RAS Engineering is a professional engineering company serving building managers and owners by providing advice, oversight restoration, and testing to ensure that all components of a build’s exterior and structure meet the standards necessary to provide quality durability and safe protection against the elements.
How to Calculate Reliable Energy Savings
for Building Envelope Retrofits
Building envelope retrofits—such as adding insulation, upgrading windows, or improving air sealing—can significantly reduce energy consumption in buildings. Accurately forecasting energy savings for these improvements is critical for building owners, engineers, and energy managers. Reliable forecasts can help with budgeting, project planning, and decision-making. Here's a comprehensive guide on how to calculate dependable energy savings forecasts for building envelope retrofits.
1. Understanding the Building Envelope and Energy Losses
The building envelope refers to the physical barrier between the interior and exterior of a building, which includes walls, roofs, floors, windows, and doors. This barrier plays a significant role in energy consumption because it impacts how much heat enters or leaves the building. Energy losses or gains through the envelope occur mainly through three mechanisms:
Conduction (heat transfer through materials)
Convection (air leakage)
Radiation (heat transfer via electromagnetic waves)
Improving the building envelope can reduce these losses, but the key is to quantify the savings before the retrofit is carried out. Here's how you can calculate reliable energy savings forecasts.
2. Step-by-Step Guide to Forecasting Energy Savings
Step 1: Conduct an Energy Audit
An energy audit is the first and most critical step in understanding how energy is being used in the building. An audit identifies key areas of energy waste and opportunities for improvement. Energy audits typically involve:
Inspection of building components (e.g., windows, insulation, air sealing, etc.)
Analysis of utility bills to identify patterns and energy usage trends
On-site testing, such as blower door tests or infrared thermography, to detect air leakage and heat loss
The audit results will give you a baseline for current energy usage and help you prioritize which retrofits offer the best potential savings.
Step 2: Simulate Building Energy Performance
Once you have audit data, it's time to model the building's energy performance. Several tools, such as EnergyPlus, eQuest, or RETScreen, allow you to simulate the building's thermal behavior and energy use. These tools take into account:
Building geometry
Materials used in the construction
Local climate conditions
Building occupancy and usage patterns
By running these simulations, you can understand how different retrofit measures, such as improving insulation or upgrading windows, will impact energy usage.
Step 3: Use Building Energy Models
After simulating the building’s current state, incorporate retrofit measures into the model. For example, if you plan to add insulation or upgrade windows, adjust the model's parameters to reflect these improvements. These changes will reduce heat loss or gain and, therefore, energy consumption. Key parameters to adjust in your model include:
R-values (thermal resistance) for walls, roofs, and floors
U-values (heat transfer coefficients) for windows
Air infiltration rates for improved air sealing
Run the simulation again with these updated values to forecast the energy savings.
Step 4: Account for Local Climate Conditions
Energy savings will vary greatly depending on the local climate. In colder climates, improvements to insulation and air sealing will yield higher savings, while in hotter climates, reflective roofing or high-performance windows may provide the most significant reductions in cooling energy.
Incorporate heating degree days (HDD) and cooling degree days (CDD)—metrics that indicate the demand for heating or cooling—into your model. Tools like ASHRAE's Climate Data Center provide local climate data, which can be integrated into your forecasts to improve accuracy.
Step 5: Consider Occupancy and Behavior
Energy usage is influenced not only by the building’s design but also by how it’s used. Occupancy levels, operating hours, and occupant behavior (such as thermostat settings) can all impact the energy savings realized from a retrofit. When forecasting savings, make assumptions about:
Building occupancy patterns
Operating hours
Thermostat settings
To make the forecast reliable, include different scenarios for how these factors might change after the retrofit (e.g., lower thermostat settings because of improved insulation).
Step 6: Calculate Payback Periods and ROI
Energy savings are only part of the picture. You also need to factor in the cost of the retrofit to calculate metrics like the payback period (how long it takes for energy savings to cover the retrofit cost) and the return on investment (ROI). Here’s a simple formula for payback period:
Payback Period (years)
=
Cost of Retrofit
Annual Energy Savings
For ROI, consider both the direct financial return and any indirect benefits, such as improved comfort, indoor air quality, and increased property value.
3. Using Historical Data and Case Studies
Historical energy savings data from similar retrofits can further validate your forecasts. If your building is comparable to others in size, construction, and usage, studying case studies of similar retrofits can provide a useful benchmark. In addition, energy-efficiency databases like Building Performance Database (BPD) or DOE’s Better Buildings Initiative provide access to retrofit outcomes across a wide range of buildings and climates.
4. Monitoring and Verifying Savings
After the retrofit, it's crucial to monitor actual energy consumption to verify that savings meet or exceed forecasts. You can install smart meters or use energy management systems (EMS) to track real-time energy use. Comparing post-retrofit energy consumption with your baseline will confirm the accuracy of your forecast and help adjust future projections.
Verification also involves adjusting for any external factors that could affect energy consumption, such as changes in weather, occupancy, or usage patterns. This process, known as Measurement and Verification (M&V), is often guided by protocols like the International Performance Measurement and Verification Protocol (IPMVP).
5. Additional Considerations for Reliable Forecasting
Incentives and Rebates: Research available local, state, or federal incentives for energy-efficient retrofits. These can improve the financial feasibility of projects and enhance ROI.
Non-Energy Benefits (NEBs): While energy savings are the primary focus, consider other benefits such as improved comfort, enhanced building durability, and better indoor air quality.
Sensitivity Analysis: Conduct a sensitivity analysis to understand how variations in key assumptions (like energy prices, weather, or occupancy) might impact your savings forecast. This helps in planning for uncertainties.
Conclusion
Calculating reliable energy savings forecasts for building envelope retrofits requires a systematic approach that combines energy audits, simulations, local climate data, and real-world case studies. The more detailed your initial analysis, the more accurate your forecasts will be, leading to better decision-making and more successful retrofit projects. By following the steps outlined above, you can ensure your energy-saving predictions are both reliable and actionable, maximizing the value of your building envelope retrofit investments.