Understanding U Values

​​U-values express the rate of heat transfer through any element of a building, such as a wall, roof or window. The units used to express U-values are watts per m² Kelvin (W/m²K). This means that if a roof had a U-value of 1.0 W/m²K, for every degree of temperature difference between the air on the surface inside the roof and the air on the surface outside, 1 watt of heat would pass through any m² of the roof.
Therefore, the lower the U-value is, the better the insulation provided by the material.

For example, here are some typical U-values for building materials:
Average glass or polycarbonate conservatory roof: U-value of 3.0 W/m²
Double-glazed window: U-value of 2.8 W/m²
Solid brick wall: U-value of 2.0 W/m²
Cavity wall: U-value of 1.6 W/m²
Average house roof: U-value of 0.35 W/m²

A Solid Conservatory Roof can achieve a U-value of between;
​0.26 W/m² and  0.14 W/m²