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 stringent prescriptive options in Appendix RC; rather than embedding the entire appendix in the base code, members voted to reference Appendix RC (option to “go to appendix”) so that jurisdictions and designers can read and apply the appendix directly. That approach was chosen to reduce complexity in the base code while preserving the Appendix RC compliance pathway.

Residential code edits passed Two additional residential proposals were voted and passed during the session: - R402.2.13 (slab insulation): The board approved an exception allowing builders to omit a slab‑edge insulation retrofit for existing slabs where the owner or site conditions prevent practical or safe installation, provided the rest of the prescriptive envelope requirements are met. The motion passed by vote (13 yes, 3 no, 2 abstain) and staff will add clarifying language and trade‑off options in the draft. - R402 top‑plate/party‑wall air sealing clarification: The board approved clarified language requiring sealing of interior walls that abut unconditioned spaces or otherwise create top‑plate penetrations to unconditioned attics or voids. The edit was intended to remove ambiguity in multifamily and attached housing when a unit separation or attic interface would otherwise be missed by a narrow interpretation. The motion passed (15 yes, 1 no, 1 abstain) and will be incorporated into the next draft.

Votes at a glance - Metric to measure home size: board chose conditioned floor area (with below‑grade weighted at 50%) over living‑space above‑grade — passed via straw poll. - Large‑home threshold: 5,000 sq ft (using the adopted equation) — passed (majority). - Extra‑large threshold: 7,500 sq ft (using the adopted equation) — passed after runoff (majority). - Extra‑large compliance approach: reference Appendix RC rather than embedding all language in the body — passed (19 yes). - R402.2.13 slab‑insulation exception: passed (13 yes, 3 no, 2 abstain). - Interior wall top‑plate sealing clarification: passed (15 yes, 1 no, 1 abstain).

Why it matters These choices change which new homes will be subject to more stringent “large‑home” efficiency requirements. Moving to conditioned floor area and explicitly including a share of below‑grade area was intended to prevent circumvention (for example, building large below‑grade spaces but avoiding above‑grade thresholds) while recognizing lower heating and cooling loads for true below‑grade spaces. Reference to Appendix RC for extra‑large homes creates a clear, higher‑stringency compliance pathway for the very largest dwellings without over‑complicating the base code.

Next steps and open issues Staff will incorporate the metric language and numeric thresholds into the next draft for public review. Board members and industry representatives flagged the need for additional clarifying text about what qualifies as conditioned/occupiable space and how finished/unfinished basements will be measured; staff and members agreed to refine definitions and to capture those clarifications in draft 2. The air‑barrier/inspection sequencing commercial item remains unresolved and will return after offline work by proposers and staff.

Speakers Kim (board chair), Carolyn (board member, large‑home proposer), Adam (staff/consultant), Andrew (builder representative), Tim (board member), Elizabeth (board member), Ron (board member), Jess, Kristen, Dan (proposer), Tom (participant).

Clarifying details - Adopted metric: conditioned floor area above grade + 50% of conditioned floor area below grade. - Large‑home threshold: 5,000 sq ft (per adopted metric). - Extra‑large threshold: 7,500 sq ft (per adopted metric); extra‑large homes to follow Appendix RC requirements by reference. - R402.2.13 slab exemption: available when existing slab makes edge insulation impractical; trade‑offs expected. - Interior wall sealing: applies to interior walls that abut unconditioned spaces (attics, voids) and to separations between units where applicable.

Authorities referenced - 2024 International Energy Conservation Code (IECC) - Appendix RC (referenced for extra‑large homes) - Appendix RG (previously discussed as strengthening option for large homes)

Proper names [{"name":"Energy Office","type":"agency"},{"name":"IECC 2024","type":"statute"},{"name":"Appendix RC","type":"other"},{"name":"Appendix RG","type":"other"}]

Searchable tags ["home-size","conditioned-floor-area","Appendix-RC","R402","large-home","extra-large-home","slab-insulation","air-sealing"]