Ecological Footprint Assessment of Building Construction

The Ecological Footprint (EF) calculations are generally performed by following the methodology defined by Mathis Wackernagel, based on top-down analysis, on macroeconomic data that estimates the footprints at various territorial levels: Earth, continents, countries, cities, etc. The present chapter establishes a reference frame, also top-down, in order to define the indicators and their relevance. The indicators have been used in the calculation of the impact of humanity on the environment, among which the EF is included. According to EF methodology, all consumptions, materials, energy, and waste absorption have their corresponding productive land requirements for their production or disposal.

The EF indicator methodology has been adapted to the peculiarities of the construction sector during the construction phase. A calculation model is presented with some innovative aspects, such as including food intake and worker mobility, or water consumption in the construction site, which are not included in the general methodology of the indicator; footprints associated with cropland, pasture and fishing appear due to the inclusion of food.

The methodology and all the steps which are part of the calculation are explained and new hypothesis are proposed, making it easy to implement the current analysis in the EF evaluation of any dwelling construction project.

The impact sources of energy and water, which consume resources directly, are analysed. Both are crucial in the EF calculation. First, for energy consumption, both fuel and electricity are examined. The transformation of these two types of consumption into EF values is performed through the existing EF methodology, although certain procedures have to be adapted to the building sector. The conversion of energy to productive territory considers forest land as the productive land necessary for the absorption of CO2 emissions resulting from burning fuel. In the energy footprint of the building, the average absorption factor obtained from urban vegetation is applied. Using the absorption and emission factors established, the energy productivity is obtained.

Secondly, the water supply EF is evaluated. Generally, all EF studies obviate this aspect due to the intrinsic difficulty of transforming water consumption data into a quantity of consumed land; a transformation is proposed. In the water EF, the forest productivity is employed, which is taken as 1,500 m³/ha/year.

This chapter analyses the impact sources that consume resources indirectly, that is, the impact is caused not by the source, but by its components. For this study, we focus on two of these components: manpower and material consumption, both of great importance in the EF calculation.

First, the manpower consumption is studied by focusing on the most determinant aspects of its impact: food and mobility. The transformation of these consumptions into EF values is performed by previously documented processes which are adapted to the specific characteristics of the building sector. Second, the EF associated to the consumption of construction materials is evaluated during the building execution process; which takes into account the energy consumption deriving from the manufacture, transport and installation of each of the materials used in the construction of buildings.

In this chapter, the environmental impact of waste and the constructed area are analysed. The waste is defined as those residues most relevant to the present model: urban waste and construction and demolition waste. For the urban waste, the generation estimates per person per year from statistical data are employed. In the case of the CDW, generation estimates come from a software tool, developed, among others, by the present authors and, which gives, according to the residential typology considered, the CDW volume expected.

Once the expected waste volumes are determined, the waste analysis is based on the methodology found in Wackernagel´s studies into the determination of its footprint. His work establishes that the footprint associated with waste disposal, emissions, and/or discharges is calculated in the same way as for the materials: the same energy intensity (embodied energy) is applied but the percentage of energy that can be recovered for recycling is deducted.

In the constructed land EF calculation, only the land used for urbanization and buildings is considered. In this case, a conversion factor is unnecessary because the units are already in terms of surface area, and the area passes from m2 to ha. The equivalence factor is that of agricultural land, since most of the infrastructure and built land are located in areas of agricultural quality.

A building and urbanization project of one hundred multifamily dwellings in Spain is studied in detail and its ecological footprint (EF) determined. The same methodology is then applied to the construction of other ten projects that include detached, semi-detached and multifamily dwellings. The impact factors are grouped into: direct consumption (energy and water), indirect consumption (manpower and construction materials), waste, and land occupied directly. The manpower impact in building construction is mainly food intake and mobility (workers commuting to the construction site).

For construction material analysis, the project bill of quantities is employed; each material quantity is transformed into its corresponding embodied energy, and expressed in terms of EF. A similar analysis, but using empirical and statistical data, is performed with the power consumption on the construction site and the workers' mobility. The waste generated, which is municipal solid waste and construction and demolition waste, is included in the analysis. Finally, the land directly occupied by the construction project also has a footprint. In summary, each element that forms part of the construction project uses resources (energy, water, manpower, materials) or generates waste, giving rise to an EF. The most important impact in all cases analysed is the embodied energy of construction materials, almost 90%, followed by the food intake by the workforce, 5-9%.

The partial and global footprints obtained are: forest, food, energy, built land, and total EF.