The building regulation minimum standard ( Document L ) applies to all new and replacement windows (unless situated within a conservation area or listed building). Windows must achieve a U value of 1.8 or less for new builds and extensions, and a U value of 2.2 or less for replacement windows. U values are a means of measuring the heat insulation properties in buildings, the lower the U value, the better the insulation.

In cases of conservation areas and listed buildings the local council conservation department may well insist upon re-instatement of single glazing. The main point to note here is that there are no hard and fast rules, and consultation with the local authority is strongly recommended.

The most cost effective unit specification to comply to building regulation is a 4:16:4 ( 24mm ) standard air filled unit with low emissivity ( Low E ) glass on the inside leaf, giving a centre pane U value of 1.9
There are many factors that contribute to the performance of any given unit specification, Most important is cavity width, and cavity fill type, which affect conductive heat losses. In addition there are various types of low E coatings whose purpose is to impede heat loss by reflecting long wave heat radiation back into the room. Then there are ‘warm edge’ spacer bars, to reduce the cold bridge effect around the perimeter of the unit.

It has been proven that 16 / 18mm cavities between panes offer the best insulation properties for air filled units. Narrower cavities start to lose insulation due heat conductivity, whereas cavities much above 20mm start to become increasingly less efficient due to heat loss via air convection occurring within the cavity itself. Increasing the glass thickness on either leaf has almost no effect on the heat insulation properties of the unit.

Air is a very good insulator, as long as it is not moving. This is why a woollen jumper keeps you warm on a cold day, by trapping a layer of insulating air close to the skin. Air is used as the basic cavity fill for standard double glazed units and suffices very well for most applications where you can achieve a 16mm cavity ( which equates to a 24mm overall unit thickness 4:16:4 ).
Argon gas can be used instead of air, and slightly improves the U value on any given unit. Or Krypton gas which offers substantial insulation gains over air fill. Both gasses cost more than air filled, and it should be noted that they have slightly narrower optimum cavity widths than those associated with air fill.

We will recommend glazing specification according the individual application, and in consultation with the customer. This may involve achieving the most cost effective glazing to comply with building regulations. Or we may have to work within given constraints. A common example is during refurbishment work, when manufacturing new sashes to fit existing narrow box frames where there is not sufficient sash thickness to accommodate a standard 24mm unit. Here we would specify a Krypton or Krypton/Xenon gas filled unit in order to achieve building regulation compliance with a cavity of just 4 or 6mm ( Unit thickness of 12 or 14mm ).

Use of safety glass, most usually toughened or laminated is another requirement of building regs in certain locations within buildings, most notably if glazing is situated within 800mm of floor level. Although it is sometimes prudent to use safety glass for security reasons in addition to those areas specified by building regulation.