Window frame materials compared

Key issues

Heat loss v Manufacturing impacts on the environment

• Heat lost through the window frame in its use phase (operational energy) is likely to have a much greater environmental impact in the frame’s life cycle than any impact generated through its production phase.

• It is important to choose a material with the least conductivity.

• Wood is the least conductive material followed by PVC and metal.

Wood, durability and environmental impact

The choosing of wood, its treatment and maintenance are crucial in reducing a window frame's environmental impact:

• Specify FSC sourced timber

• Transport adds embodied energy. Try and source UK timber whenever possible.

• For both hard and softwoods ensure that the specification explicitly excludes the use of sapwood.

• Painting wood adds significantly to its environmental impact. Either specify a naturally durable species that doesn’t need treating or select a treatment with low impact.

• If the wood is to be treated/painted, ensure that this is done in the factory prior to site. Factory painted frames double the period before the need to repaint.

• Be careful to avoid damage to frames on site. Ensure that they are not used as formwork in wall openings.

• The design of the sections will have an effect on performance. Design to maximise rapid drainage, maintain dry glazing channels and locate weather seals away from wet areas.

• Keep windows as large as practically possible. Even with wooden frames, the metal spacers between the glass panes act as cold bridges. Large panes have less perimeter length than lots of smaller panes.

Recycled content

• The overall environmental impact of metal and PVC window frames can be substantially improved by the quantity of recycled material. Both materials have significant impacts throughout their production phases particularly in their non-renewable resource use, energy use and toxic bi-products.

• Currently PVC windows contain little or no recyclate though the plastics industry is committed to improve this situation by 2010. By 2005 the industry was committed to recycling 50% of post consumer PVC windows – to date there has been no confirmation from the industry as to whether or not this has been achieved.

• Aluminium and steel windows already contain varying quantities of recyclate. If you are specifying windows with metal content do check with the manufacturer about the percentage of recyclate.

PVC

Windows are one of the most important applications of PVC. Currently around half the windows installed in the UK are PVC. The debate between the relative environmental credentials between wood and PVC frames centres around the environmental impact of the production of PVC.

• A report commissioned by the EC in 2004 titled ‘Life Cycle Assessment of PVC
and of principal competing materials’ summarized its section on a comparison between wood, aluminium and PVC window frames:

‘available studies conclude that there is no “winner” in terms of a preferable material since most of the studies conclude that none of the materials has an overall advantage for the standard impact categories.’

However: This conclusion was based on the premise that competing wooden windows used indiscriminate non-local wood sources and required maintenance levels of repainting every 3-5 years with VOC based systems. It also allowed for the assumption of a high degree of ‘closed loop’ post – consumer recycled PVC in an expanding market. PVC production analysis data was derived from highly regulated European sources.

• Another report ‘Construction Materials Report’ based on BedZED used the BRE Ecopoints system to demonstrate that the wood frame windows used in the development showed a considerable difference between wood and PVC:

• Further, an earlier report commissioned by DEFRA in 2000, ‘Life Cycle Assessment of Polyvinyl Chloride and Alternatives’ extended the usual life cycle analysis into the area of risk:

“Both the PVC and PS (Polystyrene) life cycles exhibit a greater degree of risk than the other materials considered. This is due to the nature of the industrial processes involved such as oil and gas, ethylene, chlorine, ethylene dichloride (EDC) and PVC production for PVC products and oil and gas, ethylene, benzene, ethyl benzene and styrene production for PS. The hazards associated with these processes are typically greater in impact if and when they occur, in some cases (e.g. fires and explosions in oil and gas production) having the potential to be catastrophic, resulting in the potential for a considerable number of fatalities from a single incident. Whilst the hazards may be considerable, due to regulatory controls, the likelihood of occurrence and the resulting risks are reduced.”

Some specifiers might prefer not to take the risk.

See 'Further Information' at the foot of the page.

Composites

There are many variables where synergy is achieved by using a combination of materials taking advantage of each material’s strength. The most common composite is that of wood faced with aluminium. Wood’s traditional environmental weakness of surface coating with synthetic paints is counterbalanced by the cladding of a wood section with the very much more durable aluminium. Conversely, the high environmental impact of aluminium is mollified by the small quantity used in the section. The overall environmental profile can be significantly improved by using recycled aluminium (though note that there is a worldwide shortage). Although relatively new on the scene aluminium clad timber frames are expected to have lifetimes of in excess of 40 years. PVC by comparison is around 25 years.

We would specify:

First choice: • FSC durable temperate hardwood
Second choice: • FSC temperate softwood clad with (pref. recycled) Aluminium • FSC temperate softwood treated with plant based paint systems
If you are compromised: • Use a certified softwood painted with low VOC paint
Avoid: • PVC • Aluminium without thermal breaks.

Window products:

• L413 Windows