Extra cost analyses of two apartment buildings for achieving nearly zero and low energy buildings

• EU's nZEB requirements have induced the interest for economic quantification.
• Since 2010 the cost optimal primary energy level has shifted from 145 kWh/m2 to 110 kWh/m2.
• Achieving nZEB level of 100 kWh/m2 still requires relatively high additional investments.
• In both studied cases remarkably better energy efficiency level could have been achieved with lower investments.

The nZEB (nearly zero energy building) requirements taking effect in 2021 have induced the interest about additional cost and technical solutions amongst real-estate developers, construction companies, architects and engineers. The objectives of this study are twofold, first to determine the cost-optimal energy efficiency level for two lately built apartment buildings in a cold climate of Estonia, and secondly, which are the proper measures to achieve low energy and nZEB requirement levels. The influence of high-efficiency external walls, roofs, windows, ventilation units and solar collectors on energy use and construction costs were studied by using multi-stage methodology for reducing the number of combinations. The results show that since 2010 the cost optimal primary energy level has shifted from 145 kWh/m2 to 110 kWh/m2, but achieving nZEB level of 100 kWh/m2 still requires relatively high additional investments. With initial measures, nZEB requirements were not fulfilled in the studied cases, but the solutions close to nZEB required extra investment of 65 €/m2 i.e. 4–7% compared to the actual buildings. Low energy building level needed additional investments up to 2%. In both case studies remarkably better energy efficiency level could have been achieved with lower construction costs.