# A Data-Driven Model for the Energy and Economic Assessment of Building Renovations
## 3 Results
### HVAC Upgrade ROI vs. System Age Relationship  
Figure 8 illustrates the relationship between HVAC system age and retrofit ROI for the three building clusters, with a theoretical curve demonstrating the generally increasing return potential as systems age. High-consumption buildings (dark orange) achieve the highest ROI (2.39) at a system age of approximately 15 years, outperforming both Critical-consumption buildings (red, ROI: 2.12) and Moderate-High-consumption buildings (light orange, ROI: 2.05) with similar system ages. This visualization reveals that, while system age is a significant factor influencing upgrade economics, it is not the sole determinant; building usage patterns and interactions with other systems also play crucial roles. The accompanying insights note how ROI increases with system age and how decision tree modeling predicts approximately 20% energy savings for systems older than 15 years. This information provides valuable guidance for timing HVAC system interventions to maximize economic returns. Figure 7: HVAC Upgrade analysis across building clusters. The combined economic analysis results demonstrate that retrofit intervention strategies should be highly targeted to building-specific characteristics rather than applied as generic solutions. The optimal intervention varies significantly by building cluster: Wall Insulation for Critical-consumption buildings (ROI: 2.40), HVAC System Upgrades for High-consumption buildings (ROI: 2.39), and Smart Home Systems for Moderate-High-consumption buildings (ROI: 2.48). These findings provide a quantitative foundation for strategic decision making, enabling stakeholders to allocate limited resources to interventions with the greatest economic and environmental returns. Table 1 summarizes a comparative overview of the five key retrofit interventions evaluated in this study, highlighting their economic viability across different building clusters based on ROI and payback period. Smart Home Systems emerge as the top-performing intervention in Moderate-High-consumption buildings, showing the highest ROI of 2.48 and the shortest payback period of 5.5 years. This suggests that integrating digital control technologies in buildings that already possess relatively efficient envelopes can yield substantial returns with minimal upfront investment. Wall Insulation proves to be the most impactful for Critical-consumption buildings, which typically suffer from poor envelope performance. With an ROI of 2.40 and a payback period of approximately 10.2 years, insulation improvements substantially reduce heating demands, making them economically justifiable despite their moderate cost. HVAC System Upgrades are especially effective in High-consumption buildings, offering a nearly equivalent ROI of 2.39 and a quicker payback of 8.0 years. This is likely due to the \begin{table}
\begin{tabular}{c c c c c} \hline
**Retrofit Measure** & **Building Cluster** & **ROI** &
\begin{tabular}{c} **Payback Period** \\ **(Years)** \\ \end{tabular} & **Most Effective Cluster** \\ \hline Wall Insulation & Critical & 2.40 & \(\sim\)10.2 & Critical-Consumption Buildings \\ Window Replacement & All Clusters & \(\sim\)1.20 & \(>\)15.0 & Not Recommended as \\ HVAC System Upgrades & High & 2.39 & 8.0 & Highly-Consumption Buildings \\ Smart Home Systems & Moderate–High & 2.48 & 5.5 & Moderate–High- \\ Solar Panel Installation & All Clusters & 1.15–1.27 & 13–15+ & Less Econonically Competitive \\ \hline \end{tabular}
\end{table} 
Table 1: Economic effectiveness of retrofit interventions by building clusters. Figure 8: Relationship between HVAC system age and retrofit ROI. combination of inefficient existing systems and high baseline energy use, which amplify the energy-saving impact of new HVAC technologies. In contrast, Window Replacement shows a lower ROI (-1.20) and a payback period exceeding 15 years across all clusters.