2021 IECC Section C402.1 to C402.2
The opaque envelope is everything in the building shell that is not a window or door: roofs, walls, floors over open air, and slabs. This lesson covers the insulation rules of Sections C402.1 and C402.2 for Climate Zone 2, the zone of Houston and most of the Gulf Coast. Every number below comes from the 2021 IECC Zone 2 column of Table C402.1.3 or C402.1.4, and other zones use different numbers, so always confirm the zone first (Lesson 2).
Section C402.1 requires the opaque parts of the envelope to meet the specific insulation rules of C402.2 and then one of three methods: the insulation component R-value method (C402.1.3), the assembly U-, C-, or F-factor method (C402.1.4), or the component performance alternative (C402.1.5). This lesson teaches the first two. Which method the designer picked is listed on the drawings (C103.2), and you review the building against that method only.
The table has two columns per climate zone. Commercial buildings, or portions of them, enclosing Group R occupancies use the Group R column. Everything else uses All other (C402.1.3). Three habits prevent most errors. First, "ci" means continuous insulation, uninterrupted by framing, and cavity insulation sits between framing members. Second, layers add up: cavity layers are summed for the cavity requirement and continuous layers are summed for the continuous requirement. Third, cavity R-values never count toward a continuous requirement. An entry such as R-13 + R-3.8ci asks for both parts. NR means no requirement.
In Zone 2, a roof with insulation entirely above the deck needs R-25ci in both columns. A roof with an attic or other cavity construction needs R-38. A metal building roof needs R-19 + R-11 LS (a liner system), and the table notes that the R-value method for metal buildings requires a thermal spacer block, or the design must use the U-factor method instead.
Section C402.2.1 adds installation rules. Above-deck insulation may not be thinner than 1 inch at its lowest point, at a gutter edge, drain, or scupper. Continuous insulation board goes in at least two layers with staggered joints, except where it tapers to the deck at those edges. Insulation lying on a suspended ceiling with removable tiles does not count toward the roof value. Skylight curbs are insulated to the roof level or R-5, whichever is less, unless a listed unit skylight includes the curb.
Zone 2 wall values depend on framing type, and the Group R column is stricter for three of the four types.
| Wall type (Zone 2) | All other | Group R |
|---|---|---|
| Mass | R-5.7ci | R-7.6ci |
| Metal building | R-13 + R-6.5ci | R-13 + R-13ci |
| Metal framed | R-13 + R-5ci | R-13 + R-7.5ci |
| Wood framed and other | R-13 + R-3.8ci or R-20 | R-13 + R-3.8ci or R-20 |
For wood framing the designer may choose either combination. A "mass wall" must qualify under C402.2.2: at least 35 pounds per square foot of wall surface, or at least 25 where the material weighs 120 pounds per cubic foot or less, or a heat capacity above 7 (or above 5 for material at 120 pounds per cubic foot or less) in Btu per square foot per degree F. Insulation built into concrete masonry units does not count under the R-value method, but the table allows plain concrete block walls meeting its grouting spacing and core-fill conductivity limits to stand in for R-5.7ci.
Floors over outdoor air or unconditioned space need R-30 between joists or framing in Zone 2. A mass floor needs R-6.3ci in the All other column and R-8.3ci for Group R, and it must weigh at least 35 pounds per square foot, or at least 25 where the material is 120 pounds per cubic foot or less (C402.2.3). Cavity insulation must stay in permanent contact with the underside of the subfloor, so a batt hanging low on wire is a finding.
Slab-on-grade floors show why the zone matters: an unheated slab in Zone 2 has no insulation requirement, but a heated slab does. A heated slab needs R-7.5 running 12 inches down at the perimeter plus R-5 under the full slab, and hydronic tubing or warm-air ducts in or under the slab make it a heated slab. Under C402.2.4.1, perimeter insulation goes on the outside or the inside of the foundation wall and runs down from the top of the slab by the required distance or to the top of the footing, whichever is less. Insulation extending away from the building is protected by pavement or at least 10 inches of soil. Full-slab insulation is continuous except at structural columns and service penetrations. Perimeter insulation is not required where the slab sits more than 24 inches below finished exterior grade. Radiant panels in the envelope also need R-3.5 on every face away from the heated space (C402.2.6).
Zone 2 has no requirement for below-grade walls in Table C402.1.3, which matters for a Houston basement or a small retaining-type stem wall. Where a colder zone does require insulation, it must run down to 10 feet below outside grade or the lowest conditioned floor, whichever is less (C402.2.5). If heated slabs sit below grade, the wall follows the heated slab exterior insulation requirements.
Table C402.1.4 offers a second route. In Zone 2 (All other, then Group R where different) the maximums are U-0.039 for a roof with insulation above the deck, U-0.027 for an attic roof, U-0.064 for a wood-framed wall, U-0.151 or U-0.123 for a mass wall, U-0.033 for a joist floor, F-0.73 for an unheated slab, and F-0.69 for a heated slab. A U-factor describes the whole assembly with its framing, so do not compute it as 1 divided by the R-value on the batt label. Use a tested or calculated assembly value and compare it with the table.
Worked example. A four-story wood-framed apartment building (Group R) in Harris County is Zone 2, so the wall needs R-13 + R-3.8ci or R-20. Plan A shows R-13 batts in 2x4 studs plus a board labeled R-5 continuous outside: the cavity part meets R-13 and R-5ci is at least R-3.8ci, so it passes. Plan B is 2x6 studs with R-19 batts and no board: the R-20 option is not met (19 is less than 20) and there is no continuous layer, so it fails, while R-21 batts would pass. The roof over the same building has insulation entirely above the deck and needs R-25ci. Two layers of board each labeled R-13 add up to R-26 and pass. Two layers each labeled R-10 add up to R-20 and fail.
In the field
At the thermal envelope inspection of a new four-story apartment building, look at the wall before drywall. Read the label on the batts (C303.1.1), check whether a continuous layer is actually outside the studs, and count what is there: R-13 in a 2x4 wall with no board fails, and so does R-19 alone in a 2x6 wall. Then check the roof board labels and layer count, and ask whether any floor bay over open air has R-30 in contact with the subfloor. Missing labels mean the installer's certificate must stand in for them, and the certificate lists what goes on the thermal envelope certificate (C401.3).
1. Under the R-value method of Section C402.1.3, can the R-value of cavity insulation be counted toward a continuous insulation (ci) requirement?
2. A new Group B office in Harris County will have roof insulation entirely above the deck. What minimum insulation does Table C402.1.3 require in Climate Zone 2?
3. A Group M store in Zone 2 has a truss attic with insulation at the ceiling line. What does the 'Attic and other' row of Table C402.1.3 require?
4. An inspector examines the walls of a four-story, wood-framed Group R-2 apartment building in Harris County. The framer has installed 2x6 studs with R-19 batts and no exterior continuous layer. What is the finding?
5. For a wood-framed wall in a Group B building in Zone 2, which choice meets Table C402.1.3?
6. A Group A-3 community center in Galveston County will have insulation entirely above the roof deck. The designer proposes two layers of continuous board, each labeled R-10. What is the result under Table C402.1.3 for Zone 2?
7. A four-story Group R-2 apartment building in Harris County has concrete masonry walls that qualify as mass walls. What does Table C402.1.3 require for above-grade mass walls in Zone 2?
8. A Group A-3 community center in Harris County has a heated slab with hydronic tubing in the concrete. What does Table C402.1.3 require in Zone 2?
9. A Group B office with an unheated slab-on-grade is planned in Fort Bend County. What perimeter or under-slab insulation does Table C402.1.3 require in Zone 2?
10. Slab perimeter insulation extends horizontally away from a building. Under Section C402.2.4.1, it must be protected by pavement or by at least how much soil?
11. Under Section C402.2.1.2, above-deck roof insulation must be not less than what thickness at its lowest point, such as a drain or scupper?
12. A framer lays fiberglass batts on the removable ceiling tiles of a suspended ceiling in a Group B office and reports it as the roof insulation. How does Section C402.2.1.3 treat it?
13. A small Group B office is built on piers over open air in Galveston County, with wood joists in the floor. What does Table C402.1.3 require for joist or framing floors in Zone 2?
14. A concrete wall in a Group S warehouse is made of material that weighs 110 pounds per cubic foot and the wall weighs 30 pounds per square foot of surface. Does it qualify as a mass wall under Section C402.2.2?