E3 Humidity, Vapor and Condensation

Topic

Air always contains a certain amount of water vapor, and relative humidity indicates how close that amount is to the maximum the air can hold at a given temperature before becoming saturated. When there is a humidity difference between a building's interior and exterior, that difference drives water vapor to migrate from the more humid environment toward the drier one, passing through building materials at a speed determined by each material's resistance to this diffusion: some barely impede it, while others act almost as barriers.

The problem arises when this vapor, while moving through a wall or roof, reaches a zone where the temperature has dropped below the dew point; at that stage, the vapor condenses within the building assembly itself rather than on a visible surface. This interstitial condensation dampens the insulation and damages materials—often going unnoticed until the deterioration is already advanced. The Glaser method is a tool used to anticipate this issue before it occurs; it compares the expected temperature and vapor pressure—layer by layer within the assembly already defined by the heat transfer model—to determine whether the vapor will condense at any point along its path.

The standard solution addresses the root cause rather than the symptom: installing a vapor barrier—a material virtually impermeable to vapor—on the warm side of the insulation prevents vapor from advancing into the assembly's cold zone, where it would risk condensing, thereby halting the migration before it reaches the danger zone.

Thus, this module—which begins by describing vapor migrating from a humid environment to a dry one—concludes by showing that this very movement (invisible while occurring inside a wall) determines whether insulation lasts for decades or is ruined in just a few years, often without anyone noticing until it is too late.

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