What is an example of a Class III vapor retarder?

• A. Polyethylene sheeting
• B. Gypsum board
• C. Foam insulation
• D. Vinyl wall coverings

Answer: (B)

Gypsum board is classified as a Class III vapor retarder. This means it provides a medium level of resistance to water vapor diffusion, allowing some moisture to pass through while still reducing overall moisture movement to some extent. This characteristic makes gypsum board suitable for certain interior applications, especially in spaces where a more robust vapor barrier is unnecessary but where moisture control is still a consideration.

In contrast, a Class III vapor retarder like gypsum board effectively balances moisture control and breathability in building assemblies, making it ideal for use in wall assemblies behind finishes like paint or wallpaper. This can help mitigate issues related to condensation and potential mold growth in humid environments by allowing for the necessary vapor transfer while still protecting the structural integrity of the materials behind it.