Why Is the Vapor Control Layer Important?
Mold, mildew, and rot — Excessive vapor diffusion into the building enclosure can lead to the accumulation of significant amounts of moisture, which can condense into water and result in mold and mildew growth and rot in structural and insulation materials.
Degradation of thermal performance — Accumulation of moisture in the thermal control layer due to the condensation of excessive vapor diffusion can dramatically lower the performance of many types of insulation.
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Key Concepts for the Vapor Control Layer
Vapor drive — The rates and directions of vapor drive are based on several factors:
Concentration gradient — Movement from high to low absolute humidity; related to vapor pressure.
Temperature gradient — Movement from high to low temperature.
Air pressure gradient — Movement from high to low air pressure.
Combined, these factors create a vapor drive, forcing water vapor molecules to diffuse through building enclosure materials in the direction of the vapor drive.
Vapor drive is governed by humidity, temperature, and air pressure. Accordingly, the vapor drive changes in both direction and intensity over the course of any given day. Often, relative humidity and temperature will induce a drive in one direction, overriding relative air pressure pushing in the opposing direction, as is pictured here (higher relative humidity and temperature outdoors, higher air pressure indoors).
Diffusion — The movement of water through a material. Diffusion can occur in two main ways: vapor diffusion and liquid diffusion. The water control layer addresses liquid diffusion, and vapor diffusion is addressed by the vapor control layer.