Background There is a general consensus on the need to reduce carbon dioxide (CO2) emissions. A key means of reducing equivalent CO2 emissions is by reducing energy consumption. Strategies including the implementation of energy performance certificates (Directive 2002/91/EC) can help work towards this objective. For the building sector reducing energy consumption means taking a lifecycle approach and addressing consumption at each stage, including the sourcing of materials, construction, use and end-of-life. There has been much awareness developed in recent years on the importance of energy-efficient buildings, including the use of insulation, solar panels and efficient lighting systems. However, more attention is needed on the emissions generated during the construction phase. The manufacture of concrete is currently responsible for 75% of the emissions from the construction sector. There is a need to consider alternative building systems that can reduce CO2 equivalent emissions. An interesting option is through the use of local resources with low embodied energy and a return to traditional methods of architecture. Posidonia oceanica (Neptune grass) is a seagrass species endemic to the Mediterranean. On the Balearic island of Formentera, P. oceanica has been traditionally used in multiple applications, including the thermal insulation of buildings. Objectives The REUSING POSIDONIA project aims to demonstrate the feasibility of developing a multifamily residential building with a significantly reduced ecological footprint. It aims to reduce energy and water consumption and the production of waste during the construction phase and service life of the buildings. The project proposes a model of hyper-local architecture, linking environmental issues with the cultural tradition of the region and restoring traditional local architectural techniques. It will implement and evaluate alternative construction techniques for a building with a total floor area of 1 083 m2. Specifically, it will test Neptune grass packed in re-used pallets as insulation material and non-reinforced lime concrete in the foundations. The project will also introduce measures to reduce energy, water and waste consumption. It will implement passive energy-saving systems using natural sea breezes to control heat in the summer. An efficient active system will be introduced to optimise heating in the winter using renewable energy sources – including facilities for biomass. Water consumption and waste generation will be minimised by facilities to harness rainwater and recycle waste. The project will test and assess these construction techniques to ensure that they meet the necessary technical requirements of the building. It will also ensure that they do not imply significant increases in cost or time delays for the constructor. The project thus hopes to demonstrate the feasibility of more environmentally-friendly forms of construction that can both reduce the emission of greenhouse gases and improve the quality of city landscapes. Expected results: The project expects to achieve the following results: A 50% reduction in CO2 emissions during the construction phase – equivalent to some 300 kg/m² or a saving of nearly 325 000 kg of CO2 for the test building; Demonstration of the thermal conductivity of Neptune grass-based insulation in the target range of 0.40 - 0.45 W/mK; Demonstration of resistance of at least 5 KN/m by the non-reinforced lime concrete used in the foundations; Achievement of an Energy Class A rating for the developed building – with a target of some 2.8 Kg CO2/m²/yr; Reduction in energy consumption during use of the building by 75%, reaching a maximum energy consumption of 15 kWh/m²/yr; A maximum mains water consumption of 88 l/person/day, or 60% less than standard water consumption levels; A 50% reduction in waste generation; and No more than a 5% increase in construction costs.