The DETR's proposals to revamp Part L are putting the pressure on window designers, makers and installers. How can glazing become more thermally efficient?
The government's war on waste will have a major impact on the way windows are designed and specified. If its proposals to shake up Part L of the Building Regulations are adopted, they will affect all firms that manufacture, design, install and specify windows (Timetable for change).

Under the new regulations, the allowable heat lost through windows would be reduced by 30% by 2004 (Proposed changes to U-values). These changes could spell the end for some double-glazed window units and force designers and manufacturers to rethink their output to improve thermal performance.

This will involve either improving the insulation value of the frame – by making the area of the frame smaller and increasing the better insulated glazed part of the window – or using more sophisticated types of glazing such as those with a heat-reflecting coating. It will also see new types of thermally efficient frames being introduced, as well as new styles of windows and types of glass. In some cases, it could even mean architects having to specify triple-glazed units.

Because aluminium-frame windows are constructed from a heat-conducting metal, they usually include a thermal break in the frame to prevent heat being lost from the room to outside. Most modern manufacturers use a plastic isolator between the inner and outer framing profiles, usually constructed from polyamide.

However, some older aluminium-frame designs incorporate a pour and de-bridge resin thermal break that is less efficient and is unlikely to remain in use after the proposed changes.

Window thickness

Complying with Part L will also affect the thickness of the double-glazed unit. The proposed changes will distinguish between windows with a 6 mm air gap and those with a wider spacing between the panes – the wider the gap, the more air is trapped between the panes and the higher the insulation value of the window unit. Units with a 6 mm gap between the panes will seldom provide sufficient insulation to meet the new thermal requirements.

Timber windows frequently use 4-6-4 glazing units – a construction of two 4 mm thick panes of glass, separated by a 6 mm air gap. These do not perform as well as thicker units, and timber window frames will have to be redesigned to accommodate better glazing types.

Even with PVCu frames, it will be necessary to change from the common 12 mm air gap to a 16 mm gap, unless the highest performance low-emissivity glass is used with argon gas filling, which requires the use of heavier framing sections.

The need to improve the frame's thermal efficiency will push the emphasis on to heavier frames and glazing bars. This will affect the style of windows, with single, large glass units becoming more commonplace than today's more familiar windows divided into a number of smaller glazing units with intermediate bars.

Sometimes, the requirements can be met only by increasing the number of panes of glass in the unit to create a triple-glazed unit. "How windows will have to be glazed…" on page 46 shows how windows will have to be constructed to achieve the proposed new U-value.

Fillings and coatings

The gas in the cavity between the panes also has an impact on a window's performance. The cheapest way of filling the gap between the two panes is with air. Other gases, such as argon, are better insulators, but more expensive. Argon is likely to be increasingly used in new glazing products, with a corresponding increase in cost. However, there are no benefits of using argon in thin air gaps.

One solution to increasing the thermal efficiency of the window would be to increase the insulating value by using low-emissivity glasses. These are treated on one surface with a coating that reduces the amount of light that can pass through the window in the infrared part of the spectrum – the part of the spectrum that carries heat. So, the glass helps trap heat in the room.

There are two broad types of coating available. Hard coatings are applied to the hot glass during its manufacture. Because this is part of the manufacturing process, hard coatings are cheaper than soft ones, which are applied afterwards. Soft coatings allow much lower infrared transmission and so are more efficient at trapping heat in a room.

Work is ongoing to produce better low-emissivity glass, which will be given greater impetus by the changes to Part L.

Number and size of windows

An easy solution for designers looking to reduce the amount of heat lost through glazing would be to reduce the number and size of windows in a building. But the new Part L cautions against this. Reducing the amount of glazing will cut down on the amount of natural light, pushing up artificial lighting costs. As a guide, the regulations caution against using less than 20% glass in a wall.

On the other hand, the regulations allow windows to comprise only up to 25% of the wall area because walls are better insulators than windows. Designers, therefore, have just 5% to play with. However, some green designs use sunlight passing through windows as the primary source of heat in a building. Where this is the case, solar gain will be included in U-value calculations. For example, if a greater proportion of glazing is installed on the south facade, where they will be exposed to the sun both in summer and winter, the U-values of the windows will be relaxed.

The regulations allow the U-value to be relaxed (and so reduce the units' insulation value) but never by more than 0.15 W/m2°C. However, even where solar gain is used to save heating energy, it will not alleviate the need for contractors to use higher performance windows.

The regulations do not only apply to new buildings. Whereas remedial work has previously been outside the scope of the Building Regulations, the proposed changes to Part L will bring replacement windows and doors within their scope for the first time. On the assumption that the existing walls are not likely to meet the new U-values for walls, it will be necessary to use the improved U-value for windows given in the table above.


Manufacturers and specifiers will not be the only ones affected by Part L – those who install windows will have to rethink their practices, too. The regulations propose for the first time that air leakage through the building envelope will have to be limited; that means the window will have to fit snugly into the facade.

In the past, Part F of the Building Regulations has set a minimum requirement on the amount of air needed in a room as fresh air, primarily for health reasons. The proposed changes to Part L will now set upper limits on the amount of heated or cooled air that can leak out of a building in order to conserve energy.

For commercial buildings with a floor area of more than 1000 m2, an air pressure test will have to be carried out. This will involve pressurising the building using a special fan. It will require buildings to have an air leakage rate of less than 10 m3/h/m2 of outside wall at an air pressure of 50 pascals.

Window units will easily achieve this rate but air may well leak around the window frames rather than through them. This raises an important question: if the building is found to leak more air at the junction between the window and the wall than is permitted in the regulations, who will be liable? Who will accept responsibility for correcting the fault? The window fitter, the facade installer or the architect for poor detailing of the junction?

In future, it will be necessary to ensure a better seal between the window and surrounding wall and between composite windows connected by joining mullions to form ribbon glazing. Particular attention will have to be given to the detailing of sills, which are often bespoke rather than part of the window framing system.

Areas of responsibility and details of interfaces between trades and components will need to be made clear at the outset of the project – just one of a number of changes the new regulations will bring to construction.

Timetable for change

The proposed changes to Part L of the Building Regulations will come into effect at the end of 2001 or early 2002. The plan is for the DETR to introduce the new requirements in two parts: an initial insulation value that will be tightened after the first 18 months, with a further tightening of the regulations proposed for sometime after 2005. The changes are designed to conserve energy and help the government meet its commitment to achieving a national reduction of CO2 to 20% below 1990 levels. Buildings, which produce about half of all national CO2 emissions, are an obvious target for government action. All building fabric will have to be better insulated to reduce the amount of energy lost through windows and walls. A fundamental change to the regulations is a change in the definition of building work. By widening the meaning of “material alteration” in the proposals, the government has managed to bring work on existing buildings within them. At the moment, this will only extend to replacement walls and boilers.