1. Why Water Vapor Can Turn to Liquid Inside a Wall
Air often contains invisible water vapor. When that air meets something cold enough, the vapor can turn into liquid drops. That is what happens on a window on a winter morning: warm, damp room air touches cold glass.
Many walls are made of layers, like a sandwich: the inside surface, insulation, and an outer layer facing the weather. In winter the inside layer is warm and the outer layer is cold. Building scientists note that in winter, water vapor often moves from the warm side of a wall toward the cold side. [1] So if damp indoor air gets into the wall, it reaches colder and colder material as it goes. If the material gets below the dew point of the air reaching it, water vapor can condense there.
The trouble is that this happens out of sight. A wet window can be wiped. Water inside a wall can soak the wood and the insulation for months before anyone notices. That is why a dry window is no proof that the wall behind the plaster is dry.
2. How Moisture Gets Into a Wall: Mostly by Riding on Air
You might picture vapor slowly soaking through a solid wall, like water through a paper towel. That does happen, and it is called diffusion. But one building-science paper points out that diffusion, though easy to calculate, is often an insignificant source of moisture in modern buildings. [2]
A bigger carrier is moving air. Warm indoor air that leaks outward through gaps in the wall carries its vapor with it. If the air cools below its dew point inside the wall, some of its vapor can condense into liquid. The same paper says that wintertime condensation from leaking air is a common building problem in cold climates, and that it is one of the two major sources of moisture in the above-ground walls of a building. The other major source is wind-driven rain. [2]
So a first defense is not about vapor at all. It is a layer that stops air from flowing through the wall, called an air barrier. The same sources are careful to separate this from a vapor barrier: a vapor barrier slows the movement of water vapor, while an air barrier stops air from moving, and the two jobs are easily confused. [1]
3. The Two-Part Rule: Keep Water Out, and Let It Out
An engineer who wrote one of the sources sums up the whole job of a building's shell in two requirements: keep water out, and let water out if it gets in. For liquid rain the advice is blunt: it will get in if you build where it rains, so give it a way to drain. For water vapor the principle is the same, in vapor form: keep it out, and let it out if it gets in. [1]
The same paper admits this is not as simple as it sounds, because the best way to keep vapor out can also trap vapor in. [1] Think of a rain jacket that is waterproof on the outside but does not let sweat escape. A layer that blocks vapor can also keep moisture trapped inside a wall that has already gotten a little wet.
4. What a Vapor Retarder Does, and Why It Depends on Climate
A vapor retarder is a layer built into a wall to slow the movement of water vapor. In cold climates it has traditionally been placed on the warm, inside part of the wall, so less indoor vapor reaches the cold layers. The same author argues that this idea is largely a cold-climate habit that spread to other climates without good reason. [1]
The reason is the other half of the rule. A wall also needs to dry out. In a building that is air-conditioned in summer, or in a below-ground room, the warm side can be the outside, and a vapor barrier on the inside blocks drying in the wrong direction. The paper lists several cases where a misplaced vapor barrier made a wall hold water instead of drying. [1] The right design depends on the climate, the season, the materials in the wall, and how humid the rooms inside are, so the paper recommends settings for different climate regions rather than one rule for everyone. [1]
5. How the Layers Work Together to Keep a Wall Dry
Putting it together, a well-designed wall in a cold climate is a set of jobs, not one magic layer:
- An air barrier stops warm, damp indoor air from streaming into the wall. [2]
- A vapor retarder, where the climate calls for one, slows the vapor that does seep through. [1]
- The wall still has a way to drain any rain that gets in, and a way to dry toward at least one side. [1]
The idea is not to seal everything, and not to leave everything open. It is to decide which way moisture may travel and to make sure it is never trapped. Where each layer should sit depends on the local climate, so a layer recipe that works in one country is not automatically right in another.
6. What You Can Notice at Home Without Touching the Walls
You should never open a wall or cut into one. But you can notice the clues that the story above predicts. Is one room's window much wetter than the others? Does one wall corner look colder than the others on a cold day? These can be clues to cold surfaces or moisture problems, but they do not show what is happening inside the wall. If you see a stain, a musty smell, or peeling paint, tell an adult and let them decide whether to call a professional.
If you want to go deeper, read the two papers below. They are written for builders, so some parts are technical, but they show how professionals think about this problem.