Why Building Density Matters as Much as Building Efficiency

We'll always have Paris.

We spend a lot of time on this site talking about reducing our carbon footprint and about building more energy-efficient buildings and homes. A lot of urban activists are talking about the need for more "missing middle" housing and why we have to increase density. I go on about how most of our transportation and its related emissions are just about getting between buildings, and that what we build determines how we get around.

Put it all together and you can only conclude that our built form and density are really among the most important factors when it comes to our per capita carbon emissions. After a recent discussion of this on Twitter, architect Mike Eliason pointed to a study from 2013, Cities and energy: urban morphology and residential heat-energy demand, that looked at different building forms and types, modeled them and concluded:

Compact and tall building types were found to have the greatest heat-energy efficiency at the neighborhood scale while detached housing was found to have the lowest.

This is not a surprise; David Owen wrote a book about it. We have shown other studies that came to this conclusion; my favorite has always been the Canadian Urban Archetypes Project which looked at single-family and small multifamily urban projects, finding that crappy old multifamily buildings had lower overall carbon footprints than modern subdivisions. This European study doesn't include transportation emissions like the Archetype one did, but is still fascinating.

The study looked at built forms in London, Paris, Berlin and Istanbul.

Overall, the hypothesis that different building morphologies feature distinctively different energy demands and that higher density building configurations lead to greater heat-energy efficiency was confirmed. The ratio between the least and best performing sample is greater than factor six, emphasising the significance of a better understanding of design-related impacts on heat-energy demands. The average building height and building density were found to be good indicators for heat-energy efficiency, each correlating negatively with the heat-energy demand. The surface-to-volume ratio also correlates well but positively with heat- energy demand.

The results show that detached houses have the worst energy performance, (no surprise there) followed by High Rise Apartment buildings. Compact Urban Blocks and Regular Urban Blocks generally have the lowest primary energy demand per square meter.

It's hard to separate the greys in these triangles but it's clear that the compact forms you see in Paris with floor area ratios between four and five are the most efficient. The authors conclude:

In summary, the theoretical results of this study suggest that urban-morphology-induced heat-energy efficiencies are significant. Our main analysis with fixed parameters for all variables except urban form resulted in theoretical variances in heat-energy demand for extreme cases of up to a factor 6. Differences of factor 3 to 4 were common across the most typical urban morphologies in each city and persisted for different insulation standards and climatic conditions.

In other words, we need to build well-insulated efficient buildings at missing-middle or Goldilocks densities, like they did in Paris or do now in much of Austria and Germany. Building efficiency is not enough; density apparently matters a whole lot more.