Overview
Vapor control is a different job from the rest of the wall. The weather-resistive barrier (covered in the house-wrap and weather barriers guide) stops bulk and liquid water and, paired with an air barrier, stops air movement. Vapor control deals with water in its gaseous form: water vapor diffusing slowly through solid materials, driven by differences in temperature and humidity. That is a smaller flow than a leak or a draft, but get it wrong and it condenses inside your wall and rots the framing. The metric that matters is the perm rating, which measures how much water vapor passes through a material. Lower perm means less vapor gets through. Building science sorts materials into classes: a Class I vapor barrier is impermeable, under 0.1 perm, like a polyethylene sheet or foil facing. Class II is semi-impermeable, roughly 0.1 to 1 perm. Class III is semi-permeable, roughly 1 to 10 perm, like latex paint on drywall. Anything over 10 perm is vapor-open, like most housewraps. I rank this among the easiest things in residential construction to get dangerously wrong, because the right answer flips depending on your climate, and a confident installer can rot a wall while doing exactly what worked somewhere else.
Why It Matters (and Why It Is Dangerous to Get Wrong)
Water vapor moves from warm and humid toward cold and dry. In a heating climate in winter, that means vapor pushes from your warm, humid interior outward into the cold wall cavity. If that vapor reaches a cold surface inside the assembly, it hits its dew point and condenses into liquid water. Now you have water sitting in the framing. A vapor barrier placed on the correct side stops the vapor before it cools and condenses. A vapor barrier on the WRONG side does the opposite: it lets moisture in, then traps it against a cold surface where it cannot escape. That trapped water wets the studs and sheathing, feeds mold, and rots the frame from the inside where you cannot see it. The classic disaster is interior polyethylene sheeting installed in a cooling-dominated climate. There the vapor drive is inward most of the year, so the interior poly traps moisture coming from outside and the wall stays wet. The principle that keeps you out of trouble is simple to state: every assembly must be able to dry in at least one direction. If it cannot dry anywhere, any moisture that gets in stays in, and the wall is on a clock.
Perm Ratings and Vapor Classes
The classes describe how readily a material passes vapor. Class I, impermeable, is under 0.1 perm: polyethylene sheet, sheet metal, foil facing. This is a true vapor barrier, it stops diffusion cold and gives the wall no path to dry through it. Class II, semi-impermeable, runs roughly 0.1 to 1 perm: the kraft paper facing on a faced batt is the common example, and unfaced extruded polystyrene falls here too. Class III, semi-permeable, runs roughly 1 to 10 perm: ordinary latex paint on gypsum drywall lands in this band, which is why painted drywall acts as a mild vapor retarder in many modern walls. Vapor-open means over 10 perm: most housewraps and some specialty membranes, which let the wall breathe and dry freely. Then there are variable-perm, or smart, vapor retarders. These change behavior with humidity: tight and retarding when conditions are dry, then opening up to become vapor-open when the surrounding material gets humid. That lets a wall resist vapor in winter but still dry out if it ever gets wet. Knowing which class a product is, and which way it faces, is the whole game.
Climate Drives Which Side (If Any)
Climate decides everything here, because climate decides which way vapor wants to move. In cold, heating-dominated climates the drive is outward in winter, so you put the vapor retarder toward the warm INTERIOR. That keeps humid indoor air from pushing into the cold wall and condensing. In hot-humid, cooling-dominated climates the vapor drive is INWARD most of the year, pushed by heat and humidity outside and air conditioning inside. An interior vapor barrier there is a disaster: it traps moisture coming from outdoors against the cool back of your drywall. In those climates you want the wall vapor-open to the inside so it can dry toward the conditioned space. Mixed climates are the trap, because the drive reverses with the seasons. There the common move is no interior vapor barrier at all, letting the wall dry both ways, or a smart retarder that adapts. Running air conditioning versus heating reverses the drive, which is why a detail that is correct in Minnesota will rot a wall in Houston. The IECC ties allowed vapor retarder classes to climate zone for exactly this reason. Find your zone first; it dictates the rest.
Getting It Right in Real Assemblies
In practice, a handful of products do most of the work. Kraft-faced batts carry their own Class II vapor retarder: the paper facing IS the retarder, so face it toward the warm side. In a cold climate that means facing toward the interior; flip that and you have built the wrong-side barrier that rots walls. Polyethylene sheeting belongs only in cold climates, on the interior, behind the drywall. Outside a heating-dominated zone it is a liability. Closed-cell spray foam is its own vapor retarder once it reaches a few inches of thickness, so it controls vapor from inside the cavity and changes how the rest of the wall must dry. Smart membranes are the modern default I reach for, because their variable perm gives you winter protection and summer drying without betting the wall on a single fixed number. The cardinal rule sits above all the products: let the wall dry in at least one direction, and never sandwich the assembly between two barriers. A double vapor barrier, say poly inside and foil-faced foam outside, traps any moisture that gets in and guarantees rot. In many current code-compliant walls, painted drywall is the only vapor retarder present, working as a Class III, and that is enough.
How to Choose
Start by identifying your climate zone, because that single fact decides almost everything that follows. Then map your assembly: what it is made of, and which direction it can dry. In a cold climate, an interior vapor retarder is appropriate, kraft facing or a smart membrane toward the warm side. In a hot-humid climate, skip the interior vapor barrier entirely and keep the wall open to dry inward toward the air conditioning. In a mixed climate, use a smart retarder or no interior vapor control at all, and let the wall dry both ways across the seasons. Never build a double barrier, no matter the zone. Check the IECC vapor retarder requirements for your specific zone before you commit, since the allowed classes are spelled out there. One more thing that matters more than any of this: air leaks carry far more moisture into a wall than diffusion ever will. A gap around an outlet or a top plate can move more water vapor in a day than diffusion moves in a season. Seal the air leaks first, using the air barrier detailing from the house-wrap and weather barriers guide, and vapor diffusion becomes the smaller, slower problem it should be.
FAQ
What is the difference between a vapor barrier and a vapor retarder? It is the perm rating. A vapor barrier is Class I, under 0.1 perm, and essentially stops diffusion, like poly or foil. A vapor retarder is Class II or III, roughly 0.1 to 10 perm, and slows vapor without fully blocking it, like kraft facing or latex paint. The terms get used loosely, but the class is what determines how a wall behaves.
Do I need a vapor barrier in my wall? Often no. In cold climates an interior vapor retarder helps, but in mixed and hot-humid climates an interior vapor barrier is a liability that traps moisture. Many code-compliant walls rely only on painted drywall as a Class III retarder. Find your climate zone and check the IECC before adding one.
Why is poly sheeting on the inside sometimes bad? Interior poly is a Class I barrier that blocks drying to the inside. In a cooling-dominated or mixed climate, where vapor drives inward, that poly traps moisture against the back of the drywall and the wall cannot dry, which leads to mold and rot. It belongs only in cold, heating-dominated climates.
Is a vapor barrier the same as house wrap or an air barrier? No. House wrap is the weather-resistive barrier that stops bulk water, and paired with sealed seams it stops air. Vapor control manages slow diffusion of water vapor through materials. They are separate jobs, and air leaks move far more moisture than vapor diffusion, so seal the air first.