What is the U-value?
The U-value, or thermal transmittance, tells you how much heat passes every second through one square metre of a building element for each degree of temperature difference between inside and outside. It is expressed in W/(m²·K). The lower the U-value, the better the element insulates.
How to calculate the U-value
The U-value is the inverse of the total thermal resistance, including the internal and external surface resistances:
U = 1 / (Rsi + R + Rse)
For a vertical wall the usual design values are Rsi = 0.13 and Rse = 0.04 m²K/W. A wall with a thermal resistance of 4.7 m²K/W therefore has a U-value of 1 / (0.13 + 4.7 + 0.04), which rounds to 0.21 W/(m²·K).
Many countries, including the UK, set their building regulation targets as U-values for walls, roofs, floors and windows. Always check the current limits that apply where you build.
Application
- Glass (Ug): the glazing only. As a guide, low-e double glazing is around 1.1 to 1.2 W/(m²·K) and triple glazing around 0.5 to 0.7 W/(m²·K), depending on coating, cavity width and gas fill.
- Frame (Uf): the frame profile only.
- Window (Uw): glass, frame and edge spacer together. This is the figure that matters.
| Low-e double glazing (Ug) | Around 1.1 to 1.2 W/(m²·K) |
|---|---|
| Triple glazing (Ug) | Around 0.5 to 0.7 W/(m²·K) |
Practical tips
- Compare Uw, not Ug: good glass in a poorly insulated frame still gives a mediocre Uw. Ask the supplier for the Uw of the complete window.
- Warm edge: a plastic or stainless steel spacer instead of aluminium cuts heat loss at the edge of the glass and reduces condensation.
- Installation matters: draughty joints or an uninsulated junction with the wall throw part of the benefit away through thermal bridges.
- Lower is better: do not confuse the U-value with R-values, where higher is better.
Related terms
- Rc-value
- Insulation value, Rd-value
- Surface resistance
- Thermal bridge
- Glazing
- Argon in double glazing
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