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A
Z
Embodied carbon in curtain walls
Current
2020
list Article list

Embodied carbon in curtain walls

In the third in our embodied carbon series, Frances Gannon looks in detail at the impact of our design choices on unitized curtain wall facades.

Lightweight, manufactured in factory conditions and quick to install: unitized curtain walls are a default choice for many larger and high rise buildings.  We know from the façade systems study in our last post, that curtain walls are higher in embodied carbon than other cladding systems, but often their light weight can result in big savings to the embodied carbon of the primary structure.  Curtain walls don’t perform as well thermally as other external walls, which can increase operational carbon, so a holistic and whole life review must be undertaken to make the right carbon choices for each project.

When designing a unitized facade, we are working with very high embodied carbon materials, aluminium and glass, so we must make informed design choices about their use, and maximise their long life, recyclable potential.  Our ‘sensitivity analysis’ looks at the impact of changing one factor at a time in a bay of curtain walling, so we can understand the impact of our early design choices and develop our ‘embodied carbon common sense’.

A bay study is a useful way of making a fair comparison and could be used for any part of a building.  Our ‘upfront embodied carbon’ analysis includes just stages A1 – A3, ‘cradle to factory gate’, to avoid the project specifics of transport (stage A4) and site activities (stage A5).  Curtain wall manufacturers’ Environmental Product Declarations (EPDs) are difficult to compare, as they don’t itemise the components included within the cladding bay, usually using 4mm + 4mm double-glazed units and smaller profiles than would be relevant on the scale of a larger building.  So we’ve worked with façade consultants FMDC to go back to the primary material data and make our own comparison on our own terms.

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Our lessons learned from this study are to be mindful of our fenestration patterns: mullions at 750mm centres can look very elegant but significantly increase embodied carbon and worsen thermal performance.  Equally, we can’t assume that widening mullion centres will reduce embodied carbon, if framing and glazing have to get thicker.  We should examine if a slightly deeper cladding zone would result in a more efficient aluminium profile and we must be proactive to source low carbon aluminium.  We should look to use alternative facing and lining materials wherever possible, to minimise the use of aluminium.  And we must understand that adding a decorative rainscreen to curtain walling is layering up materials for purely aesthetic effect.

For every project, we must take a holistic, bespoke and calculated approach to the design of any curtain walls, using these very high embodied carbon materials at their maximum efficiency. As an alternative in some situations, timber stick curtain wall systems are already available and maybe timber unitized or panelised systems will follow.  Or perhaps the future will be completely decarbonised aluminium production.  But whatever the future holds, the careful detailing of aluminium and glass components to minimise their upfront embodied carbon, maximise their lifespan and ensure they are demountable and reusable or recyclable when the time comes, is something we can design in right now.

Architects, engineers and designers – please test out design options for any part of a building in simple embodied carbon bay studies like this one and please share the results!  Our best chance of rapidly reducing upfront embodied carbon in construction is by sharing our research and our good ideas.