Green building’s dark secret—and new allure

© Bruce King March 28, 2018

First the bad news: net zero isn't net zero.Not even close. We have worked out how to build a house or school or even large office building so well that it can generate at least as much energy as it uses while still keeping things lit, comfortable and safe. Great progress, for sure, and what's not to like about a no-cost energy bill. But we've so far conveniently ignored the carbon elephant in the room:that huge wad of greenhouse gas emissions that accompanies the construction of any and every building project—a carbon wad often made even bigger because we made the building "net zero"!

The so-called embodied carbon or carbon footprint of a building (or any human construction) is essentially the climate impact of that thing's creation.That impact can be huge, but is usually much less than the lifetime climate impact of the operating carbon emissions—the climate cost of power, lighting, heating, and air conditioning.And so green building has focused almost exclusively on reducing operating energy, moving towards "net zero" with better insulation, windows, mechanical systems, and onsite energy harvesting.As a result, we have more and more net zero buildings and passivhaus homes, and for that matter electric cars and other transformative zero emission technologies.Many regard that as a sort of mission accomplished, the best that we can do for climate-friendly design.But that is a mistaken and dangerous understanding, for it ignores the time value of greenhouse gas and its real climate impact.

The climate effect of an emitted carbon gas is its global warming potential (GWP;1.0 for carbon dioxide, and higher, sometimes much higher, for other carbon compounds), multiplied by the time it is in the air.The embodied emissions of any building project are up there cooking before the project is even occupied, while the operating emissions—or savings from an efficient or net-zero project—are attenuated over the 40 or 60 or 100 year service life of the structure.This gives embodied emissions a time value, a heavy weighting because they are at work for much longer.That time value is further exacerbated by this pivotal moment in history:if we're going to reverse climate disruption, we need actions that are effective today far, far more than nice savings spread over the decades to come.Time is of the essence, and that makes embodied carbon emissions all the more essential.

Now, the good news.We know where most of the embodied carbon of buildings is:Portland cement, metals, foam insulation and, surprisingly, the refrigerants in freezers, coolers and air conditioners.There's plenty in other places, of course, but these are our primary targets, and plenty of people and money are chasing climate-friendly alternatives to each.We have a long way to go, but several promising trends are emerging.For the first time in history, we can build large and complex buildings entirely of wood, instead of carbon-intensive steel or concrete, and you've probably heard of this rapidly growing class of mass timber structures—a fantastic development that works well for the climate only so long as the wood is sustainably sourced.And, straw bale construction has exploded out of its humble invention a century ago in Nebraska, USA to appear in almost every country on Earth:as a highly insulating wall system, an inspiration for the advent of derivative insulation technologies, and a sort of poster child for the much broader possibilities for using agricultural by-products in construction.

This and much more led us to notice that we can not only reduce embodied carbon, but flip the dynamic around to carbon positive buildings that absorb more carbon than they ever emit.Want to know more?We wrote a book about it:"The New Carbon Architecture / Building to Cool the Climate".

Ecococon in Europe is building mass timber structures insulated with compressed straw and meeting Passivhaus standards. Photo courtesy of Ecococon