Thermal transmittance
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 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. Losses due to thermal radiation, thermal convection and thermal conduction are taken into account in the U-value. Although it has the same units as heat transfer coefficient, thermal transmittance is different in that the heat transfer coefficient is used to solely describe heat transfer in fluids while thermal transmittance is used to simplify an equation that has several different forms of thermal resistances.
It is described by the equation:
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:
...
Wikipedia