The study analysed the Whole Life Cycle Analysis (LCA) results of seven glass options in seven cities – considering the stages of production, construction, use and end of life.

LCA is understood by many as the most comprehensive tool to compare and rank different structural options in terms of carbon emissions. The research at Loughborough found that, whilst LCA is viable for most structures, it is flawed when it comes to windows and glass structures because of their short lifespan.

This is because LCA has no global standard for setting the length of the life-cycle for the calculation, which is typically the estimated life of the building. For instance, if this period is set between 40-59 years, the window is replaced only once, but if it is 60 – the window is replaced twice. This doubles its embodied carbon impact for manufacturing, making the ranking of window options this way liable.

A common practice alternative ranks window options based on their operational savings only. The study found results using this method were insufficient because advanced glazing technologies have higher carbon during production – an element which needs to be considered.

Recognising these anomalies, the Loughborough study suggests using a new indicator known as the Glass Viability Index (GVI). This new measurement effectively shows the carbon payback of a window – meaning its increased embodied carbon balanced by the annual energy savings.

Using GVI, academics from the University’s School of Architecture, Building and Civil Engineering found that some electrochromic or smart windows, popular in cities across the US, were less environmentally friendly than a normal window in some cases. Elsewhere, the study found that other technologies, such as water-filled glass, benefit from the fact they have low embodied carbon – which keeps them viable for a longer time.

The study also showed that GVI becomes even more important in the future. With the advance of renewable energy production, the carbon payback for the same window becomes longer every year.

Analysing the current trends in renewable installation, the researchers introduced a ‘dynamic GVI’ to calculate this increase of carbon payback over time. The results have shown that the viability of advanced glazing with higher embodied carbon decreases over time, with a potential year of when they become obsolete – or the GVI “Breakpoint Year”. For example, following the current trend the Breakpoint Year for electrochromic was between 2024-2042, depending on climate.

Dr Matyas Gutai, Lecturer in Architecture and Construction Technology at Loughborough University, led on the paper. He said: “One of the main reasons for an advancement in glass technologies is to help make the industry more eco-friendly. However, some of these more advanced options result in higher carbon emissions in the initial manufacturing process – which naturally goes against the reasoning for them in the first place.

“What we are suggesting is the introduction of the Glass Viability Index which will measure the necessary amount of time for any glass option to balance the increased carbon emissions with its respective operational savings.

“This will work alongside the LCA to ascertain which glass solution is best – whether that be a move to water-filled glass, smart glass or to remain as it is. For instance, if the GVI is between 6 and 11 years, then it will compromise our carbon goals in 2030 and 2035 – whilst anything above 35 years might not ever be viable. This result would see a need for alternative solutions.”

It’s hoped the introduction of GVI will help provide a uniformed standard for measuring a window’s carbon impact, which offers a fair comparison between window options and eliminates the liability of the building’s simulated lifespan – also known as Representative Study Period (RSP).