Understanding Key Air Barrier Terms: Part 3
In this final installment of our key air barriers terms, we’ll take a closer look at some timely topics that you may be hearing about more frequently in these times of increased focus on energy efficiency.
In the context of buildings, a thermal bridge is a building component that has a higher thermal conductivity than the materials around or contacting it, creating a path for heat transfer. For example, a metal stud directly contacting the external sheathing of a building acts as a thermal bridge, conducting indoor heat to the building exterior (in cold weather) or conducting outdoor heat into the building (in hot weather). This conductivity can impact the energy efficiency of the building, increasing the heating and cooling requirements, and also potentially cause condensation in wall assemblies.
Put simply, the dew point is the temperature at which air becomes saturated with water vapor. If the air is cooled further, this water vapor will condense and become liquid water. In the case of a wall assembly, if infiltrating air cools within the wall assembly below the dew point, it can condense on the wall components and collect there, setting up conditions for mold growth and potential structural damage.
Relative humidity (RH)
RH is a measure of the amount of moisture in an air-water mixture, expressed as a percentage. When humidity reaches 100 percent, condensation can occur. Technically, RH is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. This reflects the relationship between temperature and humidity; at lower temperatures, the ability of air to contain moisture vapor is decreased, at higher temperatures, it is increased.
One strategy for countering thermal bridging is the use of continuous insulation. Covering the entire exterior of a building with insulation provides a “thermal break” that prevents thermal bridges. Continuous insulation is becoming more popular in both commercial and residential construction in part because the International Energy Conservation Code (IECC) began recommending higher insulation values in colder climate zones to help improve building energy efficiency.
A hygrothermal analysis looks at the movement of heat, air and moisture flow through the building envelope, including walls, roofs and foundations. Using sophisticated software models, hygrothermal analysis enables architects and building engineers to see how a particular assembly of components will perform under a variety of temperature and humidity conditions. For example, a hygrothermal analysis may be used to see how the location of insulation relative to the air barrier will affect condensation under certain weather conditions. Hygrothermal analysis is an extremely helpful tool for taking the guesswork out of decisions regarding air barriers and other building envelope components.