Evaluación del impacto ambiental durante todas las fases de vida del edificio

The environmental impact assessment of a building throughout its life cycle is governed by European standard EN 15978:2011 (Sustainability of construction works — Assessment of environmental performance of buildings — Calculation method), which defines a modular framework of 4 macro-phases and 17 information modules. The product phase (modules A1-A3) covers raw material extraction, transport to factory, and product manufacturing; the construction phase (modules A4-A5) includes transport to site and the construction process. The use phase (modules B1-B7) encompasses the building's own use, maintenance, repairs, replacements, refurbishments, operational energy consumption, and operational water consumption. The end-of-life phase (modules C1-C4) includes demolition, waste transport, waste processing, and final disposal. An additional module (D) accounts for the net benefits of reuse and recycling beyond the system boundary. According to aggregated data from Rock et al. (2020), based on a meta-analysis of 650 building LCA studies published between 2000 and 2020, the average emission distribution is: product phase A1-A3 at 45-65% of the total in conventional buildings and 60-80% in nZEB buildings, use phase B6 (operational energy) at 25-45% in conventional and 10-25% in nZEB, and the construction, maintenance, and end-of-life phases collectively at 5-15%.

The environmental impact categories assessed in a building LCA conforming to EN 15978 include global warming potential (GWP, measured in kg CO2eq), ozone depletion potential (ODP), acidification potential (AP), eutrophication potential (EP), photochemical ozone creation potential (POCP), and abiotic resource depletion (ADPE and ADPF). In practice, GWP concentrates the greatest regulatory and market attention. Benchmark GWP values for the complete life cycle of a building vary significantly by typology and climate zone: an office building in central Europe presents values of 800-1,500 kg CO2eq/m2 over 50 years for conventional construction and 400-700 kg CO2eq/m2 for optimized construction (DGNB, 2023). A single-family dwelling in Spain ranges from 600-1,200 kg CO2eq/m2 over 50 years for standard CTE construction and 350-600 kg CO2eq/m2 for Passivhaus with low-carbon materials. The complementary standard EN 15804:2012+A2:2019 standardizes the format of Environmental Product Declarations (EPDs), which provide the environmental impact data for each material needed for the building LCA calculation.

Phases A1-A5 (production and construction embodied carbon) constitute a concentrated emission over a 1-3 year period that cannot be reduced retrospectively, unlike operational carbon which can be mitigated through renovations and electrical grid decarbonization. The materials with the greatest contribution to embodied carbon are reinforced concrete (30-50% of the total A1-A3), structural steel (15-25%), aluminum for windows and facades (5-15%), and petrochemical-based insulation (3-8%). One cubic meter of conventional reinforced concrete (C30/37 with 300 kg/m3 of CEM I cement and 80 kg/m3 of steel) emits 350-450 kg CO2eq, while an optimized concrete with 50% cement replaced by fly ash and slag, 30% recycled aggregate, and electric arc furnace steel with 90% scrap emits 180-250 kg CO2eq/m3, a reduction of 40-50%. The cement industry has set the target of reaching 430 kg CO2/tonne of cement by 2030 (compared to the current 610 kg CO2/tonne) through low-carbon clinker, alternative fuels, and carbon capture (GCCA, 2022).

The construction phase A4-A5 typically represents 3-8% of total embodied carbon but includes optimizable aspects. Material transport (A4) depends on distance and transport mode: local ready-mixed concrete (20-40 km) generates 5-15 kg CO2eq/m3 in transport, while natural stone imported from China can generate 200-400 kg CO2eq/tonne from maritime and road transport alone. The construction process (A5) includes energy consumed by site machinery (cranes, concrete mixers, welders) and waste generated. A conventional 10,000 m2 building generates between 800 and 1,200 tonnes of construction waste, equivalent to 80-120 kg/m2, of which 15-30% goes to landfill. Industrialized off-site construction reduces construction waste by 50-80% thanks to the precision of automated fabrication, material offcut reuse, and inventory control, according to WRAP data (2020). In Spain, the 150-dwelling industrialized housing project by AEDAS Homes in Boadilla del Monte (Madrid, 2023) documented waste generation of 28 kg/m2, 75% below the sector average.

The use phase (B1-B7) dominates the life-cycle environmental impact in conventional buildings, but its relative weight diminishes as operational consumption decreases. Module B6 (operational energy) represents 300-600 kg CO2eq/m2 over 50 years for a conventional office building in Spain (consumption of 150-200 kWh/m2/year with an average emission factor of 0.2 kg CO2/kWh), but only 50-120 kg CO2eq/m2 for an nZEB building (consumption of 30-50 kWh/m2/year). The progressive decarbonization of the Spanish electrical grid — the emission factor of the electricity mix dropped from 0.38 kg CO2/kWh in 2010 to 0.12 kg CO2/kWh in 2023 (REE, 2024) — continuously reduces the impact of module B6, which increases the relative proportion of embodied carbon A1-A3. The maintenance (B2) and replacement (B4) modules include emissions associated with periodic finish renewals (repainting every 5-10 years: 2-5 kg CO2eq/m2), HVAC system replacement (every 15-20 years: 15-40 kg CO2eq/m2), and facade and roof renovation (every 25-40 years: 30-80 kg CO2eq/m2).

Module B7 (operational water use) gains relevance in water-stressed regions such as Spain. An office building consumes 0.5-1.5 m3/m2/year of water, and a dwelling 100-150 liters per person per day. The environmental impact of water includes the energy for treatment and distribution (0.5-2.0 kWh/m3 depending on source and supply altitude) and wastewater treatment (0.3-0.8 kWh/m3). A study by Canal de Isabel II (2023) quantified that the urban water cycle in Madrid consumes 1.2 kWh/m3, equivalent to 0.14 kg CO2eq/m3. For a 100-dwelling building with consumption of 15,000 m3/year, this represents 2,100 kg CO2eq/year or 105 tonnes of CO2eq over 50 years. Rainwater harvesting, greywater reuse, and low-consumption fixtures can reduce potable water consumption by 40-60%, with payback periods of 5-12 years. The inclusion of module B7 in LCAs is mandatory under Level(s) but frequently omitted in practice: only 22% of building LCAs published in Europe include this module in quantified form (JRC, 2023).

Modules C1-C4 quantify the environmental impact of demolition (3-8 kg CO2eq/m2 for mechanical wrecking, 5-12 kg CO2eq/m2 for selective deconstruction), waste transport to treatment plant (2-5 kg CO2eq/m2 for distances of 20-50 km), waste processing (concrete crushing: 3-5 kg CO2eq/tonne, steel remelting: 400 kg CO2eq/tonne), and landfill disposal of the non-recyclable fraction (15-30 kg CO2eq/m2 depending on composition). The total end-of-life phase C typically represents 20-50 kg CO2eq/m2, or 3-7% of the complete life cycle, but its importance lies in determining the building's circularity potential. Module D accounts for the net environmental benefits of reuse and recycling beyond the building's system boundary: recycled steel avoids primary production with a credit of -1.4 kg CO2eq/kg, crushed concrete as aggregate avoids natural aggregate extraction with a credit of -5 to -10 kg CO2eq/tonne, and recovered timber for energy avoids fossil fuels with a credit of -1.8 kg CO2eq/kg (depending on the energy mix displaced).

The regulatory application of whole life-cycle LCA is advancing rapidly. The EU green taxonomy requires for the activity of new building construction (activity 7.1) that buildings exceeding 5,000 m2 calculate the life-cycle GWP for each phase and communicate it to investors and buyers. The European Commission's Level(s) framework establishes indicator 1.2 (Life cycle GWP) as the core sustainability indicator, with three reporting levels: level 1 (simplified estimate based on generic data), level 2 (detailed calculation with specific EPDs), and level 3 (verification by an independent third party). In Spain, the VERDE certification from GBCe includes LCA as a weighted criterion at 15% of total points, and BREEAM ES awards up to 6 credits for performing a complete LCA in accordance with EN 15978. The sector forecast is that the CTE update planned for 2026-2027 will incorporate a life-cycle GWP declaration requirement for new buildings, aligning Spanish regulation with France (RE2020), Denmark (BR18), and the Netherlands (MPG), which already apply maximum life-cycle emission limits. The probable threshold for Spain is being discussed in the range of 500-700 kg CO2eq/m2 over 50 years for housing and 700-1,000 kg CO2eq/m2 for tertiary buildings, with progressive reductions of 10% every 3 years.