The rule: a vapor retarder goes on the warm-in-winter side of the insulation.
In a heating climate that is the interior. Warm indoor air holds much more moisture than cold outdoor air, so vapor drives outward, and the retarder stops it before it reaches a surface cold enough to condense.
Why the rule reverses
In a cooling-dominated climate the drive is inward — hot humid outdoor air toward a cool dry interior. An interior polyethylene sheet is then on the cold side, and moisture condenses on its outer face, inside the wall.
That is why interior poly is standard practice in Canada and a genuine hazard in Florida.
Mixed climates have no fixed answer
Zones 4 and 5 heat in winter and cool in summer, so the vapor drive reverses seasonally. Any fixed impermeable layer is wrong for part of the year.
Two solutions:
Smart vapor retarders — membranes whose permeance changes with humidity. Closed in winter when indoor air is dry, open in summer to let the assembly dry inward. This is what MemBrain and similar products do, and it is the standard answer in mixed climates.
Design for drying, not for stopping. Use kraft-faced batts (about 1 perm, a retarder rather than a barrier) and make sure at least one side of the assembly can dry.
The distinction that matters
- Vapor barrier: 0.1 perm or less. Polyethylene, foil.
- Vapor retarder, Class II: 0.1–1.0 perm. Kraft facing.
- Vapor retarder, Class III: 1.0–10 perm. Latex paint.
Most assemblies want a retarder, not a barrier. Slowing vapor is enough; stopping it removes the assembly's ability to dry, and everything eventually gets wet somehow.
Air sealing does more than vapor control
Far more moisture moves through an assembly carried by air leaks than by vapor diffusion — commonly by a factor of ten or more. A perfect vapor barrier full of holes for wiring is worse than no barrier and a well-sealed air barrier.
If you have to choose where to spend care, spend it on the air barrier.
Where a real barrier does belong
Under a slab. 10–15 mil polyethylene directly beneath concrete, seams lapped and taped. There is no drying direction downward, and the ground is an infinite moisture source.
On a crawlspace floor. 6 mil minimum across the whole ground surface, lapped up the walls and sealed. This is the single most effective moisture measure in a crawlspace, ahead of any amount of ventilation.