Where should a vapor barrier be placed in an attic that has two layers of batt insulation?

The ideal placement of a vapor barrier in an attic with two layers of batt insulation is below the insulation.

This configuration serves a crucial purpose in managing moisture within the attic space. By positioning the vapor barrier underneath the insulation, it inhibits moisture from the living spaces below from migrating into the attic. The barrier prevents potential condensation that could occur if warm, moist air from the living areas rises and comes into contact with the colder attic environment.

In contrast, placing the vapor barrier above or between the insulation layers or on the roof sheathing could lead to moisture issues. If it were located above the insulation, moisture could still migrate up and condense on the roof sheathing or other cold surfaces, leading to mold, rot, or structural damage. Positioning it between layers could create pockets of air that may trap moisture, which is not ideal. Lastly, placing the barrier on the roof sheathing would directly limit the ability to manage moisture ingress from the building's interior, compromising the performance of both the insulation and the structural integrity of the roof system.

Thus, the appropriate location for the vapor barrier in this setup is below the insulation, effectively reducing the risk of moisture damage while promoting a healthier attic environment.