When is a vapour barrier typically used?

A vapour barrier is typically employed when the operating temperature of the system is less than the ambient air temperature. This is crucial because a vapour barrier serves to prevent moisture from the warmer, humid air from penetrating into colder surfaces where it can condense and lead to problems such as mold growth, corrosion, and material degradation.

In scenarios where the operating temperature is lower than the surrounding air, it is vital to ensure that any moisture present in the air does not come into contact with cooler surfaces that can lead to condensation. Thus, the placement of a vapour barrier is intended to mitigate these risks by acting as a shield against moisture movement, maintaining the integrity of the structure and the efficiency of insulation systems.

The other options do not align with the primary function of a vapour barrier. For instance, using a vapour barrier when operating temperatures exceed ambient air temperatures or in high-temperature installations might not be necessary, as the temperature differential is reversed, negating the need to control moisture in that context. Similarly, the performance of a vapour barrier isn't directly tied to low moisture levels, but rather to the temperature differential affecting moisture movement.