Moist air also leaks into basements through floor and slab perimeters, wall joints, cracks, windows, and around drains. Winter stack forces can act to depressurize the basement area, drawing moist air into the basement through these pathways. Moist air is subsequently directed to the upper regions of a structure. Since below-grade moisture is one of the most powerful sources of moisture in conditioned spaces, building dry foundation assemblies can be one of the most effective strategies used to control interior airborne moisture.The easiest way to control diffusion is by installing vapor impermeable materials on the side of the assembly with the highest vapor pressure.

Building codes consider materials with a permeability rating of 51 to be vapor barriers. As a general rule, designers should position vapor barriers toward the inside surface in heating-dominated climates and toward the outside surface in cooling-dominated climates. Many building codes and architectural standards require seams and holes in vapor barriers to be sealed to form a continuous, uninterrupted line of protection. However, effectiveness depends on a material’s vapor permeability and surface area covered. In other words, if 95 percent of an envelope surface is covered with a vapor barrier, the barrier is 95 percent effective as a vapor diffusion retarder.This relationship provides designers and builders with some leeway, suggesting as it does that diffusion barriers need not be installed perfectly.However, in order for air barriers to be effective, they must be continuous and durable.In some cases diffusion can drive moisture through an envelope from the inside and from the outside. A structure built in a locality where a balanced mix of heating and cooling is required is one example. A foundation assembly where significant amounts of moisture exist inside and outside the envelope is another example of a situation where moisture can diffuse in both directions.

And porous claddings like masonry and wood that become rain soaked and subsequently exposed to solar radiation give rise to vapor driving inward, even in a heating-dominated climate. This type of diffusion will affect the design of vapor barriers. Buildings should be designed such that moisture can dry toward both the inside and the outside.Box 3.2 lists design measures that could be incorporated into green school guidelines to ensure appropriate moisture management as it relates to a building’s envelope. Excellent resources for proper moisture control design include The Moisture Control Handbook: Principles and Practices fo? Residential and Small Commercial Buildings by Joseph Lstiburek and John Carmody, The Building Foundation Design Handbook, and Moisture Control in Buildings. Building commissioning can also be an effective way to identify and preempt potential moisture problems in schools. In addition to bringing potential health benefits, designing for effective moisture management will probably have benefits for the building itself.

The more durable a building is, the longer its components will last. Materials in long-lived building assemblies are replaced less frequently than those in nondurable structures. This makes dry structures resource-efficient and energy-efficient, because no replacement materials need be harvested, mined, or produced, nor is energy used to make, transport, or assemble the replacement components. Dry buildings also require fewer resources and money for repair and maintenance. For example, damp surfaces cause stains and peeling paint, which necessitate frequent repainting and cleaning. For these reasons, dry buildings may have lower life-cycle costs, in addition to offering potential health benefits for occupants.More research is needed on the moisture resistance and durability of materials used in school construction. Such research should also investigate other properties of these materials, such as their generation of bioaerosols and indoor pollutants as well as the environmental impacts of producing and disposing of them.