Vapour barriers are required in which applications?

• A. Hot applications
• B. Moderate applications
• C. Cold applications
• D. All applications

Answer: (C)

Moisture diffusion and condensation control is what this item is testing. In cold environments, the warm indoor air contains a lot of water vapour. When that vapour tries to move into the insulation and meets cold surfaces inside the building envelope, it can reach its dew point and condense. That condensation soaks the insulation, reduces its effectiveness, and can lead to mold and structural problems. A vapour barrier on the warm side blocks most of the moisture from entering the insulation, keeping it dry and maintaining its insulating performance.

In hot applications, the driving force for condensation inside the insulation is much less, so a vapour barrier is not as universally required. In moderate situations, climate details and construction design determine the need. So the primary reason vapour barriers are required is to control moisture in cold applications.