# §150.0(e): Slab-edge insulation
What it requires
Slab-on-grade floors (the concrete pour that sits directly on dirt) lose heat through the slab edge, where the concrete meets the foundation wall. §150.0(e) sets the mandatory floor for slab-edge insulation:
- R-5 minimum on the slab edge in any climate zone where slab insulation is required.
- Vertical strip, applied either at the outside face of the foundation wall (exterior method) or against the interior face of the slab perimeter (interior method).
- 24-inch depth measured from the top of the slab downward.
- Continuous coverage around the heated perimeter — no gaps for foundation vents or utility penetrations without a thermal break.
The prescriptive package in §150.1(c)1 specifies R-7 for 24" in CZ16 (the mountain zone), which is the only California climate zone where slab-edge is binding in practice. Other zones leave it as an optional performance trade-off variable.
When it applies
- Newly constructed residential in CZ16, on every slab edge.
- Newly constructed residential in CZ1, CZ2 — sometimes required at the §150.1(c)1 prescriptive level depending on the room's floor area and the trade-off path.
- Additions in CZ16 with a new slab pour.
How we use it
When your project's climate zone is CZ16, the Confirm step models the R-7 slab edge as part of the prescriptive envelope. For other climate zones, slab edge isn't surfaced on the design cards — it's only a performance-path trade-off, and our three options each clear compliance without invoking it.
The slab-edge detail is one of the line items the AHJ inspector verifies at foundation inspection (before the slab is poured), so it's a hard-stop construction detail that can't be value-engineered out late.
Common gotchas
- Slab edges that get installed under the foundation footing rather than at the slab perimeter — this misses the thermal-bridge path and fails inspection.
- Insulation board that's not rated for ground contact — standard EPS / XPS works, but the substrate has to be sealed against termites in zones where that's a concern.
- Garage slab in a CZ16 attached-garage project — §150.0(e) doesn't apply to unconditioned-garage slabs, but the line between conditioned and garage often shifts mid-design.