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More on u-values

Remember, the lower the u-value,
the better the insulation.

It's clear that many suppliers are, shall we say, stretching their claims over U-values they offer.

It's important to distinguish between the value at the centre of the pane and the whole-frame value:

With a centre-pane u-value of 1.2 or 1.3 it is possible to reach a calculated whole-frame value of 1.5 or thereabouts for a traditional boxed sash  using 20mm sealed unit double-glazing.

Remember, the current Building Regulations call for a U-value of 1.6 or less.  A larger figure demonstrates a  worse performance.

NO ultra-slim sealed-units using conventional technology - with centre pane values of around 1.9 -  are going to get anywhere near meeting the current Building Regulations, although for a while (until they fail) they will be a significant improvement over single glazing, at an equally significant cost.

Recent developments in glazing technology mean that U-Values as low as 0.8 can be obtained in a DGU light enough to be used in a sliding sash window.  As with most new technology working at the limit, these DGUs are not cheap, although unlike the welded-edge vacuum units, they ARE affordable.  Ask for details.

In the mean time, be sure to ask for a calculated U-value with your Energy Certificate.
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Updated May 2017