Doors and windows: The old rivalry between timber and PVCu has taken on a new life after BRE awarded PVCu windows an overall A grade in its latest Green Guide. Stephen Kennett listens to the thud of jaws hitting the ground

In recent months a greener-than-thou spat has broken out between the timber lobby and the PVCu industry. The argument centres on the environmental ratings awarded for certain types of windows in BRE’s latest Green Guide to Specification.

In July, a body that promotes the use of PVCu, issued a press release stating that “recent advertising campaigns by the Wood Window Alliance suggesting wood has an A rating compared with PVCu and aluminium’s D, run a very real risk of misleading their audience”. It appears the WWA tucked away the fact that the D relates to a single “climate change indicator”, rather than the overall rating, in a footnote.

Yet the leap PVCu has made in the table, from C in the previous guide to an overall A rating in this year’s, has raised eyebrows among architects and specifiers. Sian Moxon, senior architect and leader of the Green Team at Jestico + Whiles says the upgrading of PVCu windows in the guide is contentious. “We do not support the decision, considering the scarcity and finite nature of oil supply, and the forecast for future demand. We would not consider PVCu to be a sustainable material and are concerned that the improved rating is perhaps the result of lobbying from the PVCu industry.”

The BRE’s Green Guide to Specification, which was published online in June, aims to help architects and designers to select materials with strong environmental credentials. It measures the embodied impact of materials and building components for generic building types, and uses this to assign each a single rating, ranging from A+ to E.

Yet it seems the guide is also creating waves in other sectors of the window industry. A quick glance at the results in the section on non-domestic windows reveals that hardwood double-glazed windows with a water-based solvent have an A+ rating, as do pre-treated softwood windows; no surprises there. But so, too, does a PVCu window with steel reinforcement. It’s a similar story with the section on domestic windows, where PVCu achieves an A. On the other hand, powder-coated aluminium softwood windows get a D and powder-coated aluminium windows with a softwood internal frame score an E.

The Council for Aluminium in Building was reluctant to comment on the situation, saying it was a sensitive issue. However, it did indicate that it had concerns over the ratings that aluminium windows received in the guide and was currently in discussions with BRE over it.

Ian Miller, an associate with Cyril Sweett, specialises in looking at the whole-life analysis of products. He has recently completed a detailed whole-life cost assessment for Nordan, a Norwegian window maker that makes the aluminium-clad timber windows known as NTech Passive range.

Although he supports the principle of the guide, he says the rating for aluminium-clad timber windows is an issue as “specifiers take it at face value. The problem we have is that if you simplify the rating system to this level, it would appear that there are anomalies”.

As expected, an aluminium-clad timber window gets a lower rating because the aluminium is an energy-intensive material. Miller has concerns, however: “Irrespective of my work with Nordan, I still can’t understand how an aluminium-clad wooden window is getting a lower rating than PVCu when 90% of its component is wood. No disrespect to the PVCu market, but it is a plastic product, it comes from oil and it takes an enormous amount of energy to produce it.”

So how does BRE get to these results? According to Kristian Steele, a principal consultant at the firm, the guide takes a cradle-to-grave approach when assessing products. This includes measuring the environmental impact at all stages, from the extraction of raw materials to the production process, the transportation, the installation and its final disposal. These are represented in one of 13 environmental impact categories covering everything from climate change to fossil fuel depletion.

We would not consider PVCu to be a sustainable material and are concerned that the improved rating is perhaps the result of lobbying from the PVCu industry

Sian Moxon, Jestico + Whiles

The actual lifecycle assessment uses BRE’s Environmental Profiles Methodology. This was designed to provide a level playing field when comparing products and was created after an extensive consultation process with the UK construction supply sector. Steele says: “Any dissenting views could be raised.”

To make the comparison fair a “functional unit” is used – this is a double-glazed window of a fixed size and thermal performance. Trade bodies and manufacturers are then asked to provide information on a standard product to meet this specification, including the quantities of material needed for the frame, glazing, gaskets and so on, and these are used to calculate the rating.

This approach means that a triple-glazed window with a low U-value would not be given credit for its insulating properties. “The green guide measures embodied impact issues. The A to D ratings don’t look at the operational rating,” says Steele. “These are dealt with in the SAP and SBEM calculations.”

Indeed the guide clearly states that the main consideration when specifying windows is the heat loss through them and that the embodied impact should play only a small part in the overall decision.

BRE relies on the manufacturers and trade bodies for the information used in the calculations. Naturally a large multinational manufacturer will have different results to an SME, so BRE looks at the spread of makers in the UK market and takes a representative sample to reflect this.

Steele is confident of the validity of the data BRE is provided with. “In many material areas we have verified information so when we do a generic exercise we have a wealth of information that allows us to look objectively at information provided by manufacturers. So we can be quite confident the data we are provided with is accurate and that we are able to identify any errors.”

Each product is considered over a 60-year study period. Steele says: “We’re not proposing that they have a 60-year service life; we’re doing the study over a 60-year period, so some materials and specifications will require maintenance and replacement before 60 years and this replacement is built into the model.”

This means a window with a better service life performance will have a reduced environmental impact.

BRE assumes a basic service life for generic window types and any manufacturer that wants to use alternative figures has to have evidence to support it.

With credits available to designers for BREEAM and the Code for Sustainable Homes, many designers will look to the guide for materials specification advice. Not everyone is convinced. “As a practice we would not be persuaded by the guide to specify PVCu, as consideration of aesthetics and our own sustainability knowledge would deter us,” says Jestico + Whiles’s Moxon. “But, since PVCu windows tend to be cheaper than timber, timber/aluminium composite and aluminium windows, the new rating will make it harder for us to argue the case against them to contractors and clients.”