Article by Dekalbklcja
The foundation, walls, windows, and roof of a building make up an “envelope” intended to shelter people, equipment, and furnishings from the weather and from natural and manmade hazards. Windows and doors allow outside air, light, people, equipment, and supplies to enter or exit a building. Skylights allow in natural light or daylight. Building envelopes can be designed for natural ventilation, for mechanically conditioned air systems, or for some combination of these. Whether planned or not, buildings have multiple openings that allow the penetration and internal movement of air, water, and contaminants.
Building assemblies exist in a dynamic environment. Some materials have the ability to store moisture and subsequently dry without harmful affects. Excess moisture can be controlled when a healthy balance or equilibrium is maintained between the rates of entry and removal.
For example, masonry construction incorporating a drain screen in the walls can provide effective moisture control through a balance of storage capacity and high drying potential. Problems with building assemblies arise only when assemblies accumulate moisture faster than their ability to store and dry without associated degradation of performance: Because steel framing and gypsum wallboard have virtually no storage capacity, a small leak can quickly become a large problem.
A complex set of moisture-transport processes related to climate, building design, construction, operation, and maintenance determine whether a building will have excess moisture that could influence the health of the occupants. The approach used by many building scientists to understand and diagnose moisture transport is termed “source-path-driving force” analysis: For any particular case, there is a source of moisture, a pathway moisture follows, and a force that drives moisture along that pathway. If a building designer is able to control at least one of the three elements in this chain, moisture can be effectively controlled. Controlling more than one element provides for a valuable redundancy.
Effective moisture management considers the potential damage and degree of risk associated with each of the following four transport mechanisms (from most to least potent): 1. Bulk transport, 2. Capillary transport, 3. Air transport, and 4. Vapor diffusion. Bulk transport is the liquid flow of rain, snowmelt, or groundwater into a building envelope under the influence exerted by envelope under the earth( gravity, hydrostatic pressure, wind, of pressure differences or air pressure. It is the most significant moisture transport mechanism that must be addressed by designers.
Capillarity is the wicking of liquid through the pore structure of a material. Wood, concrete, brick, and mortar are able to draw water into their porous structures in a manner similar to a sponge. Below-grade building assemblies like foundation walls, footings, and slabs are particularly sensitive because they are in contact with wet soil and standing water. Water drawn through these assemblies evaporates into the inside space, elevating interior humidity levels. Above-grade components are at capillarity risk too. Rain and splashback on exterior walls can be drawn into the envelope through capillary pathways, such as overlaps in siding, pores in wood and masonry materials, and joints between otherwise nonporous materials.
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