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Air and Vapor Barrier Basics

John Huang, Ph.D., PE and John Poullain, PE

This three-hour online course provides guidance on the basics of moisture and condensation problems in buildings and the typical causes. The course discusses how vapor pressure and surface temperatures interact to cause condensation, discusses the relationship between temperature, relative humidity and moisture and shows their combined influences with an ASHRAE psychrometric chart. The course presents ways to identify the sources of moisture, mold related health effects and how various building wall cavities can protect against condensation. Sources for additional information on the course topics are provided.

Air transported moisture is a very destructive mechanism. It is a bigger problem than vapor diffusion because diffusion, a slow process, does not contribute the vast amount of moisture that is air transported. Air often has a considerable amount of moisture in vapor form and as it moves from inside to outside or outside to inside in a building the vapor moves along with it. Air movement is caused by air pressure differences from wind or stack effects on the building and also depends on whether itís the heating or cooling season time of year. If a surface within the wall or ceiling cavity is below the dew point temperature, condensation will form, water may drain to the bottom of the wall, mold will grow and insulation and structural damages may occur from repeated cycles. Also condensation may collect behind the siding, absorb chemicals from the siding and stain it. If the siding becomes wet enough the paint will blister.

Wall assemblies are designed to perform several functions primarily for energy efficiency and for weather proofing the building and its interior. The problems caused by condensation and water must also be considered in order to construct a sound building and solve these problems. Condensation in a building or wall will depend on several factors: climate (hot-humid or dry, mixed, marine or cold), exterior and interior temperatures, relative humidity, annual precipitation and if itís the heating or cooling season. Wall systems as shown in the text have been developed to meet various climatic conditions, i.e. hot, cold, very cold or subarctic. Wall systems vary with different types of exterior materials (brick, stucco, siding or precast concrete), wall constructions (wood, steel or concrete block) but similar types of air and vapor barriers and drainage planes are used.

Air barriers are distinguished from vapor barriers because they control the flow of air by resisting differences in air pressure. Water vapor will naturally move from higher pressures to lower pressures through air barriers but is resisted by vapor barriers depending on the perm rating. Effective barriers must be installed properly at all doors, windows and any other openings in the building envelope to completely enclose the air within a building. Many air barriers are also vapor barriers and vice versa.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

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