U-Value
A U value is a measurement of the overall heat energy transfer rate (under standard conditions) through a particular section of construction material, e.g. roof, walls, glazing etc.
It is defined as the rate of heat flow in watts (W) through an area of 1 square metre (m) for a temperature difference across the structure of 1 C degree centigrade or Kelvin (K).
Typical U values in W/m2K
Solid brickwork, no insulation 2.5
Cavity wall, no insulation 1.6
Cavity wall with insulation 0.5
Single glazing 5.7
Double glazing 2.8
Double glazing with low-u coating 1.8
Double glazing rating C 1.6