**3. What does being nearly zero implies in the context of a building?**

The discussion thus far identifies why we need to reduce emissions and why we need to become highly energy efficient, especially when it comes to the building industry. The fundamental goal here is to neutralize resource consumption by reducing energy needs and harnessing renewable resources for energy production. This approach will produce buildings that offset the total amount of energy used by the building annually with the amount of renewable energy that can be captured on-site or via renewable energy providers [14]. The concept of net zero buildings (NZEB) was first discussed internationally in 2008 [15] and has been refined over time by the International Energy Agency (IEA) with almost 20 nations globally via the Task 40 initiative. The European Union was similarly discussing the definitions through its EPBD initiative that finally resulted in coining the term nearly zero energy buildings (nZEB) [16–18]. EPBD's recast directive establishes that the nearly zero or significantly less amount of energy required during the operation of the building should be catered to via energy derived from renewable energy sources (on-site or generated nearby). In Europe, the application of the nZEB model has become a requirement since December 31, 2018, for all public buildings. This application has slowly percolated to all new buildings from 2020 onwards. The United States Department of Energy (DOE) further classified zero energy buildings based on their total life cycle energy. This definition included building energy (on-site building energy consumption—heating, cooling, ventilation, indoor and outdoor use, lights, plug loads, process energy, elevators, intra-building transportation, etc.), energy consumed in transportation of primary fuels, thermal and electric losses in generation plants, and loss of energy during transport of energy to the building site [19]. A holistic spin on energy balancing of the building is thus proposed. It is also important to note that as opposed to autonomous zero energy buildings that can generate and consume equal amounts of energy to sustain themselves, nZEBs can connect to the external electricity grid provided that the annual energy export is equal to the annual energy import.

Accordingly, this chapter focuses on the added value of an nZEB by retrofitting the existing stock of buildings by reducing their energy needs and employing appropriate physical improvements to enhance its efficiency standards. In retrofitting existing buildings, three fundamental principles need to be adhered to—reuse, reduce, and sequester. Re-use in this context implies the use of recycled materials, paying specific attention to the end-of-life re-use properties of the materials used during retrofitting, and the idea of designing with an aim for deconstructing. Reducing implies carefully optimizing materials used during the renovation to selectively opt for low carbon materials [20]. Sequestering in the case of retrofitting involves the provision of carbon sequestering locations coupled with materials that can sequester carbon, such as bioplastics, the use of mycelium insulation, recycled plastic, and biomaterials-based carpeting, and 3D printed wood made from sawdust, to name a few. Ideally, retrofitting to reach a nearly zero energy building status can be clubbed into two design strategies—passive and active [19]. Passive strategies incorporate material properties, urban positioning/orientation, envelope design, and shading, to name a few. On the other hand, active strategies deal with improvements within HVAC systems, energy-efficient lighting, etc.

On the material front, though, one needs to comprehend the notion of "Embodied Energy." Embodied energy is typically associated with the total impact of material greenhouse gas emissions during its entire life cycle. Lifecycle covers the dimensions of a material's extraction, manufacturing, transportation, construction, maintenance,

**Figure 1.** *CO2 emissions of new build and refurbishment (image source: AECOM, 2021 [22]).*

and disposal. If we take the new construction route, it is estimated that embodied carbon alone will be responsible for 72% of the carbon emissions between now and 2030 [21]. Embodied carbon cannot be rectified because it is embedded within the building once it has been erected. It is thus crucial to address the embodied carbon issue during the design or before the retrofitting stage is actualized on any building site (**Figure 1**).

Besides this, the basic building materials which are prevalently used in the construction sector—concrete, steel, and aluminum, are together responsible for 23% of total building emissions in themselves. Portland cement, the primary ingredient for making concrete, is responsible for releasing 40% CO2 during the burning of fossil fuels for its manufacturing and emits 60% CO2 during its processing phase. Similarly, the production of steel is a significant determinant of how much CO2 it generates. Typically, basic oxygen furnaces responsible for producing steel rely on burning fossil fuels—coal or natural gas to melt iron ore, thus contributing to CO2 emissions at a large scale. Better material alternatives or the alteration of production technologies of such fundamental materials used in the building industry are thus crucial. Embodied energy becomes specifically vital if seen from the context of developing nations, witnessing a boom in the building construction sector at an exponential rate.

Having gained some perspective on the concept of nearly zero energy buildings, the big question then is how do we translate this theoretical thinking into reality via retrofitting existing buildings? The following section provides some perspective on the same.
