Which side does a vapor barrier go on?

Toward the warm, humid side: the interior in cold heating climates, the exterior in hot humid ones, and mixed climates favor forgiving vapor retarders over true barriers. The rule is warm-in-winter side in the north.

Is plastic sheeting in walls a good idea?

In cold climates behind interior drywall, polyethylene has a legitimate history. In mixed and cooling climates, and in any wall with exterior foam, interior poly can trap inward-driven moisture and grow problems. Smart retarders now cover the ambiguous cases.

What is the difference between a vapor barrier and an air barrier?

Vapor barriers slow diffusion through materials; air barriers stop bulk airflow through gaps. Moving air carries far more moisture than diffusion, so air sealing outranks vapor control in most assemblies.

Do crawl spaces need a vapor barrier?

Yes, on the ground: heavy polyethylene over exposed soil is the one placement nearly every climate agrees on, since earth exports moisture continuously into the structure above.

Vapor Barriers: Where They Belong and Where They Cause Trouble

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

Few building components are installed with more confidence and less understanding than plastic sheeting, and vapor barrier placement is where good intentions grow wall problems: a barrier on the wrong side traps moisture exactly where the assembly needs to dry. The governing ideas are two, moisture diffuses toward cold, and assemblies must dry somewhere, and the climate decides the rest. This reference covers the physics, the placement rules, the classic mistakes, and the crawl space case everyone agrees on.

The physics: diffusion, drives, and drying

Water vapor diffuses through materials from warm-humid toward cold-dry, and where it reaches a surface below its dew point inside an assembly, it condenses there, invisible, chronic, and feeding the mold and rot economy in the sheathing and studs. Vapor retarders exist to slow that diffusion on the humid side before it reaches cold surfaces; permeance ratings grade them from true barriers (polyethylene, foil) through retarders (kraft facing, certain paints) to freely breathing materials. The counterweight principle is drying: every wall gets wet eventually, by diffusion, a flashing lapse, or construction moisture, and survives by drying toward whichever side stays open, which is why the cardinal sin is sealing both sides, poly inside plus impermeable layers outside, building a bag that holds whatever water ever enters.

The climate rule

Cold heating climates: the interior is the warm humid side all winter, so the retarder faces in, kraft-faced batts or, in severe cold, interior poly behind the drywall, with the exterior kept relatively permeable so the wall dries outward, the classic northern assembly. Hot humid climates: the drive reverses, outdoor humidity diffusing toward air-conditioned interiors, so vapor control faces out and the deadly detail is interior poly or vinyl wallcoverings, which become condensing surfaces on the back of cool drywall, a documented southern failure mode. Mixed climates, most of the country, see the drive reverse seasonally, and the modern answer is forgiveness rather than force: no interior poly, vapor-retarding paint or smart membranes (materials whose permeance rises when humid, letting walls dry inward in summer while retarding in winter), and assemblies designed to dry at least one direction always. Exterior foam sheathing rewrites the local math by warming the sheathing above dew point, which is why foam-clad walls specifically prohibit interior poly, drying inward being their design.

The air-barrier hierarchy

The quiet headline of modern building science: bulk airflow through gaps moves vastly more moisture than diffusion through materials, a winter’s diffusion through a drywall sheet amounting to cups while the same wall’s electrical penetrations leak quarts on air currents. The ranking follows, air sealing first, always, with vapor control the refinement, and it explains the era’s inspection findings: walls with immaculate poly rotting at outlet penetrations, and attic assemblies whose moisture arrived through the ceiling’s holes, not its paint. It also rehabilitates the humble: sealed drywall with ordinary paint is already a modest vapor retarder and a fine air barrier, which in mixed climates is most of the answer.

The placements everyone agrees on

Crawl space ground cover: heavy polyethylene (6-mil minimum, overlapped and taped, run up piers) over exposed soil, in every climate, because earth exports moisture continuously and the sheet on the ground is the one barrier with no wrong side, the foundation of the encapsulation approach. Under concrete slabs: poly beneath new slabs, standard practice, the reason old slabless-barrier basements sweat and efflorescence blooms. Behind showers and tubs: the wet-area membranes and boards of modern practice, vapor control where bulk water lives. The placements to interrogate instead: poly ceilings below ventilated attics in mixed climates, vinyl wallpaper in cooling climates, and any retrofit that adds an impermeable layer to a wall whose other side is already sealed, the double-bag being the finding that keeps remediators employed.

The bottom line

Retarder toward the warm humid side, cold climates in, hot climates out, mixed climates forgiving; never both sides sealed; air sealing before vapor theology; and poly on every crawl space floor without debate. The wall that can dry survives its mistakes, and the one that cannot keeps them.