The 2022 Habitat Competition Spotlights Climate-Positive Concrete Housing

The winners of the 2022 Habitat Competition—organized by the Association of Collegiate Schools of Architecture (ACSA), which represents architecture schools in the U.S. and Canada, Habitat for Humanity, and The National Ready Mixed Concrete Association (NRMCA)—have been announced. This year's challenge was for "students, working individually or in teams, to explore a variety of design issues related to the use of concrete in design and construction of a Habitat home." See the winning entries here.

The competition is indeed a challenge because one could make the case that it is pretty hard to make low-rise concrete housing climate positive, given the upfront carbon footprint of concrete. Making cement accounts for 5% of carbon emissions, and low-rise housing is really the low-hanging fruit, given that the vast majority of it is built of wood frame and there are so many options that are more carbon positive. So what have these students done that justifies the use of the material?

Architecturally, I find the West Region Winner (Abby Loftus of Savannah College of Art and Design) to be the most interesting and wonderfully presented. The program is for low-income housing in a high-rent ski town in Colorado.

It's a very attractive terraced housing scheme. The jurors liked it for many of the same reasons, noting that "the project is well presented through beautiful renderings and responds to a difficult topography with a wonderful design solution."

Alas, above grade, it is made from conventional insulated concrete forms (ICF), a polystyrene and concrete sandwich with possibly the highest upfront carbon emissions of any form of construction. Below grade, the terracing is achieved with massive concrete retaining walls. In a previous post discussing ICFs, I concluded that "wherever there is a substitute, we should not be using concrete or petrochemicals in green building" and that foam and concrete sandwiches should not be on the menu.

The Central Region winners (Stella Coble & Michelle Powell of the University of Texas at Austin) developed "a sophisticated design that walks you through the environmental conditions in relation to the site, the inhabitants, and the material focus of this competition, namely concrete" and "references how concrete can be deployed for residential construction using tilt-up construction." But the concrete mix is very interesting, chosen to reduce upfront emissions.

"On a material level, concrete panels are made from liquid waste carbon injected concrete and minimize the amount of cement emissions with clinker partially substituted with raw limestone to lower its carbon footprint. In addition to concrete tilt-up panels, the project also uses concrete in precast structural tees and cast-in-place foundations and slabs. It also uses corrugated roofing made from 100% recycled product and terracotta clay blocks, sourced from San Antonio, which are used in the garage breezeway."

The North Region Winner (Bella Scott of the University of Illinois, Urbana-Champaign) is nestled among the wood-frame houses of Vancouver. This Canadian city is at the heart of the North American wood revolution.

But, hey, it is a concrete competition, so the project is built out of "Ashcrete," which is "a concrete substitute consisting of recycled fly ash and other industrial waste. The substance is generally stronger and able to last longer than traditional concrete, allowing the building to exist longer in the moderate Vancouver climate." The problem here is that fly ash is sourced from coal-powered generating plants, and there are none in British Columbia, so it has to be imported from Alberta and is in short supply. Scientists in British Columbia are now looking at ways to replace fly ash with crushed glass because "we know the price of fly-ash is going to increase in the future."

The South Region Winner (Ryan Bramlett & Nikolas Makela of the University of Colorado Denver) is in New Orleans, which raises a different set of issues. Actor Brad Pitt could tell you about the problems building with wood frame construction here, whereas concrete might well be a more resilient choice.

"Efficient and sustainable homes that can offer a level of protection against natural disaster lowers both initial and lifecycle costs for housing units in a historically disenfran­chised community. Resiliency is reflected in layers of programmatic el­evations which, alongside the longevity and inherent strength of precast concrete systems, creates a foundation to design home place resilient to the site’s inherent threats."

The competition raises important questions about where the use of concrete is appropriate. I think it is fair to say that fly ash concrete is not appropriate in Vancouver, that better mixes of concrete with carbon cure-type injection of carbon dioxide make sense everywhere, and that the climate and flooding conditions in New Orleans make it a very special case. But I am hard-pressed to figure out how any of these can be called "climate positive."

As someone who teaches sustainable design at Toronto Metropolitan University, I am not sure what I would do if my students were asked to enter this competition. I would certainly insist they answer the question: Should we be building low-rise housing out of concrete in the first place? Define climate positive! And I also wonder, should the Association of Collegiate Schools of Architecture be participating in this at all?