Thermal Transmittance
Thermal transmittance (or U-value) is a measure of the thermal performance of non-homogeneous, building structures.
It is defined in BS EN ISO 7345 as:
"Heat flow rate in the steady state divided by area and by the temperature difference between the surroundings* on each side of a system."
* In thermal metrology the 'temperature of the surroundings' are defined in terms of the 'environmental temperature' - which is defined in ISO 8990 Annex A
Although thermal transmittance has the same units as thermal conductance (C), it differs in that it is applicable to non-homogenous structures (e.g. windows and doors) and it includes the heat transfer processes associated with the heat transfer into the structure from the warm environment and the heat transfer out of the structure to the cold environment.
For more information please contact: Ray Williams
- Hot box test elements - details of types, sizes and construction details.
- NPL has a measurement facility to measure the thermal performance of pipe insulation capable of measuring the thermal conductivity or thermal transference of pipe insulation.
- Measuring the thermal performance of insulating products utilising reflective surfaces.
- Description of NPL's hot box facility for measuring the thermal performance of large non-homogeneous structures.
- Members of the Thermal Performance Group are currently participating on the following BSI CEN and ISO working groups associated with this work.
- NPL can manufacture glazed calibration panels as specified in BS EN ISO 12567-1 Annex C1.