Board adopts new large and extra‑large home metrics and thresholds; adds Appendix RC pathway and approves two residential code edits

After prolonged discussion and multiple straw polls, the Energy Office’s Low Energy and Carbon Code board on Thursday changed how the residential code will define “large” and “extra‑large” homes and approved two residential amendments: one easing slab‑edge insulation requirements where an existing slab prevents practical compliance, and one clarifying required air sealing at interior wall top plates that abut unconditioned spaces.

Definition and metric The board voted to move away from the 2024 IECC’s living‑space‑above‑grade‑plane metric and instead adopt “conditioned floor area” as the metric used to determine whether a home triggers additional large‑home requirements. To account for the lower energy intensity of below‑grade conditioned spaces, the board adopted an equation that measures a home’s size as: all conditioned floor area above grade plus 50% of conditioned floor area below grade. That equation won a majority on a straw poll after members discussed ease of enforcement and fairness for homes with large basements.

Size thresholds Using the new metric the board set the numeric thresholds as follows: - Large homes: 5,000 square feet (conditioned floor area above grade + 50% of conditioned area below grade yields the reported figure) — adopted by majority vote. - Extra‑large homes: 7,500 square feet (using same metric) — adopted by majority vote after a runoff between competing values.

The board also decided that extra‑large homes would be subject to the more…