Water Vapor Control Basics

Diffusion/Vapor Pressure — by BSC — last modified 2007/04/10

Diffusion is the movement of heat or moisture along a concentration gradient. Although this is correct scientific definition, most people in the building industry are concerned only with water vapor diffusion, and the potential for it to cause condensation wetting during cold weather. This is a misplaced focus. Diffusion of water vapor is important, but its role in drying is the most significant, and the potential for diffusion-driven condensation during hot weather is also important. To understood vapor diffusion and the proper use of vapor control layers like vapor barriers and vapor retarders one needs information about vapor in air, how vapor moves, and the laws of diffusion.

Vapor Permeance — by BSC — last modified 2007/04/10

 

Convection/Air barriers — by BSC — last modified 2007/04/10

Although there are many reasons to control air movement within and through buildings, the water vapor that is transported along with air is a major concern. Depending on the direction and rate of airflow, as well as the conditions along the flow path, air flow can cause condensation or avoid, wetting enclosures or dry them, and add humidity to a space or remove it.

Interstitial Condensation — by BSC — last modified 2007/04/10

Hidden condensation in voids, cavities and gaps, can, if undetected, accumulate over time and result in major mold, corrosion, decay, and freeze-thaw damage. Interstitial condensation is primarily caused by the leakage of uncontrolled air flow, but vapor diffusion and even night-sky radiation can also be the culprit. Once one has identified and understood the source and cause, solutions to the problem tend to be clear.

Drying — by BSC — last modified 2007/04/10

A durable and healthy building must balance the wetting that inevitably occurs with the same amount of drying to avoid accumulation and certain failure over time. Different climates and different assemblies offer the potential for vastly different quantities of drying. Drying by drainage, surface evaporation, diffusion, air flow and ventilation can all be employed to provide the drying required. However, enclosures and building systems must be carefully designed and properly constructed to ensure this drying occurs without compromising the many other functions of the building and its enclosure.

Airspaces — by zope — last modified 2006/02/26

 

Moisture storage — by zope — last modified 2006/02/26