What Are U-Values?

March 27, 2019

U-values are a measure of how thermally efficient your windows and doors are…

The lower the U-value, the better the performance. The minimum U-value required by building regulations is 1.6Wm2K.

The most important thing to understand is that this refers to the WHOLE UNIT, not just the glass. Many companies will quote the centre-pane U-value of their glass. However, this is disingenuous if not put into context.

Our preferred timber products are supplied with high quality double glazing units which have a centre pane value of 1.0. This is considered the optimum before upgrading to triple glazing, which can achieve 0.8. Many other companies use cheaper glazing units which score 1.2 or worse.

But the question to ask your supplier is “what is the whole unit U-value of my window?” This takes into account all the U-values of the product components combined. If they can’t tell you, they are not a specialist window manufacturer. They are probably a general joinery firm who are just as happy making a staircase.

Ideally your quotations will give you these whole unit U-values in writing. The sample below shows a comprehensive specification of a timber traditional weighted box sash window (see our gallery) which achieves an excellent whole unit U-value. This product is fitted with a Krypton-filled slimline 14mm glazing unit.

Window U-values

Mass-produced products, such as aluminium and uPVC, will often be advertised as being ‘A+ Rated’, ‘A Rated’, ‘B Rated’ or ‘C Rated’ (the latter being the minimum requirement to comply with Building Regulations). These products must have been tested and rated by the BFRC. You can check this on their website. If your product has not been tested, it cannot be advertised with a rating.

Also, if a product is advertised as having, say, ‘A Rated Glass’, this is disingenuous. Glazing units cannot have a rating, only a U-value. Ratings can only be assessed for a whole unit.

Bespoke timber products are generally not tested, precisely because of their bespoke nature, in which case getting a U-value per product in writing is important.

See below a useful link to a ‘Build-It’ article on the subject:


We are always here to help, so email us if you have a project and need some information.

Robert Goldsbrough

Robert Goldsbrough

Robert has been developing and building period & contemporary homes since 1995, and has installed all types of external home improvement products.