | R-5ci or R-13 |
| 4 (except Marine) | R-10ci or R-13 |
| 5 and Marine 4 | R-15ci, R-19, or R-13+5ci |
| 6 | R-15ci, R-19, or R-13+5ci |
| 7 and 8 | R-15ci, R-19, or R-13+5ci |
"ci" means continuous insulation. In "13+5ci" the first number is cavity insulation and the second is continuous, per the IECC footnote.
The PNNL Building America Solution Center table shows how this changed: 2015 and 2018 used 10/13 in zone 4 and 15/19 in zones 5–8; the 2021 edition rewrote them as 10ci or 13, and 15ci or 19 or 13+5ci. Your state may still be on an older edition.
Picking an assembly
- Rigid foam against the concrete keeps the wood framing on the warm side and away from cold, damp concrete.
- Unfaced batts in a stud wall need the foam behind them in most climates to avoid condensation on the concrete.
- Fiberglass batts against bare concrete without a moisture plan are the classic source of hidden mold.
Worked example
In climate zone 5, the example's 116 ft of 8 ft exterior wall is 928 sq ft. Meeting R-15 with continuous foam takes about 31 4 × 8 boards including 5% for cuts. At HomeAdvisor's $0.30–$6.75 per sq ft, insulation costs $278–$6,264 installed; the wide range covers everything from batts to spray foam.
Where basement inspections usually fail
- Bedroom windows that are too small or too high, especially sliders and double-hungs measured by the frame instead of the opening.
- Window wells under 36 in deep from the wall, or deeper than 44 in with no ladder.
- Ceilings that dip under 6 ft 4 in at a duct or beam once drywall and flooring go in.
- Stairs with a short top or bottom riser after new flooring changes the floor levels.
- Missing fire blocking at the top of furred walls and around soffits.
Many cities publish one-page handouts on these items, like the St. Paul, Boise and Des Moines sheets cited on this site. Ask for your city's version before you frame.
Before you call for an inspection
- Measure every egress opening with the sash open and write the numbers on the frame.
- Measure ceiling height at the lowest duct or beam after the ceiling material is up.
- Check riser heights top and bottom after flooring.
- Leave insulation and vapor details visible until the insulation inspection if your town does one.
- Have the permit card and approved plans on site.
Common questions
What R-value for basement walls in zone 5?
R-15 continuous, R-19 cavity, or R-13 cavity plus R-5 continuous under the 2021 IECC.
Related calculators
Sources
- https://basc.pnnl.gov/information/2009-2021-iecc-and-irc-minimum-insulation-requirements-new-homes
- https://www.homeadvisor.com/cost/basements/