Thermal transmittance, also known as U-value, is the rate of transfer of heat (in watts) through one square metre of a structure divided by the difference in temperature across the structure. It is expressed in watts per square metre per kelvin, or W/m²K. Well-insulated parts of a building have a low thermal transmittance whereas poorly-insulated parts of a building have a high thermal transmittance.
where Φ is the heat transfer in watts, U is the thermal transmittance, T1 is the temperature on one side of the structure, T2 is the temperature on the other side of the structure and A is the area in square metres.
Thermal transmittances of most walls and roofs can be calculated using ISO 6946, unless there is metal bridging the insulation in which case it can be calculated using ISO 10211. For most ground floors it can be calculated using ISO 13370. For most windows the thermal transmittance can be calculated using ISO 10077 or ISO 15099. ISO 9869 describes how to measure the thermal transmittance of a structure experimentally.
Typical thermal transmittance values for common building structures are as follows:
In practice the thermal transmittance is strongly affected by the quality of workmanship and if insulation is fitted poorly, the thermal transmittance can be considerably higher than if insulation is fitted well.
When calculating a thermal transmittance it is helpful to consider the building's construction in terms of its different layers. For instance a cavity wall might be described as in the following table:
| Thickness | Material | Conductivity | Resistance = thickness / conductivity |
|---|---|---|---|
| - | outside surface | - | 0.04 K·m²/W |
| 0.10 m | clay bricks | 0.77 W/m·K | 0.13 K·m²/W |
| 0.05 m | glasswool | 0.04 W/m·K | 1.25 K·m²/W |
| 0.10 m | concrete blocks | 1.13 W/m·K | 0.09 K·m²/W |
| - | inside surface | - | 0.13 K·m²/W |
In this example the total resistance is 1.64 K·m²/W. The thermal transmittance of the structure is the reciprocal of the total thermal resistance. The thermal transmittance of this structure is therefore 0.61 W/m²·K.
(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.)
It is possible to allow for mortar joints in calculating the thermal transmittance of a wall, as in the following table. Since the mortar joints allow heat to pass more easily than the light concrete blocks the mortar is said to "bridge" the light concrete blocks.
| Thickness | Material | Conductivity | Resistance = thickness / conductivity |
|---|---|---|---|
| - | outside surface | - | 0.04 K·m²/W |
| 0.10 m | clay bricks | 0.77 W/m·K | 0.13 K·m²/W |
| 0.05 m | glasswool | 0.04 W/m·K | 1.25 K·m²/W |
| 0.10 m | light concrete blocks | 0.30 W/m·K | 0.33 K·m²/W |
| (bridge,7%) | mortar between concrete blocks | 0.88 W/m·K | 0.11 K·m²/W |
| 0.01 m | plaster | 0.57 W/m·K | 0.02 K·m²/W |
| - | inside surface | - | 0.13 K·m²/W |
The average thermal resistance of the "bridged" layer depends upon the fraction of the area taken up by the mortar in comparison with the fraction of the area taken up by the light concrete blocks. To calculate thermal transmittance when there are "bridging" mortar joints it is necessary to calculate two quantities, known as "Rmax" and "Rmin". Rmax can be thought of as the total thermal resistance obtained if it is assumed that there is no lateral flow of heat and Rmin can be thought of as the total thermal resistance 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.
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: