Thermal transmittance

The thermal transmittance of a material (such as insulation or concrete) or an assembly (such as a wall or window) is expressed as a U-value.

Choice of materials and quality of installation has a critical impact on the window insulation results.

(Note that this example is simplified as it does not take into account any metal connectors, air gaps interrupting the insulation or mortar joints between the bricks and concrete blocks.)

Rmax can be thought of as the total thermal insulance obtained if it is assumed that there is no lateral flow of heat and Rmin can be thought of as the total thermal insulance obtained if it is assumed that there is no resistance to the lateral flow of heat.

The U-value of the above construction is approximately equal to 2 / (Rmax + Rmin) Further information about how to deal with "bridging" is given in ISO 6946.

To take the effects of workmanship-related factors fully into account it is necessary to carry out a thermal transmittance measurement.

[4] ISO 9869 describes how to measure the thermal transmittance of a roof or a wall by using heat flux sensor.

Schematic of ISO and ASTM compliant thermal transmittance measuring system.
Example of measurement system for thermal transmittance according to ISO 9869 and ASTM C1155, model TRSYS.