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Example data table
| Scenario | Area (sq ft) | R existing → R new | HDD | Fuel | Price | Estimated yearly savings |
|---|---|---|---|---|---|---|
| Detached garage workshop | 450 | 2 → 13 | 2,200 | Electricity | $0.18/kWh | $220–$450 |
| Attached garage, moderate climate | 350 | 3 → 19 | 1,600 | Natural gas | $1.40/therm | $120–$260 |
| Cold region, larger envelope | 650 | 2 → 30 | 4,800 | Propane | $3.20/gal | $500–$1,100 |
Formula used
This tool estimates yearly heat loss using a steady-state approximation:
- U = 1 / R
- Q = U × A × HDD × 24 (BTU per year)
- Q_total = Q × (1 + L) (adds leakage adder L)
- Delivered = Q_total / η (system efficiency η)
- Units = Delivered / BTU_per_unit
- Cost = Units × Price
NPV discounts future net savings and subtracts the project cost.
How to use this calculator
- Enter total wall and ceiling area you will insulate.
- Fill in existing and new insulation values.
- Add local HDD and a leakage adder if needed.
- Select fuel type, price, and heating efficiency.
- Set project cost, escalation, years, and discount rate.
- Press Calculate savings and review results above.
- Export results using CSV or PDF buttons.
Surface Area And R-Value Inputs
Insulation savings scale with exposed surface area and the change in thermal resistance. The tool converts R to U using U = 1/R, then estimates annual heat loss from local heating degree days. For example, moving from R-2 to R-13 cuts U from 0.50 to 0.077, reducing conductive loss by about 85% for the insulated assembly directly.
Weather And Garage Temperature Assumptions
Heating degree days represent how often outdoor temperatures fall below a base setpoint, commonly 65°F. If you actively heat the garage, HDD is a strong predictor of fuel use. A colder region with 4,800 HDD can show more than double the load of a 2,200 HDD region when area and insulation levels are similar. If the garage is kept cooler than the house, consider using a smaller effective HDD.
Fuel Cost, Efficiency, And Unit Conversions
Annual delivered heat is adjusted by system efficiency to reflect real energy purchases. The calculator converts BTU into kWh, therms, or gallons using heat content values. If efficiency improves from 80% to 95%, the same building load requires about 16% less fuel, compounding insulation benefits. When fuel price increases, yearly savings rise proportionally.
Payback, NPV, And Multi-Year Planning
Simple payback divides project cost by net annual savings, after subtracting any annual maintenance. The analysis period adds price escalation and discounts future savings to compute net present value. A positive NPV indicates the upgrade beats the discount rate. Extending the period from 10 to 20 years often raises NPV meaningfully for durable insulation retrofits. A higher discount rate reduces NPV and pushes break-even later.
Air Sealing And Carbon Impact Metrics
Garages are frequently leaky, so the air-leakage adder approximates additional losses as a percentage of conduction. Tightening gaps can reduce the adder and lift savings without changing R-value. Emissions avoided are estimated from an emission factor per energy unit; set it to zero if you only track financial outcomes. Tracking tonnes of CO₂ helps compare insulation to other upgrades.
FAQs
What insulation parts matter most in a garage?
Start with the largest exposed surfaces, usually the ceiling and exterior walls. Add air sealing around doors and penetrations. Higher R-values help most in colder climates or when the garage is heated regularly.
How do I choose heating degree days?
Use an annual HDD value for your city at a 65°F baseline, or a local weather dataset. If you heat the garage to a lower temperature than the home, reduce HDD to reflect that lower setpoint.
Why does efficiency change the savings?
The model converts heat loss into purchased energy using system efficiency. A less efficient heater must buy more fuel for the same heat delivered, so insulation saves more energy units and more cost.
What does the air-leakage adder represent?
It approximates extra heat loss from drafts and uncontrolled ventilation. If you weatherstrip the garage door and seal cracks, lower this percentage to reflect improved tightness and better comfort.
How should I interpret NPV and break-even year?
NPV discounts future savings and subtracts the upfront cost. Positive NPV suggests the upgrade meets your discount rate. Break-even year is when cumulative undiscounted savings exceed the project cost.
Can I use this for cooling savings too?
This version focuses on heating using HDD. For cooling, use cooling degree days and an efficiency metric like SEER or COP. You can still approximate with lower HDD or run a separate cooling model.