A standard insulating glass unit with a 16mm air gap incorporating one pane of Pilkington K Glass™, in an appropriate timber or PVCu frame, has been shown to achieve a measured U value in the range of 1.8 to 1.9 W/ m 2 K.
The new building regulations for replacement windows and conservatories came into effect on 1st April 2002 in the UK. The first conservatory company tried to frighten me not to use independent builders. They said that only specialist companies like themselves were able to comply to the new rules.
The truth was simpler than that. To comply with new energy management requirements for meeting the Kyoto Protocol, new standards have been introduced. U-values, which indicate the amount of heat loss, are now required to be 2.2 or better (lower, that is).
Cavity walls have U-value of 0.35, compare to a double-glazed window (with a 16mm cavity) of 2.7. Special glass called Pilkington K (or low emissivity) reduces it further to 2.0. Inject with argon gas, and the U value is reduced to 1.9.
A south-facing window will cause the U value to reduce to 0.1.
U values are also affected by the slant. Vertical windows have lower heat loss than sloped ones. A glass roof pitched at 45 degrees will have U values of 1.9 for argon-filled K glass, compared to 2.2 for K glass (non-argon filled). A vertical double glazed glass unit with 20-mm cavity (also called a 28-mm sealed unit) will have a U value of 1.5 and 1.8 for argon K and non-argon K respectively.
Compare this to a single glazed unit with a U value of 4.8. No wonder it's warmer and quieter after I had replaced my sash windows with 28-mm sealed units. Those air-filled windows (not Pilkington K-glass and argon-filled cavities) have a U-value of 2.7, which is too high by the new rules.
11 April 2002 Thursday
Pilkington K Glass™, which usually forms the inner pane of a double glazing unit, allows less heat to escape through your windows than ordinary glass. Thanks to a special energy saving coating which lets the sun's rays through, but reflects heat from fires and radiators back into your home. Heat which ordinary double glazing allows to escape.
U-values at Pilkington's site.
Windows that achieve proposed Part L - U values.
A non-statistically based observation from the entire table is that a UPVC window, with a 4/16/4K air unit would typically achieve U1.9. Using argon would improve U by about 0.1, an insulating spacer would improve U by a little less than 0.1, and the use of soft coat instead of hard coat low E would improve U by 0.2 - 0.3, depending on emissivity. It is not felt there is enough measured data to make observations about the effect of cavity width.