General construction: how buildings go together
Ventilation and indoor air basics
Buildings need a steady supply of fresh air and a way to remove moisture and pollutants. This guide covers ventilation types, where it matters most, and how it relates to air sealing.
Buildings need a steady supply of fresh air and a way to remove moisture and pollutants. This guide covers ventilation types, where it matters most, and how it relates to air sealing.
Why ventilate
People, cooking, bathing, cleaning products and building materials add moisture and pollutants to indoor air. Ventilation dilutes and removes them, which protects health and the building.
Types of ventilation
- Natural: windows, vents and stack effect. Free, but weather-dependent and uncontrolled.
- Exhaust: fans pull air out of kitchens and bathrooms; replacement air enters through leaks or vents.
- Supply: a fan pushes filtered air in.
- Balanced, with heat recovery: equal supply and exhaust through a core that recovers heat or coolness.
Spot ventilation
Kitchens and bathrooms need extraction that discharges outside, not into the roof space. Duct size, length and bends reduce flow, so the fan rating on the box is not the real flow.
Standards
Minimum ventilation rates are set by local code or standards such as ASHRAE 62.2 for homes. Ask the designer which applies to your project.
Tight building, planned air
The tighter the shell, the more deliberate the ventilation must be. See air sealing and HVAC basics.
Quick answers
- Do I need mechanical ventilation? Depends on the building tightness and code. Tight buildings generally need it.
- Is opening windows enough? Often not reliably, since it depends on weather and habits.
- Where should exhaust fans discharge? Outdoors, never into an attic or soffit cavity.
Keep reading
- Air sealing the building envelope
- HVAC basics for small buildings
- Moisture and condensation in walls
- Energy efficiency: the envelope first
Always check the project drawings and specification, and confirm requirements with the engineer of record and the local authority.
Questions people ask
Do I need mechanical ventilation?
Depends on the building tightness and code. Tight buildings generally need it.
Is opening windows enough?
Often not reliably, since it depends on weather and habits.
Where should exhaust fans discharge?
Outdoors, never into an attic or soffit cavity.
Sources and further reading
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