Calculator Inputs
Example Data Table
Sample scenario to show how inputs map to outputs.
| Windows | Leak/window (CFM) | Reduction (%) | HDD | CDD | Cost/window | Rebate | Net annual savings | Payback |
|---|---|---|---|---|---|---|---|---|
| 12 | 10.0 | 45.0 | 4200 | 900 | $18.00 | $25.00 | $112.15 | 1.70 yrs |
Formula Used
- Total airflow reduced (CFM) = windows × leak per window × (reduction % ÷ 100).
- Annual heating BTU saved = 1.08 × CFM × HDD × 24 × heating multiplier.
- Annual cooling BTU saved = 1.08 × CFM × CDD × 24 × cooling multiplier.
- Cooling kWh saved = cooling BTU ÷ (SEER × 1000).
- Heating energy saved depends on fuel:
- Electric: kWh = heating BTU ÷ 3412 ÷ efficiency.
- Natural gas: therms = heating BTU ÷ (100,000 × efficiency).
- Propane: gallons = heating BTU ÷ (91,500 × efficiency).
- Oil: gallons = heating BTU ÷ (138,500 × efficiency).
- NPV = Σ (cashflowᵧ ÷ (1 + discount)ᵧ) with year 0 as the investment.
- Simple payback = investment ÷ net annual savings (year 1).
How to Use This Calculator
- Count the windows you plan to seal and estimate leakage per window.
- Enter your expected reduction percent after weatherstripping or caulking.
- Add local HDD/CDD values and your heating and cooling performance settings.
- Use your latest utility bills to set electricity and fuel rates.
- Enter the sealing cost, rebates, and optional annual maintenance.
- Choose an analysis period, escalation, and discount rate for finance metrics.
- Press Calculate Savings to view results above the form.
- Use Download CSV or Download PDF for reporting.
Leakage behaves like continuous ventilation
Air leakage around windows behaves like a small, always‑open vent. Reducing CFM cuts the volume of conditioned air that must be replaced, improving comfort near glass and trim. In this calculator, total airflow reduced equals windows × leak per window × reduction percent. Estimate leak by feel, smoke, or seasonal draft notes, then refine after upgrades on the worst windows first. Focus on sash locks, meeting rails, and trim gaps.
Degree days convert airflow into seasonal load
Seasonal energy impact is estimated with degree days, which approximate how many hours your home fights outdoor temperatures. Heating BTU saved uses 1.08 × CFM × HDD × 24, then applies a heating multiplier for wind and stack effect. Cooling BTU saved uses the same structure with CDD and a cooling multiplier for humidity, infiltration patterns, and longer compressor runs. Keep multipliers at 1.00 unless conditions are extreme.
Fuel pricing and efficiency shape heating value
Heating savings depend on both fuel price and equipment efficiency. The model converts BTU saved into the unit you buy: kWh for electric, therms for natural gas, and gallons for propane or oil. It then divides by seasonal efficiency to reflect delivered heat. Key factors include 3,412 BTU per kWh, 100,000 BTU per therm, 91,500 BTU per propane gallon, and 138,500 BTU per oil gallon.
SEER controls cooling kWh per BTU avoided
Cooling savings are valued in electricity because air conditioning is typically electric. The calculator converts cooling BTU into kWh using SEER, where kWh ≈ BTU ÷ (SEER × 1,000). A higher SEER lowers kWh per BTU, reducing the dollar value of each BTU avoided. In hot climates with high CDD, modest sealing can still matter, especially when rates include demand or time‑of‑use pricing.
Cashflow outputs support upgrade comparisons
Financial outputs translate energy savings into decision metrics. Net annual savings subtracts optional maintenance from gross heating plus cooling savings. Simple payback divides upfront investment by year‑one net savings for a quick screen. NPV discounts each future year using your discount rate and applies escalation to represent changing energy prices. The break‑even year and IRR help compare sealing to insulation, equipment, or window replacement options.
FAQs
1) Which inputs usually change results the most?
Net savings is driven by reduced CFM, local HDD/CDD, fuel choice, efficiency, and utility rates. If you are unsure, start by updating rates and degree days first, then refine leakage and reduction percent.
2) How can I estimate leak per window without test data?
Use a simple draft check: feel for cold air on windy days, or move a tissue or smoke source near sash and trim. If you have blower-door data, allocate a small share to windows. Start conservative and adjust after sealing.
3) Why does heating efficiency affect savings?
Lower efficiency means you must buy more energy to deliver the same heat. When you reduce the delivered load through sealing, the model converts BTU avoided into purchased units and divides by seasonal efficiency.
4) Should I include window replacement costs here?
Use this tool for sealing and weatherstripping costs. Full window replacement changes insulation value, solar gain, and durability. If replacement is planned, compare its incremental cost against this sealing option, using the same rates and financial assumptions.
5) What discount rate should I use?
A practical choice is your after-tax return expectation or borrowing rate for home upgrades. Use a higher rate for strict payback goals and a lower rate for long-lived comfort improvements. Keep it consistent across projects when comparing NPVs.
6) How accurate is the degree-day approach?
Degree-day methods are good for screening because they capture seasonal temperature exposure. Accuracy depends on realistic CFM estimates and how often HVAC runs. Use multipliers to reflect windy sites and validate with before-and-after bills when possible.