Data on nearly zero energy buildings (NZEBs) projects and best practices in Europe

This data article refers to the paper “Assessing Nearly zero energy buildings (NZEBs) development in Europe” [1]. Data linked with this article relate to collected best practices NZEBs throughout Europe. Data on building geometry, year of construction or renovation, primary energy consumption, saving percentages, renewable production, heating demand are provided. Data allow an overview of the status of most commonly implemented efficiency measures and renewables in NZEBs. In particular, data are available in relation technologies, such as heating, domestic hot water, lighting, renewable sources, ventilation, cooling. Heat recovery efficiency data are also collected. U-values are detailed for roofs, walls, floors, windows. Further data can be visualized in relation to technologies costs, cost of construction and maintenance.


Value of the Data
• The data are useful to follow the implementation of NZEBs and related technological measures. • The data can be used to have quantitative information on NZEBs in Europe in terms of energy consumption, efficiency measures and costs. • The data give insight on retrofit solutions for envelope, appliances, and systems in NZEBs.
• The data support energy efficiency and energy policies related to buildings.
• The data can be useful for the development of NZEBs, comparison with other building types, retrofit intervention, or further analysis.

Data Description
Buildings are of strategic importance of European policies aimed at limiting greenhouse gas emissions [10] . Although European policies encouraged the construction sector to move towards NZEBs [11] , the majority of NZEBs are still demonstration projects, indicating that a full implementation of the concept is not yet reached [12] . Identifying best practices help the NZEBs diffusion. Reported data relate to NZEBs projects and best practices in European Member States as collected from different sources [2][3][4][5][6][7][8][9] .
Data on NZEBs best practices are attached to this paper in the form of an excel spreadsheet (named "NZEBs"). It is composed of different sheets: In Sheet 1 (named "NZEBs best practices"), the following information is available for each collected best practice building: • Building/project name and category (e.g. office, hotel) • Member State location (country in which the building is located) • Year of construction or refurbishment (year in which the building was constructed or renovated) Implemented technologies (systems installed in the building to cover the energy needs, e.g. heat pumps) • Primary Energy Demand (PED) (kWh/m 2 y) (amount of energy that must be generated to satisfy the total energy demand of the building) • Renewables (RES) (%) (percentage of renewable energy in the building) • Floor area (m 2 ) (area of the building) • Space heating demand (kWh/m 2 y) (amount of heating required to heat the building) • Building Type (e.g. residential, non-residential) In Sheet 2 (named "Technologies"): • Average RES share in best practice example buildings by MS (average of the renewable energy present in the buildings) • Average primary energy demand in best practice example buildings by MS (average of the primary energy demand) • Average space heating demand in best practice example buildings by MS (average of the space heating demand) • Data source In Sheet 6 (named "Heat recovery"): • Building/project name • Member State location; • Heat Recovery Efficiency (efficiency of the installed heat recovery system) • Average heat recovery efficiency per MS (average of the above) • Data source In Sheet 7 (named "Systems"): • Implemented technologies • Number of best practice buildings where a technology was installed (buildings having a certain technology) • Relative percentage (derived percentage of the building having a certain technology) In Sheet 8 (named "U-values"): • Building/project name • Member State location • Data source • U-value for walls, roofs, floors, windows (or heat transfer coefficient: measure of heat loss through a building shell element, e.g. walls, roofs, floors, windows) • Average U-Value for NZEB exemplary buildings per MS (average value of the above)

Experimental Design, Materials and Methods
The 51 buildings included in the data are located in 20 Member States (BE, FR, IT, IE, AT, SE, UK, EE, DE, FI, DK, SI, NL, BG, HR, PL, MT, PL, LU, HU). Italy and Austria represent the largest part of NZEBs of the sample (12% each), followed by France and Germany (10% each). Various types of buildings are included in the sample, like educational, commercial, office and residential buildings from 19 Member States. The 43% of the buildings are educational buildings (schools, nurseries, child-care facilities, universities). Some of them are pilot projects. The 33% are residential buildings (family houses, apartments). The 12% are office/administration buildings. Finally, there are also hotels and mixed used urban projects. The building/projects included have been constructed during the period from 2002 to 2019. However, for some of them the construction year is not available.
Data allow visualize the average primary energy demand values (including renewables) of NZEBs best practices per country, as shown in Fig. 1 .
In the collected NZEBs best practices, the projects with available on energy demand data are 28, ranging from the École François Mitterand in France ( −7.5 kWh/m 2 y) to the Plus Energy Settlement in Kleehäuser , Freiburg in Germany (152 kWh/m 2 y) (2006) [6] . Data are higher for Member States with colder climate. An exception to this is the Green Lighthouse in Denmark (3 kWh/m 2 y) [2] . Some variations to this pattern may be explained by the differences in construction years and consequently in legislation and technology development.
Data allow also visualise the most diffused NZEBs technologies. As example, a NZEB best practice building using geothermal heat pumps for heating is the Primary School in Bielawa in Poland. An example of NZEB using ambient air heat pumps for heating is the Technical University Sofia in Bulgaria. Finally, an example of a building using water heat pumps is the Faculty of Agriculture, University of Osijek in Croatia [2 , 3] . Reported energy services differ within the collected building sample. As example, heating is available in all buildings (e.g. solar thermal, gas and pellet boiler, heat and water pump, district, cogeneration, and floor heating), cooling (natural and mechanical) is reported in 17 buildings, and domestic hot water (e.g. heat pump, solar thermal, district, boiler) in 36.
Main costs associated with technologies are available. As example, Table 1 is obtainable from the data linked to this paper [5] . It summarizes the main costs associated to heat pumps in 4 Member States (France, Bulgaria, Spain and Italy). Total cost includes installation cost, product   cost, initial design cost, annual maintenance cost as well as operational energy costs. Same kind of tables can be obtainable for other technologies from the provided data. Data are also collected for specific technologies. For boilers, there are NZEBs pilot education buildings in Europe using this technology for space heating [2 , 3] . Examples are the École François Mitterand in and the Hill Primary School in the United Kingdom. For biomass boilers, Elementary School and Kinder-garten Albrechtsberg in Austria is a best practice example of a pilot education building using biomass boiler for space heating and domestic hot water.
Data linked to this paper allow to visualise NZEBs that use exclusively passive space cooling methods. Elementary School and Kinder-garten Albrechtsberg in Austria [2] use night ventilation for space cooling while NZEB multi-family house F3 Brdo , Ljubljana, uses external shading, manually operated roller blinds [8] . In the collected best practices, there are also NZEBs combining mechanical cooling with natural cooling solutions. Green Lighthouse in Denmark uses both heat pumps a well as the night ventilation for cooling [3] . On the other hand, NZEBs, especially in warmer climates, use often mechanical cooling solutions ( Fig. 2 ). The Leaf House in Loccioni, Italy uses heat pumps and radiant floors for cooling [6] . The Technical University Sofia , Bulgaria uses a VRF system [2] .
The majority of the collected buildings with available information about ventilation includes heat recovery systems ( Fig. 3 ). Heat recovery is an air-to-air heat or energy recovery system  which works between two sources at different temperatures. Current heat recovery systems are able to recycle about 60-95% of wasted energy, which is promising [13] . In the collected projects best practices, the heat recovery ranges from 70% to 96%. An example of this case is the École François Mitterand in France [2] where heat recovery systems are used in winter while natural ventilation is achieved through windows during summer months. The office building in Žminj, Croatia, is ventilated by a forced recuperation ventilation system combined with a heat exchanger [2] . The Multifunctional School Houthaven in the Netherlands uses heat recovery units for ventilation [2 , 3] . The ENERPOS University complex in Saint Pierre, France has ceiling fans as well as a VRV system [6] .
In relation to heat recovery efficiency in collected NZEBs best practices ( Fig. 4 ), the Beddington Zero Energy Development in London heat recovery system presents an efficiency of 70% [6] . The Bushbury Hill Primary School in the United Kingdom has a mechanical ventilation system with heat recovery efficiency by 80% during winter, while it is based on windows during summer [2] . Finally, the Green Home Nanterre in France has a decentralised ventilation system of 96% heat recovery [4] . Data are also collected for efficient insulation, an important passive method to reduce energy demand and achieve high energy savings [14 , 15 , 16] . In Fig. 5 , the average U-values per Member State for every building element are presented as collected in NZEBs best practices with available data for walls, roofs, grounds and windows.
U-values range from 0.08 W/m 2 K (NZEB After School day care center in Belgium) to 0.4 W/m 2 K ( Faculty of Agriculture, University of Osijek in Croatia) [2] . The U-values for roofs range from 0.07 W/m 2 K ( Primary School in Budzów in Poland) to 0.26 W/m 2 K ( Research Centre of the Technical University of Sofia in Bulgaria) [2] . For floors, U-values range from 0.10 W/m 2 K ( Beddington Zero Energy Development in London and in Plus Energy Settlement in Weiz-Gleisdorf in Austria) to 0.41 W/m 2 K ( Leaf House in Loccioni in Italy) [6] . For windows, U-values range from 0.27-0.40 W/m 2 K (in Maardu Day Care Center in Estonia [9] to 1.76 W/m 2 K (in École François Mitterand in France) [2] .
Among examples that do not impose the RES requirement in their national NZEB standards, there are the Primary School Mariagrün in Austria and the Bushbury Hill Primary School in the United Kingdom [3] . Regarding the studied NZEB buildings/projects, 19 out of 51 have available data for their RES contribution. This share ranges from 21% to 105%. The Plus Energy School Hohen Neuendorf in Germany uses biomass, CHP and PV to cover the 21% of its energy needs while the École François Mitterand in France includes 406 m 2 of PV to cover an equal share [3] . On the contrary, RES cover the 105% of the primary energy needs of the Väla Gård in Sweden (all energy use included, excl. PV generation) [4] . Best practice examples using solar thermal energy for space heating are the Lehtomäki day care center in Finland [9] and the NZEB multi-family house F3 Brdo in Ljubljana, Slovenia [6] . For example, the Plus Energy School Hohen Neuendorf in Germany has 400 m 2 of photovoltaics [2] .
In relation to lighting, Multifunctional School Houthaven in the Netherlands uses presence detectors while Sustainable Building Energy Information Centre Debrecen in Hungary includes both presence and daylight detectors in its lighting. Best practice examples are the Luthaa Nursery in Finland (LED lamps) and the Childcare Centre Cologno Monzese in Italy (energy saving lamps) [2 , 3] .
Regarding the costs due to additional NZEB technologies, the lowest ones are identified in Bushbury Hill Primary School in the UK, a building based on passive solution for cooling and ventilation in summer [2] . In Luthaa nursery in Finland and in Kleehäuser , Freiburg in Germany the additional costs are only 10% [2 , 6] . On the contrary, the additional costs are 50% (or 2600 €/m 2 ) in Green Lighthouse in Denmark, a building that uses geothermal, solar thermal and PV as renewables, district heating, heat pumps and LED lights [2] . In addition, the impact of NZEB technologies on investment costs in Väla Gård is calculated to be around 61% (6% RES, 25% HVAC, 1% DHW, 12% ventilation, 11% heating and 6% windows). The building includes ventilation with heat recovery, ground source heat pumps and PV [4] . Fig. 6 shows the average project and lifecycle maintenance costs of collected best practices NZEBs per Member State.
The data from 24 NZEBs show that the project costs range from 682.3 €/m 2 in A-32 building, Salburua in Spain [AZEB, 2018] to 2624 €/m 2 in Moretti More in Italy [4] . Data for maintenance costs are available in 11 buildings/projects from [4] . These data range from 366 €/m 2 in Brussels Rings to 1106 €/m 2 in Isola nel Verde in Italy [4] . The average maintenance costs in these projects are 750 €/m 2 . A broad scale shift towards NZEBs requires an effort in the current market. Costeffective integration of efficient solutions and renewables remain a major challenge.

Ethics Statements
No ethics fields involved.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.