Key Concepts for the Thermal Control Layer
In order to ensure that your thermal control layer functions as intended, there are a few important concepts to understand:
There are three methods of heat transfer — Sensible heat is that which can be measured by a thermometer, and it moves in three ways:
Conduction — The transfer of heat energy directly through a material. As heat energy from the source excites the molecules in the adjacent region of the material, they in turn excite all connected molecules. This transfer occurs in any direction, and the rate of heat flow is based on the material’s conductivity, its thickness, and surface area, and the difference in temperature on either side of the material. The thermal control layer is primarily there to control conduction.
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Convection — The transfer of heat through a fluid (including air). Heated fluids become less dense and rise, creating convection loops. The air control layer (see below) controls convection.
Radiation — The transfer of heat through air or space. Radiant energy moves equally in all directions, and only in straight lines.
Difference in temperature — Heat moves from areas of higher concentration to areas of lower concentration; that is, from warmer places to cooler places. The rate at which heat moves is influenced by the difference in temperature, known as the AT (delta T). If it is significantly colder on the exterior of the building, there will be a stronger “drive” to the heat flow than if temperatures were similar.
Insulation — Controlling the conductive flow of heat through the building enclosure requires the use of materials that are good insulators; that is, materials that resist the conductive flow of heat. Every surface of the building that is exposed to the exterior will require some degree of insulation to prevent conductive flows.
U-value and R-value — Measuring and expressing the effectiveness of an insulation material is done in two ways. The conductivity of a material is expressed as a U-factor, and the lower the number the more resistant the material is to the conduction of heat (that is, lower numbers are better insulators). The thermal resistance of a material is expressed as R-value, and the higher the number the more resistant the material to the conduction of heat. R-value is calculated as the inverse of U-value: (R = 1/U).