When should a vapour barrier not be installed?

The correct response highlights the importance of moisture control in building construction. A vapour barrier should not be installed when the insulation is wet because it can trap moisture within the wall assembly. This trapped moisture can lead to a host of issues, including mold growth, degradation of the insulation's thermal performance, and potential damage to structural elements.

In situations where insulation is already wet, introducing a vapour barrier can prevent adequate drying and exacerbate the problem, potentially compromising the integrity of the building over time. Installing a vapour barrier is crucial for moisture management, but doing so in the presence of existing moisture creates unfavorable conditions, emphasizing the need for a dry environment before installation.

Other scenarios, such as air-tight construction, may involve careful consideration regarding vapour barriers but do not inherently preclude their installation if moisture management is adequately addressed. Similarly, warm weather does not automatically negate the need for vapour barriers if moisture conditions are properly managed. However, it is the state of the insulation—wet versus dry—that fundamentally determines the appropriateness of installing a vapour barrier.