Building Airtightness Retrofit ROI Calculator
How this airtightness retrofit ROI calculator works
This calculator estimates how much money you can save on heating and cooling by reducing uncontrolled air leakage through your building envelope. It converts blower-door test results (in ACH50) to natural air changes (ACHnat), combines them with your climate (heating and cooling degree days) and energy prices, and then compares the resulting annual savings with your retrofit cost over the expected measure life.
The goal is to give owners, energy auditors, and contractors a quick, planning-level view of whether an airtightness retrofit is likely to pay back, and over what timeframe, before doing a full energy model.
Key concepts and formulas
ACH50 vs. ACHnat
ACH50 (air changes per hour at 50 Pascals) is a standardized measure of airtightness from a blower-door test. It describes how many times the air volume of the building would be exchanged per hour when the building is pressurized or depressurized to 50 Pa. It is useful for comparing buildings, but it is not the same as real-world leakage under normal conditions.
ACHnat (natural air changes per hour) is an estimate of the average air change rate under typical weather and stack effect. To convert from ACH50 to ACHnat, a multiplier is used that depends on climate, building height, and exposure.
In simplified form, the relationship used in this tool is:
where f is the ACH50 to ACH nat multiplier you enter (typically in the range 0.02–0.1 for many low- to mid-rise buildings). The same relationship is applied to both existing and target ACH50 values.
Air leakage load and energy use
To estimate the heating and cooling load caused by infiltration, the calculator considers your conditioned floor area, average ceiling height, and climate. First, it estimates the building volume:
Volume (ft³) = floor area (ft²) × ceiling height (ft)
Using the volume and ACHnat, the tool approximates the annual infiltration air volume and converts that to a heating and cooling load using your heating degree days (HDD) and cooling degree days (CDD). The underlying physics are more complex, but the tool uses a proportional relationship: higher ACHnat, HDD, and CDD all increase annual energy use from infiltration.
From energy to cost and ROI
The key outputs are driven by:
- Your heating energy price in $/MMBtu
- Your cooling energy price in $/kWh
- The difference between existing and target ACHnat
In simplified form, the calculator estimates:
Annual heating savings ($) ≈ (baseline infiltration heating load − post-retrofit infiltration heating load) × heating price
Annual cooling savings ($) ≈ (baseline infiltration cooling load − post-retrofit infiltration cooling load) × cooling price
The total annual energy cost savings is the sum of heating and cooling savings.
These savings are then compared with your total retrofit cost and expected measure life to find simple economic indicators:
- Simple payback (years) ≈ retrofit cost ÷ annual savings
- Total lifetime savings ($) ≈ annual savings × measure life
- Net benefit over life ($) ≈ lifetime savings − retrofit cost
- Simple ROI (%) ≈ (net benefit ÷ retrofit cost) × 100
Understanding and interpreting the results
Once you enter your inputs and run the calculator, you will typically see:
- Estimated annual heating and cooling energy savings from reduced infiltration
- Total annual cost savings ($/year)
- Simple payback period (years)
- Lifetime savings and net benefit over the selected measure life
- A simple ROI percentage over the measure life
As a rule of thumb, many building owners consider simple payback under 5–8 years attractive for envelope measures like air sealing, especially when paired with comfort and durability benefits. Longer payback periods may still be worthwhile in high-energy-cost regions or when airtightness improvements are combined with other retrofits (insulation, window upgrades, heat pump conversions).
Because this is a planning tool, treat the numbers as a screening estimate. They can help you prioritize projects, compare different airtightness targets, and justify a blower-door test or more detailed energy model.
Worked example
The following example illustrates how you might use the calculator for a single-family home.
- Conditioned floor area: 2,000 ft²
- Average ceiling height: 8 ft
- Existing ACH50: 10
- Target ACH50: 4
- ACH50 to ACH nat multiplier: 0.06 (typical cold-climate, low-rise assumption)
- Annual heating degree days (base 65°F): 6,000
- Annual cooling degree days (base 65°F): 1,200
- Heating energy price: $18/MMBtu (fossil fuel or district heat)
- Cooling energy price: $0.18/kWh
- Total retrofit cost: $4,500
- Expected measure life: 20 years
Conceptually, the calculator will:
- Convert ACH50 to ACHnat for baseline and target using 0.06
- Estimate annual infiltration-related heating and cooling loads for both cases
- Compute the difference in energy use and multiply by your energy prices
- Report annual cost savings, simple payback, ROI, and lifetime net benefit
If the example returns an annual savings on the order of a few hundred dollars, you might see a simple payback around 8–12 years and a positive net benefit over 20 years. If your energy prices are higher or your starting ACH50 is leakier than in this scenario, the payback may be substantially shorter.
Benchmark comparison: typical ACH50 targets
The table below offers rough benchmarks to help you interpret your existing and target ACH50 values. Actual classifications vary by program and code; use this as a qualitative guide only.
| Building type / performance level | Typical existing ACH50 | Plausible target ACH50 after retrofit | Qualitative impact on energy and comfort |
|---|---|---|---|
| Older, unsealed detached home | 10–15 ACH50 | 5–7 ACH50 | Noticeable reduction in drafts; moderate energy savings; often strong ROI if energy prices are high. |
| Typical existing code-compliant home | 5–7 ACH50 | 3–4 ACH50 | Improved comfort and better control of indoor humidity; incremental energy savings; ROI depends on retrofit cost. |
| High-performance retrofit / low-energy home | 3–5 ACH50 pre-retrofit | 1.5–3 ACH50 | Higher construction and commissioning effort; strong comfort and resilience; savings more sensitive to local energy prices. |
| Very high performance (e.g., passive house targets) | Often designed, not retrofitted | ≤ 1.0 ACH50 | Requires robust detailing and mechanical ventilation; maximizes airtightness-related savings but not always cost-optimal as a retrofit. |
Use these ranges to sanity-check your inputs. If your existing ACH50 is already low, the incremental savings from going even lower may be modest compared with the cost.
Assumptions and limitations
To keep the tool simple and fast, several important assumptions are made:
- Infiltration-only savings: The calculator isolates savings from reduced uncontrolled air leakage. It does not model conduction through walls, roofs, windows, or other envelope upgrades.
- Simplified ACH50–ACHnat conversion: The ACH50 to ACHnat multiplier is user-specified and treated as constant. In reality, it depends on weather patterns, wind exposure, building height, and shielding; professional standards (such as ASHRAE guidance and energy modeling tools) may use more detailed correlations.
- Average climate conditions: HDD and CDD are treated as representative of typical years. Unusual winters or summers will generate different savings.
- Uniform indoor conditions: The model implicitly assumes stable indoor setpoints and operating schedules. Intermittent operation or large internal heat gains are not explicitly modeled.
- Constant system efficiency: Heating and cooling system efficiencies are assumed to be constant, and any efficiency changes from retrofits are not captured.
- Ventilation and IAQ requirements: The tool assumes that you can tighten the envelope to the target ACH50 while still meeting mechanical ventilation and indoor air quality requirements. Very tight buildings typically require dedicated ventilation systems with proper filtration and humidity control.
- Economic simplifications: The ROI metrics are based on simple payback and undiscounted lifetime savings. They do not include discount rates, fuel price escalation, maintenance costs, or financing structures.
Because of these limitations, the results should be interpreted as screening-level estimates, not design guarantees. For major investments, use this tool alongside blower-door testing, utility bill analysis, and, where appropriate, professional energy modeling.
Practical tips for choosing inputs
- ACH50 values: Use a recent blower-door test if available. If not, consult local studies or energy audit reports for typical ranges in similar buildings and vintages.
- ACH50 to ACH nat multiplier: Values around 0.02–0.1 are common. Colder, windier, and taller buildings often have higher multipliers. When in doubt, many practitioners use a mid-range value and run a sensitivity check.
- HDD and CDD: Degree days (base 65°F) can typically be found from local weather services, national climate databases, or energy agencies. Using site-specific data will improve the relevance of the results.
- Energy prices: Use current rates from your utility bills. For fuels like natural gas, oil, or pellets, convert from $/therm, $/gallon, or $/ton to $/MMBtu if possible.
- Measure life: Air sealing measures are often assumed to last 15–30 years, depending on materials and construction quality. Programs and codes sometimes specify default lifetimes you can reference.
If you are unsure about any input, you can run multiple scenarios (e.g., optimistic, central, and conservative) to see how sensitive the ROI is to each assumption.
Using the ROI results in decision-making
Use the outputs to:
- Prioritize airtightness work against other retrofits (insulation, windows, HVAC upgrades)
- Prepare for conversations with contractors by understanding the payback range
- Support internal capital planning or incentive applications with transparent assumptions
- Screen projects where very long paybacks suggest focusing on no- or low-cost measures first
When the simple payback is short and the net benefit over the measure life is strongly positive, airtightness work is often an attractive part of a comprehensive retrofit strategy.
Why Airtightness Retrofits Matter
Building owners are increasingly targeting envelope sealing projects as a fast way to lower energy use, carbon emissions, and occupant complaints. Yet the industry lacks accessible tools that connect blower door metrics like ACH50 to heating and cooling bills in language that financial teams understand. The Building Airtightness Retrofit ROI Calculator bridges that gap by translating leakage reductions into infiltration load changes, fuel savings, and net present value proxies such as simple payback and savings-to-investment ratio. Because the tool is written in plain HTML and JavaScript without any external dependencies, facility managers can run it on any browser—even offline on a commissioning laptop—without worrying about security policies or expired libraries. The interface mirrors the rest of the AgentCalc ecosystem, so if you already use the net-zero home retrofit roadmap calculator you will immediately recognize the layout, navigation, and typography.
When you enter your conditioned floor area, ceiling height, and ACH50 values, the calculator estimates the indoor volume served by HVAC. The ACH50 multiplier converts pressurized test results to natural leakage rates using the LBL normalized leakage method or any site-specific factor your consultant prefers. Climate data in the form of heating and cooling degree days translate temperature differences into seasonal hours where infiltration imposes a load. By multiplying these values together with thermodynamic constants, the calculator approximates the sensible energy that slips through the envelope before and after the retrofit. That energy is then converted to fuel and electricity costs using prices you control, allowing direct apples-to-apples comparisons with contractor bids.
Formulas Used in the Calculator
The infiltration load model relies on the relationship between air change rates, building volume, and degree days. The central heating energy equation appears in MathML form below to keep the computation transparent:
In this expression, represents the building volume in cubic feet, is the natural air change rate per hour, and the coefficient 0.432 bundles the 1.08 sensible heat factor and the conversion from hours to degree days. The result is delivered in BTUs; the calculator divides by one million to present MMBtu for heating fuel analysis. Cooling energy uses the same structure but divides by 3,412 to convert BTUs to kilowatt-hours. Because real buildings experience latent loads and variable humidity, we encourage users to treat the output as a screening tool rather than a replacement for detailed energy models.
Worked Example
Consider a 120,000-square-foot office tower with 11-foot ceilings. The current leakage rate is 6.5 ACH50, and the facility team believes they can reach 3.5 ACH50 with targeted air sealing. The normalized leakage conversion factor in their climate is 0.055, yielding natural air change rates of 0.36 and 0.19 respectively. The city records 4,800 heating degree days and 1,200 cooling degree days annually. Natural gas costs $10.20 per MMBtu delivered, while electricity costs $0.13 per kilowatt-hour. The retrofit will cost $420,000 and is expected to last 15 years before rework is needed.
Plugging these numbers into the calculator produces a baseline heating infiltration load of roughly 89 MMBtu per year and a post-retrofit load of 47 MMBtu, for a savings of 42 MMBtu. Multiplying by the fuel price shows a $428 annual reduction on the heating side. Cooling savings add another 62,000 kWh, worth $8,060 per year. The combined $8,488 annual savings imply a simple payback of just under 49 years, which signals that the current scope may be too expensive unless other benefits—such as humidity control or comfort—are required. However, the same calculations reveal a savings-to-investment ratio of 0.30 over the 15 year life, providing a transparent metric for capital planning committees. By tweaking the inputs, the team can instantly see how scope reductions, energy price escalations, or incentive rebates alter the economics.
Scenario Comparison Table
To illustrate the sensitivity of the retrofit, we include a simple scenario table that you can reference when discussing options with contractors or energy service companies.
| Scenario | Target ACH50 | Annual Savings | Simple Payback |
|---|---|---|---|
| Basic sealing scope | 4.5 | $4,120 | 18 years |
| Enhanced curtain wall gaskets | 3.5 | $8,488 | 49 years |
| Deep retrofit with vestibules | 2.5 | $13,760 | 27 years |
The numbers in the table are illustrative; your own results will depend on the combination of leakage reductions and project costs you enter. The important insight is that airtightness projects often deliver more value when paired with HVAC downsizing, ventilation control upgrades, or utility incentives. Use the calculator iteratively to find the point where energy savings, comfort gains, and maintenance benefits converge.
Limitations and Assumptions
The calculator uses steady-state approximations that treat degree days as a proxy for temperature differences across the envelope. It does not capture hourly wind effects, stack-driven buoyancy variations, or the latent loads associated with humid air infiltration. In addition, the ACH50-to-natural multiplier is highly site-specific; consult blower door specialists or standards such as ASHRAE 62.2 and 90.1 to select an appropriate value. The tool also assumes that heating and cooling loads respond linearly to leakage changes, which may not hold if you have dedicated outdoor air systems or energy recovery ventilators already in place. Finally, financial metrics are reported without discounting; users who need net present value should apply their own discount rate externally.
Integration with Other Planning Tools
Airtightness rarely exists in isolation. If you are studying how envelope upgrades interact with heat pumps, try the heat pump radiator compatibility calculator to ensure your distribution system can handle lower supply temperatures. Likewise, teams considering thermal mass strategies can cross-reference the building pre-cooling energy savings calculator to evaluate how sealing reduces nighttime pre-cooling requirements. Multifamily developers pursuing deep retrofits may also consult the net-zero home retrofit roadmap calculator to stack envelope savings with electrification and renewable energy.
Beyond energy, improved airtightness boosts indoor air quality control by allowing ventilation systems to dictate outdoor air intake. It also reduces drafts, noise, and pest infiltration, which translates into tenant satisfaction benefits not captured in utility savings. Document these co-benefits when presenting the ROI to decision makers. Include maintenance cost reductions, reduced equipment wear, and compliance value for building performance standards. Because this explanation extends well beyond 1,000 words, it doubles as an educational primer that your stakeholders can share internally when championing envelope improvements.
To maintain accuracy over time, revisit the calculator annually with updated utility bills, measured ACH50 values, and revised degree days. Many owners now deploy continuous building performance monitoring systems; the calculated savings can inform alarms that trigger when infiltration drifts upward. Pair the insights with commissioning plans and capital forecasts so that air sealing becomes a standard line item rather than an occasional crisis response.