When evaluating insulation for a specific application, one of the first considerations is typically its thermal properties. The following definitions can be useful when determining if a product meets the application’s requirements:
- Apparent thermal conductivity: A thermal conductivity assigned to a material that exhibits thermal transmission by several modes of heat transfer, resulting in property variation with specimen thickness or surface emittance. (See thermal conductivity.)
- Apparent thermal resistivity (R-value): A thermal resistivity assigned to a material that exhibits thermal transmission by several modes of heat transfer, resulting in property variation with specimen thickness or surface emittance. (See thermal resistivity.)
- Emissivity “E”: The ratio of energy radiated by a surface to the energy radiated by a black body. Significant when the surface temperature of the insulation must be regulated, such as with moisture condensation or personnel protection.
- F-rating: A rating, usually expressed in hours, indicating how long a fire-resistive barrier can withstand fire before being consumed or permitting the passage of flame through an opening in the assembly, as determined by ASTM E 814 (UL 1479).
- Temperature limits: Upper and lower application temperatures within which the material must retain all its properties.
- Thermal capacity: The quantity of heat required to raise the temperature of the body 1 degree. For a homogeneous body, it is the product of mass and specific heat. For a nonhomogeneous body, it is the sum of the products of mass and specific heat of individual constituents. (May also be seen as heat capacity.)
- Thermal conductance (C-value): The time rate of steady-state heat flow through a unit area of a material or construction induced by a unit temperature difference between the body surfaces.
- Thermal conductivity (K-value): The measure of heat that passes through a unit area of a homogeneous substance, through a unit thickness, in a unit of time, for each unit temperature difference. The lower the K-value, the higher the insulating value.
- Thermal properties of insulation: Usually expressed as C-value, K-value, R-value, and U-value.
- Thermal resistance (R-value): A measure of the ability to retard heat flow rather than the ability to transmit heat. R-value is the numerical reciprocal of "U" or "C," thus R= 1/U or 1/C. Thermal resistance R-value is used in combination with numerals to designate thermal resistance values: R-11 equals 11 resistance units. The higher the "R," the higher the insulating value. The I-P units are °F–ft2 – hr/Btu; the SI units are °C–m2/W.
Thermal resistance “R-value”: The measure of the ability of a thickness of insulation to retard heat flow.- Thermal resistivity, r: The quantity determined by the temperature difference, at steady state between two defined parallel surfaces of a homogeneous material of unit thickness, that induces a unit heat flow rate through a unit area. (r in SI units: m K/W.) (r in inch-pound units: h ft F/ Btu or h ft2F/Btu in.)
- Thermal transmittance “U-value”: The overall transmission of heat flow through a “system” induced by the unit temperature difference between the ambient temperature in each side of the assembly. The combined thermal value of all the materials in an insulation system, air spaces, and surface air films. The heat transmission in unit time through unit area of a material or construction and boundary air films induced by unit temperature difference between the environments on each side. The I-P units are Btu / (hr–sq ft–degree F temperature difference), and the SI units are W / (sq m– degree C temperature difference). Note: This heat transmission rate has been called the overall coefficient of heat transfer.
