Whole House Ventilation

Last updated April 1, 2025
By Ian Story

This article discusses ventilation requirements for the living spaces of residential buildings (group R occupancies) in Washington State.

Both the IRC and IBC require ventilation for all habitable rooms(IRC R303.1, IBC 1202.1/1202.5). Per the original code, this can be via natural ventilation or mechanical ventilation. However, the Washington State amendments separately require a mechanical whole-house ventilation system (WA Amendments to IRC: R303.4, WA Amendments to IBC: 1202.5), which leaves the natural ventilation option as an optional bonus.

The IRC and IBC refer to separate code sections for mechanical ventilation (IRC M1505 / IMC 403), but the Washington State amendments have again modified these sections:

• IRC whole-house ventilation requirements (WA State Amendments) … See Section M1505.4
• IBC whole-house ventilation requirements (WA State Amendments)

The text varies slightly between the two codes, but most of the essential requirements are the same. The biggest difference is between R-3 occupancies (single-family residences / duplexes / townhomes) and R-2 occupancies (apartment buildings / triplexes / 4-plexes): R-2 occupancies are required to use a balanced supply/exhaust system, which will typically be an HRV. The minimum required ventilation rate is based on floor area and number of bedrooms. For continuous ventilation, use the following table:

There are equations to reduce the airflow for intermittent ventilation, but we usually just specify continuous fans.

Ventilation requirements can be met by one of the following systems:

• Exhaust fans only (makeup air is assumed to be drawn in via negative pressure and small air leaks in the building envelope). Note: this is our go-to solution for buildings that don’t want to (and aren’t required to) upgrade to an HRV/ERV. Every residential building already has to have exhaust fans installed in the bathrooms, laundry rooms, and kitchen, so there is little extra cost to setting one or two of these fans for continuous operation.
• Supply fans only (stale air is assumed to exhaust through positive pressure and small air leaks in the building envelope). I have never used this option.
• Balanced supply/exhaust system. Typically this option is combined with an HRV/ERV.
• Air handler integrated system (fresh air is incorporated into the central air handler and distributed via ducts).