# §150.0(q): Radiant barrier
What it is
A radiant barrier is a reflective foil layer applied to the underside of the roof deck (or laminated to the deck sheathing itself). It reflects long- wave radiation from the hot roof back outward, cutting attic temperatures by 10–30°F in summer and reducing the cooling load that the HVAC system has to remove.
§150.0(q) makes a radiant barrier mandatory in most California climate zones, with two carve-outs.
When it applies
Required by §150.0(q) in:
- CZ2, CZ4, CZ5, CZ6, CZ7, CZ8, CZ9 — coastal and inland warm zones.
Not required (and in some cases explicitly suppressed):
- CZ1, CZ16 — cold zones where the radiant barrier provides no useful cooling benefit and can complicate winter moisture management. Suppressed in our model.
- CZ10, CZ11, CZ12, CZ13, CZ14, CZ15 — hot zones where the §110.10 cool-roof requirement applies. The cool-roof assembly conflicts with the Title 24 Rule 856 standard-design expectations for a radiant barrier, so the model suppresses RB in these zones. The compliance verification report carries an "RB suppressed per Rule 856" note.
- CZ3 — mild coastal; radiant barrier optional, not commonly required.
How we use it
We pick the radiant-barrier-on-or-off setting from your project's climate zone automatically. The construction library has both with-RB and without- RB assemblies, and the right one is chosen based on the CZ.
The High-Performance Envelope option honours the same suppression rules even when it tightens the rest of the envelope — the §110.10 cool-roof interaction in CZ10–15 takes priority over the RB upgrade.
See the per-climate-zone landing pages for the specific RB setting that applies to your project, e.g. Climate Zone 12 or Climate Zone 4.
Common gotchas
- Specifying a radiant barrier in CZ10–15 thinking "more is better" — it actually fails the standard-design comparison because the §110.10 cool-roof assembly doesn't model with an additional RB.
- Painted-foil radiant barriers that lose reflectivity within a year — the §150.0(q) spec requires emittance ratings that hold up under attic conditions.
- Conditioned-attic designs where the roof deck is the air barrier — the radiant barrier still applies on the structural side and gets confused with the spray-foam insulation layer below.