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Control vapour and manage moisture

Warm moist indoor air condensing inside walls causes rot and mould. The fix depends on climate and wall type, so copy a detail proven in your climate.

How

1. Know the classes (US IRC): Class I is 0.1 perm or less. Class II is above 0.1 and up to 1.0 perm. Class III is above 1.0 and up to 10 perm.
2. The IRC requires a Class I or II vapour retarder on the interior side of frame walls in climate zones 5, 6, 7, 8 and Marine 4. Mild and hot climates generally do not need one (DOE).
3. IRC exceptions include basement walls, the below-grade part of any wall, and walls of materials not damaged by moisture or freezing. A responsive vapour retarder is allowed inside in any zone.
4. Do not trap moisture: avoid two impermeable layers on both sides of a wall.
5. Natural walls must breathe: the US straw bale appendix bans Class I and II retarders on bale walls and Your Home advises against cement render on straw.
6. Shed rain first: overhangs, flashings, drip edges, a drained cavity behind cladding where needed (NZ) and ground clearance.
7. Manage indoor moisture with kitchen and bathroom extraction (thermal module).

Sources

1. Vapor Barriers. US Department of Energy, Building Science Education, accessed 2026-11
2. 2015 IRC Appendix S: Strawbale Construction. International Code Council (copy hosted by Ecological Building Network), accessed 2026-11
3. Straw bale. Your Home, Australian Government, accessed 2026-11
4. Construction systems (Smarter Homes guides). Building Performance, New Zealand Government (MBIE), accessed 2026-11
5. Vapor Retarders. Insulation Institute (NAIMA), accessed 2026-11

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