IRC N1102 / IECC: Building Thermal Envelope

The thermal envelope is the insulated shell of your house: the ceiling, the walls, the floor, and the windows and doors that separate conditioned air from the outdoors. IRC Chapter 11 (Section N1102), which mirrors the International Energy Conservation Code (IECC), sets the minimum insulation and window performance for that shell. In Phoenix, where summer afternoons hit 115 degrees, these numbers decide how hard your air conditioner works and how high the August bill runs.

What N1102 requires for Arizona (zone 2B)

The Phoenix metro sits in climate zone 2B, the hot-dry zone, and the code's prescriptive table gives a set of minimum R-values (the measure of how well insulation resists heat flow) plus window limits. For zone 2 the headline numbers are: a ceiling of about R-38, a wood-frame wall of R-13 (or R-0 cavity plus R-10 continuous), and an exposed floor of R-13. Slab-edge insulation is not required in this zone. Here is the table note:

Where Section R402.1.3 requires R-49 insulation in the ceiling, installing R-38 over 100 percent of the ceiling area requiring insulation shall be deemed to satisfy the requirement for R-49.

The window numbers matter most here

In a cooling climate, the windows do more damage than the walls. N1102 caps two window properties. The U-factor, which measures heat conduction, must be 0.40 or lower. The solar heat gain coefficient (SHGC), which measures how much of the sun's heat passes through the glass, must be 0.25 or lower for glazed fenestration. That low SHGC is the Arizona-specific lever: a 0.25 SHGC window blocks three-quarters of the sun's heat, which is why low-E, spectrally selective glass is standard on Valley custom homes and why west-facing glass without it bakes a room.

What it means in practice

For example, a custom home in Fountain Hills built to the prescriptive path gets R-38 blown into the attic, R-13 batts in the 2x4 walls (or a 2x6 wall with more), and low-SHGC dual-pane windows rated at or below 0.40 U-factor and 0.25 SHGC. A house can also pass on a performance path, where a software model trades a better wall against a slightly worse window, but the prescriptive numbers above are the simple route most builders use. Air sealing and a proper vapor barrier strategy round out the envelope, though in the dry zone 2B climate, interior vapor retarders are generally not required.

How it connects

The envelope works with the mechanical and slab sections of the home. The slab itself is governed by IRC R506 concrete slab-on-ground floors , and the R-value concept here is the same one explained in the R-value glossary entry . Strong insulation also lets you size the air conditioner smaller, which lowers both the upfront equipment cost and the monthly bill.

Edition note

The zone 2 prescriptive R-values (R-38 ceiling, R-13 wall) and the 0.40 U-factor / 0.25 SHGC window limits are the same in the 2018 and 2021 IRC/IECC. The 2024 IECC pushes the envelope and equipment harder in some zones and adds new options, so a city on the 2024 code may enforce tighter targets. Because Arizona cities adopt the energy code on their own timelines, confirm the edition your jurisdiction enforces on the Arizona building code editions by city page.

Source

Read the table on the official ICC site: 2021 IRC N1102.1.3 on codes.iccsafe.org . The I-Codes are copyrighted, so only the short ceiling-insulation note is quoted here; the rest is our explanation.