rooflight u-values simplified

The U-value is the measurement of heat lost through a roof window. The lower the U-value, the better the rooflight is at keeping heat in.

There are three ways to measure the heat loss of a flat roof window as explained below – Ug, Uw and Ud. The glass Ug-value tends to be the lowest U-value and many rooflight companies only quote this figure which in our eyes is essentially cheating. Building regulations inspectors are not concerned with the rooflight glass U-values, they are more interested in ‘combined product’ U-values.

Clients should also bear in mind that triple glazed rooflights are not necessarily better than double glazed rooflights for the UK climate when you factor in ‘solar gain’ which is free heat harvested from the sun in the spring, autumn and winter.