Thermal insulation / Part L compliance

The requirement for thermally insulating windows is becoming more and more important within the construction industry to meet the demands of government policy, the conservation of fuel and power and ultimately the future of the planet.

Steel Window Services are committed to energy conservation and sustainable development and have invested heavily in thermal testing and simulation to offer a range of finely tuned products offering outstanding thermal insulation properties.

All thermally insulating mild steel products have been fully independently tested (full test results available upon request) to establish their U values (i.e. the amount of energy that is lost through the window).

Although having a low U value is not the complete solution for having an energy conserving window, it is a very important consideration in both replacement windows and new build. Some products, particularly the W30 range of windows, have been design-optimised to give outstanding Window Energy Ratings under the B.F.R.C. (British Fenestration Ratings Council) scheme. The ratings under this accredited scheme take into account not only the U value of the window but also the air tightness and the solar gain (i.e. the amount of "free" heat energy absorbed through the window). There is also a statutory requirement to comply with Document L of the Building Regulations where U values are the main criteria of compliance.

Outlined here is an easy to use guide to help you establish which glass types you will need to specify, with particular products, to attain the best or required U values of the system once installed. You can then be assured that compliance with regulations is fully met and that your building has the most cost effective thermal solution required.

It is recommended that you contact Steel Window Service's Technical Department with your exact requirements to ensure compliance and performance is equally met, especially when fine tuning glass specifications for additional aspects such as solar gain, reflection, security, privacy, fire-rating, etc.

Exemptions are likely to occur for replacement windows in buildings that are listed, in a designated conservation area, are a scheduled ancient monument, in a designated building of architectural or historical interest, in a national park, in an area of outstanding natural beauty, or are built using traditional construction with permeable fabric that both absorbs and readily allows the evaporation of moisture. This is to preserve the historic nature of the building, and local authorities are likely to accept either like-for-like replica replacement windows or as near to existing as possible but using insulating glass units with a centre pane U value (Ug) of 1.2 W/m²K or better (with reference to Document L of the Building Regulations).

Ug Centre pane U value of glass
Uw Combined frame and glass U value (no spacer bar, i.e. single glazed)
Uw (stand.) Combined frame and glass U value using standard aluminium spacer
Uw (warm) Combined frame and glass U value using warm edge spacer bar
Uw (steel) Combined frame and glass U value using steel fire rated spacer bar
W.E.R. British Fenestration Ratings Council (BFRC) Window Energy Rating
ND No specific data available at time of press
Configuration not possible / applicable

* 1.7 W/m²K fully glazed / 1.9 W/m²K fully panelled / 1.8 W/m²K ½ panelled - ½ glazed
All U values measured in W/m²K
All double glazing units have a soft coat low E coating to face 3 from outside
All triple glazing units have soft coat low E coatings to faces 2 and 5 from outside
W40 triple glazed windows have both IGU cavities filled with 90% Krypton gas/10% Air
W40 double glazed windows have IGU cavities filled with 90% Argon/10% Air
W20 and W30 windows have IGU cavities filled with 90% Krypton gas/10% Air

Standard units as follows:   SMW+ - 4/4/4 (12mm)   W20 - 4/8/4 (16mm)
(options are possible)   W40 (double) - 4/16/4 (24mm)   W40 (triple) - 4/6/4/6/4 (24mm)

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