Lotteau, Loubet and Sonnemann (2017) showed, for French collective residential concrete buildings of two idealized form families, that building shape dominates embodied energy (EE) and embodied carbon (EC) per unit floor area, and that the compactness ratio (envelope area over floor area) is a promising but form-family-dependent proxy, with structural density still an exogenous model input. They posed as open problems the generalization of this relation: parameterized life-cycle-inventory datasets or simple proxies valid across a wider range of real geometries, structural dimensioning computed from geometry, robustness across construction systems and national datasets, trade-offs across additional impact categories, and combination with simplified use-phase models up to neighborhood scale. The generalization problem remains open.

Background

Life-cycle assessment (LCA) is the standard framework (ISO 14040/14044) for quantifying a product's environmental impacts across its whole life cycle. Applied to buildings, LCA distinguishes operational impacts (energy for heating, cooling and lighting during use) from embodied impacts — those of producing, transporting and disposing of construction materials. Embodied energy (EE) is the primary energy consumed in these pre-use and end-of-life stages, and embodied carbon (EC) the associated greenhouse-gas emissions. As regulations have reduced operational energy, EE and EC have grown in relative importance, and they are committed early: decisions about a building's overall massing — shape, height, footprint — are taken at the sketch-design stage, before any detailed material specification, yet lock in much of the embodied impact. Quantifying shape effects at that stage is hard because conventional LCA needs a detailed bill of quantities.

Lotteau, Loubet and Sonnemann (2017, Energy and Buildings 154:1–11) addressed this for French collective residential buildings in reinforced concrete. They decomposed a building into functional elements — generic component categories such as intermediate floors, load-bearing structure, partitions, facades and roofs — whose quantities can be computed from a small set of geometric parameters, and combined them with French (INIES) and ecoinvent life-cycle inventory (LCI) data, i.e., datasets of material and energy flows per unit process. Using two parametric generic form families, "block" and "court" (courtyard), and Sobol sensitivity analysis (a variance-decomposition technique that attributes output variance to individual input parameters and their interactions), they found that shape dominates EE and EC per unit floor area within the a priori design space accessible to designers: the number of storeys alone explains about 54% of the EE variance, while footprint dimensions, storey height and the structural density (the quantity of structural material per unit of built floor area, treated as an exogenous input parameter) each explain roughly 6–10%; within this space, shape can matter more than the composition of the functional elements. Contribution analysis identified intermediate floors, partitions and the structure as the main contributors to both EE and EC. The compactness ratio C=Aenv/AfloorC=A_{\mathrm{env}}/A_{\mathrm{floor}} (total envelope area divided by total floor area; higher values mean a less compact building) proved a good univariate proxy for both EE and EC — but the fitted linear compactness–impact laws differed between the block and court form families, so their validity for real, mixed geometries is unproven. Foundations were dimensioned from geometry (excluding them changes impacts by only a few percent), but the superstructure was not: structural density remained an input rather than a computed result. A supplementary analysis quantified foundations and underground parking: one basement parking level adds roughly 10% EE and 12% EC for a four-storey reference building, an effect diluted to a few percent for tall buildings.

The study is thus bounded by one country (France), one structural system (reinforced concrete collective housing), two idealized form families, and two impact categories (EE and EC). In its conclusions the authors state what would be needed to turn these case-specific findings into a general early-design capability, and those directions — jointly, not separately — constitute the open problem.

Problem Statement

The open problem posed by the source study is whether its shape–impact relation can be generalized from one country, one structural system, two idealized form families, and two impact categories into a validated, geometry-sensitive early-design assessment capability. Concretely: (i) can the compactness–EE/EC relation be developed into generalized parameterized LCI datasets or simple morphological proxies whose predictive validity extends across a wider range of real building geometries (courtyards, setbacks, wings, mixed forms), rather than only the block and court idealizations; (ii) can structural density be computed inside the model from geometry (spans, heights, loading), with foundations and underground parking fully and generally accounted for, instead of being an exogenous input; (iii) how much of the shape–impact relation is an artifact of the chosen functional-element definitions and of French LCI data — i.e., how do the results change across construction systems and countries; (iv) which shape-related trade-offs appear when the model is extended beyond EE and EC to additional impact categories; and (v) can simplified morphology-sensitive models of use-phase contributors (heating, cooling, lighting) be combined with the embodied assessment so that building shape is evaluated on both embodied and operational counts from building to neighborhood scale? These five directions serve one common objective: a validated method by which the embodied — and, combined, total — environmental consequences of building-shape decisions can be quantified generally at early design stages, not case-by-case. All five directions (i)–(v) are jointly required; the objective is a model, dataset, or proxy whose geometry sensitivity and validity domain are demonstrated on real buildings beyond the original form families, structural system, and national dataset (at minimum across two further construction systems and national LCI datasets), whose structural density is computed from geometry rather than supplied as an input, which extends to impact categories beyond EE and EC, and with the coupling to use-phase assessment shown at least at one integrated scale. Directions (iii) and (iv) are part of that target: how much of the shape–impact relation is an artifact of the chosen functional-element definitions and of French LCI data, and which shape-related trade-offs appear when the model is extended to at least one additional impact category beyond EE and EC.

References

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