glazing

The energy performance of windows has improved enormously over the past twenty years.  One of the most notable improvements was the introduction of low-E glass, gas filling of the cavaties, warm edge spacers and the adoption of wider cavaties. 

Building Regulations have tightened the demands made of our glazing systems and currently a U-value of 1.8 or less is demanded.  The very best windows available on the market achieve U-values less than half this amount; they are invariably triple-glazed and tend to be imported (Germany, Sweden).  There is still very low demand for triple glazing in the UK and few manufacturers exist here.  If you want to go for a low energy window (U-value of 0.8 or less) you'll most likely have to import it and it will look modern.

Remember that the U-value method of measuring energy efficiency, whilst fine for walls, roofs and floors is more clumsy for windows because of their ability to take heat in.  The British Fenestraiton Rating Council (BFRC) has developed an alternative method of ranking windows and there is an increasing use of energy rating symbols, from A down to g.  As well as assessing the U-value of the whole frame assembly, this rating also considers the heat absorbtion ability of windows during daytime and also assesses frames for airtightness.

Glazing Options

Standard double glazing comes in widths of either 18mm or 24mm - the deeper the unit, the better the insulation, both thermal and accoustic.  Low E-coatings, providing extra levels of thermal efficiency, usually now come as standard.

The cavity between the two opanes can be filled with inert gas (usually argon) which increases thermal efficiency.

Other options include self-cleaning glass and low-iron glass - this is exceptionally clar and see-through. 

Electro-opaque gas (costs about £1000 per m2) is expensive but attractive.  A current constantly runs through it to keep the glass clear.  When a switch is flicked, the current is turned off and the lgass becomes instantly opaque, not letting any light through.

Triple-glazing is a good, but expensive, option for those looking for extra thermal efficiency.