Attic Air Sealing Heat Loss Calculator
Estimate the heat loss through attic air leakage and the potential energy savings from proper air sealing, based on your home's characteristics and local climate.
Typical leaky home: 8–15 ACH50. Tight home: 1–3 ACH50.
Recommended target: 3–5 ACH50 for existing homes.
Find your local HDD at NOAA or energystar.gov. Example: Chicago ≈ 6500, Atlanta ≈ 2800.
Natural gas: ~$1.00–$2.00/therm. Electric heat: convert kWh cost accordingly.
Gas furnace: 0.80–0.97. Heat pump COP: 2.0–4.0.
Formulas Used
1. House Volume: V = Floor Area × Ceiling Height (ft³)
2. Natural ACH (LBL Model): ACHn = ACH50 ÷ N, where N = 20 (average shielding, 1-story equivalent)
3. Infiltration Airflow: CFM = (ACHn × V) ÷ 60
4. Sensible Heat Loss Rate: Q̇ = 1.1 × CFM (BTU/hr per °F ΔT) — from air heat capacity 0.018 BTU/ft³·°F × 60 min/hr
5. Annual Heat Loss: BTU/yr = Q̇ × HDD × 24 hr/day
6. Annual Energy: Therms = Annual BTU ÷ 100,000
7. Annual Cost: Cost = (Therms × $/therm) ÷ Efficiency (AFUE or COP)
8. Simple Payback: Years = Sealing Cost ÷ Annual Savings
Assumptions & References
- LBL infiltration factor N = 20 is the standard default for average wind exposure and single-story homes (Sherman & Grimsrud, 1980; ASHRAE 136).
- Air heat capacity constant 1.1 BTU/hr·CFM·°F is the ASHRAE standard value at sea level (0.075 lb/ft³ × 0.24 BTU/lb·°F × 60 min/hr ≈ 1.08, rounded to 1.1).
- Heating Degree Days (HDD) base 65°F per NOAA/DOE convention.
- 1 therm = 100,000 BTU (natural gas standard).
- Attic air sealing addresses the primary bypass paths: top plates, recessed lights, plumbing/electrical penetrations, and attic hatches.
- Estimated sealing cost of $0.15/sq ft is a midpoint estimate; actual costs vary by contractor, region, and complexity.
- This calculator estimates infiltration heat loss only; conductive losses through insulation are separate.
- References: ASHRAE Fundamentals Handbook; DOE Building America; Lawrence Berkeley National Laboratory; Energy Star Home Sealing Guide.