The environmental performance of zero‑emission buildings in a fossil‑free energy system

Buildings can play an important role in reducing GHG emissions through increased energy efficiency. The European Commission mandates that all new buildings should be “zero emission build-ings” (ZEB), aiming at a zero GHG emission building stock by 2050. The extent to which ZEB can contribute to reduced GHG emissions, however, varies between countries, due to different energy systems. It is also important to consider other environmental effects to avoid that climate benefits come with unintended consequences. Here, we explore the life-cycle environmental performance for a ZEB in a case where electricity and heating are largely fossil-free. The assessment concentrates on i) environmental impact of the use stage in relation to the product stage, ii) the interrelation between different energy sources, with focus on household electricity, and iii) the performance for more impact categories than primary energy use and climate change. While our results generally support the use of ZEBs from an environmental perspective, they also show that the climate benefit in this setting is marginal. However, given that energy systems are connected and energy savings in one place can reduce the demand for fossil energy elsewhere, the climate benefit of ZEBs is likely underestimated. Besides methodological implications for future studies, this indicates that current EU policy is promising, as incentives for implementation of ZEBs are unaffected by domestic effects.


Introduction
In 2015, the UN Paris Agreement was formulated, to "strengthen the global response to the threat of climate change in the context of sustainable development and efforts to eradicate poverty" (United Nations, 2015).Consequently, on 11 December 2019, the EU Commission (EC) launched 'A European green deal' (EGD), as one of its prioritised strategies (European Commission, 2019).The EGD states that EU shall reduce its emissions of greenhouse gases by 55% (relative to 1990´s emissions) no later than 2030 and have zero net emissions of greenhouse gases (GHG) by 2050.These goals are legally binding for the Member States through the 'European Climate Law' (European Commission, 2021) and represent increased ambitions from the previous goal of 40% GHG reductions by 2030.
According to the recently revised EU Energy Performance of Buildings Directive (EPBD) (European Parliament, Council of the European Union, 2024), buildings are responsible for 40% of the EU energy consumption and 36% of energy-related greenhouse gas (GHG) emissions.At the same time, 75% of Union buildings are energy-inefficient.Addressing energy efficiency in buildings is therefore imperative for reaching GHG emissions reduction targets in the EU.
Following nearly 50 years of policy development concerning energy efficiency of buildings in the EU (Economidou et al., 2020), the EPBD now prescribes that from 2030, all new buildings should be "zeroemission buildings" (ZEB).Before 2030, new buildings should be at least "nearly zero-energy buildings" (NZEB).Moreover, Member States should incentivize "deep renovation", facilitating the transformation of existing buildings to NZEBs or ZEBs, aiming at a zero GHG emission building stock by 2050.
A NZEB is defined in EPBD as "a building that has a very high energy performance, […] where the nearly zero or very low amount of energy required is covered to a very significant extent by energy from renewable sources, including energy from renewable sources produced on-site or energy from renewable sources produced nearby."A ZEB is defined in short as a building "with very low energy demand, zero onsite carbon emissions from fossil fuels and zero or a very low amount of operational greenhouse gas emissions".Although the definition of NZEB does allow for a small amount of energy for heating from nonrenewable sources, both the NZEB and ZEB definitions imply energy-efficient buildings heated with renewable energy sources.The latter indicates that the EC wishes to address and limit total GHG emissions from a life cycle perspective.
Although the ZEB concept has been used for quite some time (Panagiotidou & Fuller, 2013), a globally agreed definition was lacking until the amendment of the EPDB in 2018 (D'Agostino et al., 2021).Until the revision of EPBD in 2024, "ZEB", however, referred to a "Zero Energy Building".Prior to this, Passive Houses (PH) was the dominating paradigm for lowenergy buildings and the voluntary PH standard and certification has been an important driving force for transforming the building sector towards lower energy use.The conceptual similarity between these three concepts is that they require very high energy performance.The key difference is that NZEB and ZEB require the use of renewable energy to meet the energy needs while PH have no such restrictions.While NZEB.ZEB, and PH adhere to different standards, they rely to a large degree on passive technologies for lower energy use, such as insulation and three-glazed windows (D'Agostino et al., 2021).
A heated building will have an environmental impact during its entire lifetime, i.e., the product stage, construction process stage, use stage, and endof-life stage.The product stage includes raw materials supply, transport, and manufacturing of materials.The construction process stage covers the processes from the factory gate of the different construction products to the practical completion of the construction work.The use stage covers the period from the practical completion of the construction work to the point of time when the building is deconstructed/ demolished.The end-of-life stage of a building starts when the building is decommissioned.
Life Cycle Assessment (LCA) has been increasingly applied in building research in the last decades, when analysing the environmental performance of buildings (Anand & Amor, 2017;Chastas et al., 2016;Hollberg et al., 2021;Nwodo & Anumba, 2019).The life cycle approach gives extra 'credits' in different certification tools, such as LEED and BREEAM (Sartori et al., 2021), and is also emerging in national building regulations, e.g. the Swedish regulation on climate declaration for buildings (Swedish Parliament, 2021).The regulation is based on calculated values of greenhouse gas emissions of the product stage and the construction process stage of new residential buildings.Several LCA studies on low-energy buildings show that the global warming potential (GWP) and primary energy use are greater in the product stage and the use stage than in the construction process stage and end-of-life stage, and that GWP in the product stage can be as large as, or even larger than, GWP from energy use during the use stage over the lifetime of the building (Blengini & Di Carlo, 2010;Liljenström et al., 2014;Erlandsson et al., 2018;Dodoo & Gustavsson, 2013).Consequently, the relative importance of material choices for the overall environmental impact has increased.Furthermore, a significant variation has been reported concerning GHG emissions in the use stage for lowenergy buildings, depending on whether heat and electricity is from external sources (i.e., district heating and electricity grids) or internal (i.e., using solar energy solutions for heat and power) (Vares et al., (2024) 17:75 Page 3 of 17 75 Vol.: (0123456789)

2019
).Overall, it makes sense to focus on reducing the energy demand in the use stage, promote the use of renewable energy sources, and reduce the use of PE and fossil fuels.
The effects of energy savings in the use stage on GHG emissions largely depends on the energy source.Although passive houses have been shown to have a generally lower climate impact than conventional buildings, a passive house heated with electricity from non-fossil energy sources does not necessarily perform better than a conventional building from a climate perspective (Brunklaus et al., 2010).The choice of end-use heating system can be equally important for the climate performance as the use of passive technologies (Dodoo & Gustavsson, 2013).Using energy use as a proxy for climate performance for NZEB and ZEB can thus be problematic, especially if the heating source has low emissions of GHG; if energy savings (to reduce GHG emissions) are achieved by using more materials (i.e., insulation), total GHG emissions may then increase.Furthermore, household electricity, an important heat source in low-energy houses, has been shown to account for the greatest share of operational GHG emissions in the use stage, especially in a house heated with a renewable energy source (Dodoo & Gustavsson, 2013).Still, household electricity is often left out when evaluating a building's environmental impact, even in the EPBD.Furthermore, although energy use and GWP are important issues from an environmental perspective and high up on the policy agenda, focusing environmental assessments on only one, or a few, impact categories provides an incomplete picture of the total environmental performance.For example, bioenergy is considered a renewable energy source according to the EU directive, but combustion of biomass emits, nitrogen oxides (NOx), particles, sulphur dioxide (SO 2 ), carbon oxide (CO), and hydrocarbons (HC), for example (Strömberg & Herstad Svärd, 2012), with effects on climate, human and environmental toxicity, acidification, and eutrophication (Swedish Environmental Protection Agency, 2019).
Here, we explore the life-cycle environmental performance for a ZEB in the specific case where the energy source for space heating has very low GWP.The assessment concentrates on i) environmental impact of the use stage in relation to the product stage over a 50-year period, ii) the interrelation between different energy sources, with focus on the role of household electricity, and iii) the effect of using passive techniques to achieve high-energy performance of buildings for more impact categories than primary energy use and climate change.

Methodology
The article is based on a single-case LCA study consisting of one functional unit of analysis, a woodenframed ZEB in Östersund, Sweden (lat.63°N), built to Swedish passive house standard 2010 (Swedish Centre for Zero Energy Houses, 2012; Svensson, 2013) The environmental impact of the ZEB is then compared to a standard conventional building at the same location.

Functional unit
The house in the study is a semidetached house for two families.Each apartment has an area of 160 m 2 , divided into two floors (Fig. 1).
The high-energy performance is achieved by passive techniques, i.e., a well-insulated, airtight thermal envelope, and Heat Recovery Ventilation, HRV.The foundation is a slab-on-ground construction with 400 mm insulation of cellular plastic sheets.The walls are constructed with a primary bearing framework with vertical lightweight wood and Masonite studs, to reduce thermal bridging, and cellulose fibre insulation between the studs.A second vertical and third horizontal framework, with wooden studs and glass wool insulation, are mounted on each side of the primary framework, resulting in a 513 mm wall with a total of 415 mm insulation.The roof is insulated with 582 mm cellulose fibre and pitched both externally and internally (Fig. 1).The external roofing is fabricated of steel sheets.The calculated average thermal transmission coefficient (Um) is equal to 0.172 (W m −2 K −1 ).For further details regarding the ZEB, see Svensson (2013) and Danielski et al. (2013).
The main source of space heating is district heating from the local biomass-based CHP.The space heating is supplied by floor heating in the bathrooms and by water heating coils in the HRV unit.A wood stove is installed in the living room for peak heating demand and for the convenience of the residents (Danielski et al., 2013).

Study boundaries
An assessment was conducted of the product stage and the use stage, with focus on the latter, for the ZEB.The average final energy demand over the lifetime of 50 years for heat (including the wood stove) and household electricity, was based on separate measurements for space heating, domestic water heating, household electricity and for auxiliary electricity including electricity for the pumps and the ventilation system during one year (2012-2013), as reported by Danielski, et al. (2013;2016).Extensively used materials and materials crucial for meeting the passive house standard were included in the study, as specified in Table 1.The selection was based on Svensson (2013) but including 'Assembly materials, nails and screws' and excluding 'Construction process stage'.
The environmental impact of the construction process stage was assumed not to be specifically dependent on passive techniques.Both the construction process stage and the end-of-life stage have a very small relative impact, compared to the product stage and the use stage (see e.g.Erlandsson et al., 2018).The construction process stage and the end-of-life stage were therefore not included.

Allocation
The study has an attributional approach using yearly-average data to assess lifecycle environmental impacts.The allocation 'cut-off by classification model' was applied for the environmental impact of co-products based on product prices, i.e., economic allocation.The environmental burden was allocated to the primary producer.For a detailed description of the model, we refer to Ecoinvent, (2024).

Impact categories and characterisation models
The studied environmental impact categories are (a) climate change, and (b) primary energy use (PE), based on the EU directive (European Parliament, 2018).To provide a more complete picture of the total environmental performance, (c) ecotoxicity, (d) human carcinogenic toxicity, (e) human non-carcinogenic toxicity, (f) eutrophication, and (g) acidification were also included.The impact was calculated using lifecycle impact assessment (LCIA) data from the Ecoinvent database v3.3; a widely used database in LCA studies, for such characterisation (Wernet et al., 2016).Default values were used unless otherwise specified.The characterisation models for this assessment were: Climate change: IPCC 2013 Baseline model of 100 years (GWP 100 ) (IPCC, 1994).Global Warming Potential (GWP) is a characterization factor that describes the radiative forcing impact of one mass based unit of a given GHG relative to that of carbon dioxide over a given period of time.GWP 100 refers to a GWP over a period of 100 years.It is a very common characterization factor for LCA and is quantified in terms of kg CO 2 equivalents (CO 2 -eq).

Primary energy use (PE): Cumulative energy demand (CED)
.The goal of the CED is to calculate the total primary energy input for the generation of a product (Röhrlich et al., 2000) The value is determined by the amount of energy withdrawn from nature (Hischier & Weidema, 2010) and is quantified in terms of MJ equivalents (MJ-eq).
Labelled best practice for evaluation of toxicity in the International Reference Life Cycle Data System (ILCD) (Hauschild et al., 2013), and designed to describe the fate, exposure, and effects of chemicals.The characterisation factors (CFs) in the USEtox model are labelled recommended and interim, reflecting the level of reliability of the calculations in a qualitative way, where the reliability of the interim factors are lower than the recommended factors (Rosenbaum et al., 2008).The USEtox team

Data collection and assumptions
The Ecoinvent database offers datasets that are, to some extent, regionally adapted.In this study, LCIA datasets adapted to Swedish conditions were chosen when available (e.g.energy data for the use stage).If such data were not available, European data were used.Global data were used when necessary.For the amount of building materials (based on blueprints and interviews) and annual energy consumption see Svensson (2013) and Danielski et al. (2013).

Product stage: materials
As the actual amount of materials delivered to the building differs from the blueprints, adjustments were made to the amount of materials reported by (Svensson, 2013).
Because of the standardisation of product dimensions, for example, products often need to be cut to measure on the building site.The extra material, henceforth called ´spills´, have to be taken into consideration when assessing the product stage environmental impact.Examples of spills are waste from cutting gypsum boards, studs, flooring, etc.The amount of spills is different for different materials and building techniques.Josephson & Saukkoriipi (2005) estimated the amount of spill to vary between 2-10%.Since no records of the spills are available for the ZEB, an overall average of 8% increase of the volume of materials from the foreground inventory was assumed.
Assembly methods of, e.g.plastic foils or roofing plates, where function requires overlap, requires a greater amount of material than what can be measured from blueprints.To correct for this, overlap values from the respective manufacturers were used.
Assembly material made from metal e.g.nails, nailing plates, brackets and fixings was added.
Cellulose insulation is applied with different densities in walls and roof structures.The manufacturer´s recommended values were used to calculate absolute amounts.

Use stage
Space heating energy for the ZEB consists of two parts: (a) the active space heating from the CHP power plant and the stove, and (b) the passive space heating from persons, sunlight, and appliances.Around 40% of the heat demand in the ZEB is covered by household electricity (Danielski, 2016).Electricity thus plays an important role in heating this building, as household appliances, powered by electricity, become heat sources.Another component of space heating is pre-heating of ventilation air.In the assessed ZEB, there is a water-based preheater coil in the ventilation.Energy for pre-heating of ventilation air is included in space heating.
The Swedish electricity mix was used for the evaluation of the impacts from household and operational electricity, based on LCIA data from the Ecoinvent database.

Comparison to a conventional building
To assess the effect of passive techniques, a calculation of the environmental impact for a conventional building was made as a reference.The conventional building was assumed to be located at the same site as the ZEB with the same source for space heating.The differences between using passive techniques (as in the studied ZEB) versus conventional techniques are in the thermal performance, including airtightness, of the thermal envelope, and in heat recovery from exhaust air by a Heat Recovery Ventilation unit (HRV) (Danielski, 2016).
A ZEB has a higher thermal performance and therefore a lower energy demand in the use stage than a conventional building.On the other hand, a ZEB uses more material in the product stage.Passive techniques thus affect both the product stage and the use stage.
The thermal properties of the thermal envelope for the ZEB and the standard conventional building are listed in Table 2.
A building erected in 2010 had to fulfil the legal demands regarding maximum specific energy use at 150 kWh/m 2 and maximum average heat transfer coefficient, Um at 0,5 W/m 2 K (The Swedish National Board of Housing Building and Planning, 2008).However, a survey of the Swedish building stock up until 2005, conducted by the Swedish National board of Housing, Building and Planning (2009) of 1800 buildings in 30 Swedish municipalities, showed average U values for outer walls 1996-2005 at 0.18 W/ m 2 K, roofs at 0.12 W/m 2 K and windows at 1.90 W/ m 2 K. (Swedish Board of Housing Building and Planning, 2009).These values were interpolated to 2010 levels, based on the trend over time.U-values of the foundation and the doors was chosen from the building code for small buildings (The Swedish board of Housing Building and Planning, 2008).Based on the U-values and dimensions above, a more realistic Um value for the conventional building was calculated (Table 2).The dimensions of the construction and the amount of materials were adjusted accordingly.
The conventional roof was assumed to have less thick insulation than the ZEB and the dimension of the glulam roof beams was adjusted based on structural load.The walls in the conventional building have wooden studs and less thick insulation.The foundation has less thick underlying insulation.
Passive techniques include improved air tightness.Air leakage through the thermal envelope is commonly assessed by measuring the leakage at a 50 Pa pressure difference between the interior and exterior (q50).For this ZEB, no measured data for the air leakage is registered.The air leakage value for the ZEB (Table 2) is therefore represented by the Swedish passive house criteria for maximum air leakage 2012 (Swedish Centre for Zero Energy Houses, 2012).
Airtightness was also included in the building code but as a functional parameter without a value.A study of 100 air leakage measurements by Svensson & Hägered Engman (2009) showed that the majority of conventional buildings have a q50 value between 0.3 and 0.8 l/sm 2 Aom.A value of 0.6 l/sm 2 A om was thereby assumed for the conventional building.
U m is the average heat transfer coefficient, including the effect of thermal bridges.The impact of thermal bridges on Um varies from 5-20%, depending on the insulation-grade and design of the building (Pettersson, 2018).Here, we assume an 8.5% increase of Um for the ZEB (Danielski et al., 2013) and a 15% increase for the conventional building (Pettersson, 2018).
Table 2 Thermal properties of the thermal envelope and air leakage for the conventional building and the ZEB.BBR16 refers to legal requirements.Vol:.( 1234567890) HRV was uncommon in new Swedish buildings in the first decade of 2000.A conventional building would most likely have had an exhaust air ventilation without heat recovery (The Swedish National Board of Housing, Building and Planning, 2009).Less ducts are needed for an exhaust air only system, and the appliance is simpler, so exhaust air ventilation uses less material in the product stage.On the other hand, the heat demand is higher in the use stage.We assume that half of the material from the HRVunit and the ventilation ducts in the ZEB are needed for the conventional building ventilation.It is further assumed that the heat demand increases corresponding to the recovered heat in the ZEB ventilation.The benefit of recovery of heat and the operational energy use from the HRV-unit are estimated based on data from the Swedish Energy Agency.
The roofing material only affects the product stage and has no effect on the thermal performance of the building.The studied ZEB has a steel sheet roofing, which was uncommon for Swedish buildings at the time.For the conventional building, we assume concrete tile roofing, corresponding to the overwhelming majority of buildings from that time with roof inclination equal to or larger than 14 degrees (The Swedish National Board of Housing, Building and Planning, 2009).
The relative difference in impact between the conventional building and the ZEB, ΔI, is calculated as ΔI = (I conventional -I ZEB )/ I ZEB for the thermal envelope (te), HRV, and the roofing (r), respectively, for both the product stage and the use stage.Changes in the thermal envelope, roofing, and HRV are independent, so the difference in impact based on such changes can be added or subtracted individually.
Based on the above, environmental impacts (see "Impact Categories and Characterisation models") were calculated for four different variations of the conventional building (Table 3).Note that this is only done.

Zero energy building
The product stage (raw materials supply, transport, and manufacturing of products) and the use stage (operation and maintenance), are close to equal for the Climate change, Eutrophication and Acidification categories.The use stage has a larger relative impact in CED, ecotoxicity, and non-carcinogenic toxicity categories.(Fig. 2).
The product stage has a clearly larger relative impact only in the human carcinogenic toxicity category, where metals constitute 50% of the product stage impact (Fig. 2; Table 4; Fig. 4d).Metals contribute to almost 18% of the product stage impact in the change category, 33-50% in the toxicity categories and 21-27% in the acidification and eutrophication categories (Table 4; Fig. 3; Fig. 4b,  d, f).
Insulation is another overall large contributor, as expected in a well-insulated building.Wood fibreboards and glued laminated timber (GLT) are also among the 'top five' contributors to the product stage impact in all the studied categories.Concrete/ cement represent roughly the same amount in m 3 as GLT, but only reaches "'top five'" in one category, climate change.The substantial impact of metal in climate change, eutrophication, and acidification, is apparent.The metal sheet roofing is a substantial impact contributor in the metal category in the product stage.(Table 4; Fig. 3).
The relatively large impact of the primary energy and eco-toxicity categories in the use stage is mainly attributed to household and operational electricity (Fig. 2; Fig. 3; Table 5).As for human non-carcinogenic toxicity, half of the use stage impact is due to space and water heating from the CHP plant, and a third from electricity (Table 5; Fig. 4f).Electricity is the biggest contributor in the climate change and primary energy categories (Fig. 3), while space heating is the biggest contributor in the eutrophication and acidification categories.
Energy use from space heating and household electricity is of the same magnitude (Table 5).The major source of environmental impact in the use stage varies between environmental categories.
Regarding energy from the wood stove, it has a higher impact per energy unit than the energy from district heating in all environmental impact categories.It also has a higher impact than electricity per energy unit in the climate change, human noncarcinogenic toxicity, eutrophication, and acidification categories (Table 5).While it supplies 20% of the energy for active space heating, the wood stove accounts for 65% of the associated GWP in the climate change category (Fig. 3 and Table 5).Although the impact from the district heating is low in the climate change category, it is the biggest contributor to eutrophication and acidification in the use stage (Fig. 3).
The relatively high energy use for space heating compared with DWH is due to the cold climate.

Toxicity
The toxicity results indicate that the toxicity impact is mainly associated with the use stage and that the household electricity is a large contributor in all assessed categories (Fig. 4).The product stage impact is small with the exception of metals in the human toxicity carcinogenic category, where metals constitute 50% of the impact.The sensitivity analysis of the toxicity data reveal a notable difference in impact, both regarding magnitude and distribution, when interim CFs are disregarded (Fig. 4).
The magnitude is reduced with a factor of 10 2 -10 5 for the human non-carcinogenic toxicity categories, 10 2 -10 4 for the human carcinogenic toxicity, and 10 6 -10 8 for the ecotoxicity category.The results from the toxicity impact calculations should, therefore, be interpreted with caution.The exception is for metals, contributing to a large proportion of the material impact in the toxicity categories, both with and without interim CFs.Comparisons between the ZEB and the conventional building The results indicate that the ZEB, in this particular setting, performs better than a conventional building in all the assessed impact categories and in all four variations of the conventional building, as presented in Table 4 (Table 6; Fig. 5).The differences in the climate change category are, however, small.Toxicity was excluded from the comparison based on the outcome of the sensitivity test.
In general, compared with all four variations of the conventional building, the ZEB has similar GWP but substantially lower primary energy demand, eutrophication, and acidification (Fig. 5).The environmental benefits of the ZEB are generally greater compared to variation 1 and 3 than to variation 2 and 4.This is explained by greater impacts associated with metal roofing in scenario 3, which to some extent is counterbalanced by the benefits of using HRV in variation 4.

Discussion
The relative impact of the use stage compared to the product stage differs between the environmental impact categories (Fig. 2).The product stage stands for about half of the total impact in the climate change category and about 20% in the primary energy category, thus in line with findings from other studies of low-energy buildings in cold climate e.g.(Dodoo & Gustavsson, 2013;Liljenström et al., 2014;Erlandsson et al., 2018).This was, however, unexpected.Given the low GWP of the energy source used for active space heating and the subarctic climate (requiring relatively more insulation), the product stage was expected to be responsible for a greater climate change impact relative to the use phase.One reason is that household electricity was included in the assessment.Even though the ZEB is not actively heated with electricity, passive heating from household electricity accounts for 40% of the heating demand.Household electricity use is not affected by the thermal properties of the building envelope and thus sets a "lower limit" for the level of environmental impact in the use stage, when passive techniques are used.In the ZEB, more than 45% of the use stage GWP and 60% of the use stage CED would remain, even if the energy for space heating and hot water would be generated on-site, with zero GWP and zero CED.Dodoo & Gustavsson (2013) showed that household electricity accounts for the greatest share of the operational primary energy use and CO 2 e emissions in the use stage, and that the share was largest (about 75%) in a passive house heated with a renewable energy source.The heat from electrical appliances is indeed a significant source for space heating in low-energy houses.Still, household electricity is often left out when evaluating a building´s environmental impact.According to the EPBD (European Parliament, Council of the European Union, 2024), this electricity is also not mandatory to include in final energy use accounting.However, as shown here, leaving the household electricity out of an environmental impact calculation could mask the actual environmental impact of the use stage.Another reason why the use-share has a relatively high climate change impact is that the wood stove produces heat with a disproportionally large impact, especially in the climate change category.The total GWP impact from the stove is almost double the impact of space heating using district heating.
Passive techniques tend to imply a larger amount of material in a building´s thermal envelope compared to a conventional building, which results in greater (absolute) environmental impacts from the product stage.For ZEB to be an environmentally beneficial alternative, the increased environmental impacts from the product stage need to be compensated by lower environmental impacts from the use stage.The most obvious example is the use of insulation, i.e., cellular plastic sheets (foundation) and cellulose fibre and glass wool insulation (walls and ceiling), which has substantial environmental impacts (Pelsmakers et al., 2015;Hurtado et al., 2016;Reynolds, 1991).The environmental impacts of using additional insulation thereby need to be counterbalanced by the reduced impacts of using less energy.The extent to which reductions in energy use results in GHG emissions savings depends largely on the heating system (Dodoo & Gustavsson, 2013).
As passive techniques only reduce active space heating, a ZEB and a conventional building will differ most in categories where the active space heating has a high environmental impact.Compared to electricity, active space heating has a relatively high impact in the acidification and eutrophication impact categories, and a relatively low impact in the climate change and primary energy categories (Fig. 3, Table 5).Furthermore, although the energy use for active space heating in the conventional building is three times larger than in the ZEB in this study, the reduction of GWP by passive techniques can just barely compensate for the increased GWP from the product stage after 50 years of service life.However, passive techniques reduce primary energy, acidification, and eutrophication impact by about one third (Fig. 5, Table 6).This shows that, although a ZEB in this setting does not necessarily perform better than a conventional building regarding climate change mitigation, it can potentially have a substantial effect on reducing other environmental impacts.
Zero emission buildings can contribute to meeting climate change mitigation targets by increased energy efficiency (Wang et al., 2019).By reducing the overall energy demand in the system and thereby reducing the demand for fossil energy, they can also contribute to increasing national and regional energy security.However, while our results generally support the use of ZEBs from an environmental perspective, they also indicate that the climate benefits in a country with a renewable energy system are limited.Electricity grids are, however, connected and a surplus of (cheap) renewable energy in one country is generally used to replace (more expensive) fossil fuels elsewhere.This suggests that the estimated climate benefits of ZEB, in this setting, are underestimated.From a methodological perspective, an alternative could be to use the European electricity mix, or even coal as this is typically used on the margin.This would result in substantially greater GHG emissions savings (Mosterio-Romeo et al., 2014).This highlights that ambitions to reach domestic mitigation targets require a focus on measures that provide domestic benefits, which is not necessarily the same as the actual benefits.Such a focus thus risks targeting suboptimal solutions.From this perspective, the current EU policy direction is promising, as incentives for implementation of ZEBs are unaffected by domestic climate benefits.Finally, it should be noted that this study concerns new buildings.There is also a notable potential for climate change mitigation by increased energy efficiency resulting from the retrofitting of existing buildings (Drouilles et al., 2019;Piccardo et al., 2020) The environmental performance of such measures should be subject to further studies, similar to this.

Conclusions
The ZEB, in this particular setting, performs better than a conventional building in all the assessed impact categories and in all four scenarios.It has similar climate impact but substantially lower primary energy demand, eutrophication, and acidification, compared with the conventional house.The limited climate benefit may, however, be considered an underestimation since the study does not consider that energy efficiency measures in a country with a largely renewable energy system can result in fossil fuel substitution elsewhere.This highlights that ambitions to reach domestic mitigation targets can be problematic, as they require a focus on measures that provide domestic benefits, which is not necessarily the same as the actual benefits.The product stage and the use stage show similar impacts in the climate change, eutrophication and acidification categories, while the use stage has a larger relative impact in the CED, ecotoxicity, and non-carcinogenic toxicity categories.The relatively limited climate change impact of the product stage relative to the use phase was unexpected but is likely explained by the inclusion of household electricity in the assessment, since passive heating from household electricity accounts for 40% of the heating demand.
The results support that ZEB can contribute to meeting climate change mitigation targets by increased energy efficiency, while reducing other environmental impacts and increasing energy security, both nationally and regionally.To improve the relative environmental performance further, the environmental impacts from the product stage need to be further decreased to ensure that it is fully compensated by the lower environmental impacts from the use stage.This requires development of new materials and new construction methods.Further studies are also needed on the environmental performance of the retrofitting of existing buildings to meet NZEB or ZEB standards.

Fig. 2
Fig. 2 Comparison of impact between the product stage (total material) and the use stage (50-year total use phase)for the ZEB

Fig. 4
Fig.4 Categorised values for different toxicity impact categories distributed over product stage material-and use stage energy categories

Table 3
Four variations of the conventional building where tested in the assessment.Ic = Environmental impact of conventional building, IZEB = Environmental impact of ZEB, Vol.: (0123456789)

Table 4
LCIA results for the construction stage for the chosen environmental impact categories

Table 5
LCIA results for the use stage for the chosen environmental impact categories

Table 6
LCIA results for the ZEB and the conventional building in four scenarios with different variations of the conventional building Vol:. (1234567890)