Thermal transmittance is the rate of transfer of heat through matter. The thermal transmittance of a material or an assembly is expressed as a U-value. The thermal insulance of a structure is the reciprocal of its thermal transmittance.
Example of measurement system for thermal transmittance according to ISO 9869 and ASTM C1155, model TRSYS.
In the context of construction, the R-value is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window or a complete wall or ceiling, resists the conductive flow of heat. R-value is the temperature difference per unit of heat flux needed to sustain one unit of heat flux between the warmer surface and colder surface of a barrier under steady-state conditions. The measure is therefore equally relevant for lowering energy bills for heating in the winter, for cooling in the summer, and for general comfort.
Installed faced fiberglass batt insulation with its R-value visible (R-21)
Heat flux measurement set-up
Heat flux measurement results
Aerogel is an extremely good thermal insulator, which at a pressure of one-tenth of an atmosphere has an R-value of R-40/m, compared to R-3.5/m for a fiberglass blanket.