A vapor retarder is a building material that slows moisture vapor diffusion to reduce condensation and moisture-related damage.

A vapor retarder is a material used in construction to slow the movement of moisture vapor through building assemblies. Unlike vapor barriers, which are nearly impermeable, vapor retarders allow limited vapor diffusion, helping control condensation while still permitting assemblies to dry under certain conditions. This balance is critical for long-term building durability.

Common vapor retarder materials include kraft-faced insulation, smart membranes, coated gypsum board, and certain paints or finishes. Vapor retarders are classified by permeability ratings—Class I, II, and III—based on how much vapor they allow to pass through. The appropriate class depends on climate zone, building use, and assembly design.

From a code and building science perspective, vapor retarders play a key role in moisture management and energy efficiency. Modern building codes often favor vapor retarders over true vapor barriers, especially in mixed and warm climates. When properly coordinated with air barriers and insulation, vapor retarders help reduce moisture-related failures while supporting healthy indoor environments.

Some modern “smart” vapor retarders can change permeability based on humidity levels, allowing walls to dry more effectively when moisture is present.

• Poly Vapor Barrier
• Retarder
• Vapor Barrier