Why are vapor barriers required?
Take your cold soda can or glass of ice water: what happens to the exterior of that container when sitting on your table? It develops condensate. Why? Because the temperature of the container falls below the dew point temperature given the relative humidity of the surrounding air. So water vapor in the air around the object condenses, or the air molecules come together forming water droplets.
In buildings, condensation is caused when warm air begins to cool while it travels through building materials like insulation and drywall. For example, if you have a wide temperature differential between the outside (20 degrees & snowing) and the inside of a home (heated to 72 degrees), condensation occurs. The net effect is damaged drywall and mold.
To counteract this in colder climates, a vapor barrier is placed behind the drywall so condensation will occur there and not on the drywall. We need vapor barriers in California Climate Zones 14-16. In some climate zones, an air barrier allows currents of air to help prevent water vapor from forming. A vapor barrier is likely required if the building is cladded with absorptive material and if the structure is in United States climate zones 4C, 5 through 8. If you are building in a colder climate, check local laws or consult with your architect if a vapor barrier is required.
For more information on how the construction of your building envelope and other assembly features can affect the lifespan of your building, please contact VCA Green below:
Moe Fakih, Principal
2012 IECC Climate Zone Map. Image Source: International Code Council