What's in a U-Value?

When you’re buying new windows and doors, you’ll often see references to the U-Value, which is essentially a measurement of their thermal efficiency and how well (or how badly) they stop heat transferring from inside to out. 

Every element in the fabric of a building has a U-Value attached to it and if you want to improve the sustainability or energy performance of your home, then the lower the U-Value the better. A window with a U-Value of 1.0 W/m2K, for instance, would lose heat at half the rate of a window with a U-Value of 2.0W/m2K.

The U-Value is a mathematical calculation measured in Watts (W/m2K) – that’s the rate of heat flow through 1m2 of a material, where there is a difference in temperature from inside to out of 1 degree (Kelvin or Centigrade). 

Manufacturers and architects use online U-value calculators to work out the performance of different building elements using their Thermal Resistance, and intelligent modelling software to work out the overall building performance using the U-Value of all the different elements. For windows and doors, that calculation has to take into account three different elements – the frame, the glass and the spacer bar within the glazed unit and each of those can have a big impact on the U-Value achieved, so it is the combination which is all important.

The government has recently introduced updates to the Building Regs which have reduced the maximum U-Values allowed. For windows and doors, it is now 1.2 for installations in new homes and 1.4 for replacements and refurbishments. Rest assured, AluK’s products can comply with ease.