Calculating U-Value of Windows and Doors


U-values are generally used to describe the thermal performance (heat loss) for a section of construction - such as a walls, roofs, windows and doors. U-value is used to measure how effective elements of a buildings fabric are as insulators.  

U-values measure how effective a material is an insulator. The lower the U-value is, the better the material is as a heat insulator.


A windows thermal performance is a windows’ whole U value not just the value of the centre pane U value. U values are determined by reference to EN14351-1 and window type is a single frame and fixed sash


Simplified heat loss through any given surface is calculated using: 

Heat loss = U x A x dT


U = U-value (W/m2K) Thermal transmittance

A = Area of surface (m2)

dT = Temperature difference inside to outside ( K )

Units of heat loss = Watts

The formula Heat Loss = ∑ ( U x A x dT )