Quick answer: "Moisture barrier" can mean four different things. A water control layer (weather-resistive barrier, membranes, flashing) keeps rain and liquid water out. An air barrier stops air leaking through the building, which carries far more moisture than diffusion does. A vapor retarder slows water vapour diffusing through materials; codes classify them by permeance: Class I ≤ 0.1 perm (polyethylene, foil), Class II 0.1–1.0 perm (kraft-faced insulation, some paints), Class III 1–10 perm (latex paint). A thermal layer keeps surfaces warm enough not to condense. Choose and place each one for the climate and the whole assembly; a vapor barrier in the wrong place traps moisture.
Quick answer
- Air leakage moves much more moisture than vapor diffusion: get the air barrier right first.
- Vapor retarder classes: I ≤ 0.1 perm, II 0.1–1.0, III 1–10.
- Cold climates: vapor control on the warm (interior) side; hot-humid climates: avoid interior vapor barriers.
The four control layers
| Layer | Controls | Examples |
|---|---|---|
| Water control | Rain, liquid water, groundwater | Weather-resistive barriers, membranes, flashing, sub-slab drainage |
| Air control | Air leakage (and the moisture it carries) | Sealed sheathing, membranes, tapes, gaskets |
| Vapor control | Vapor diffusion | Polyethylene, foil, kraft facing, vapor-retarder paints, smart membranes |
| Thermal control | Heat flow; keeps surfaces above dew point | Insulation, continuous exterior insulation |
In building science, these are usually listed in order of importance: water first, then air, then vapor, then thermal control.
Vapor retarder classes
| Class | Permeance | Examples |
|---|---|---|
| I (vapor barrier) | ≤ 0.1 perm | Polyethylene sheet, foil, sheet metal |
| II | > 0.1 to ≤ 1.0 perm | Kraft-faced batts, some vapor-retarder paints |
| III | > 1.0 to ≤ 10 perm | Latex paint, some membranes |
(1 US perm ≈ 57 ng/(s·m²·Pa).)
Where to put vapor control
| Climate | Typical approach |
|---|---|
| Cold | Vapor control on the interior (warm) side; avoid a second impermeable layer outside |
| Mixed | Class II or III, or smart (variable-permeance) membranes |
| Hot and humid | Avoid interior vapor barriers (no poly behind drywall); vapor drives inward in summer |
| Slabs on grade | Vapor retarder under the slab; see concrete moisture barriers |
| Crawl spaces | Ground vapor retarder; see crawl space vapor barrier installation |
Related: concrete moisture barriers, crawl space vapor barrier installation.
Local building codes specify vapor retarder requirements by climate zone; follow them.
Common mistakes
- Two vapor barriers in one wall (inside poly plus impermeable outside sheathing), trapping moisture between them.
- Interior poly in hot-humid climates, where summer vapor drives inward and condenses on the back of the poly.
- Treating a vapor retarder as an air barrier without sealing seams and penetrations.
- Relying on the barrier and ignoring rain leaks, which are the most common moisture source.
Where indoor humidity control fits
Even a well-built envelope gets wet if indoor humidity is high for long periods: in pools, kitchens, industrial wet processes, crowded buildings, and in winter in cold climates where indoor moisture drives outward. Keeping indoor RH at a level suited to the climate and use reduces the moisture pushing into walls and roofs. The US EPA notes that moisture control is the key to mold control (US EPA, Mold course chapter 2). For high-humidity buildings, see our industrial dehumidifiers and indoor pool dehumidification.
FAQ
Frequently asked questions
What is the difference between a vapor barrier and a vapor retarder?
A vapor barrier is a Class I vapor retarder (≤ 0.1 perm). "Vapor retarder" covers Classes I to III.
Is an air barrier the same as a vapor barrier?
No. An air barrier stops air leakage; a vapor retarder slows diffusion. Some materials do both if sealed.
Where should a vapor barrier go?
In cold climates, on the warm (interior) side of the insulation. In hot-humid climates, usually not on the interior.
Do I need a vapor barrier under a concrete slab?
Usually yes, especially under moisture-sensitive flooring.
Sources
- US EPA, Mold course chapter 2.
- International Residential Code, vapor retarder classes and requirements.
