Capillary break vs vapor barrier
The difference between a capillary break and a vapor barrier: a capillary break stops liquid water wicking up through material. A vapor barrier slows water vapour diffusing through an assembly.
Side by side
| Axis | capillary break | vapor barrier |
|---|---|---|
| What it stops | Liquid water moving through pores against gravity. | Water vapour diffusing through a material with the pressure gradient. |
| Where it goes | Between footing and foundation wall, or under a slab. | On one face of the insulation — which face depends on the climate. |
| What happens if it is missing | Ground moisture feeds the structure continuously. Interior drying never resolves it. | Vapour accumulates in the assembly seasonally, and may or may not dry out. |
| What happens if it is wrong | There is no wrong side — it is present or it is not. | On the wrong face it traps moisture instead of excluding it, and makes things worse. |
| Can it be added later? | Rarely, without lifting the structure. Retrofits work around it instead. | Yes, during any assembly renovation — and often what causes the problem. |
One is about the ground, the other about the seasons
If a basement wall is damp at the base year-round and no leak explains it, you are looking at a missing capillary break. If a wall is dry most of the year and wet in one season, or if growth appeared after insulation was added, you are looking at a vapour problem. The two rarely need the same fix.
Why a missing capillary break cannot be dried out
Capillary action is continuous. It does not depend on rain, a leak or a season — it draws water from wet ground into porous masonry and keeps drawing as long as the ground is wet, which in most places is most of the time.
A dehumidifier in that basement removes water from the air, which lowers the humidity, which increases the rate at which the wall gives up water, which increases the rate at which the ground replaces it. The machine runs continuously and the wall never dries, because the wall is not the reservoir — the ground is.
The fixes are exterior: managing surface water away from the foundation, and interior finishes that tolerate a damp wall rather than trapping it. Framing and drywall against that wall is the failure mode this describes.
Why a vapor barrier on the wrong face is worse than none
Vapour moves from warm to cold. In a heating-dominated climate that is outwards in winter, so the retarder belongs on the warm inside face. In a cooling-dominated climate with air conditioning it is inwards through summer, and an interior barrier becomes the cold surface vapour condenses on.
Mixed climates get both, which is why modern practice leans towards assemblies that can dry in at least one direction rather than towards more impermeable layers.
The practical signal is timing: growth that appears on the inside face of an exterior wall after a retrofit, particularly on a wall that was fine for decades, is usually an assembly that has lost its ability to dry.
Questions people ask
- Does polyethylene sheeting under a slab count as both?
- It functions as both there — it blocks capillary rise and vapour diffusion from the soil. Under a slab the two problems share one solution; in a wall assembly they do not.
- My basement smells musty but the walls feel dry. Which is it?
- Either can produce it. Feel the wall at the base and again at chest height: a moisture gradient that is strongest low down points at ground water rather than at a vapour problem.
- Is a vapor barrier the same as a vapor retarder?
- In practice the words are used interchangeably. Building codes distinguish classes by permeability, and "retarder" is the more accurate word for most of what gets installed.
Other pairs that get confused
- water activity vs relative humidity
- barrier EIFS vs drainable EIFS
- polybutylene vs PEX
- moisture risk vs mold risk score
Both terms are defined in the glossary.