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Committee issues two formal interpretations on code scope and whole‑house ventilation
Summary
The committee approved two formal opinions: (1) the IRC/IMC cross‑reference applies as applicable under R403.6; and (2) for 2018 IMC/IRC ventilation language, fresh air must be ducted to each habitable space and group R‑2 multifamily expects balanced systems while other occupancies may use supply/exhaust/ERV combinations.
On March 14, 2025, the Mechanical Ventilation, Plumbing and Energy Code Committee approved two formal interpretations clarifying how existing code language should be applied by local jurisdictions and permit reviewers.
First, the committee adopted Opinion 25‑MARCH‑01 responding to a Bremerton/Ruston question about the 2018 and 2021 Washington State Energy Code reference in R403.6. Staff recommended, and the committee accepted, that the cited language intends the applicable provisions of the International Residential Code (IRC) to apply when a project falls within the scope of the IRC and the International Mechanical Code (IMC) to apply when it does not. Committee members recorded the motion and approved the interpretation.
Second, the committee adopted a clarification on 2018 IMC/IRC mechanical ventilation language that had caused inconsistent local interpretations. Staff proposed and the committee approved a position answering two linked questions: whether all dwelling units require a balanced supply‑and‑exhaust whole‑house system and what configuration satisfies the “ducted outdoor ventilation air to each habitable space” requirement.
The committee-approved guidance states that all fresh outdoor air required by the code must be delivered (ducted) to each habitable room. For group R‑2 multifamily dwelling units, the intent is to have balanced systems that provide both supply and exhaust (or an ERV/HRV that provides both). For other dwelling types covered by the IRC, a strictly balanced mechanical system is not universally required; systems may consist of any combination of supply fans, exhaust fans, or ERV/HRV units, provided that fresh outdoor air is supplied to each habitable space and applicable adjustment factors are applied when systems are not balanced. The guidance also referenced exceptions for joined spaces and a prior opinion (20‑06) addressing distributed versus non‑distributed delivery.
During the discussion, local building officials and code consultants said the 2018 code rewrite removed earlier trickle‑vent and inlet language and left ambiguities about whether exhaust‑only configurations could meet the ducted‑air requirement. Staff and committee members said the practical interpretation is that exhaust‑only systems cannot meet the ducted‑fresh‑air‑to‑each‑space requirement; supply elements or ERV/HRV systems (or a combination with local exhaust fans that are part of the whole‑house system) are needed to provide ducted outdoor air to each habitable room.
The committee moved and approved both opinions in committee. The motions to adopt the written opinions were made by committee members and seconded on the record; committee members voted aye and the opinions were approved and scheduled for publication as formal interpretations.
