The calculation
The heat escaping through the ceiling depends on the attic area, the R-value of the insulation and how cold your region is. We calculate the heat lost with your current insulation, then with R-51, and the difference gives the kilowatt-hours saved each year.
kWh per year = area (sq. ft.) × (1 ÷ (current R + 2) − 1 ÷ (51 + 2)) × degree-days × 1.8 × 24 ÷ 3,412
- The “+ 2” adds the resistance of the ceiling itself (drywall and air films, about R-2).
- Degree-days measure the cold of a normal winter, adding up each day the gap between 18 °C and the average outdoor temperature.
- 1.8 × 24 ÷ 3,412 converts everything into kilowatt-hours.
The savings in dollars are those kilowatt-hours multiplied by 11.142¢, the price of electricity under Hydro-Québec’s Rate D beyond 40 kWh per day, the price that applies to heating in winter. The amount is rounded to the nearest $10 per year. Over 20 years, it is simply 20 times that amount, at today’s rate, without counting rate increases.
The data
| Data | Value used | Source |
|---|---|---|
| Electricity price | 11.142¢/kWh | Hydro-Québec, rates in effect April 1, 2026 |
| How cold your region is | heating degree-days below 18 °C from the weather station closest to your address, or for your region in the R-value calculator | Environment and Climate Change Canada, 1981-2010 climate normals |
| Target R-value | R-51 | what we install in the attic |
| Current insulation, R-value calculator | the R-value calculated from your insulation and its depth | your measurement |
| Current insulation, price calculator | R-24 | the median of the existing insulation recorded at our customers’ homes (8 inches) |
| Area | the area measured by satellite or entered by you; in the R-value calculator, the attic size you pick | your home |
A few degree-day examples: Montreal 4,363, Quebec City 5,141, Gatineau (Ottawa station) 4,477, Sherbrooke (Bromptonville) 4,683, Trois-Rivières 4,826, Rimouski 5,074, Saguenay (Bagotville) 5,636.
What the estimate doesn’t count
- Air sealing. Our work also seals air leaks in the ceiling, which adds to the savings. We don’t count it.
- Your heating system. The calculation assumes electric heating, baseboards or electric forced air. With another system, the savings in dollars will be different.
- Your home and your habits. The thermostat, windows, walls, the winter of the year and rate increases all change the real bill.
For information only
These amounts are estimates provided for information only, based on averages. They are not a guarantee of savings or a promise of results, and they are not part of your quote. Isolation à Rabais does not guarantee any amount of savings. Your actual savings depend on your consumption and the condition of your home. The Hydro-Québec rate is updated every April 1.
To find out exactly what your current insulation is worth, use our R-value calculator.