U-Values and WERs
The insulation value of a sash window (or any other structural component, for that matter) - its U-Value - is usually expressed as its EMISSIVITY, the units of which - for those of a technical bent - are Watts/(M2K) (Watts lost through 1m2 for every degree Celsius).
The only thing you really need to understand is that from 2003 until October 2010 the Building Regulations in England and Wales required new windows and doors to have a U-value NOT EXCEEDING 2.0 (1.8 in Scotland). This figure was revised downwards to U=1.6 (in both England and Scotland) by the new Building Regulations introduced on the 1st of October 2010.
An unimproved single-glazed window is likely to have a calculated value of around 5.0 or worse. Such a window will effectively have a internal surface temperature close to that of the outside air. If it's -5C outside, there will probably be ice on the inside: Not much better than leaving the door open.
With coated glass and gas-filled sealed units the centre-pane U-value of Sealed-Unit Double-Glazing can be reduced to U=1.1 or even better, depending on how much you are prepared to spend. However, don't confuse this with the WHOLE-WINDOW U-value, (WWV) where the insulation properties of the frames themselves are also brought into the calculation.
TAKE CARE: Not every business bothers to make this distinction, but ALL SupaSash windows doors and conservatories refer to their WWV.
In terms of room comfort and fuel burn, a U-value of 1.6 is an awful lot better than a U-value of 5.0, and with SupaSash frames you can bring your traditional window up to the highest standard of the 21st Century.
Window Energy Ratings (WERs):
The whole picture has been complicated by the somewhat controversial issue of WERs, which form part of the amended Building Regulations introduced in October 2010. In effect, the idea of Ratings - on the face of it a simple common-sense, easy-to-understand matter - is complicated by the introduction of G-Values - or the energy GAINED by the window from sunlight - the SOLAR GAIN. The problem is, it isn't easy to work out whether this is a benefit in any particular location. Solar gain is great in the winter - think of big south-facing windows gathering the sunlight - but not so hot (pardon the pun) in summer, where TOO MUCH heat might be gathered. So shutters of some sort, or PASSIVE-STACK ventilation, seem to be required to make the most of this angle. Unless you have a stately home with a long run of floor-to-ceiling rising sash windows, this idea is probably best left to designers of new buildings, where the summer ventilation issues may be addressed from the start.
For most vernacular UK dwellings (particularly those with small windows and barely adequate natural light, such as typical Victorian/Edwardian terraces) my own assessment is that the U-Value is the thing to concentrate on. And don't let anyone tell you otherwise unless they can make out a very good case for it!
The obvious truth is that the U-value option is recognised by the Building Regulations of 2010, so, for us, it's business as usual, with calculated U-values and a certificate provided with every installation.
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