What's in a U-Value?

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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. 

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.