Carrer Nou is Spain's 1st multi-residential certified Passivhaus project, achieving EnerPHit Demand Method certification. The project won the Grand Prize for Sustainable Renovation in the Spanish Green Solutions Awards 2018 , reaching 2nd place in the International version of the same awards.
The building is located in the dense city centre of Girona, at 76 metres above sea level, 30 kilometres inland from the Mediterranean Costa Brava. The climate is humid subtropical (Koppen classification Cfa), with cool winters and hot summers. According to the ES0031a-Gerona PHPP (Passive House Planning Package) climate file, average ambient temperatures in the heating and cooling periods are 9.6 ºC and 20.3 ºC respectively, with absolute maximums and minimums over the last five years (2012-2016) of 41.2 ºC and -7.3 ºC respectively.
The chosen EnerPHit certification route was the Energy Demand Method. Limiting values for the local climate (Warm-temperate) and final values achieved for the certification, are shown in the PHPP results.
Passive design strategies: due to local legislation, no thermal intervention was permitted on the outside of the building. This required careful analysis of construction details and hygrothermal performance to eliminate the risk moisture damage and thermal discomfort to occupants. The passive design strategies are explained below.
Thermal envelope and airtight layer: the thermal envelope and airtight layer was designed on a per apartment basis. This was decided due to the fact that the entrance foyer on the ground floor presented considerable challenges in terms of airtightness and thermal detailing. The lift and stairwell were therefore left out of the thermal envelope.
Thermal insulation: the existing walls (out > in) consist of 20 mm cement render, 290 mm perforated brick, and 15 mm gypsum plaster (325 mm in total). To meet the required heating and cooling demands, thermal transmittance of external walls had to meet U ≤ 0,19 W/m2K. To reach this without losing excessive floor space, while allowing for a service void to install electrical services, the chosen wall insulation system was 80 mm PIR insulation boards, dot and dab bonded to the existing walls, followed by a 48 mm mineral wool insulated service void between steel studs, and a 13 mm gypsum plasterboard finish (141 mm in total). The floor of the lowest apartment was insulated with 50 mm PIR boards, followed by a 20 mm mortar layer, 3 mm acoustic membrane and 17 mm ceramic tiles or timber flooring.
The attic floor was newly built, with a CLT timber structure, insulated externally with 140 mm of blown wood wool between timber battens, and 60 mm of high density wood fibre insulation rendered to the outside. A vapour permeable, air tight and reflective radiant barrier was installed in the roof (under the timber battens and ventilated clay tiles) in order to reduce radiation transmission gains in the summer.
Reduction of thermal bridges: the principal challenge was to reduce thermal bridges and increase minimum surface temperatures on the intermediate floor/ceiling and external wall junctions [Figure 9]. The chosen solution was to insulate floors and ceilings with 50 mm of insulation, to a distance of 1 metre inwards from the external walls. This reduced the linear thermal bridge coefficient from Ψ = 0,47 W/mK to Ψ = 0,28 W/mK, while maintaining a minimum internal surface temperature of ϑ Si = 16,67 ºC.
Airtightness: the primary airtight layer on external walls was the PIR insulation boards. They were taped at joints, and then taped at the floors with a latex-mineral membrane that was installed for acoustics. At the ceilings the PIR boards were taped to the existing plaster. Prior to installation the plaster was inspected and repaired before being deemed fit for use as the airtight layer on ceilings. Window frames were taped to the PIR boards. Roller blind casings were sealed with insulation boards and taped. Service penetrations (electrical, waste water, ventilation etc.) were all taped on their way through the airtight layer. Preliminary blower door tests revealed leaks around window blind casings, at ventilation and waste water service penetrations, at cable penetrations for window blind motors, and due to insufficient sealing of the joints between window panes and frames. Following corrections, the final airtightness test results ranged between N50 0.78/h to 1.03/h.
Protection from moisture damage: the internal insulation system required analysis of potential moisture damage with transient hygrothermal calculations to EN 15026 with the WUFI Pro tool. Transient hygrothermal simulations were run for a 10-year period, for the northern façade, to assess temperature and relative humidity at the critical interface between the existing wall and internal insulation, showing RH < 80% and no risk from moisture damage.
The building is well protected from driving rain and wind by neighbouring buildings, with only two façades exposed to the elements. However, the external cement render was repaired and re-painted, to stop the entrance of air and driving rain. Similarly, prior to installing the installation on the walls, the existing internal plaster layer was inspected and repaired to eliminate the entrance of outdoor air and moisture. Special attention was paid to the detecting and correcting air leaks through external walls during the preliminary airtightness testing.
Active design strategies: The PHPP calculation showed that passive cooling with no natural night ventilation put overheating frequency at 20 %. With a conservative calculation of 0,6 ach of night ventilation, overheating frequency dropped to 7 %. However, with the potential problem of noise due to the proximity of bars and restaurants with outdoor tables, an active cooling system was designed. Supply air heating was calculated to meet peak heating demands in the winter, but was insufficient for the summer cooling. Therefore a hybrid supply air heating and cooling system with radiant ceiling panels was designed for each apartment, with a Passivhaus certified heat and humidity recovery mechanical ventilation unit, and a heating/cooling/dehumidification coil on the supply air stream. 19 m2 of radiant ceiling panels provide the remaining power that is not covered by the supply air. Individual monobloc air source heat pumps per apartment provide thermal energy generation. In cooling mode, the control system measures the dew point temperature in each room, modulating ventilation flow rates, and cooling coil and ceiling panel water temperature, to prevent surface condensation and meet zone set points. The control system allows for remote monitoring and optimisation of performance and operation.
DHW production is centralised with solar thermodynamic heat pump system and four 500L storage tanks, located outside the thermal envelope. DHW circulation pipes runs are insulated with 35mm of insulation and run entirely outside of the thermal envelope.
Conclusions: With a final specific construction cost of approximately 1.185 €/m2 of gross floor area, the project suggests that deep energy retrofitting to Passivhaus standard “in one big jump”, is technically and economically viable in Spain for existing multi-storey residential urban buildings. To increase the uptake of EnerPHit projects of this kind, cost optimisation and the refinement of simple, replicable and robust technical solutions- above all in relation to internal insulation systems- will be important factors in years to come.