Energy
What are NZEB buildings?
ZERO-CONSUMPTION BUILDINGS
In the transition towards a sustainable socio-economic model, we are redefining what we have known until now as a consumer society, which is sustained by generating energy to feed ourselves, manufacture goods, or move around.
The construction sector accounts for nearly 60% of greenhouse gas emissions worldwide. Therefore, proposing a new building model based on a zero-emission lifecycle will be a key factor in the global energy transformation and resource management.
Across all sectors and organizations involved in this transformation, it is already taken for granted that the 2020–2030 decade we are immersed in will be the turning point. We face the great challenge of achieving a large-scale model shift in all sectors, and the construction sector, due to its cross-cutting nature and impact on all sectors, plays a leading role.
In this regard, the term NZEB—Nearly Zero Energy Buildings (or EECN, Edificios de Consumo de Energía Casi Nulo, in Spanish)—is gaining prominence. This concept is already present, to a greater or lesser extent, in practically all building regulations of the European Union member states.
What are NZEB buildings?
NZEB buildings respond to the challenge posed by transforming the built environment and are defined as Nearly zero-energy buildings, that is, Edificio de Consumo de Energía Casi Nulo (EECN). [1]
In 2014, the European Commission began defining this type of building as:
“A building that has a very high energy performance. The nearly zero or very low amount of energy required should be covered to a very significant extent by energy from renewable sources, including energy from renewable sources produced on-site or nearby.”
European Commission, Nearly zero-energy buildings, 2020
However, NZEB buildings are just a starting point in a process that must consider factors beyond carbon emissions, or more ambitious goals that replace ‘Nearly’ with ‘Net’, i.e., zero-emission buildings (Net zero-energy buildings).
In fact, a new building concept called NZIB already exists, which goes a step further by considering the building’s entire lifecycle. This is the so-called Nearly Zero Impact Building (or EICN – Edificio de Impacto Casi Nulo, in Spanish), which considers not only energy generation and consumption but also emissions throughout the building’s entire lifecycle, as well as water, material, and waste management—essential factors to consider according to the principles of circular economy in architecture.
While the NZEB concept focuses on energy aspects, considering passive and active systems to achieve occupant comfort, NZIB focuses on the impact of the built environment, considering the building’s entire lifecycle.
In an NZEB building, we will first propose passive systems where we make the building work through bioclimatic architecture strategies. In winter, we capture sun and store it thanks to thermal mass systems and good insulation; in summer, we protect ourselves from the sun and generate cross-ventilation. All this aims to achieve adequate thermal comfort and minimize dependence on active HVAC systems. If these are necessary due to adverse climates or locations, they will be powered, whenever possible, by energy from renewable sources.
Although the NZEB building concept focuses on the building’s energy performance and is defined as a very high-efficiency construction with very low energy consumption, whose demand is met by renewable energies, it is true that many NZEB regulatory frameworks already introduce concepts focused on analyzing the building’s useful life and its impact on the environment, with the aim of initiating the decarbonization of the built environment.
The total carbon footprint calculation of a building must consider the carbon consumption throughout its entire lifecycle, from planning, construction, and use to the end of its useful life. However, it is true that many of the regulations we will analyze later focus on energy performance and the building’s self-consumption capacity—that is, producing the energy it will consume—with the aim of decarbonizing the built environment. These factors are very focused on the building’s useful life. Conversely, in certain situations, reference is also made to the use of low-carbon materials and construction processes.
EU Regulatory Framework
NZEB regulation is very recent but mandatory in most EU countries, where each member state has adopted different regulatory frameworks, all focused on achieving a zero-consumption built environment.
The first references to energy efficiency already appear in the 1951 Treaty of Paris, in which 104 of the 194 signatory countries specifically committed to promoting the energy efficiency of the building stock to meet climate change mitigation objectives. However, currently, only 62 member states have energy codes and specific building regulations. [2]
On May 19, 2010, a Directive of the European Parliament and of the Council on the energy performance of buildings entered into force, under the title Energy Performance of Buildings Directive. [3]
It presents requirements regarding the methodology for calculating the energy performance of buildings, to later apply minimum performance requirements for new constructions, renovations of existing buildings, the building envelope and installations, as well as energy certifications, periodic inspections, and control systems.
More specifically, this regulation establishes a deadline with the express requirement that all new buildings constructed after December 31, 2020, must comply; and in the case of public buildings, the regulation came into force in 2018, with some exceptions for protected buildings, places of worship, or buildings used sporadically.
This is a generic directive issued by the European Union with the aim that each country specifies its own criteria and indicators. A guidance document is established that sets common criteria that each member state must use as a basis to draft its own national plan to implement the NZEB model in its country and comply with the directive. [4]
“Each Member State shall establish a long-term strategy to support the renovation of its national stock of residential and non-residential buildings, both public and private, transforming them into highly energy-efficient and decarbonized building stocks by 2050, facilitating the cost-effective transformation of existing buildings into nearly zero-energy buildings.”
European Commission, Guidance document for national plans for increasing the number of nearly zero energy buildings, 2013
In 2016, the European Commission presented a new package of measures aimed at providing a stable legislative framework to facilitate the energy transition.
To meet the commitments of the Paris Agreement, the “Clean Energy for All Europeans” proposals aim to help the EU energy sector become more stable and competitive. The package has three main objectives: prioritizing energy efficiency, achieving global leadership in renewable energies, and providing a fair deal for consumers. [5]
In 2018, the European Council approved a new Directive revising the 2016 one, aiming to encourage renovation by promoting cost-effective renovation projects and emphasizing that non-decarbonized buildings are highly inefficient. It proposes an annual renovation rate of 3% of the existing building stock, which would allow for total decarbonization by 2050.
Furthermore, it simplifies inspections of heating and air conditioning systems and insists on the need for long-term monitoring of their evolution through the implementation of progress indicators, with measurable targets for 2030 and 2040, and the specification of national strategies.
But the most important point is that it lays the foundations for the definition of NZEB buildings, establishing milestones that every national plan must include based on its national, regional, or local conditions. Basically, it involves a numerical indicator of primary energy use expressed in kWh/m2 per year and intermediate targets to improve the energy performance of new buildings. [6]
Additionally, the Commission commits to publishing a report on the progress made by Member States every three years. The latest ‘status report’ was published in 2018 and highlights Spain as one of the few European Union countries that has not yet included a legal document on its application of the NZEB building definition. [7]
Spain finally published the revision of the new Technical Building Code belatedly, at the end of 2019, which came into force in mid-2020 with certain extensions due to COVID.
Spanish National Plan
Spain established a National Plan in November 2014, which includes a report prepared by the Directorate General for Architecture, Housing, and Land analyzing the consumption distribution of the residential and non-residential building stock.
It does not yet include the definition of NZEB buildings with the numerical indicator required by the European Union, claiming that this detailed definition is expected to materialize at a later stage. [8]
In 2018, the so-called State Housing Plan 2018-2020 was drafted, structuring a series of programs to promote the rental housing stock, aid for people facing eviction, accessibility, rural renewal, youth, and energy efficiency and sustainability.
This program to promote energy efficiency and sustainability improvements in housing simply establishes that in Spain, a nearly zero-energy building is any building, new or existing, that meets the regulatory requirements established in the Basic Document “DB HE Energy Saving” of the Technical Building Code, regarding the limitation of energy consumption for new buildings. This definition is already included in Royal Decree 732/2019, which modifies the Technical Building Code, approved by Royal Decree 314/2006.
The promotion program does not quantify a maximum annual demand but imposes a reduction in the overall annual heating and cooling energy demand of the dwelling compared to the pre-intervention situation, ranging between 20-35% depending on the climate zone. If such a reduction is achieved, owners may be eligible for a subsidy that in no case may exceed €12,000 or 40% of the investment. [9]
The 2019 update of the Technical Building Code incorporates the definition of “Basic Requirement H0: Limitation of energy consumption,” which is defined for the first time as a basic requirement and sets a limit on primary energy consumption in buildings, which must largely come from renewable sources.
Thus, reference is made to two different parameters: non-renewable primary energy consumption and total primary energy consumption. Both depend on the climate zone we are in. For the consumption calculation, external solicitations—which are climate actions—and internal solicitations—which are thermal loads—are considered, along with the operational conditions of the different rooms according to their use.
If we consider the case of Barcelona, which would be climate zone B, we have a limit of 25 kWh/m2 per year of non-renewable primary energy for new buildings and extensions, and 56 kWh/m2 per year for total primary energy, including renewable and non-renewable.
If we compare it with the Passivhaus Standard, one of the world benchmarks for measuring and classifying a building’s level of passivity, closely related to its energy efficiency, it establishes three energy limitations: heating demand must be less than 15 kWh/m2 per year, cooling demand must be less than 15 kWh/m2 per year, and primary energy demand must be less than 120 kWh/m2 per year. This last value is the one that interests us, as it includes the energy used for heating, cooling, hot water, and electricity.
Thus, we realize that the CTE establishes much lower values than the Passivhaus Standard for private residential use, since in the worst case, in the most unfavorable winter climate zone and multiplied by the coefficient 1.15 for non-peninsular territory, it would barely exceed 130 kWh/m2 per year.
Among other things, the CTE also refers to the conditions of the thermal envelope to limit the need for primary energy and achieve thermal well-being, establishing a limit on the thermal transmittance of the different envelope elements through the so-called Global Heat Transfer Coefficient through the thermal envelope. The requirements for covering hot water and electricity demand refer to generation through renewable sources, this time without establishing any mandatory percentage. [10]
Design Strategies for NZEB Buildings
So far, we have addressed the requirements imposed by the different regulations, but how is this consumption reduction effectively achieved in design?
The main idea is simple: we reduce energy demand through architectural strategies to achieve high levels of energy efficiency, allowing us to cover the remaining reduced demand with renewable energies.
Following the principles of bioclimatic architecture, when designing a building, aspects such as the geographical location and climatic conditions of the building’s surroundings, the immediate environment, resource availability, and the building’s intended use must be analyzed and considered. [11]
Thus, to reduce energy consumption in buildings, we can apply the following design strategies:
– Reduce energy demand through passive, architectural, and design measures.
– Increase the performance of building installations and equipment to reduce their consumption.
– Promote energy production using renewable energy sources.
– Measure and monitor energy consumption, energy production, and environmental parameters to understand how, when, and where we consume energy.
– Improve the consumption habits of building users.
Next, we will organize the different design measures based on whether they are passive or active measures. We describe a passive measure as one that does not require energy to function and is part of the dwelling’s own design. In contrast, active measures involve primary energy consumption to be effective.
Passive Measures
Passive strategies are closely linked to bioclimatic construction, which involves designing and constructing the building in a way that is closely tied to the climate of its location, protecting it from inclement weather and taking advantage of climatic benefits.
Pre-existing Conditions and Solar Orientation
The first passive strategy to apply when designing an NZEB building is to consider the environment where it will be located and its orientation, adapting it as much as possible to the site. In this sense, it is essential to consider the main factor: solar orientation.
Good solar orientation will allow us to capture heat and sunlight throughout the day. In the northern hemisphere, we prioritize south-facing orientation, and failing that, southeast or southwest orientation, which allows us to maximize capture throughout the day.
The sun provides heat and light in winter, so we seek strategies to create openings on the south facade that allow us to capture this heat with well-insulated glazing that does not generate thermal losses.
In summer, we will want to let in sunlight but not its heat. That is why we will seek strategies to protect ourselves from the sun while allowing light to pass through. One option is to work with porches or pergolas that block the incidence of the vertical summer sun. These pergolas can be opaque or vegetal, a strategy that improves thermal comfort by including plants that, through the process of photosynthesis and water evaporation, absorb calories from the environment.
Other protections we can use to block direct sunlight but not light are adjustable horizontal slat protections, which tilt according to the room’s needs, allowing more or less light inside.
Thermal Insulation
The thickness of thermal insulation is always greater in the design of buildings that prioritize energy efficiency, as is the case with NZEB buildings. Of course, this thickness is established based on the climate zone and will be closely related to the material we use; straw or wood fiber insulation is not the same as polystyrene insulation.
It is also important to note that when we focus solely on efficiency issues, we neglect the ecological impact of construction. Polystyrene insulation has a very high positive ecological footprint, while wood fiber or cork insulation usually has a negative ecological footprint, meaning that its growth and production process not only does not emit CO2 but absorbs it.
In this sense, it is worth emphasizing again the importance of combining the NZEB term, referring exclusively to building efficiency, with the NZIB term, referring to its ecological impact.
Absence of Thermal Bridges
A thermal bridge is a discontinuity in the building’s thermal envelope that causes a reduction in thermal resistance compared to the rest of the enclosure. Thermal bridges can occur at the junction of floor slabs with the facade, the facade with the roof, pillars with the facade, or around openings, among others. That is, when construction details are not well resolved and there is a direct temperature exchange between interior and exterior due to the absence of insulation.
Current building regulations increasingly aim to achieve airtightness in buildings, and having an unresolved point through which there is thermal exchange between interior and exterior is a serious drawback for guaranteeing comfort in the indoor environment. This is not only due to the uncontrolled temperature variation through it but also due to the pathologies that can be generated.
A thermal bridge can cause condensation due to the temperature difference between interior and exterior. Our interiors are usually warm, whether naturally or artificially conditioned, and when the outside temperature drops, any cold surface in a warm environment can generate condensation of ambient humidity. Since warm air has a much higher hygroscopic capacity—meaning it can hold more moisture than cold air—it is easy for the cold surface to condense this moisture in the form of small droplets.
The danger of this condensation is not just aesthetic, as in many cases it occurs inside walls, in non-visible and unventilated areas. This condensed moisture becomes internal pathologies that flourish as mold or fungi on walls or ceilings: a serious health risk for occupants as they inhabit an unhealthy indoor environment.
Thermal Mass
Thermal mass is the ability of a material to store heat and release it when the outside environment is colder relative to the interior temperature. This characteristic allows certain high-density materials to capture and store heat during the day thanks to solar radiation and release it hours later when the outside temperature drops.
Materials with thermal mass are usually high-density materials such as brick, earth block, concrete, or stone. Thermal mass will depend on the density and thickness of said material, but also on its relationship with insulation.
Traditionally, insulation was placed on the interior, leaving the thermal mass wall—which is capable of accumulating heat—on the exterior. In NZEB construction, following passive architecture principles, we will place the insulation on the exterior. That is, we keep the thermal mass wall that accumulates solar heat during the day on the interior and protect it from the outside to maintain this heat for many hours.
Of course, there are many different construction systems, and in each case, we must analyze the most suitable strategy. In fact, nowadays, with the proliferation of prefabricated lightweight frame constructions, whether in wood or steel—again, we should consider the NZIB concept regarding the ecological impact of the latter—we find that thermal mass is compromised, as a wood or steel structure has no capacity to store heat.
In these cases, the solution is to include elements that provide thermal mass to the whole, such as a high-density interior floor or wall.
Ventilation and Airtightness
Natural ventilation of a building is a basic principle of traditional architecture for achieving good indoor air quality.
To ensure cross-ventilation, it is necessary to plan openings on different facades of the building, preferably north-south to promote airflow due to temperature difference. If necessary, as in very compact floor plans where it is not possible to generate these effective ventilations, we can introduce elements such as courtyards, atriums, or setbacks that allow optimizing the ventilation of all rooms, both day and night.
The other side of the coin is airtightness, a concept that already appears in the most recent national building regulations following European directives.
Betting on a ‘thermos building’ strategy means keeping in mind the minimum standards for air changes per hour. To achieve good airtightness, it is necessary to create an uninterrupted, airtight envelope. This is achieved through good planning of construction details, strict on-site control, and airtightness tests upon completion of the work.
Even so, it is important to note that airtight materials must be hermetic but also breathable, meaning they prevent air passage but allow controlled water vapor passage to enable vapor transpiration and avoid condensation inside the walls.
Thus, an NZEB building must be airtight to prevent air leaks that alter the interior comfort temperature, but at the same time breathe to ensure a healthy indoor environment. To achieve this, it is necessary to guarantee constant air renewal, which regulations already require to be automated, so it does not depend on the user’s good behavior in their daily ventilation practices.
To ensure air renewal, we have several more or less technical strategies. We can delegate renewal to a heat recovery machine that extracts stale air and introduces fresh air, using the heat from the indoor air to preheat the renewal air.
Another option is through strategies more related to bioclimatic architecture, such as galleries or courtyards that preheat the air naturally through the greenhouse effect and introduce it inside through simple renewal systems, such as air vents connected to a CO2 sensor or micro-ventilation in doors and windows.
Active Measures
Active measures provide the NZEB building with the necessary annual primary energy through systems that maintain indoor comfort and health conditions, prioritizing the use of renewable energy and self-consumption, which can meet the production of both thermal and electrical energy for heating, DHW, lighting, and electrical equipment consumption.
Renewable Energy Generation
There are different renewable energy sources: photovoltaic solar energy, solar thermal energy, wind energy, geothermal energy, aerothermal energy, biomass energy, and the use of residual energy.
– Photovoltaic panels: allow transforming solar energy into electricity, generating current from capturing solar radiation. Yields are increasingly higher, making it easier to maintain a building with a relatively low number of panels. Furthermore, today panels are manufactured in different formats, such as sheets or facade panels, tiles, or in recent years, flexible elements that adapt to a multitude of surfaces.
– Wind turbines: allow harnessing wind resources, also called low-power wind energy or mini-wind, as opposed to the large scale of large windmills, as it produces electrical energy below 100 kW. These are turbines that can be installed on a building’s roof and transform the kinetic energy of the wind—energy of movement—into electrical energy.
– Solar or thermodynamic panels: unlike photovoltaic panels, thermodynamic solar panels only generate thermal energy, harnessing the heat provided by solar radiation to transfer it directly to DHW production and building climate control. Therefore, it will also be necessary to have an electrical energy source for the NZEB building’s demand.
– Geothermal energy: thermal energy that harnesses the stable temperature of the earth a few meters below the ground surface (from 2 m), which remains relatively constant throughout the year (15 °C – 19 °C). The installation consists of buried pipes in the form of a well or extended horizontally, as detailed in the article house with geothermal energy.
– Aerothermal energy: thermal energy from harnessing air energy to cover the heating, cooling, and domestic hot water (DHW) demand of buildings, as we expand on in the article house with aerothermal energy.
– Biomass: combustion thermal energy whose CO2 emission balance (emissions absorbed during plant growth and those released during combustion) is practically neutral, as explained in the article building a house with biomass.
– Use of residual energy: thermal energy from specific activities or processes inside buildings (kitchens, laundries, etc.). It can be used as an energy source for climate control or DHW production in the same buildings.
Without a doubt, NZEB is a concept that is already becoming the prevailing construction model, whether driven by regulation, user ecological awareness, or interest in saving energy costs in construction.
It is a model we must urgently move towards, implementing it at all levels of the construction sector. Furthermore, in the coming years, we must also incorporate the NZIB concept, which is increasingly important in a global world of finite resources that must advance towards a circular economy where awareness of resource use, local, sustainable, and long-lasting consumption prevails.
BIBLIOGRAFÍA