**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/m²/K. Well-insulated parts of a building have a low thermal transmittance whereas poorly insulated parts of a building have a high thermal transmittance.

- Φ = A × U × (T
_{1}- T_{2})

where Φ is the heat transfer in watts, U is the thermal transmittance, T_{1} is the temperature on one side of the structure, T_{2} 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:

- single glazing: 5.7 W/m²K;
- single glazed windows, allowing for frames: 4.5 W/m²·K;
- double glazed windows, allowing for frames: 3.3 W/m²·K;
- double glazed windows with advanced coatings: 2.2 W/m²·K;
- triple glazed windows, allowing for frames: 1.8 W/m²·K;
- well-insulated roofs: 0.15 W/m²·K;
- poorly insulated roofs: 1.0 W/m²·K;
- well-insulated walls: 0.25 W/m²·K;
- poorly insulated walls: 1.5 W/m²·K;
- well-insulated floors: 0.2 W/m²·K;
- poorly insulated floors: 1.0 W/m²·K;

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.

Read more about Thermal Transmittance: Calculating Thermal Transmittance, Measuring Thermal Transmittance

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