We hear about them all the time in the building industry, but what exactly is a U-value?

A U-value is a number that measures the amount of energy (i.e. heat) lost through a particular material. U-values are measured in W/m²K.

Take an ‘example’ window with a U-value of 2.0 W/m²K …  This means that for every degree in temperature difference between the inside and outside, that 2 Watts of energy is transmitted through each square metre of the window.

The lower the number, the less heat is lost, therefore the more thermally efficient the material is. So products with lower U-values are better insulating products.


Example U-Values;

A solid brick wall has a U-value of approximately 2.0 W/m²K

A normal cavity wall with insulation has a U-value of approximately 0.18 W/m²K

A single pane of glass has a U-value of approximately  5.0 W/m²K

A non-insulated aluminium window (if you remember the old aluminium windows and doors from the 1960’s?!) has an overall U-value of approximately 4.0 W/m²K

A typical PVC framed standard double glazed window has an overall U-value of 1.4 – 1.6 W/m²K


With modern technology in both materials and production methods, it is now possible to get the U-value of windows and doors down to even below the 1.0 W/m²K figure. Triple glazing adds another whole dimension to reducing the U-value of windows and doors – see Triple Glazing – Is it really worth it?

When choosing products for your project, be aware that the figures that get quoted can be misleading! Sometimes you will be advised the U-value of one component (e.g the glass), and not the overall figure.  Make sure you ask for the overall U-value, for the combined glass & frames!

Document L (2000) of UK Building Regulations requires all replacement windows and doors to have a U-value of no greater than 1.6 W/m²K, and glazing in new build projects, to have U-values no greater than 1.4 W/m²K

Double glazed units have a centre pane U-value of approximately 1.1 W/m²K but do be aware that this is not the overall value – and it’s the overall figure which is the important one! When combined with the frame and calculated properly, the overall U-value of most ‘standard’ double glazed doors & windows is now around 1.6 W/m²K.  There are various ways to improve the U-values of folding doors and windows, some of which are less expensive than others!

Please do get in touch with us to discuss the options that we can supply for you, to meet your requirements while keeping to a sensible budget!