Window Draft & Air Leakage Savings Calculator
Estimate your annual heating and cooling cost savings by sealing window drafts and air leaks using the infiltration heat loss method.
Formulas Used
1. Total infiltration airflow:
Q (CFM) = Nwindows × (CrackLengthin / 12) × LeakRateCFM/ft
2. Sensible heat transfer rate:
Q̇ (BTU/hr·°F) = QCFM × 1.08
(1.08 = specific heat of air 0.018 BTU/ft³·°F × 60 min/hr)
3. Annual heating energy lost:
HeatLoss (BTU) = Q̇ × HDD × 24
4. Annual cooling energy gained:
CoolGain (BTU) = Q̇ × CDD × 24
5. Heating cost savings:
Gas: HeatLoss / 100,000 / AFUE × $/therm
Electric: HeatLoss / 3,412 / COP × $/kWh
Oil: HeatLoss / 138,500 / AFUE × $/gal
Propane: HeatLoss / 91,500 / AFUE × $/gal
6. Cooling cost savings:
COPcool = SEER / 3.412
kWhcool = (CoolGain / 3,412) / COPcool
CoolSavings = kWhcool × $/kWh
7. Simple payback: SealingCost / TotalAnnualSavings
Assumptions & References
- Air infiltration rates (0.1–0.5 CFM/linear ft of crack) are based on ASHRAE Standard 90.1 and NFRC 400 window air-leakage classifications.
- Sensible heat factor of 1.08 BTU/hr·CFM·°F assumes standard air density at sea level (ASHRAE Fundamentals Handbook, Chapter 18).
- Degree-day method (HDD/CDD base 65°F) per ASHRAE 90.1 Appendix D and DOE EnergyPlus documentation.
- Fuel energy content: Natural gas 100,000 BTU/therm; Electricity 3,412 BTU/kWh; Heating oil 138,500 BTU/gal; Propane 91,500 BTU/gal (EIA).
- CO₂ emission factors: Electricity 0.386 kg CO₂/kWh (US average, EPA eGRID 2022); Natural gas 11.7 lb/therm; Oil 22.4 lb/gal; Propane 12.7 lb/gal (EPA GHG equivalencies).
- Model assumes 100% of infiltration is eliminated after sealing (conservative upper bound). Real-world savings typically range 50–80% depending on workmanship.
- Latent (humidity) loads and solar gain through gaps are not included; actual savings may be higher in humid climates.
- SEER to COP conversion: COP = SEER / 3.412 (per AHRI Standard 210/240).