How to read the R-value table
Find your climate zone in the first column, then read across for each part of the building envelope. The numbers are the minimum R-value of the insulation itself under the prescriptive R-value path of the 2021 International Energy Conservation Code (Table R402.1.3). Where a cell lists several options separated by "or", any one of them complies.
Wall entries written as two numbers joined by a plus sign combine cavity insulation and continuous insulation (ci). For example, the zone 5 option 20+5ci means R-20 in the cavity plus R-5 continuous. Mass walls (concrete, block, log) list two values: the higher one applies when most of the insulation is on the inside of the wall.
The slab column gives both an R-value and a depth: R-10 extending 2 ft or 4 ft down from the top of the slab edge (or under the slab), depending on the zone. Zones 1 and 2 have no slab requirement.
Finding your climate zone
The IECC divides the United States into eight thermal zones, plus moisture regimes (A moist, B dry, C marine). Zones run from 1 in south Florida and Hawaii to 8 in interior Alaska. The state table gives the zone covering most of each state's population and the other zones found there; it is a summary only. Look up the county in Table R301.1 for the binding answer.
Batt thickness and fit
A 2×4 wall cavity is 3-1/2 in deep, which holds R-13 or high-density R-15 batts. A 2×6 cavity is 5-1/2 in deep and takes R-19, R-20 or high-density R-21. An R-19 batt is about 6.25 in thick, so it is compressed in a 2×6 wall and performs a little below its label. Attic levels such as R-49 and R-60 are usually reached with blown insulation or two layers of batts laid crosswise.
Common R-value mistakes
- Using a state's main zone for a county in a colder zone.
- Compressing batts behind wiring and plumbing, which reduces their R-value.
- Treating these minimums as targets for an existing home; they apply to new construction and many additions and alterations.
- Assuming the 2021 table is in force. Many jurisdictions still enforce the 2015 or 2018 IECC, or local amendments.
The table reproduces the 2021 IECC residential prescriptive requirements. Compliance can also be shown with the U-factor alternative or a performance path, which may allow different combinations.