Best Practice > Glazing

Windows and doors will always be the thermal weak point in your house – even when they are closed!

With current fabric U-values and a normal 10 to 20% glazing ratio, the heat loss from your windows can be at least equal to that from your walls. When aiming for an overall highly insulated building, it makes sense therefore to give as much attention to the windows and doors as to the walls and roof. The most important part of the opening is, of course, the glazing.

Double glazing with a hard low E coating has become the standard these days for a new build and replacement. Specify, as a minimum, dual sealed units bearing the kitemark symbol and meeting BS5713: 1979. Glazing units should be fitted to BS 6262, in accordance with the Glass and Glazing Federation standards. In fact there are a number of ways in which you can improve on standard double glazing, which are discussed a few paragraphs below.



PVC, or polyvinyl chloride (the ‘u’ indicates ‘ unplasticised’) is used widely in buildings these days. From conservatories, windows and doors to fascia boards, barge boards, soffits, rainwater goods (gutters, hoppers and downpipes), internal and external piping, flooring, wiring insulation, uPVC is everywhere. Its advantages are versatility, good strength to weight ratio, low maintenance and of course, cost competitiveness. Hollow chambers within the uPVC window frames provide an additional degree of insulation. This design feature, coupled with increased recyclability and long lifespan of uPVC has seen this product being given an A rating in the Green Guide online (on the Building Research Establishment (BRE) website).


Though widely used in commercial, industrial and public buildings, aluminium would be the least common option in a domestic setting, as it is usually not seen as compatible with most traditional styles of dwelling. However, aluminium can be a very appropriate material in a high-tech new build design. It is possible to obtain aluminium window frame systems, which comprise aluminium (for durability and low maintenance) on the outside and timber (for aesthetic properties) on the inside.

Timber (thermal break):

Overall good environmental profile (not tropical hardwoods). Good quality systems are likely to be the top of your price range compared to other systems. Timber frames are sympathetic to the architecture of period properties and are therefore most suitable for retrofit. Timber frames do however require regular maintenance i.e. repainting. The most cutting-edge manufacturing techniques for timber window frames comprise of a continuous thermal break running the whole length of the centre of the frame. The system dramatically increases the thermal resistance of the window frames resulting in U-values as low as 0.7W/m2K (when used in conjunction with the most advanced triple glazed units with soft low E coatings and insulated 16/19mm spacers).



Sometimes glazing U-values can be measured mid pane. In the case of units with multiple glazing this does not take into account the weakest point (thermally) of the unit  –  that is the edge, where a spacer is incorporated to maintain the pane separation. Spacers are usually made of aluminium, which conducts heat, thereby creating a cold bridge at the edge of the unit. However, sealed units are available using insulated spacers which reduce the cold bridge and improve the overall thermal performance of the unit.
Insulated spacers, which divide the panes of glass in multiple glazing units provide an additional thermal break as opposed to the old style of using aluminium spacers which would have given rise to higher edge heat losses.

Pane separation:

Sealed double glazing units have tended to be available in two standard formats, 6mm pane separation and 12mm. The 12mm option has become the norm as it is easily accommodated in most framing systems and gives a better thermal performance. The improved thermal performance of the units with the wider pane separation is due to the extra thickness of insulating air. The optimum pane separation has been found, by theory and experiment, to be around 19mm. When you go over 19mm, the extra width of the cavity allows circulating convection currents to be set up which start to accelerate the heat loss. A 19mm pane separation would of course require a considerable depth of rebate in the frame. However, 16mm units are readily available and can be accommodated within most frames, especially for fixed (non-opening) sashes. The 16mm units should involve little if any over cost, as the only additional expense would be for a small amount of extra material in the spacer at the edge of the unit.


Multiple layers:

An option favoured in Scandinavian countries due to the severe climate is to go for a triple or even quadruple glazing layers within the sealed unit. The more glazing layers you have the more layers of insulating air (or inert gas), and thus the better the thermal performance. Of course a law of diminishing returns applies, whereby you get less value from each successive glazing layer. Also, the frames need to be very deep in order to accommodate successive glazing layers and optical transparency starts to be affected. However in the context of an overall highly insulated design multiple glazing is worth looking at.

Low E soft/hard:

Low emissivity (or a low E) glazing has a very thin layer of metallic coating applied to the surface. This has the effect of reflecting back some of the heat from inside the house, which would otherwise be lost through the glazing. The coating does reduce the level of solar gains transmitted through the glazing, though this effect is not significant compared to the heat loss saved. The low E coated surface is positioned on the cavity side of the inner pane in a double glazing layer.

There are two types of low E coating, hard and soft, the soft coating giving a lower U-value. The only other difference essentially is that soft coated glass cannot be toughened or laminated after application, these processes needing to be carried out prior to coating, whereas hard coated glass can be toughened or laminated post-treatment.

N.B. Low E glazed units will come with stickers indicating which side should face inwards / outwards. It is most important that this instruction is followed.

Inert gas (evacuated):

Air is a fairly good insulator, but inert gases are better due to their low thermal conductivity. The most commonly used inert gas in double (and multiple) glazing units is Argon, Krypton and Xenon (in increasing order of thermal performance) are also found. The amount of inert gas in the cavity will reduce by a few percent each year, due to the fact that seals are never 100% perfect – it is simply not possible due to the size of the gas molecules to produce a perfect seal at the molecular level. This decay will gradually reduce the thermal insulation of the unit, though the gases are not harmful either to people directly or to the environment.

U-values – (centre pane versus overall):

Manufacturers will quote a U-value for a given glazing unit. This should take into account heat losses from the edge of the unit. However sometimes a centre pane U-value will be given which will be lower but will give a false impression of the overall thermal performance. Make sure if you are comparing glazing systems that you are comparing like with like.

W E R/B F R C:

Window Energy Ratings (WER) were launched in 2004, providing an accessible and robust method of measuring product improvements and promoting energy efficiency to consumers by allowing comparison of the overall energy performance of different window systems. The rating scheme, which is now provided by the British Fenestration Rating Council (BFRC) is similar to that for white goods using colour-coded letter ratings from A (most efficient) to G (least efficient). Energy is also expressed in terms of kWh/m2/yr and figures given for air tightness and solar transmission. Most standard systems will have a negative rating, indicating net heat loss throughout the year. However, some of the higher performance systems can have a positive figure, indicating net heat flow into the building. At present the system is voluntary, though in time it may replace U-values as a method of expressing energy performance of glazing systems.

A window on the future:

Two up-and-coming systems which may have an impact in the near to mid-term future are, firstly, electrochromic glazing, which can change its light transmission, and even its U-value characteristic in response to an electric current. Secondly is evacuated glazing. Instead of using air or inert gas between the panes, a vacuum is formed, which gives the best possible barrier to heat loss by convection. Achieving a high level of vacuum and high quality seal are the main challenges with this technology (currently being researched at the University of Ulster, Jordanstown campus). A key benefit of evacuated glazing is that pane separation does not matter — it can be as little as you like within manufacturing limitations. Therefore, the overall thickness of the unit can be greatly reduced, compared to multiple layer and increased pane separation systems. Even compared to standard, double glazing units, thus enhancing applicability, especially in a retrofit situation.

Orientation/Glazing ratio:

Orientation (the direction your house faces) is something you will only be able to influence if you are building from scratch. Even then you will be limited by constraints such as planning permission, access and position of adjacent dwellings. There may also be other considerations such as privacy, views, etc, which you wish to take into account. Such competing factors need to be reconciled with each other to arrive at an optimum design.

Ideally to make best use of heat and light from the sun your house should ‘face south’, i.e. the house should address available solar gains in terms of its layout and the area and position of glazing. So you would have higher levels of glazing on the south façade, and your main daytime occupied areas on the south side of the house. Nor does the house have to face bolt south, 25° or so either side will not make that much difference.


South facing glazing, even single glazing, is neutral over the year in terms of the balance of energy gained and lost. South facing double glazing can be a significant net contributor to heating the building over the year. However, care must be taken not to allow excess solar gains leading to overheating. South facing vertical glazing has the property of being to an extent self-regulating with regard to solar gains. In the summer when the sun angle is higher, a greater proportion of the solar radiation is reflected off the glazing, whereas in winter, with lower sun angles, more of the radiation is transmitted through the glazing. Also overhangs and reveals can be sized to exclude high summer sun angles but allow solar gains to enter the house during the heating season.

Daylight factor:

Day lighting and natural ventilation are key aspects of passive solar design – just as important as utilisation of solar heat gains and retention of heat in the building. Please refer to the Lighting section for more in depth information on calculating the daylight factor.


For existing buildings, films can be applied to glazing to reduce the amount of solar gains entering the building. In addition to this, double glazed units have been designed for retrofitting into existing timber frames. Provided the frames are of sufficient depth, the rebates can be extended to accommodate the extra depth of a double glazed unit.


Most external doors are made of solid timber or UPVC. In both cases, with no glazing the U-value is of the order of 3.0W/m2K, while half glazed (double glazing, standard class), gives around 3.1W/m2K and a fully glazed around 3.3W/m2K. All such systems these days can be provided with good levels of draught stripping and there is little to choose in thermal terms between the two materials.

If you are looking for greater energy efficiency there are door systems on the market which offer improved thermal performance without all the environmental dis-benefits of UPVC and tropical hardwoods. Steel or fibre glass panel doors, with a polyurethane foam core can give U-values as low as 0.3 to 0.5W/m2K for a solid door, increasing in proportion to the area of glazing.

Choice of glazing type will become more significant, the larger the glazed area. Low E double glazing is recommended as standard. Pane separation is likely to be restricted by the depth of rebate, which can be accommodated within the thickness of the door.

Note that the thermal performance of your door will be irrelevant if it is open! Use of draught lobbies (see section on Air tightness) on main entrances can greatly reduce the amount of heat lost when entering and leaving the house. The effectiveness of the draught lobby depends on only having one door open at any time which will be influenced by the floor area of the lobby – the minimum requirement is 2m2.

Roof Lights

In a pitched roof, a roof light is a simple and low cost choice of window (compared to a dormer window), which is easily installed in the plane of the roof pitch. Essentially all that is involved is the replacement of tiles/slates, battens and roofing felt with a proprietary roof light, ensuring that lead flashing is correctly installed around the roof light to provide a weatherproof seal. Usually it will be necessary to remove part of at least one rafter to accommodate a roof light of sufficient size. Extra horizontal timber members are then inserted to brace between the rafters on either side of the gap. A roof light can also be incorporated in a flat roof, where particular care is needed to prevent water ingress.

While the vertical glazing has the property of being self shading to some degree, inclined glazing will be much more prone to high solar gains and thus risk of overheating. This should be taken into account when sizing your roof light. Because the roof light ‘sees ‘much more of the sky you will not need as large an area as you would for a conventional vertical window, in order to provide the same level of daylight and solar heating gains. Also, less of the solar radiation will be reflected back and is the case with vertical glazing.

Heat losses from inclined glazing can be relatively high, as more heat is lost by direct radiation to the night sky. Your roof light should therefore have at least the same thermal performance as you would specify for new windows, if not higher, though this has not tended to be the case in practice. Contrary to common belief it is perfectly possible to get triple glazed roof windows.