Nearly Zero-Energy Building (NZEB)

What is a Nearly Zero-Energy Building (NZEB)?

A Nearly Zero-Energy Building (NZEB) is a residential or non-residential structure with very high overall energy performance and a very low quantity of energy requirements. The definition, introduced by the EU Energy Performance of Buildings Directive (EPBD) in 2010 and recast in 2018, mandates that the nearly zero or very low amount of energy required must be covered to a significant extent by renewable energy sources, preferably produced on-site or from nearby sources.

The term nearly zero is deliberately imprecise because each EU member state sets its own numerical thresholds for primary energy use, typically ranging from 40 to 160 kWh/m²/year depending on climate zone, building type, and national policy. This flexibility reflects different climatic conditions and economic contexts across Europe, though it also creates a 4-5 times variation in what qualifies as NZEB between member states.

Since 1 January 2021, all new buildings in the EU—residential and non-residential, publicly or privately owned—must be constructed to the NZEB standard. Public buildings had to comply earlier, from January 2019. In Slovakia, this mandate is enforced through Act 378/2019 on the energy performance of buildings and Decree 35/2020, with technical requirements specified in the national standard STN 73 0540-2+Z1+Z2:2019 (thermal protection of buildings).

How do NZEBs meet their energy requirements?

NZEB design integrates three core elements: optimized building envelope, efficient systems, and renewable energy generation. The process begins during early design and requires collaboration between architects, engineers, and clients to ensure energy performance targets are embedded throughout the building lifecycle.

Building Envelope Optimization: The foundation of NZEB is minimizing energy demand through high-performance insulation, triple-glazed windows with low U-values (typically 0.15–0.25 W/m²K), and airtight construction. This reduces heating, cooling, and ventilation loads compared to standard buildings.

Efficient Systems: Heat recovery ventilation (HRV) systems capture warmth from exhaust air to preheat incoming fresh air, reducing heating demand. High-efficiency heat pumps (air-source or ground-source) replace fossil-fuel boilers. Smart building controls optimize operation based on occupancy and weather. These measures reduce operational energy consumption by 70% or more compared to 2006 baseline standards.

Renewable Energy Integration: The remaining energy need is met primarily through on-site renewable sources: photovoltaic (PV) solar panels for electricity, solar thermal collectors for domestic hot water, or biomass systems. Some NZEBs also connect to district heating systems supplied by renewable energy. This net-zero energy balance means the building produces as much energy as it consumes annually.

Component	NZEB Requirement	Typical Performance Metric
External wall U-value	≤0.18 W/m²K	High insulation, minimal thermal bridges
Window U-value	≤0.20 W/m²K	Triple glazing with low-emissivity coatings
Building airtightness	≤3.0 air changes/hour at 50 Pa	Sealed construction with mechanical ventilation
Primary energy indicator	Varies by member state (40–160 kWh/m²/year)	Slovakia: typically 60–80 kWh/m²/year for residential
Renewable energy share	Majority of final energy from renewables	Solar PV, heat pumps, biomass, district renewables

What are the EU and Slovak legal requirements for NZEBs?

The NZEB mandate originates from the Energy Performance of Buildings Directive (EPBD), most recently recast as Directive EU/2024/1275 (adopted May 2024). The directive requires all new buildings to achieve the NZEB standard from January 2021 onwards and introduces a successor standard: Zero-Emission Buildings (ZEBs—buildings with no on-site fossil-fuel carbon emissions) required from 2028 for public buildings and 2030 for all others.

In Slovakia, this framework is implemented through national law. Act 378/2019 on the energy performance of buildings, which entered force in March 2020 alongside Decree 35/2020, transposed the 2018 EPBD recast. These define NZEB in Slovak law and align with the European Commission's 2016 NZEB recommendations. Since April 2025, Slovakia's new Building Act 25/2025 (zákon o výstavbe) provides the procedural framework for permits and building control, though NZEB technical requirements remain in the energy performance decree and the national thermal standard STN 73 0540-2.

The Slovak national National Plan for Increasing the Number of Nearly Zero-Energy Buildings (established under EPBD requirements) sets out gradual tightening of minimum performance thresholds. The energy classification system grades new buildings from A0 (best) to G; from 2021 onwards, new buildings must achieve at least class A0 (representing NZEB performance). This aligns with the global primary energy indicator calculation method, which measures total primary energy use including delivered energy for heating, cooling, hot water, ventilation, and lighting.

How does NZEB differ from Passive House?

Although often confused, NZEB and Passive House are distinct standards with different origins and philosophies. Both deliver high-performance buildings that exceed conventional standards, but they approach the problem differently.

NZEB is a regulatory framework mandated by EU law. Each member state defines its own numerical thresholds, leading to wide variation in what qualifies as NZEB. NZEB explicitly requires renewable energy generation to offset energy consumption, making it a net-zero energy balance standard. It is flexible and allows many implementation pathways.

Passive House is a supra-national voluntary certification standard with precise, climate-independent criteria: a heating demand of ≤15 kWh/m²/year and cooling demand of ≤15 kWh/m²/year, airtightness ≤0.6 air changes/hour at 50 Pa, and specific criteria for thermal comfort. Passive House prioritizes minimizing energy demand through exceptional envelope design and heat recovery before adding renewable systems. Over 25 years of operational experience, design tools, training, and quality verification support Passive House compliance.

Criterion	NZEB	Passive House	Low-Energy House
Heating demand	No fixed limit; varies by climate	≤15 kWh/m²/year	Typically 30–50 kWh/m²/year
Primary energy indicator	40–160 kWh/m²/year (member state dependent)	Typically 40–60 kWh/m²/year	60–120 kWh/m²/year
Renewable energy required	Yes, majority of needs covered	Encouraged but not mandatory	Not typically required
Airtightness	≤3.0 air changes/hour at 50 Pa	≤0.6 air changes/hour at 50 Pa	≤5.0 air changes/hour at 50 Pa
Status	Legal minimum (EU mandated)	Voluntary certification (higher performance)	Intermediate standard (some EU regions)

A Passive House certified building will typically exceed its national NZEB standard and serves as a proven pathway for member states to exceed their NZEB requirements with confidence. Many architects and builders in Slovakia use Passive House as a design reference even for NZEB-compliant projects because the standard provides detailed guidance and quality assurance mechanisms.

What are the costs and practical challenges of implementing NZEB?

Construction costs for new NZEB buildings are typically 10–15% higher than conventional standards due to enhanced insulation, renewable energy systems (particularly solar PV), mechanical ventilation with heat recovery, and control systems. However, lifecycle economic analysis strongly favours NZEB: operational energy costs are 70–80% lower over the building's 50-year lifespan, and the European Commission's cost-optimal methodology confirms that NZEB represents the most cost-effective investment in most EU climates.

Practical challenges remain significant, particularly in Eastern European countries including Slovakia:

• Skills and knowledge gaps: The building sector has limited familiarity with NZEB design and verification. Proper implementation requires integrated design collaboration early in the project, not a retrofit approach.
• Quality and commissioning: Airtightness and thermal bridge management demand careful construction detailing and quality control. Blower door testing and thermographic inspection are essential but not always prioritized. Commissioning of heat recovery ventilation and control systems is often inadequate, leading to underperformance relative to design.
• Climate-specific design: NZEB solutions developed for Northern (heating-dominated) climates differ significantly from those needed in Southern regions. Slovak designers must adapt international best practices to the Continental climate.
• Financing and incentives: Slovakia introduced a state subsidy of €8,000 (as of 2019) for new homes meeting NZEB standards, but uptake remains limited compared to renovation support programmes.
• Renewable energy integration: On-site PV systems require suitable roof orientation, structural capacity, and grid connection arrangements. Some older residential areas face constraints for district renewable heating integration.

How is NZEB applied in Slovak residential design?

In Slovakia, NZEB requirements for new residential buildings are anchored in STN 73 0540-2+Z1+Z2:2019 (thermal protection standard) and implemented via energy performance certificates and energy audits. The standard specifies U-values for external walls, roofs, windows, and thermal bridge details tailored to the Continental climate zone, where heating dominates over cooling.

Slovak NZEB residential projects typically combine passive solar gain (south-facing windows), thermal mass (concrete or masonry cores), high-performance insulation (20–30 cm mineral wool or wood fibre), triple glazing, mechanical ventilation with heat recovery, and ground-source or air-source heat pumps for heating and hot water. Roof-mounted solar PV is the most common renewable energy solution for single-family homes and apartment buildings, though integration with collective heating systems (where available) is promoted.

Common Slovak terminology includes budova s takmer nulovou spotrebou energie (building with nearly zero energy consumption), and designers often reference the Global Primary Energy Indicator (combined indicator) when calculating compliance. The Energy Performance Certificate (EPC) for each building displays its energy class (A0–G); all new residential buildings since 2021 must achieve A0.

A common misconception is that NZEB and Passive House are identical; while many Slovak designers now build to Passive House standards as a means of reliably exceeding NZEB thresholds, the two standards serve different purposes. Passive House provides a design methodology and certification guarantee, whereas NZEB is the legal compliance floor. Equally important is recognizing that NZEB is not a final endpoint: the 2024 EPBD recast introduces Zero-Emission Building requirements from 2028, which will phase out any remaining fossil-fuel heating in new buildings, pushing the sector towards heat pumps and district renewable systems as universal practice.