The Engineer: Carbon tension
How much energy efficiency matters depends on where you are.
August 31, 2023 By Claudio Sacilotto
In my last article, I discussed that the impending climate emergency is precipitating a revolution in the building industry and that our society needs to be more carbon-efficient. Our building industry needs to rely less on fossil fuels, and we have been doing this with smart design and using efficient insulation materials. The approach to net-zero design will enable buildings to sustain themselves as our society tries to minimize operational carbon. But to be truly carbon-efficient, we also need to consider the carbon footprint of the materials and methods we use in the construction of the building. What’s needed is an objective analysis of the overall process. Some of us refer to this as cradle-to-grave assessment.
I used to blatantly ignore environmental detractors who have used distorted arguments to make their point. They’d question whether the environmental impact of erecting a wind turbine outweighs the free energy it gets from the wind. Or if the carbon footprint of constructing a hydroelectric dam (earthworks, deforestation, concrete, etc.) outweighs the benefit of providing green electricity. Does an electric vehicle use more embodied carbon in its aluminum body than the operational carbon that it claims to save? The internet has lots of answers, but I needed to figure it out myself and remember to stay objective.
And our industry is not immune to these arguments. The one hotly debated issue I’ve been hearing about lately is the carbon effect of adding a third pane of glass to an IGU (hence making it a triple pane IGU) instead of using the default standard dual pane. Is triple glass the answer to carbon-efficient glass insulation? I’ve run the calculations many times and researched other computations. And my final answer is…it depends.
The answer is not obvious and it may surprise you. To calculate this properly, a life cycle analysis needs to be conducted to evaluate the carbon cost of the third pane of glass and compare that to the operational efficiency gains. How much insulation improvement is it contributing? It becomes a more difficult calculation when you consider other factors, such the effect on solar heat gain that is impeded with that third pane. Perhaps there will be improvements to air conditioning efficiencies but worsen the passive heating potential of the building. Will a third pane darken the interior necessitating more lighting?
The fenestration industry has always maintained that opting for tri-pane glass windows instead of dual pane glass sounds like a sensible choice to reduce your operational carbon footprint. However, how much this matters depends on where you are located. In an electrically heated home in Quebec (where the electrical grid is predominantly green hydropower), the embodied carbon of that extra third pane will overwhelm the operational carbon savings. A predominantly green heating grid is not emitting a significant amount of carbon dioxide, therefore the carbon payback (embodied carbon footprint with respect to operational carbon savings) may not make sense. I am, of course, purposely ignoring other important aspects that tri-pane glass windows bring, such as improved thermal comfort and improved noise reduction.
Install those same windows in Alberta or Nova Scotia where heating energy is predominantly fossil fuel-based and that third pane is now paying for itself quickly, from a carbon perspective.
The embodied carbon of glass is consistent with most manufacturers worldwide and has not varied much over the years. Despite operational improvements, float glass manufacturing uses fossil fuels as it’s the only viable method to melt the raw material in furnaces that can reach as high as 3,000 F. The life cycle assessment of glass may yield a high carbon footprint, but there are not many other suitable alternatives to glass. Human well-being relies on regular exposure to light and dark each day. We need a view of the outdoors from our environmentally separated spaces. Essentially, we cannot live without windows and windows are made from glass. •
Claudio Sacilotto is director of engineering at Novatech Patio Doors in Woodbridge, Ont.
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