In plain English, the U-factor is how much warmth an object transfers from its warm side to its cold side. It is given in W/m²K (Watts per m² Kelvin). The lower the value, the better the insulation.
A U-factor of 0.6 W/m²K is one of the best possible today and increases from there. Triple glazing for example, typically produces U-Factors between 0.4 and 0.8 W/m²K.
The following terms are used in the overall U-value formula:
- Uf describes the heat transmittance coefficient of the frame
- Ug indicates the temperature loss through the glass plate
- ψg defines the energy loss through the edge compound (warm edge)
- Uw describes the total U-factor of the overall window
Heat Gain and Loss
Heat flow is unavoidable when two objects with different temperatures come into contact. Think of wrapping your cold hands around a hot coffee mug. Windows, doors and skylights gain and lose heat in three different ways:
- Direct conduction through the glass surface or frame
- Heat radiating in and out of a house via interior objects like people, furniture and walls
- Air leaks
Solar radiation plays an important role in heating interior spaces. Sunlight enters via the glass panes and is absorbed by things like carpets, curtains and furniture. This stored heat is later slowly released, warming up the air in the room.
Special glazing ensures that long wave radiation (i.e. the heat) does not leave by reflecting it back inside. This helps maintain a consistent temperature and prevent the need for additional heating. However, this has no effect on visible light which can pass back and forth unobstructed.