A vapor barrier is a material that slows or stops water vapor from diffusing through walls, roofs, and floors. Its resistance is measured in perms, and Class I materials such as polyethylene sheeting block vapor almost completely. In cold climates it goes on the warm interior side of the insulation; in hot, humid climates it belongs toward the exterior, or is left out entirely.

Few building products cause as much confusion, or as much hidden damage, as this thin sheet of plastic. Installed on the correct side of an assembly, a vapor barrier prevents condensation inside walls and roofs. Installed on the wrong side, it traps moisture, feeds mold, and rots framing from the inside out.

This article covers what a vapor barrier actually does, how perm ratings and vapor retarder classes work, and exactly where the layer belongs in walls, roofs, crawl spaces, and slabs depending on your climate.

What Does a Vapor Barrier Actually Do?

Water vapor moves through a building assembly in two ways: by air leakage and by diffusion. Air leakage carries far more moisture. A small gap around an electrical box can move a hundred times more water vapor into a wall cavity than diffusion through the entire drywall surface. Diffusion is the slower process, where individual vapor molecules pass directly through solid materials, driven by differences in temperature and humidity.

A vapor barrier addresses only the second mechanism, diffusion. An air barrier addresses the first. The two jobs are often confused because some products, such as taped polyethylene or fully adhered membranes, can perform both roles. They remain separate functions, and a wall needs its air barrier detailed continuously even when no vapor barrier is required at all.

The technically correct term is vapor diffusion retarder, since most materials slow vapor rather than stop it. The U.S. Department of Energy uses this term in its Building Science Education guidance on vapor barriers, which notes that in most mild and hot U.S. climates a dedicated barrier is unnecessary, while colder regions do need one. In everyday practice, "vapor barrier" survives as the catch-all name.

Perm Ratings and Vapor Retarder Classes

Vapor resistance is measured in perms. One perm equals one grain of water vapor passing through one square foot of material per hour, per inch of mercury in vapor pressure difference. The lower the perm rating, the tighter the material.

Building codes in the United States sort materials into three classes based on that rating. The table below summarizes the system used in the International Residential Code and in building science practice.

Vapor Retarder Classes at a Glance

The following table shows each class, its perm range, and typical materials:

Class	Perm Rating	Example Materials	Behavior
Class I	0.1 perm or less	Polyethylene sheet, aluminum foil, sheet metal, rubber membrane	True vapor barrier, nearly impermeable
Class II	Over 0.1, up to 1.0 perm	Kraft-faced fiberglass batts, some smart membranes in dry conditions	Semi-impermeable retarder
Class III	Over 1.0, up to 10 perms	Latex or enamel paint on gypsum board	Semi-permeable, allows meaningful drying
Vapor open	Over 10 perms	Housewrap, unpainted gypsum, mineral wool	Permeable, dries freely in both directions

The full test procedure is published by ASTM International as ASTM E96/E96M, Standard Test Methods for Gravimetric Determination of Water Vapor Transmission Rate of Materials. When you compare product data sheets, confirm the perm values come from the same test method before treating them as equivalent.

A newer category, the smart vapor retarder, changes permeance with humidity. Polyamide-based membranes act like a Class II retarder when the air is dry in winter, then open up to 10 perms or more when humidity rises, letting the wall dry toward the interior in summer. This adaptive behavior makes them a safer choice than polyethylene in most mixed and cold climates.

Where Does a Vapor Barrier Go in a Wall?

The short rule: a vapor barrier goes on the warm-in-winter side of the insulation in cold climates, toward the exterior in hot and humid climates, and often nowhere at all in mixed climates. Vapor drives from warm, humid air toward cold, dry air, and the barrier must face the side where the moisture load originates.

Building scientist Joseph Lstiburek summarizes the goal in Building Science Corporation's digest BSD-106: Understanding Vapor Barriers: keep water out, and let it out if it gets in. An assembly that can dry in at least one direction survives mistakes. An assembly wrapped in impermeable layers on both sides does not.

Cold Climates (Zones 5 and Higher)

Interior air is warm and humid in winter, so vapor pushes outward into cold wall cavities. Place a Class I or Class II retarder on the interior side, directly behind the drywall, with insulation outboard of it. Kraft facing, a smart membrane, or in very cold and subarctic regions polyethylene, all work here. Keep the exterior side of the wall vapor open, or add enough exterior rigid insulation to keep the sheathing warm and condensation-free. Panelized systems behave differently: the OSB facings of structural insulated panels already act as Class II retarders, so these walls need careful joint sealing rather than an added sheet.

Hot and Humid Climates (Zones 1 to 3)

The vapor drive reverses. Outdoor air at 90 degrees and 75 percent relative humidity pushes moisture inward toward interior spaces cooled to 72 degrees. Any vapor control belongs toward the exterior, and the interior finishes must stay permeable so the wall can dry inward. Vinyl wallpaper and interior polyethylene are the classic failures here. Many assemblies in these zones need no dedicated vapor barrier at all, just a good drainage plane and airtight construction.

Mixed Climates (Zone 4)

Vapor drive flips seasonally, so committing to an impermeable layer on either side creates a wrong-side condition for half the year. Class III interior finishes, standard painted drywall in other words, combined with a ventilated cladding gap usually perform best. This is where rainscreen cladding systems earn their keep, since the air gap lets the assembly shed both rain and diffused vapor. Smart membranes are the low-risk upgrade when insulation levels are high.

Vapor Barriers in Roofs, Crawl Spaces, and Slabs

Walls get the attention, but the other assemblies follow the same physics with a few twists.

• Vented attics: the ceiling air barrier matters far more than any vapor layer. In cold climates a Class II or III retarder at the ceiling is typical; ventilation above the insulation removes what diffusion lets through.
• Unvented (conditioned) roofs: vapor control depends on the foam type and climate; impermeable membranes on both faces of the assembly are the main hazard to avoid.
• Crawl spaces: the ground releases water vapor continuously, so a Class I polyethylene ground cover is standard in nearly every climate.
• Slabs on grade: a sub-slab vapor barrier, typically 10 to 15 mil polyethylene under the concrete, blocks ground moisture that would otherwise wick into flooring adhesives and finishes.

Material choice interacts with the whole envelope strategy. Vapor-open wall systems built with hempcrete or mineral wool are designed to buffer and release moisture, and adding a plastic sheet to them defeats the concept. Indoor humidity control matters too, since natural and mechanical ventilation removes the cooking and bathing moisture that would otherwise load the assembly in the first place.

How to Choose the Right Vapor Control Layer

Work through these questions in order for any wall or roof you are detailing:

1. Which climate zone is the project in, and which direction does the dominant vapor drive point?
2. Is the air barrier continuous? Fix air leakage first; it moves more moisture than diffusion ever will.
3. What perm ratings do the existing layers already have? Sheathing, foam, and facings often provide the control layer without an added product.
4. Can the assembly dry in at least one direction in every season? If not, swap a Class I layer for a Class II smart membrane or a Class III finish.
5. Does the local code mandate a specific class for your zone? Codes set the floor, and inspectors will check.

Building codes and vapor retarder requirements vary by jurisdiction. Confirm assemblies with your local authority and a licensed design professional before construction.

Putting It All Together

Bottom Line: A vapor barrier is a diffusion control layer rated at 0.1 perm or less, and its position depends entirely on climate: interior side in cold regions, exterior side or omitted in hot and humid ones. Get the air barrier continuous first, keep at least one drying path open, and reach for smart membranes when the vapor drive changes with the seasons.