Low Energy Timber Windows and Glazing

Timber window manufacturers have focused hard on their products and have been rethinking, redesigning and reconsidering. Consumers might be forgiven for thinking twice before choosing timber windows. They certainly earned themselves a bad reputation in the sixties and seventies and this reputation for poor quality in workmanship and material opened the door for, first aluminium windows and then later, uPVC. Thankfully, the quality of timber windows has increased greatly but dispelling the perception that timber windows need intensive maintenance regimes that could carry hidden costs is not so easy to fix. Lately, not just the materials and workmanship have changed, the industry has learned from its rivals and introduced modern production and finishing techniques, including offsite pre-finish­ing and quality assurance schemes.

From an environmental perspective, drivers such as the Environmental Protection (Prescribed Processes and Substances) Regulations of 1991, have led to a shift by the industry to using 'safer', preservative treat­ments and coatings, with some going even further and producing 'eco' windows specifically for this sector of the market. Manufacturers have also paid heed to performance expecta­tions, adopting an 'holistic' approach to the specification of all materials used, and giving due attention to better component design.

Now, window manufacturers build into the design a very wide range of factors. For instance, which timber species should be used, chosen on the basis of its durability, dimensional stability and above all its sustainability and environmental impact, with many now offering Forest Stewardship Council or Pan European Forestry Council certification as standard across their range. Certainly, at the 'green building' end of the timber window spectrum there have been some great advances in the UK with at least two new facilities starting production of high quality, high environmental standard and super efficient ranges.

The wood coatings' sector has also responded to environmental concerns (and legislation changes of course) by reducing solvent content and developing high-solids' coatings and improved water-borne formulations, with lower dirt retention characteristics. However, the expectation of higher performance and sustainability carries with it its own demands. Pre-finishing of external joinery has many advantages which offer the end user a very high quality product. But in order for the end user to benefit from that quality, there needs to be a parallel responsibility by the whole construction industry (the builder in particular) to handle and use this pre-finished joinery in a manner which reflects its value.

There are ways of ensuring that the products (windows and doors) are given adequate protection until the point of hand-over. This may require the adoption of such practices as installing windows into openings built around pre-fabricated formers. One of the most recent developments in the industry has been the introduction of energy performance certification, similar to that given to domestic appliances (see below).

It is clear that the joinery and wood coating industries are responding positively to change through a process of integration and closer co-operation. It is only by further extending this approach, working with the installers, going beyond the manufacturing operation into the building process, and above all, meeting and addressing the requirements and concerns of the consumer, that timber windows will secure their rightful place back on our buildings.

Passivhaus windows

A development for the UK has been the introduction of windows manufactured to the German Passivhaus standard. Not only do these incorporate the most advanced triple glazing, with typical glazing U-values of around 0.6W jm2K, but to meet the stringent comfort criteria of the Passivhaus standard, solid timber frames are not sufficient. As a result various innovations incorporating thermal breaks in the timber frames have been devel­oped. A number of ranges are now available in the UK, mostly manufactured in Europe,

and some certified by the Passivhaus Institute as complying with its requirements. Arguably these represent the limit of thermal performance available with currently available glazing and frame technologies. They will be required for those designing to the Passivhaus standard, as well as offering solutions for those working to higher levels of the Code for Sustainable Homes.

Turning to glazing, on new homes, double glazing is just about standard now but many existing properties still need to be upgraded. With our increasing love of large glazed areas, it may well pay to consider enhanced double glazing, or even triple glazing, if we want to keep those wide open views but avoid large heating bills, and minimise carbon emissions. Whilst double-glazing can reduce heat loss through windows by up to 50%, our love of large glazed areas is undermining these reductions. The technology for high and ultra-high performance glazing is well established, and the uptake is improving all the time but until newer advanced technologies, like vacuum glazing, become commercially viable, which is likely to be some years yet (see Volume 2 for a description), there is not huge scope for improvement on the current best practice.

An air gap between the glass of 16mm and 20mm is considered to be the optimum, and the difference between the two is negligible. Below 16mm the direct heat transference between the panes reduces the effectiveness and over 20mm air convection between the panes that has a similar effect. The basic standard for sealed unit double glazing, as required by the Building Regulations, would be a 28mm thick unit combination as such: standard float glass / 20mm air gap / hard coat Low E. This would have a U-value typically of about 1.8Wjm2K.

Low-e coatings

Low-e (Iow emissivity) coatings are a micro­scopically thin metal oxide or semiconductor film applied on one or more surfaces of the glass, usually on a face between the panes of a double glazed or triple glazed unit. Double glazing, using low-e coated glass, gives energy conservation properties equivalent to standard triple glazing. The coating is usually applied on the outer face of the inside pane (or outer face of both inner panes in triple glazing), facing the air cavity. These coatings work by reflecting long wavelength heat generated within the room (radiators and heating appliances), back into the building, whilst at the same time allowing short wavelength, solar energy (from daylight and sunshine) into the room. The incoming short wavelength solar energy is re-radiated by internal building surfaces at longer wavelengths, which are then re-reflected by the coating back into the room.

Low-e coated glass looks identical to ordinary clear glass, as the coating is almost invisible. Its effect on light transmission and reflection is hardly noticeable. It can be used everywhere, from the largest office block application to domestic conservatories, windows and doors and whilst designed for double glazing, it can also be used as the inner pane in secondary glazing, although hard coat low-e would be recommended for this application as it is tougher and more resistant to scratches. The advantages include the following;

  • improved insulation
  • reduced heating bills
  • reduced carbon dioxide emissions
  • reduced condensation
  • reduced cold spots and down draughts
  • takes advantage of the sun's heat

Gas fills and glazing bars

Aragon and other gases

Instead of air; argon, krypton or xenon gas can be injected between the panes. These gasses have better insulation properties than air and contribute to much better overall insulation. For instance, with an argon fill the U-value would be reduced by over 30%. The gasses only displace a proportion of the air in the unit and it is generally accepted that the double glazed unit should achieve a 90% fill gas-to-air concentration. This concentration will gradually reduce with age, at a rate estimated from 0.5 to 1 % per year. Units filled with argon do not degrade significantly until they reach 75% concentration, which adds up to about a 20 year performance durability, after which the unit will perform the same as an air filled unit (this may vary depending on the type of spacer bar used).

Not all double glazing manufacturers are yet able to offer double glazing with a gas filling. Contact the GGF1 for details of those that can. Other gasses, such as krypton and xenon, can be used but they are harder to source and more costly. Chris Herring, of the Green Building Store- who sell a range of exception­ally high specification windows said "Given optimal cavities, argon filled units give virtu­ally the same performance as those filled with the denser and much more expensive gases Krypton and Xenon. Their advantage is that they enable the cavity of the glazing unit to be reduced without affecting performance. Given the expense, their use is best limited to retrofit units which are required to fit narrow glazing situations. High performance argon filled triple glazed units, for example 4-18-4-18-4 with two soft coat low E coatings can achieve almost 0.5 W/m2K centre pane value".

Spacer bars

Advanced technology spacer bars are now becoming an essential part of any double glazed unit that intends to achieve seriously low U-values. There are now a number of different types available with the foam rubber 'Superspacer' from Edgetech IG3 being the most common. Herring said "I am not convinced there is much to choose between the advanced spacer bars, they all seem to perform well, with pretty marginal differences. The important thing is to get away from metallic spacers to true warm edge spacers. Our Ecoplus range of windows use the Superspacer, which we have found to be perhaps the spacer with the widest take-up in the UK. However our Ecoclad range normally use Swisspacer or Thermix4, which are more widely available in Eastern Europe where they are manufactured."

Whichever of these advanced spacer bars you choose it will significantly reduce the heat loss around the edges of the units. Most people will have witnessed the condensation that forms around the edges of double glazed units, well the advanced spacers significantly reduce or eliminate this (condensation on glazing may vary with a number of environmental factors as well as technical factors to do with the composi­tion of the unit). However, the most significant savings using these types of spacer bar will be achieved on windows where small (Georgian) type glazing units are used due to the edge to area ratio. However, their use on all sizes of double glazing units will enhance the energy saving potential of the window. Warm edge spacers can also reduce sound too - up to 2 decibels (according to Edgetech) compared to aluminium spacer bars.

Comparing low-e glazing

Hard coat - also known as pyrolytic coating, this coating is applied at high temperatures and is sprayed onto the glass surface during the float glass process.


  • the coating is durable, which allows for ease of handling and tempering
  • can be tempered after coating application
  • can be used in single glazing applications
  • utilizes passive solar heat gain.


  • higher U-values, compared to soft coat, low-e, products
  • higher solar heat gain coefficient, compared to soft coat, low-e, products
  • hard coat glass also has the possibility of a slight haze, which can be visible at certain angles.


K glass is made by Pilkington.6

NOTE: Pilkington do supply a type of soft coat low E glass known as Optitherm but in the UK most of their Low-E supply is in the form of Pilkington K Glass.

Soft coat - also known as ‘sputter coating’, this is applied in multiple layers of optically transparent silver, sandwiched between layers of metal oxide in a vacuum chamber. This process provides the highest level of performance and a nearly invisible coating.


  • high visible light transmission with optical clarity - minimal color haze
  • ultra-low emissivities, giving optimum winter U­values
  • up to 70% less UV transmission, compared with standard clear glazing.


  • soft coat low-e must be used in a double glazed unit; the soft coating is sensitive to handling
  • most soft coat, low-e, products require tempering the glass prior to the coating application
  • edge removal of the coating is required to ensure a proper seal in an insulated unit
  • more expensive than hard coat, low-e, glass.


Saint-Gobain (Planitherm Total)5

Pilkington (Optitherrn)6

Interpane (Iplus)7