How important are embodied energy and embodied carbon?

This two-part article appeared in Green Building Magazine in Spring and Summer 2012.

I looked at the relative importance of embodied and operational energy and carbon – very much a subject of ongoing debate. I found that operational emissions were still relatively ‘bigger’ and should remain a priority – though embodied emissions should certainly not be ignored, and indeed may be more significant than sometimes appreciated, if their timing is taken into account. This is because they tend to happen ‘up front’, leading to more cumulative emissions in our current era.

However, I found reassuring advice from a number of sources that in general there should be no need to compromise operational performance when designing buildings, even if you are set on minimising the embodied impact too – in other words, fabric efficiency needs not have a high embodied cost. Indeed, as Mark Siddall (of LEAP) pointed out, low embodied impact and low operational impact can go hand in hand when it comes to the basics: compact, uncomplicated, modestly-sized buildings.

(There may be an exception when it comes to attemtping to “cut” operational emissions by adding renewables, which may have a significant embodied impact, despite the lifetime “savings”.)

Although rules of thumb can never substiute for ‘doing the numbers’, deriving figures for the embodied impact is not straightforward, with competing calculation methods and more and less transparently-derived data on offer, and little consensus. Progress is being made though, including some work that has been done since these articles first appeared; I gave a few examples of projects that were just getting under way a year ago, which may well be worth following up now.

In the second part of the article I also considered another sort of “offsetting” – the idea that the carbon sequestered in biological building materials such as timber and hemp might be subtracted from, or claimed as credits against, the operational emissions of a building.

I was not  (and continue not to be) keen on this idea, resting as it does on the presumption that ‘replacement’ plants will be grown elsewhere (ie probably not on the roof of the building!) – and to be punctilious, it also assumes that those plants would not have grown, had the materials not been used. This is the same line of argument that has led me to challenge the presumed carbon neutrality of biomass burning – see other posts on this site.

However, there is no need to cite the sequestered carbon to justify the use of many ‘natural’ materials (notably timber), as they can have a remarkably low embodied impact compared to their manufactured alternatives – though as suggested above, glib assumptions are never a substitute for checking the figures wherever you can.

The article is in two pdfs, to view or download here.

Embodied energy – a ticking time bomb? Part 1

Embodied energy – a ticking time bomb? Part 2