2.1.3. Building Envelope and Materials Used

The primary existing structural systems and materials used in the studied buildings are listed in Table 2. As illustrated in Figure 2a, during the renovation, additional insulation was attached directly to the existing external wall structure; then, new plaster was painted over the insulation for protection. For calculation, the same renovated wall type was used for all the case buildings, but the thickness of the added insulation varied according to the information found on the project website and provided by the project team. Not all case buildings had renovated roofs. For those buildings that did, three additional layers were added: a new polyurethane insulation layer, a light gravel layer, and lightweight concrete (functions as insulation). In addition, a new double layer bitumen membrane was added for waterproofing (refer to Figure 2b and Table 2). The existing exterior windows and doors were mainly made of wooden frames, and all case buildings had them replaced with more energy-efficient exterior windows and doors.


**Table 2.** Existing structural and envelope systems and materials used.

**Figure 2.** Building envelope renovation. (**a**) is the exterior wall details, (**b**) is roof details.

#### 2.1.4. Building Service Systems

The information on existing building service systems—including heating, ventilation, and plumbing systems—was provided by the project team and extracted from the project website [16]. The renovations applied to each case building were also obtained from the project report, which is publicly accessible information [16]. The breakdown description for each individual building can be found in Table 3 and is explained in the following section.

### *2.2. Renovation Strategies and Measures*

The technical improvements applied to the projects to reduce the operating energy demand are listed below as R1 through R15. Tables 3 and 4 list the applied renovation techniques for each case building.

#### **Table 3.** Renovation techniques.


**Table 4.** Applied renovation techniques for each individual case building.


### *2.3. Embodied Energy and Carbon Emissions Calculation*

The software One Click LCA™, developed by the Finnish private company Bionova Ltd., was chosen for this study [21]. The software complies with EN 15987 and EN 15804 standards [17], and EN 15804 is a guideline for Environmental Product Declarations based on the ISO 14044 standard. One Click LCA includes the building material database, which is European original and Finland specific [22]. In this project, life cycle carbon emissions were calculated for individual case projects. To normalize the added embodied energy, only renovated components were included: the building envelope, heating/ventilation system, and lighting system. Structural systems and other building service systems were excluded, as they were not changed. Furniture and interior finishes were excluded as well. The life stages included in this study were A through C. As illustrated in Figure 3, stage A is the product and construction stage and includes A1 through A5. A1 through A3 is usually called "cradle to gate," and A1 through A5, "cradle to site." Stage B is the use stage, and stage C is the end-of-life stage. A1 through C3 are typically named

"cradle to grave." The data used to create the LCA model were extracted from original construction documents provided by the project team and the author's visual inspection on site. Information extracted from the EU-GUGLE website, publications, presentations, and other available information can be found online. A life span of 50 years was used for the calculation, and the product service life was set as the default; for example, wood panels, the roof, and windows were set to be replaced once during the building's lifetime (50 years), and doors were set to be changed twice during the building's lifetime. In addition, in the One Click LCA database, transportation carbon was included during the production stage [23].


**Figure 3.** Building life cycle stages.

### *2.4. Proposed Sustainability Index*

In this study, sustainability of the renovation project was measured by the balance (trade-off) between reduced carbon emissions (through operating energy savings) and added carbon emissions (through added building materials and systems). The framework proposed by Moran et al., 2017 was adopted and modified to calculate the sustainability of a retrofit solution, using Equation (1) through Equation (4) [23]:

$$\text{SC}\_{n} = \frac{a \text{CES}\_{n} + b \text{EMB}\_{n} + c \text{ECO}\_{n}}{k} \tag{1}$$

where *a*, *b*, and *c* are weighting factors for each of the respective categories; *k* is ∑(*a*, *b*, *c*); OES*n* is the life cycle carbon reduction due to the operating energy savings of case project *n*, measured by the operational cost savings (CO2-eq/m2); EMB*<sup>n</sup>* is the life cycle carbon increase due to the embodied energy added, measured by the carbon emissions equivalent (CO2-eq/m2); and ECO*<sup>n</sup>* is the economic impact of case project *n*, measured by the operational cost savings (USD/m2). The calculation of OES*n*, EMB*n*, and ECO*<sup>n</sup>* can be expressed mathematically as Equation (2) through Equation (4):

$$\text{OES}\_{\text{ll}} = \sum\_{m=1}^{q} \left[ \left( \frac{\text{occ}\_{m,\text{ll}}}{\left( \frac{\sum\_{n=1}^{p} \text{occ}\_{m,n}}{p} \right)} \right) w\_{\text{m}} \right] \tag{2}$$

$$\text{EMB}\_{\text{ll}} = \sum\_{m=1}^{q} \left[ \left( \frac{emb\_{m,n}}{\left( \frac{\sum\_{n=1}^{p} emb\_{m,n}}{p} \right)} \right) w\_{\text{m}} \right] \tag{3}$$

$$\text{EMB}\_{n} = \sum\_{m=1}^{q} \left[ \left( \frac{\text{ec}\_{Om,n}}{\left( \frac{\sum\_{n=1}^{p} \text{ec}\_{m,n}}{p} \right)} \right) w\_{m} \right] \tag{4}$$

where *oesm*,*<sup>n</sup>* is the life cycle carbon reduction indicator *m* for case project *n*; *m* stands for the different operating energy, electricity, and district heating; *embm*,*n* is the life cycle carbon increase indicator *m* for case project *n*; *m* represents the building elements, such as the exterior wall and windows; *ecom*,*<sup>n</sup>* is the economic indicator *m* for case project *n*; *wm* is the weighting applied for each indicator depending on the category's importance; *q* is the number of the indicators evaluated in each carbon emissions reduction, carbon emissions increase, and economic category; and *p* is the total floor area measured. The sum of the weightings (∑(*wm*)) applied to indicators in each category must add up to 1.

As can be seen from Equation (1), each of the three categories for the sustainability score can be given a different weighting (a, b, c) depending on the importance of the category. The importance of the categories for different stakeholders involved in the energy retrofit projects can vary. For example, the energy supplier and building operators' priority is most likely the operating energy savings. However, for environmental protection agencies that are involved in permit review, the added embodied carbon is equally critical since it can lead to unintended environmental impacts. For building owners, the operational cost may be the primary reason for choosing retrofit solutions. In Section 3 the impact of different weightings on the sustainability index are demonstrated.

#### **3. Findings**

Table 5 shows the basic data for the studied case buildings. We will first discuss the results of the overall life cycle carbon emissions and contributors to added embodied carbon. Then, we will explain the results of applying the proposed sustainability index to the studied cases. Figure 4 illustrates the normalized whole life cycle carbon emissions of the studied case buildings (Figure 4a is the normalized carbon emission by floor area and Figure 4b is the normalized carbon emission by unit counts), and Figure 5 shows the trade-off between the reduced operating carbon and added embodied carbon.


**Table 5.** Case building data.

**Figure 4.** Normalized carbon emissions by life cycle stage.

**Figure 5.** Life cycle carbon emissions trade-off.

#### *3.1. Life Cycle Carbon Emissions*

As showed in Figure 4, case 1 ranked first, with the highest life cycle emissions due to high energy use during the B6 stage, followed by case 5 and case 3 as the top life cycle carbon emitters, also due to the high energy demand in their use life stage. For all case buildings, the B6 stage was the dominant life cycle stage for carbon emissions, contributing around 57–83% of the total life cycle carbon emissions. These findings validate the importance of further reducing the use stage energy demand through a deep energy retrofit. Figure 4 also shows that the second highest life cycle stage contributing to life cycle carbon emissions was A1–A3, the product stage, or "cradle to gate" [24]. The fractional contributions from the remaining life cycle stages were negligible. When we normalized the life cycle emissions by the number of units included in the studied buildings (refer to Figure 4b), the results were different: Case 1 ranked first with the highest life cycle carbon emissions per unit, more than 50% higher than the emissions from case 4 (ranked second) and 60% higher than case 8 (ranked third). Since we can use the number of units as a proxy for the number of occupants in the building, we can interpret the normalized results by unit numbers as an indicator of inequality regarding the carbon emissions burden that each occupant imposes on the larger environment. For example, case building 4 has fewer occupants (refer to Table 4); however, each occupant is responsible for higher life cycle carbon emissions compared to other case buildings' occupants (except case 1). In the future, we suggest using the number of units to normalize the life cycle carbon emissions, which can better reveal the inequality among different buildings and building occupants. Despite the difference, normalized life cycle carbon emissions by unit also demonstrate that the B6 use phase is the dominant life cycle stage and needs further reductions.

As illustrated in Figure 5, for embodied carbon, case 1 had the highest increase per floor area, followed by case 2 and case 6. For the offset embodied carbon through an operating energy reduction, case 2 had the highest life cycle carbon reduction, followed by case 1 and case 7. Regarding the balance between a carbon increase and offset, all case buildings had negative life cycle carbon, which is an indicator that an energy retrofit is effective in reducing the life cycle carbon emissions of existing buildings. However, if we only look at the offset carbon emissions through an operating energy use reduction, case 1 ranks first, followed by cases 2 and 6. Despite the highest life cycle carbon emissions (refer to Figure 4), as showed in Figure 5, case 1 had the highest life cycle carbon reduction compared to the base condition before the energy retrofit. These different ranking results illustrate how evaluating sustainability using different portions of the life cycle of carbon can have different results; hence, the decisions based on the analysis results may vary.

#### *3.2. Embodied Carbon: Building Materials and Systems*

A1–A3 had the second highest life cycle stage contributing to life cycle emissions; this contribution was mostly related to the selection and production of building materials, components, and systems (refer to Figure 4). Then, we looked at the life cycle carbon derived from building materials and systems during the production and construction life stages. Figure 6a shows normalized carbon emissions (per floor area) during A1–A5 stages, which we defined as embodied carbon. The external wall was the dominant category contributing to embodied carbon, and the elevator core ranked second. The only outlier was case 2, where the elevator and roofing systems were the main contributors to the added embodied carbon. Figure 6b shows that the results of normalized carbon by unit counts were different: Case 1 ranked first with the highest per unit carbon emissions from building systems and materials used for the energy retrofit, followed by case 7 and case 4. Again, these different results demonstrate the need to potentially use occupants or unit counts as a normalization unit.

#### *3.3. Sustainability Index Calculation and Visualization*

The sustainability score was first calculated using equally weighted OES, EMB, and ECO. As illustrated in Figure 7, the X-axis represents the sustainability score (unitless), and the Y-axis is the energy use intensity after renovation, measured in kWh/m2. The size of the bubble represents the total floor area of the case building, measured in m2. For example, case 8 had a gross floor area of 6060 m2, represented by the biggest circle. In general, the higher the sustainability score, the less the overall life cycle carbon emissions emitted from the renovated building, and the more sustainable the building.

**Figure 6.** Embodied carbon emissions by element.

**Figure 7.** Sustainability score.

Figure 7 demonstrates two important findings. First, size is not correlated with the sustainability score, as the largest building (case 8) and the smallest building (case 1) had similar sustainability scores that were extremely different from the other cases. Second, energy use intensity (EUI) might not be a good measure of sustainability. In current practice, EUI is often used to measure the energy efficiency of a building and consequently the sustainability of the building. If EUI is a good indicator of sustainability, then the EUI (Y-axis) and sustainability score (X-axis) would be negatively correlated. However, they do not appear to have a clear correlation. For instance, case 7 had the lowest EUI but also the lowest sustainability score. This can be explained by the trade-off between reduced carbon emissions and increased embodied carbon. As shown in Table 5, case 2 had the highest embodied emissions increase with the second lowest operating carbon reduction. Therefore, low EUI does not necessarily mean more sustainability. In addition, case 3 had the highest EUI after renovation, which may indicate that case 3 still has much space to improve the current energy performance. However, case 3 also had the third highest sustainability score due to its poor previous energy performance followed by a large operating energy reduction achieved through the renovation (refer to Table 5). Therefore, the added embodied carbon (from the renovation) was well adjusted by the large offset of an operating carbon reduction.

As shown in Table 6, the proposed sustainability score can provide us with a more comprehensive understanding of how sustainable the renovation works are from a life cycle carbon emissions perspective, providing a more robust estimation of global warming potential related to building renovation. Only focusing on the operating energy may provide an incomplete, sometimes even opposite, interpretation to measure the effectiveness of a building energy retrofit, which is clearly demonstrated in case 7. Case 7 had the lowest energy use intensity after renovation. Based on the current commonly used measurements and criteria, it is considered very energy efficient and even has the potential to achieve net zero energy if there are renewal energy sources onsite, such as solar or wind energy. However, case 7 ranked the second lowest for the proposed sustainability score, mainly due to the trade-off between added embodied energy and reduced operating energy. From a long-term perspective, case 7 can produce more life cycle carbon than the other cases, and a large portion of such emissions are embedded in the building materials, components, and systems used in the renovation.


**Table 6.** Sustainability score.

#### **4. Discussion and Sensitivity Analysis**

#### *4.1. Sensitivity Analysis*

As demonstrated from this study, EUI is not always the best measure of sustainability because of its emphasis on operating energy. The overall goal of sustainable development is to reduce carbon emissions, mitigating the impact on climate change. Reducing the operating energy demand is only one important part—integrating the counting of added embodied carbon emissions due to an energy retrofit can generate a more holistic view of sustainability. Further, the evaluation of sustainability is not only impacted by the trade-off between operating carbon emissions and embodied carbon emissions but is also influenced by the stakeholders' priorities.

To test the validity of the sustainability score calculation, we used three sets of different weights to represent different stakeholders' values. For building owners or operators, operating energy savings is prioritized and related to operation cost savings; therefore, we gave the higher weights to OES and ECO to represent the building operators' view. The results are illustrated in Figure 8a. The second set of weights was for climate change policymakers. We assumed they will prioritize a life cycle carbon emissions reduction; thus, the balance between OES and EMB should be weighted equally but higher than ECO. The results are illustrated in Figure 8b. The third set of weights represented current common practices of sustainable building renovations, which are primarily related to an operating energy reduction (measured by EUI), so we gave much higher weights to OES and lower and equal weights to EMB and ECO. The results are illustrated in Figure 8c.

The results demonstrated that the sustainability index can vary depending on the priorities the decision makers give to operating carbon, embodied carbon, and operating cost. Furthermore, how sustainability is measured can have a determining impact on whether a life cycle carbon emissions reduction can be achieved. A policy maker focused on climate change and overall carbon neutrality will have a much different evaluation of sustainability compared to building owners, operators, and designers (as demonstrated in Figure 8a–c), as Figure 8a,c are more similar to each other than they are to Figure 8b. This can be problematic if the existing sustainability measurement continues to be used in the building and construction industry, as it focuses on operating energy use. There are some countries integrating embodied energy and carbon calculations and considerations in building codes, such as Norway Standard 3720 [25], but an overwhelming majority of countries have not made the embodied carbon calculation a mandatory requirement for new construction and renovation.

**Figure 8.** Sensitivity analysis. (**a**) is the sustainability score with equal weights; (**b**) is the sustainability score by climate change policy maker; (**c**) is the sustainability score by building owners; (**d**) is the before and after renovation operating energy use.

#### *4.2. Significance of the Study*

The significant features of this study, compared to previous studies on the sustainability of renovation projects, are as follows:


#### *4.3. Limitations of the Study*

There are three limitations of this study: First, since the LCA database we used is location-specific, we were unable to verify the results using other LCA tools because each tool uses a different database for evaluating the life cycle environmental impact and carbon emissions. Second, we were unable to get further information on why certain renovation techniques were applied to certain buildings but not others; therefore, the motivation for the renovation strategies was not clear to us. Third, structural and other building systems were excluded from this study; the inclusion of the whole building system might reveal additional insights on the source of embodied carbon emissions derived from all building materials and systems.
