Air barriers, vapor control and condensation · Lesson 1 of 4

Air barriers, vapor control and condensation: principles and terminology

What condensation needs, how moisture moves through a wall by air and by diffusion, why leaks usually matter more in cold weather, the special cases where they do not, and who is responsible for each decision.

You will be able to: Define dew point, frost point, vapor drive, diffusion and air transport, say why air leakage usually causes cold-weather condensation, separate an air barrier from a vapor retarder, and state who owns each decision.

13 min · 9 slides · 11 questions

All lessons in this module

Slide 1 of 9

What condensation needs

Condensation happens when a surface drops below the dew point of the air it is exposed to.

Lowering the indoor humidity lowers the dew point of the indoor air; warming the surface lifts it above the dew point. Either move reduces the risk.

Building America Solution Center names the interior face of the sheathing as the typical primary condensing surface of a cold-climate wall.

Sources

Condensation Control in Walls - Cold Weather (last updated 2021-09-26) - US DOE Building America Solution Center (PNNL)
Building Science Digest 163: Controlling Cold-Weather Condensation Using Insulation (effective November 15, 2011) - Building Science Corporation (John Straube)

Videos for this lesson

What this video shows: Building Science Corporation on why heat, moisture and air move the way they do, and where vapor condenses. Verified from the video’s own description.

Video: Cup of Joe - Second Law of Thermodynamics by BuildingScience-com Corporation, on YouTube. Chosen from the video's published description.

Glossary

Dew point: The air temperature at which air holding a given amount of water vapor is saturated. A surface colder than the dew point of the air touching it can collect condensation.
Frost point: Below 0 °C, vapor deposits as frost at the frost point, which is warmer than the dew point of the same air. The AFS Envelope tool screens against it when the indoor dew point is below freezing.
Diffusion: Water vapor moving through a material from a higher vapor concentration to a lower one, slowed by the material’s permeance.
Air transport: Moisture carried as vapor by air that moves through gaps, cracks and openings in the enclosure from higher to lower air pressure.
First condensing surface: The surface in a wall where humid indoor air or vapor is most likely to condense; in a cold-climate wall the primary condensing surface is typically the interior face of the sheathing.
Air barrier: A system of materials designed and constructed to control airflow through the enclosure. It is a different layer from a vapor retarder, which controls diffusion.