Building Airtightness Retrofit ROI Calculator

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:

$$Q_{\mathrm{heat}}=0.432\times V\times {\mathrm{ACH}}_{\mathrm{nat}}\times {\mathrm{HDD}}_{\mathrm{65}}$$

In this expression, $V$ represents the building volume in cubic feet, ${\mathrm{ACH}}_{\mathrm{nat}}$ 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.