Vapor Retarder
A layer that slows water vapor moving through a building assembly.
Quick Answer
A vapor retarder is a material or coating that slows the diffusion of water vapor through a wall, roof, or floor assembly. Building codes classify them by permeance into Class I, II, and III. Placement depends on climate and the assembly, because the wrong location can trap moisture rather than prevent condensation.
The Full Picture
Water vapor moves through materials by diffusion, from areas of higher vapor pressure toward lower, and it can condense into liquid when it meets a cold surface. A vapor retarder reduces that diffusion so the amount of moisture reaching condensing surfaces inside an assembly stays low enough for the assembly to dry out.
Retarders are rated by permeance, measured in perms. Model building codes use classes: Class I is nearly impermeable, such as polyethylene sheet or sheet metal, Class II is semi-impermeable, such as kraft facing or some coatings, and Class III is semi-permeable, such as latex paint on gypsum. The right class depends on climate zone and the assembly's other layers.
Placement is where mistakes occur. In cold climates, the retarder is generally on the warm, interior side of the insulation. In hot-humid climates, the concern reverses and an interior vapor barrier can trap moisture driven inward from outside. Mixed climates favor smart or variable-permeance retarders that allow seasonal drying.
Vapor control is often confused with air control. Most moisture damage in walls comes from air leakage carrying vapor, rather than from diffusion alone, so a vapor retarder cannot substitute for a continuous air barrier. The two layers are designed together, and codes limit where an assembly may use a Class I retarder to avoid creating a double vapor barrier.
Real Examples
Common Misconceptions
People assume: Every wall needs a vapor barrier on the inside.
Actually: It depends on climate and assembly. In some climates an interior vapor barrier can trap moisture and cause problems.
People assume: A vapor retarder stops air leaks.
Actually: Vapor diffusion and air leakage are different mechanisms. A vapor retarder doesn't replace an air barrier unless it is also continuous and sealed as one.
Frequently Asked Questions
What is the difference between a vapor barrier and a vapor retarder?
Vapor barrier is the older term, implying that vapor is fully blocked. Codes now use vapor retarder, classified by permeance, since few assemblies need a complete block.
What are the vapor retarder classes?
Class I is 0.1 perm or less, Class II is more than 0.1 up to 1.0 perm, and Class III is more than 1.0 up to 10 perms, per the International Building Code and Residential Code.
Where does a vapor retarder go?
Generally on the warm side of the insulation in cold climates. Hot-humid and mixed climates need different strategies, so placement follows the climate zone and a moisture analysis.
Can an assembly have two vapor barriers?
It can, but it is risky because moisture that gets between them cannot dry. Codes and designers limit this and generally allow drying toward at least one side.