An approach to optimal nearly zero-energy buildings under Finnish and Spanish conditions

Europe has stablished the path towards nearly zero-energy buildings (nZEB), soon required in every new construction and large renovation in existing buildings. Regarding to this, the European energy performance of buildings directive (EPBD) proposes to search for cost-optimal building designs. The current study explores a great number of single-family house configurations, consisting on different energy-saving measures and energy-supply systems. In order to do this, a multi-stage methodology is used to reduce the number of needed simulations, performed by the Dynamic Building Energy Simulation model (DBES). The studied cases consist on single-family houses in Finland and Spain. Starting from reference buildings in these countries, different envelope parameters, heat recovery units, heating/cooling systems and renewable energy sources were considered. Results reveal cost-optimal solutions with primary energy consumption close to 125 kWh/m2a in Finland and 122 kWh/m2a in Spain. In order to achieve nZEB level, i.e., to reduce that consumption to 50 kWh/m2a, 20 m2 of PV-panels are needed in Spain to generate electricity. However, this value rises to 50 m2 in Finland. Global annual costs remain similar, or lower in the case of Spain, to those of the reference buildings. It has been proved that improving the insulation of the thermal envelope beyond current regulation requirements is not cost-efficient. Low installation-cost heating systems (e.g. air-to-air heat pumps) are the base of cost-optimal solutions, under the financial parameters considered in this study. Although, more efficient systems (e.g. ground source heat pumps) could soon reach the cost-optimal solutions if their costs keep decreasing