| Scenario | Area (ft²) | ACH50 (Before → After) | HDD / CDD | Upgrade cost | Estimated annual savings |
|---|---|---|---|---|---|
| Starter sealing | 1,500 | 9.0 → 6.5 | 2,500 / 900 | $650 | $120–$220 |
| Mid-level retrofit | 2,000 | 8.0 → 4.0 | 3,000 / 1,200 | $1,200 | $250–$520 |
| Deep air-seal | 3,000 | 7.0 → 2.5 | 4,500 / 1,500 | $3,000 | $600–$1,200 |
- Volume (ft³) = Area (ft²) × Height (ft)
- ACH50 ↔ CFM50: ACH50 = (CFM50 × 60) ÷ Volume
- ACHnat = ACH50 ÷ N (N-factor approximates weather exposure)
- CFMnat = (ACHnat × Volume) ÷ 60
- Heating load (BTU/yr) = 1.08 × CFMnat × 24 × HDD
- Cooling load (BTU/yr) = 1.08 × CFMnat × 24 × CDD × (1 + latent%)
- Fuel and electricity use are adjusted by efficiency, then multiplied by local prices.
- NPV sums escalated annual savings discounted over the analysis years.
- Enter your floor area and average ceiling height.
- Select the leakage method and input current and target values.
- Choose an exposure preset, or enter a custom N-factor.
- Use local HDD/CDD, then enter energy prices and equipment ratings.
- Add upgrade cost, years, discount rate, and escalation assumptions.
- Press Calculate Savings to see results above the form.
- Use the CSV/PDF buttons to export your latest run.
Infiltration baseline and leakage targets
Air leakage is entered as ACH50 or CFM50, then converted to natural leakage using an exposure N-factor (typically 15–25). For a 2,000 ft² home with 8 ft ceilings, volume is 16,000 ft³. Dropping from 8.0 ACH50 to 4.0 ACH50 cuts estimated ACHnat from 0.40 to 0.20 when N=20.
Weather-driven loads and degree days
The calculator estimates sensible infiltration load with 1.08 × CFMnat × 24 × degree days. With HDD 3,000 and CDD 1,200, reduced airflow lowers both heating and cooling BTU. A 15% latent add-on is applied to cooling to reflect humidity-driven energy use in many climates. If you live in a dry region, setting latent to 0–5% can better match typical utility bills.
Energy prices and equipment performance
Heating cost depends on fuel and efficiency. Gas, propane, and oil use AFUE; electric resistance uses kWh; heat pumps use COP. Cooling uses SEER to convert BTU to kWh. If electricity is $0.18/kWh and gas is $1.60/therm, tighter envelopes often shift the biggest savings toward the dominant seasonal load. Raising SEER from 14 to 18 lowers the cooling share, so sealing becomes more heating-driven in colder zones.
Financial metrics: payback and NPV
Annual savings equals “before” minus “after” infiltration cost. Simple payback is upgrade cost divided by first-year savings. For longer decisions, the tool discounts and escalates savings over the analysis period. Example: $400/year savings, 10 years, 5% discount, 2% escalation yields a present value near $3,200 before subtracting project cost. A higher discount rate shrinks NPV, while higher escalation increases it, so run two cases.
Quality checks and real-world adjustments
Use local degree days from a nearby station and match efficiency to your equipment label. If target leakage is higher than current, savings can be negative and the chart will show that. For air-sealing, verify combustion safety and consider balanced ventilation so comfort gains do not reduce indoor air quality. Many projects also reduce drafts, which users value even when fuel prices are low.
1) What leakage number should I use if I have no test?
Start with 7–10 ACH50 for older, leaky homes and 3–5 ACH50 for newer construction. Treat it as a baseline, then refine after testing or an audit.
2) What does the N-factor change?
It converts ACH50 to estimated natural leakage. Sheltered homes usually need a higher N (lower natural leakage). Windy or exposed sites use a lower N, increasing modeled costs and savings.
3) Where do I find HDD and CDD?
Use degree-day summaries for your nearest city or weather station. If you only have monthly values, add them to annual totals before entering them.
4) Why can savings be negative?
If your target leakage is higher than your current leakage, the “after” airflow increases. That raises heating and cooling loads and flips the savings sign.
5) How does SEER affect the cooling result?
Higher SEER means fewer kWh per BTU of cooling, so the cooling cost portion drops. With very efficient cooling, air-sealing savings may be driven more by heating.
6) Do I need ventilation after air sealing?
Often, yes. Tightening reduces fresh-air entry and can change combustion appliance behavior. If you seal aggressively, verify safety and consider balanced ventilation to maintain indoor air quality.