Thermal Transmittance - Measuring Thermal Transmittance

Measuring Thermal Transmittance

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 meters. 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 100 mm in diameter and perhaps about 5 mm 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 periods of around two weeks at least. For ground floors the heat flux meters may need to be left in place for over a year (due to the massive heat storage in the ground).

Generally, thermal transmittance measurements are most accurate when:

• The difference in temperature between the inside and outside of the building is large.
• 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 a long period of time.
• Several spot measurement points are used rather than just one or two.