Embodied GHG of missing middle: Residential building form and strategies for more efficient housing

Abstract

This research addresses two critical problems facing communities today: the growing demand for housing and the need to reduce material consumption to mitigate the impacts of climate change. Material production and use accounts for more than 25% of annual global greenhouse gas (GHG) emissions and must be reduced to meet the Paris Climate Agreement's 2°C target. At the same time, increasing urban populations are accelerating the demand for housing and construction materials. Strategies for supplying more materially efficient housing are urgently needed. Here, we quantify the impact of residential form on embodied emissions. Specifically, we look at the reduction potential of missing middle (low-rise multi-unit) housing, compare missing middle to single-family and mid/high-rise buildings, and identify opportunities for optimizing efficiency within forms. Forty-two new material quantifications are calculated using an ontology based on MasterFormat and UniFormat. Minimum, maximum, and most likely GHG emissions factors are used to convert material mass to CO₂eq. We observe embodied GHG of missing middle buildings varying between 5540 and 39,600 kgCO₂eq/bedroom. On average, multi-unit missing middle buildings have significantly lower embodied GHG per bedroom than single-family and mid/high-rise buildings, but variability within forms is greater than between forms, indicating a large potential to reduce embodied GHG through building design. Best-in-class design strategies include reducing substructure size and indoor parking, limiting mid/high-rise slab thickness, and choosing low-GHG insulation products. Building missing middle homes in the 1st quartile of embodied GHG efficiency with minimum insulation emissions factors could reduce future embodied residential emissions in Ontario, Canada by 46.7%.