The zone crosswalk behind unvented-attic insulation (Table 1202.3.1)

If you detail unvented attics or unvented rafter assemblies on the San Diego coast, the code just fixed the lookup table you were trusting to pick your condensation-control numbers.

Here's the thing: this isn't new law, and not a single R-value percentage moved. The January 2026 California Building Code errata did something tiny and easy to miss — it added one missing zone to one row of a crosswalk table. But it's a crosswalk people actually calculate with, so the missing cell mattered.

What actually changed

Table 1202.3.1 is the bridge between two different climate-zone systems. The unvented-attic insulation rules in Section 1202.3 are written in IECC climate zones; the zone a California designer actually knows is the California Energy Code zone. Table 1202.3.1 — “IECC Versus California Energy Code Climate Zone Comparison” — is how you translate one into the other.

The errata added California Energy Code Zone 7 to the IECC 3 (marine) row. That row's California column now reads 1, 2, 3, 4, 5, 6, 7, 9, 12, 16, and the row's county list includes San Diego. Coastal San Diego is California Energy Code Zone 7 — and until this correction, it was missing from the row that points it at the right IECC zone.

Verified against the live code text of CBC Table 1202.3.1 and BSC Information Bulletin 26-01 (p.3): “California Zone 7 is added to Energy Code comparison table column on the row for IECC Zone 3 (marine).” Effective January 1, 2026.

How the lookup actually works

This is the part that bites if you skip a step. The condensation-control percentages don't live in Table 1202.3.1 — that table only translates zones. The numbers live in Table 1202.3, “Insulation for Condensation Control,” which is indexed by IECC zone. So the chain is:

1. Start with your California Energy Code zone. Coastal San Diego = Zone 7.
2. Run it through Table 1202.3.1. CA Zone 7 now sits on the IECC 3 (marine) row — that's IECC zone 3C.
3. Read Table 1202.3 for 3C. It falls in the “1, 2A, 2B, 3A, 3B, 3C” row at 10%.

That 10% is the minimum R-value of air-impermeable insulation expressed as a percentage of the assembly's total R-value. It applies in the cases that route through Table 1202.3 — where air-permeable insulation is used (Item 5.1.2) or where air-permeable and air-impermeable insulation are combined (Item 5.1.3).

Verified against CBC Table 1202.3 (IECC 3C → 10%) and Section 1202.3, Items 5.1.2 and 5.1.3, 2025 edition with January 2026 errata.

The trap the crosswalk closes

Here's why the missing cell was dangerous. Look at Table 1202.3 and you'll see a row for Zone 7 at 60%. That “7” is an IECC zone — and per the footnote to Table 1202.3.1, IECC zones 1, 7 and 8 don't occur in California at all. A coastal-San-Diego designer who saw “Zone 7” on the energy-code side and jumped straight to the “7 = 60%” row in Table 1202.3 would size the air-impermeable layer at six times what the code actually asks for. The correct answer — IECC 3C, 10% — is a different row entirely.

That's the whole job of the crosswalk: keep you from reading your California Energy Code zone number as if it were an IECC zone number. The errata just made sure coastal San Diego is on it.

One precision note: the “2B and 3B tile roof only = 0 (none required)” line in Table 1202.3 does not reach 3C. A marine zone-3 assembly carries the 10% regardless of roof covering.

If your work touches the wall side of the same coast, the rainscreen rule flows from the very same Zone-7-on-IECC-3-marine fix — see Stucco on the California coast. That one is the wall-covering trigger; this one is the attic-insulation lookup.

None of this changes the underlying requirement, and local jurisdictions can amend on top of the state code — so confirm your zone assignment and any local amendments with the building department before you finalize the assembly.

What CrossBeam does with it

CrossBeam reads an unvented-attic or rafter detail against the climate zone the project is actually in, translates the California Energy Code zone into the right IECC zone, and pulls the condensation-control percentage from the row that genuinely applies. It's the kind of two-table lookup that's easy to short-circuit — and easy to flag before a plan checker does.