Which component should be installed after sealing air leaks but before installing insulation?

• A. Install insulation
• B. Install vapor retarder in correct location
• C. Identify and seal air leaks
• D. Inspect for gaps and seal

Answer: (B)

Moisture control through proper sequencing is the idea here. You first stop air leakage to reduce drafts and the transport of humid indoor air through the cavity. After those leaks are sealed, the next step is to install a vapor retarder in the correct location to limit the diffusion of water vapor from the warm indoor side into the cooler wall cavity, where it could condense.

Placing the vapor retarder before the insulation helps ensure a continuous, well-sealed barrier on the interior side, so interior moisture doesn’t diffuse into the cavity and potentially cause condensation inside the wall. In cold climates, this means putting the vapor retarder on the warm side of the insulation (typically the interior side) so it functions effectively while allowing the assembly to dry toward the interior if needed.

If insulation were installed first, it becomes harder to correctly position and seal the vapor retarder around all penetrations and seams, and moisture could become trapped or diffuse behind gaps, reducing long-term performance.