What type of insulation would most effectively act as a vapor retarder?

Polyethylene sheeting is highly effective as a vapor retarder due to its low permeability to moisture. Its primary function is to inhibit the flow of water vapor, making it ideal for preventing moisture issues in building assemblies. When installed correctly, polyethylene sheeting creates a barrier that prevents humid air from penetrating walls and ceilings, thus protecting the structure from condensation and potential mold growth.

Other insulation types, such as mineral wool and fiberglass, primarily serve as thermal insulators rather than vapor retarders. While rigid foam does have some moisture resistance, it doesn’t provide the same level of vapor retarding ability as polyethylene sheeting. The effectiveness of vapor retarders is crucial in ensuring the longevity and durability of the building by controlling moisture movement.