Whilst calculation of thermal transmittance can readily be carried out with the help of software which is compliant with ISO 6946, a thermal transmittance calculation does not fully take workmanship into account and it does not allow for adventitious circulation of air between, through and around sections of insulation. To take the effects of workmanship-related factors fully into account it is necessary to carry out a thermal transmittance measurement.
ISO 9869 describes how to measure the thermal transmittance of a roof or a wall by using heat flux sensor. These heat flux meters usually consist of thermopiles which provide an electrical signal which is in direct proportion to the heat flux. Typically they might be about  in diameter and perhaps about  thick and they need to be fixed firmly to the roof or wall which is under test in order to ensure good thermal contact. When the heat flux is monitored over a sufficiently long time, the thermal transmittance can be calculated by dividing the average heat flux by the average difference in temperature between the inside and outside of the building. For most wall and roof constructions the heat flux meter needs to monitor heat flows (and internal and external temperatures) continuously for a period of 72 hours to be conform the ISO 9869 standards.
Generally, thermal transmittance measurements are most accurate when:
*The difference in temperature between the inside and outside of the building is at least .
*The weather is cloudy rather than sunny (this makes accurate measurement of temperature easier).
*There is good thermal contact between the heat flux meter and the wall or roof being tested.
*The monitoring of heat flow and temperatures is carried out over at least 72 hours.
*Different spots on a building element are measured or a thermographic camera is used to secure the homogeneity of the building element.
When convection currents play a part in transmitting heat across a building component, then thermal transmittance increases as the temperature difference increases. For example, for an internal temperature of  and an external temperature of , the optimum gap between panes in a double glazed window will be smaller than the optimum gap for an external temperature of .
The inherent thermal transmittance of materials can also vary with temperaturethe mechanisms involved are complex, and the transmittance may increase or decrease as the temperature increases.