When installing a vapor barrier in a fully insulated wall, how deep can it be installed?

The correct choice regarding the installation of a vapor barrier in a fully insulated wall is that it can be installed no more than one-third of the thickness of the wall. This recommendation is grounded in building science principles that address the management of moisture within wall assemblies.

Vapor barriers are crucial in controlling humidity levels and preventing moisture accumulation within wall systems. If a vapor barrier were installed too deep within a wall—beyond one-third of its thickness—it could lead to trapping moisture within the insulation, creating conditions for mold growth and compromised structural integrity. Installing the barrier within the specified depth helps ensure that it effectively mitigates moisture while allowing for some level of vapour permeance which is essential for the overall durability and performance of the wall assembly.

Furthermore, guidelines are often based on the concept of ensuring proper drying potential for the materials involved. Within one-third of the wall thickness, the barrier can effectively serve its purpose without adversely affecting the wall’s ability to manage moisture levels efficiently.