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U-value (thermal transmittance)
The U-value shows how many watts of heat pass through one square metre of a building element when the temperature difference across it is one degree.
The U-value is measured in W/(m²·K). The lower it is, the better a wall, roof, floor or window retains heat. It depends on the thickness and thermal conductivity of the materials, so extra insulation or better windows lower the U-value.
Why does this matter when choosing heating? A building’s heat loss is calculated by summing the area of each element times its U-value times the difference between indoor and design outdoor temperature, plus ventilation and infiltration losses. This calculation gives the heat pump capacity required.
- Windows usually have a higher U-value than insulated walls, so large glazed areas increase heat demand.
- Thermal bridges – lintels, balcony slabs, foundation junctions – can add significantly to losses.
- The building’s energy performance certificate or design documents often state the U-values of elements.
If you have the building design or an energy performance certificate, attach it to your enquiry – our engineer will assess heat loss more accurately and select the right Mitsubishi Electric heat pump.