Formula used
This tool estimates floor heat-loss savings using a degree-day method, then converts thermal savings into delivered energy and money.
| Step | Formula | Notes |
|---|---|---|
| 1) Effective HDD | HDDeff = HDD × fΔT | fΔT adjusts for reduced floor temperature difference. |
| 2) Annual heat loss | Q = U × A × HDDeff × 24 ÷ 1000 | Outputs kWh thermal per year. |
| 3) Thermal savings | Qsave = (Qbefore − Qafter) × freal | freal captures behavior and controls. |
| 4) Delivered energy saved | Esave = Qsave ÷ η | η is system efficiency (use 3.0 COP as 300%). |
| 5) Annual cost savings | S = Esave × Price | Uses your price per delivered kWh. |
| 6) Payback | Payback = Net cost ÷ S | Net cost = project cost − rebate. |
| 7) NPV | NPV = Σ St/(1+r)t − Net cost | St grows using your escalation input. |
How to use this calculator
- Enter floor area and choose the correct unit.
- Provide your current insulation as U-value or R-value.
- Select how you describe the upgrade: target U/R, or material plus thickness.
- Add your local heating degree days and keep the floor temperature factor realistic.
- Enter system efficiency and your delivered energy price per kWh.
- Add project cost and any rebate to estimate net cost and payback.
- Use discount rate and escalation to understand long-term value (NPV).
- Submit to see results, then download CSV or PDF if needed.
Example data table
These sample cases show how savings change with climate, price, and insulation performance.
| Scenario | Area (m²) | U before | U after | HDD | Saved heat (kWh) | Annual savings | Payback (yrs) |
|---|---|---|---|---|---|---|---|
| Typical retrofit | 80 | 0.90 | 0.25 | 2,500 | 2,340 | $312 | 5.2 |
| Cold climate focus | 110 | 1.10 | 0.20 | 4,200 | 7,584 | $1,379 | 1.7 |
| Mild climate, high prices | 65 | 0.75 | 0.30 | 1,400 | 688 | $203 | 6.9 |
Inputs That Drive Annual Savings
Floor area, climate, and insulation level set the baseline. Convert your heating degree days using the floor temperature factor; for example, 2,500 HDD and 0.75 becomes 1,875 effective HDD. With 80 m², every 0.10 W/m²K drop in U-value cuts about 360 kWh of annual heat loss before real‑world adjustments. Use quotes from installers to refine costs and expected thickness locally.
Interpreting U-Value Improvements
U-value measures heat flow; lower is better. A change from 0.90 to 0.25 W/m²K across 80 m² at 1,875 effective HDD reduces seasonal floor losses from roughly 3,240 to 900 kWh thermal. If you input material data, 120 mm with λ=0.037 adds about R=3.24 m²K/W; at 95% performance that becomes about 3.08.
Converting Heat Loss to Money
Thermal savings become delivered energy by dividing by heating efficiency. Saving 2,340 kWh thermal with a 90% boiler equals about 2,600 kWh of delivered fuel. At $0.12 per kWh, that is about $312 per year. For a heat pump at 300% (COP 3.0), the same thermal saving is about 780 kWh of electricity.
Payback, NPV, and IRR Signals
Payback is net project cost divided by annual savings. A $1,800 project with a 10% rebate costs $1,620 net; at $312 yearly savings, payback is about 5.2 years. Over 25 years with 6% discounting and 2% price escalation, present‑value savings can exceed $5,000, giving an NPV around $3,400 and a double‑digit IRR.
Quality Checks for Real Results
Use the real‑world savings factor to reflect thermostat behavior, drafts, and workmanship; 85% is conservative for leaky homes. Increasing the temperature factor from 0.75 to 0.90 raises estimated savings by 20%. Set an emissions factor to translate energy into carbon; at 0.20 kgCO₂/kWh, 2,600 kWh saved avoids about 520 kgCO₂ yearly.
FAQs
Where can I get heating degree days for my area?
Check your local weather service, utility portal, or energy audit report. Use annual HDD with a base near 18°C for broad planning. If you only have monthly HDD, sum them for a yearly value.
Should I enter U-value or R-value for existing floors?
Use whichever you know. If you have R-value, the calculator converts it using U = 1 ÷ R. Include all layers you are confident about; unknown layers can be left out to stay conservative.
Why can efficiency be above 100 percent?
Heat pumps move heat instead of creating it. Their COP can be 2.5–4.0, which you enter as 250–400%. The tool then converts thermal savings into the electricity you would have used.
What does the real‑world savings factor do?
It scales theoretical savings to reflect behavior, controls, and workmanship. Use 80–90% if you expect rebound heating or drafts, and 100–110% if you also improve air sealing and control settings.
Does this estimate cooling savings too?
No. The model uses heating degree days, so it focuses on heating season benefits. In mixed climates, insulation can also reduce cooling load, but that requires cooling degree days and different assumptions.
How accurate are the payback and NPV numbers?
They are planning metrics, not guarantees. Accuracy depends on your U-values, HDD, price, and efficiency inputs. Use your own quotes and recent bills, then treat results as a comparison across options.