R402.1.2 Insulation and Fenestration Requirements

The IECC R402.1.2 insulation and fenestration requirements set the minimum R-values for ceilings, walls, floors, and foundations and the maximum U-factors and solar heat gain coefficients for windows, skylights, and glazed doors in new residential construction. Every number is tied to your climate zone, and the table numbering and specific values have shifted between the 2015, 2021, and 2024 editions of the International Energy Conservation Code. Before you order materials or submit for permit, confirm which edition your local building department enforces, because that single detail changes what you’re building to.

Find Your Climate Zone First

The IECC divides the country into eight numbered climate zones. Zone 1 covers the hot, humid conditions of southern Florida and Hawaii; Zone 8 covers subarctic northern Alaska.1International Code Council. 2021 International Energy Conservation Code – Chapter 3 CE General Requirements Most zone numbers also carry a moisture designation: A for moist, B for dry, and C for marine. Zone 4A in the mid-Atlantic triggers different requirements than Zone 4C in the Pacific Northwest. Zones 7 and 8 have no moisture designation because the cold dominates thermal performance regardless of humidity.

You identify your zone by finding your county in Table C301.1 or by asking your building department. Every insulation value, every window rating, and every air-sealing threshold flows from that zone assignment. Get it wrong and your entire permit package is built on the wrong numbers.

Prescriptive Insulation R-Values

The prescriptive table in R402.1.2 is where most builders start. It sets minimum R-values for every major envelope component:2International Code Council. 2015 International Energy Conservation Code – R402.1.2 Insulation and Fenestration Criteria

  • Zone 1: ceiling R-30, wood-frame wall R-13, floor R-13. No basement, slab, or crawl space insulation required.
  • Zone 2: ceiling R-38, wood-frame wall R-13, floor R-13. No foundation insulation required.
  • Zone 3: ceiling R-38, wood-frame wall R-20 or R-13+5, floor R-19. Basement and crawl space walls R-5 continuous or R-13 cavity.
  • Zone 4 except Marine: ceiling R-49, wood-frame wall R-20 or R-13+5, floor R-19. Basement and crawl space walls R-10 continuous or R-13 cavity. Slab R-10 to 2 feet.
  • Zone 5 and Marine 4: ceiling R-49, wood-frame wall R-20 or R-13+5, floor R-30. Basement and crawl space walls R-15 continuous or R-19 cavity. Slab R-10 to 2 feet.
  • Zone 6: ceiling R-49, wood-frame wall R-20+5 or R-13+10, floor R-30. Basement and crawl space walls R-15 continuous or R-19 cavity. Slab R-10 to 4 feet.
  • Zones 7 and 8: ceiling R-49, wood-frame wall R-20+5 or R-13+10, floor R-38. Basement and crawl space walls R-15 continuous or R-19 cavity. Slab R-10 to 4 feet.

Where the floor requirement is R-30 or R-38, an exception allows you to fill the framing cavity completely as long as you reach at least R-19. Heated slabs add R-5 to the slab-edge value in every zone. If you install insulation in a cavity narrower than the batt’s labeled thickness, the compressed material must still deliver at least the R-value shown in the table.

The 2024 edition changed several of these. Ceiling insulation in Zones 2 and 3 dropped from R-49 back to R-38. Slab depth in Zones 4 and 5 was cut from 4 feet to 3 feet. The 2024 code also added new floor options that let you combine cavity and continuous insulation rather than using cavity-only. These differences change material lists and assembly details, so verify the edition before you buy.

Reading Wall Notation and Mass Walls

When the table lists a wall value like “13+5,” the first number is cavity insulation between the studs and the second is continuous insulation installed over the exterior sheathing.2International Code Council. 2015 International Energy Conservation Code – R402.1.2 Insulation and Fenestration Criteria Wood studs conduct heat far more readily than the insulation between them, so a wall packed with R-20 batts performs below R-20 at every stud. Rigid foam or mineral wool board on the outside breaks that thermal bridge.

In Zones 1 and 2, R-13 cavity insulation is enough with no continuous layer. Zones 3 through 5 give you a choice: R-20 cavity-only or R-13 cavity plus R-5 continuous. Zones 6 through 8 require both: R-20 cavity plus R-5 continuous, or R-13 cavity plus R-10 continuous. Adding layered R-values is simple addition. R-13 cavity plus R-5 continuous gives R-18 total. If you use a product without a well-established R-value, the manufacturer must provide test data following approved ASTM methods before you can count it toward compliance.3eCFR. 16 CFR 460.5 – R-value Tests

Concrete, brick, and concrete-masonry-unit walls have their own column because thermal mass stores and releases heat differently than framing. Mass wall R-values run lower than wood-frame requirements in the same zone. A Zone 3 mass wall needs R-8 when most of the insulation is on the exterior, versus R-20 for wood-frame.2International Code Council. 2015 International Energy Conservation Code – R402.1.2 Insulation and Fenestration Criteria When more than half the insulation sits on the interior side, the required value rises (R-13 for that same Zone 3 wall) because interior insulation reduces the benefit of the mass.

Basements, Crawl Spaces, and Slabs

Basement walls use a two-option format throughout the table. “15/19” means R-15 continuous on either side, or R-19 cavity on the interior. R-13 cavity combined with R-5 continuous also satisfies “15/19.” Crawl space walls carry the same values as basement walls in each zone, but only for unvented crawl spaces. Vented crawl spaces rely on floor insulation above them as the thermal boundary.

Slab-on-grade foundations need no insulation in Zones 1 through 3 unless the slab is heated. Zone 4 requires R-10 perimeter insulation extending at least 2 feet below grade, rising to 4 feet in Zones 6 through 8 under the 2015 and 2021 editions.2International Code Council. 2015 International Energy Conservation Code – R402.1.2 Insulation and Fenestration Criteria Heated slabs in any zone add R-5 to the base requirement. The insulation can run vertically along the slab edge or horizontally beneath the perimeter, as long as it reaches the required depth or length.

Windows, Skylights, and Doors

R402.1.2 also sets maximum U-factors and SHGC limits for fenestration. Under the 2021 IECC:4International Code Council. 2021 International Energy Conservation Code – Chapter 4 RE Residential Energy Efficiency

  • Zones 0 and 1: window U-factor 0.50, skylight U-factor 0.75, SHGC 0.25.
  • Zone 2: window U-factor 0.40, skylight U-factor 0.65, SHGC 0.25.
  • Zone 3: window U-factor 0.30, skylight U-factor 0.55, SHGC 0.25.
  • Zones 4 and 5: window U-factor 0.30, skylight U-factor 0.55, SHGC 0.40.
  • Zones 6 through 8: window U-factor 0.30, skylight U-factor 0.55, no SHGC requirement.

Lower U-factor means less heat escapes the glass, which is why colder zones push the number down to 0.30. SHGC controls how much solar radiation enters through the glass. Hot southern zones hold SHGC at 0.25 to keep cooling loads manageable. Zones 6 and above drop the cap because solar gain helps offset winter heating. Skylights in Zones 0 through 3 may exceed the 0.25 SHGC floor as long as they stay at or below 0.30. Windows installed above 4,000 feet elevation or in windborne debris regions get a slightly relaxed U-factor of 0.32 in Marine Zone 4 and Zones 5 through 8.

Inspectors verify compliance by reading the manufacturer’s label on each unit. Missing or mismatched labels fail inspection, and reordering fenestration at that stage is one of the most expensive corrections a project can face. Keep the labels on until the energy inspection is signed off.

Air Sealing and the Blower Door

Insulation alone doesn’t make a tight envelope, and R402.1.2 is read alongside the air-sealing requirements that make those R-values work. A continuous air barrier must be installed across the entire envelope, with every break and joint sealed.5International Code Council. 2015 International Energy Conservation Code – R402.4.1.1 Installation Air-permeable insulation like fiberglass batts cannot serve as the sealing material, because it slows heat but not air movement.

Specific components inspectors look at closely:

  • Dropped ceilings and soffits must have insulation aligned with the air barrier; attic access openings and knee wall doors to unconditioned space must be sealed.
  • Sill-plate-to-foundation junctions and top plates must be sealed. Corners and headers must be filled with insulation rated at least R-3 per inch.
  • Rim joists must include both the air barrier and insulation.
  • Duct shafts, utility penetrations, and flue shafts opening to exterior or unconditioned space must be sealed.
  • The gap between window and door jambs and the surrounding framing must be sealed.

After assembly, a blower door test measures actual leakage. Under the 2021 IECC, detached single-family homes and townhouses cannot exceed 4.0 air changes per hour at 50 Pascals. Some jurisdictions and compliance paths tighten this to 3.0 ACH50 in Zones 3 through 5. A typical blower door test runs roughly $150 to $450 depending on house size and local market. Failing means finding and sealing leaks and retesting, which costs far less before drywall than after.

Three Ways to Prove Compliance

Hitting every prescriptive value is the simplest route, but it isn’t the only one.

Prescriptive Path With Trade-Offs

The code allows a total-UA approach that calculates overall heat loss through the entire envelope. If one component comes in below the table value, you can compensate by exceeding it elsewhere, as long as the design’s total heat loss doesn’t exceed what a fully code-compliant building would produce. The Department of Energy’s free REScheck software automates this calculation.6Building Energy Codes Program. REScheck Input your dimensions, insulation levels, and fenestration ratings; if your design’s total UA is at or below the baseline, the software generates a passing report that most building departments accept.

Simulated Performance Path

Section R405 lets you model the whole building’s energy performance using approved software such as REM/Rate or Ekotrope. The proposed design must show annual energy cost equal to or less than a reference building that meets all the prescriptive requirements. REScheck doesn’t work for this path. Every mandatory provision (air sealing, mechanical insulation, duct testing) still applies regardless of how well the simulation scores.

Energy Rating Index Path

Section R406 allows compliance through a home energy rating score on an index from 0 to 100, where lower is better. Under the 2021 IECC, maximum ERI scores range from 51 to 55 depending on climate zone. A home at or below the zone threshold passes. This path is popular with production builders who use a consistent design across many units. All mandatory provisions still apply.

Additions, Alterations, and Converted Spaces

R402.1.2 isn’t just for new construction. Additions must meet the same standards as new buildings, though existing unaltered portions of the home don’t have to be brought up to current code.7International Code Council. 2021 International Energy Conservation Code – Chapter 5 RE Existing Buildings An addition complies if it meets the code on its own, if the whole building meets the code together, or if the building with the addition uses no more energy than the existing structure did before.

Alterations work similarly. The work you’re doing must meet current code, but you aren’t required to open walls you aren’t touching. Converting unconditioned space into living area (finishing a garage or attic) triggers full new-construction compliance for the converted space, including insulation, air sealing, and fenestration. Homeowners finishing a bonus room are often caught off guard by this.

What Inspectors Catch

The same failures come up again and again. Insulation compression is the most frequent. R-19 batts stuffed into an R-13 cavity, or wiring and plumbing crushing batts against the sheathing, drops installed R-value below the labeled rating. Inspectors check for full contact between insulation and air barrier, and any visible gap or compression gets flagged.

Knee walls are a chronic problem. Insulation goes in but the air barrier behind it is missing or incomplete, letting conditioned air bleed into the attic. Missing fenestration labels create preventable delays, because the inspector can’t confirm U-factor and SHGC match the approved plans without the sticker. On the air-sealing side, the most common blower door failures trace to unsealed top plates, electrical penetrations, and recessed lighting fixtures that aren’t rated for insulation contact.

Failing an energy inspection means removing finished material to access the deficiency, correcting it, and scheduling a reinspection. For insulation deficiencies discovered after drywall is up, tearing out and replacing can run several thousand dollars on a typical home. That makes the pre-drywall inspection, where most jurisdictions check insulation and air barrier installation, the point at which the project is really made or lost.