**1. Introduction**

In Europe, the existing building stock is more than 50 years old, and about 40% of the existing residential buildings were constructed before the 1960s, when building regulations for energy consumption were limited [1]. Consequently, around 75% of the existing building stock in the European Union (EU) is energy inefficient [2]. In the United States, most existing houses were built before the establishment of the Building Energy Codes Program in 1992 by the U.S. Department of Energy, Washington, DC, United States [3]. These older buildings represent about 68% of the national residential building stock and are typically energy inefficient due to air leakage and inadequate insulation [2]. The National Renewable Energy Laboratory has identified approximately 34.5 million homes with wood studs that have no wall insulation [4]. Overall, in both the United States and Europe, a large portion of residential buildings will need some type of renovation, retrofit, or upgrade in the next five to 10 years.

There have been large investments in energy efficiency-related renovation in the global market. In the period 2012–2016, in the EU, more than EUR 200 billion were invested in energy renovations for residential buildings. In the next decades, energy renovation will become the key determiner for achieving the carbon-neutral goal. The European Climate Foundation has outlined three key areas for the building industry to maintain its trajectory toward zero emissions. One of the areas is reducing energy demand—specifically the operating energy demand—through renovation of the building stock [5]. In the United

**Citation:** Hu, M. Beyond Operational Energy Efficiency: A Balanced Sustainability Index from a Life Cycle Consideration. *Sustainability* **2021**, *13*, 11263. https:// doi.org/10.3390/su132011263

Academic Editors: Carlos Morón Fernández and Daniel Ferrández Vega

Received: 31 August 2021 Accepted: 5 October 2021 Published: 13 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Sates, more than USD 279 billion could be invested in building retrofits, resulting in more than USD 1 trillion in energy savings over 10 years, equal to a savings of about 30% of the annual electricity used in the United States (Rockefeller Foundation 2012, New York City, NK, The United States) [6]. The above figures highlight the significant impact building retrofits already have within the EU and American economies. There is space for tremendous potential and growth in both the European Union and the United States.

Currently, the building sector responds to the need for energy retrofits by focusing on an operating energy use reduction. Increasing numbers of companies have announced their commitment to the net zero carbon goal, based on the assumption that operating energy savings leads to an overall carbon emissions reduction and a healthy environment. However, the question remains: Is the current energy-centric renovation approach sustainable? Several studies have looked at the relation between operating energy and embodied energy. Dodoo et al. [7] found that an increase in the thermal mass in the building envelope reduced the cooling load, and hence the operating energy demand; however, such a renovation increases embodied energy considerably. Ellura et al. [8] adopted a life cycle approach for studying net zero energy building, and results showed that when the addition of embodied energy was included in the whole life cycle emission count, the net zero building performance largely shifted away from the nearly zero energy goal. Hu studied energy-efficient renovations in comparison to existing buildings, and showed that the new construction had greater environmental impact potential due to the new building materials added. Such results raised concerns of focusing only on an operating energy reduction while overlooking the added environmental impact [9].

In recent years, there have been studies focusing on the trade-off between embodied and operating carbon. Crawford et al. [10] studied the impact of different building materials of eight residential construction assemblies; a theoretical generic building was used as a base building. Rossello-Batel et al. [11] studied the relation between reduced heating demand and the embodied energy of different building typologies and building envelope options. They found that adding additional insulation in the façade can reduce energy demand to one third of the existing condition while having the highest increase in embodied energy. Stephan et al. [12] also found an increase in insulation in passive houses could reduce the heating demand in the winter, but such a decrease was offset by the higher embodied energy embedded in the insulation materials. With the increase in research on the relation between embodied and operating carbon, most studies were conducted on theoretical conditions using simulated data. Studies using data from actual renovated buildings are limited due to inaccessibility of the data.

The importance of understanding the trade-off between operating and embodied energies and their related carbon emissions has been gradually recognized by practitioners and researchers. Consequently, creating a comprehensive and holistic measurement of sustainability for building energy retrofits has become an emerging research topic. However, there have been very few studies and efforts on this topic, and proposed measurements vary greatly. For example, Bakar et al. [13] proposed using an energy efficiency index as an indicator for measuring building energy performance. Such an index is calculated as the ratio of the energy input to the factor related to the energy-using component. The embodied energy was included as one related factor and measured by the weight of the raw material used. Varusha et al. [14] suggested using the EE factor to quantify the trade-off between the embodied and operating energies of a building. The EE factor is calculated as the ratio of operating energy to embodied energy of a proposed building design against the ratio of a base building based on the ASHRAE 2016 benchmark. Triana et al. [15] proposed a sustainability index in the building life cycle energy use that includes life cycle energy consumption, life cycle carbon emissions, thermal comfort hours, and cost of the building energy life cycle. Those four values are added together and then divided by four to get the sustainability index. However, there is no sustainability index proposed especially for a building retrofit yet.

To respond to such a research gap, the aim of this study is to reveal the importance of considering embodied energy in current energy retrofit practices since the most

energy-efficient building is not necessarily the most sustainable building. Consequently, a comprehensive measure of the sustainability of a renovation project is proposed to measure the effectiveness of a building energy retrofit by integrating the life cycle assessment. This paper uses eight actual energy retrofit projects to demonstrate the trade-off between operating energy and embodied energy. The renovation techniques applied to the eight buildings include a building envelope retrofit, a building heating and ventilation system renovation and upgrades, a lighting system upgrade, and other renovation techniques. This study contributes mainly to the body of knowledge of sustainability by (1) highlighting the importance of embodied energy consideration in energy retrofit projects, (2) presenting a new measure for a sustainability index for renovation projects, and (3) testing the proposed sustainability index and evaluating the sensitivity of the results by applying them to real projects.
