Some Notes on the Pomponi Study on Optimal Sustainable Density

A study by Francesco-Pomponi (professor at Napier) and colleagues is gaining at lot of attention.

Decoupling density from tallness in analysing the life cycle greenhouse gas emissions of cities in Urban Sustinability (part of the Nature family).

Achieving optimal use of space and maximal efficiency in buildings is therefore fundamental for sustainable urbanisation. There is a growing belief that building taller and denser is better. However, urban environmental design often neglects life cycle GHG emissions. Here we offer a method that decouples density and tallness in urban environments and allows each to be analysed individually. We test this method on case studies of real neighbourhoods and show that taller urban environments significantly increase life cycle GHG emissions (+154%) and low-density urban environments significantly increase land use (+142%). However, increasing urban density without increasing urban height reduces life cycle GHG emissions while maximising the population capacity. These results contend the claim that building taller is the most efficient way to meet growing demand for urban space and instead show that denser urban environments do not significantly increase life cycle GHG emissions and require less land.

A frequent criticism of the mantra that denser is more sustainable is that denser is more energy efficient is that when you add in lifecycle costs of buildings taller is not necessarily more sustainable. The study is right to add in lifecycle costs and does so by a novel method of decomposing FAR from a single scalar value into a vector of height and land coverage components.

The study is also novel in using parametric methods of 5,000 sample 1 km grids. The result though does not look appealing in terms of open space access or daylight and sunlight, neither of which were analysed, though they often are using parametric methods.

The fault in the study lies in it looking at the 1 sq km grids in isolation and not considering transport costs or land values. You need to analyze an urban system and not just isolated grid square (and probably use hex bins to reduce axial spatial distortion). The study tell us about building energy use but not about optimal urban planning strategies and whether slightly or even a lot taller in some areas is an optimal urban planning strategy if it minimises city energy use