Heat can transfer in various ways, such as conduction, convection, or radiation; and it moves from areas of high to low temperatures independent of orientation. Conduction is the flow of heat through solid materials, such as window frames or metal studs, whereas convection is the transfer of heat through a gas or liquid, such as air, and occur naturally. Natural convection in buildings is the result of differing air densities, while forced convection can be generated by mechanical systems of variations in exterior and interior pressures. Radiation is the transfer of heat due to emission of electromagnetic waves. Thermal transfer is typically controlled by incorporating insulating components, limiting thermal bridges, as well as regulating solar radiation. The use of an air barrier system can also impact the thermal efficiency of an enclosure by limiting heat transfer through air movement.
Code requirements for air barrier systems are a comparatively recent addition unlike provisions related to thermal or vapor control. An air barrier system, as defined by the Air Barrier Association of America (ABAA), is considered to be a “combination of air barrier assemblies and air barrier components, connected by air barrier accessories that are designed to provide a continuous barrier to the movement of air through an environmental separator.” It is important to understand air permeance is the transfer through the material, whereas air leakage is movement through deficiencies in the material, assembly, or system. Air transfer also occurs from areas of high to low pressure, which can be created by wind, stack effect, and fan pressure in HVAC systems. Air movement is typically considered to move significantly larger volumes of water vapor through or within the enclosure than vapor diffusion and is a concern from an infection control perspective, enclosure performance parameter, and operating costs associated with maintaining the elevated humidity. This highlights the importance of an air barrier system in building enclosure design and construction.
Vapor diffusion and air transport tend to function independently of one another, as vapor diffusion can occur without air movement. Water vapor diffusion is also driven by variations in vapor pressure and moves vapor from areas of high pressure (warm and humid) to low pressure (cold and dry) to reach equilibrium. Vapor control in building enclosures is achieved using a vapor retarder (vapor impermeable materials that are installed and integrated to limit vapor migration). Vapor retarders are currently used primarily in cold climates and in spaces with higher indoor RH. However, if the vapor retarder is not airtight, large volumes of moisture can be transported through air movement.
Interior operating conditions
Typically, indoor building environments operate according to use and type. While some building types, such as healthcare, laboratories, and museums may have been specifically designed to accommodate a certain amount of active interior humidification, many existing buildings were not designed with that intent or ability.
A great article. There should be more articles like this.