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Window Energy Ratings and U-Values Explained


There are two ways of complying with ‘Part L’ of the building regulations; Windows Energy Ratings (WER’s) or Whole Window U-Values.

1) WER’s take into account solar gain and heat retention. These ratings are used for replacement windows, but are not an accepted rating for new builds.

2) Whole Window U-Values focus more on heat retention. U-Values are an accepted rating for both replacement windows AND new builds.

Plastic frame suppliers will normally have the information required when supplying frames with WER’s.


What is a ‘U-value’?

A U-value is a measure of thermal resistance through a material. A material with high insulation properties will have a low U-value. As the insulation reduces, the U-value will increase.

Can you supply me with an A-rated unit?

Not exactly. Sealed units on their own cannot be given a Window Energy Rating or Whole Window U-Value, however we can calculate the Centre Pane U-value. The Centre Pane U-value can vary depending on the glass type and gas fill. At Topline we can provide double and triple glazed units to the Centre Pane U-value you need to achieve.

What’s the difference between a Centre Pane U-value and a Whole Window U-value?

The Centre Pane U-value focuses solely on the efficiency of the sealed unit. It is only when this information is combined with the u-value of the frame that we can calculate a Whole Window U-value. It is important that both the unit and the frame are thermally efficient, as a high specification sealed unit glazed within a poorly insulated frame (or vice versa)  will not achieve a compliant WER or Whole Window U-Value

How effective is warm edge spacer bar?

Warm edge spacer bar does work, but has very little affect on the Centre Pane U-value of a sealed unit. However in comparison to standard steel spacer bar it sufficiently reduces the amount of heat loss through the window, making the window and sealed unit together much more thermally efficient. This therefore reduces the Whole Window U-value.

What effect does Low Iron glass have?

Using Low Iron glass will increase the solar gain of the sealed unit. This means that although it will have no affect on the Centre Pane or Whole Window U-value, it will improve the WER (remember, WER’s take into account solar gain as well as heat retention).

However, for timber frame manufacturers to supply WER frames, they would need to have each of their different wooden windows tested. Most joiners find the expense involved in testing prohibitive and time consuming, but need to have their frames rated in order to comply with building regulations and keep up with the plastic frame suppliers.

This is where we come in. We have access to software that allows us to run a number of calculations to ensure compliance in line with the Whole Window U-Value approach.

This is a service we offer to all of our timber frame manufacturers/supplier customers. We can provide you with a form to fill in the relevant information about the design of your wooden frame. Combining this information with the sealed unit specification,  we can run those details through our simulator and provide you with all the relevant  paperwork in order to comply

To discuss WER’s and Whole Window U-values in more detail please contact our office team.   

Centre Pane U-Values

Centre Pane U-Value Results when using ‘K’ Glass Centre Pane U-Value Results when using Soft Coat Planitherm