Which type of vapor retarder should not be installed on the attic floor or at the ceiling?

A Class I vapor retarder, such as polyethylene sheeting, is designed to prevent moisture from migrating through building assemblies. When installed on the attic floor or at the ceiling, a Class I vapor retarder can trap moisture in the insulation or within the building structure. This can lead to condensation issues, mold growth, and other moisture-related problems because the moisture generated within the home has no means of escaping.

In contrast, Class II and Class III vapor retarders are semi-permeable and can allow for some moisture diffusion, which can be beneficial in attic spaces. Open-cell foam, while having vapor retarding properties, also allows for moisture to pass through in a controlled manner. For this reason, installing a Class I vapor retarder in these areas is not advisable as it can lead to detrimental moisture management issues within the attic space.