| Scenario | Area (ft²) | R old → new | Annual savings | Payback | NPV (15y) |
|---|---|---|---|---|---|
| Baseline home | 250 | 1 → 7 | $210 | 5.0 yrs | $620 |
| Hot attic ducts | 320 | 2 → 10 | $340 | 3.2 yrs | $1,480 |
| Light commercial | 900 | 1 → 9 | $1,050 | 2.1 yrs | $5,900 |
- Measure duct surface area in unconditioned spaces.
- Enter existing and added insulation R-values.
- Estimate heating and cooling temperature differences.
- Add realistic runtime hours for heating and cooling.
- Set utility rates, system efficiencies, and project cost.
- Press submit to view results and export reports.
Duct loss drivers in unconditioned spaces
Supply ducts running through hot attics and cold crawlspaces can shed energy continuously during system runtime. Conduction loss scales with duct surface area, temperature difference, and insulation level. This calculator models that relationship using a U‑factor (1/R) and separates heating and cooling seasons for clearer budgeting.
How R-value improvements translate into dollars
Energy saved is estimated as (1/Rold − 1/Rnew) × Area × ΔT × Hours. Heating savings are converted to therms using your heating efficiency, while cooling savings convert to kWh using COP and 3,412 Btu per kWh. Changing utility rates instantly updates annual savings and payback.
Typical planning ranges you can test
For many homes, duct surface area often falls between 180–400 ft², while meaningful wrap upgrades commonly add R‑4 to R‑8. Seasonal ΔT assumptions frequently range 15–35°F, and combined runtime may land near 1,200–2,000 hours per year depending on climate and equipment sizing.
Interpreting payback, break-even, and NPV
Simple payback compares net upfront cost to first‑year savings. Break-even year uses cumulative nominal savings with your escalation rate. Net present value discounts future savings using your discount rate, helping compare this upgrade to other efficiency measures. A positive NPV indicates the savings stream exceeds cost on a discounted basis.
Data quality checklist for stronger estimates
Measure duct length and perimeter to refine area, then confirm insulation ratings on existing wrap. Use attic or crawlspace temperature readings to improve ΔT estimates. If ducts are leaky or poorly sealed, raise the leakage factor to reflect compounded losses, and consider sealing before insulating for best results.
1) What does the leakage factor represent?
It scales conduction-based savings to approximate extra impacts from leakage and delivery losses. Use 1.00 if unknown. Use higher values when ducts are in very harsh spaces or known to be leaky.
2) Can I use this for electric heating instead of gas?
Yes. Enter gas price as 0 and treat heating efficiency as 1.0, then convert heating savings externally if needed. Alternatively, interpret heating Btu savings as electric kWh using an appropriate COP for heat pumps.
3) How do I estimate duct surface area quickly?
Approximate area as perimeter × length for each duct run, then sum supply and return. For round ducts, area ≈ π × diameter × length. Include trunk lines and major branches in unconditioned areas.
4) Why are heating and cooling calculated separately?
Temperature differences, runtime hours, and conversion efficiencies differ by season and system type. Separating them helps you see which season drives value and aligns savings with the correct utility rate.
5) What useful life should I choose?
Many wraps and sealed jackets last 10–20 years when protected from moisture and damage. Choose a shorter life for exposed outdoor runs or harsh access areas, and longer when properly supported and protected.
6) Why can NPV be negative even with savings?
High upfront cost, low runtime, low ΔT, or a high discount rate can reduce discounted value. Improving inputs, reducing installed cost, or adding incentives can move NPV positive and shorten break-even time.