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Insulation Energy Savings Explained: What R-20 to R-60 Actually Saves on a Simcoe County Gas or Propane Bill, the Diminishing Returns, and the Payback Without the Brochure
Insulation & Air Sealing
What Insulation Actually Saves: The Arithmetic for an Attic Going From R-20 to R-60, by Fuel, With the Diminishing Returns Left In
Every insulation brochure says “save up to 30% on your heating bill” and every one of them is counting on you not asking “up to 30% of what, on which house, from which starting point”. The honest version is arithmetic, and it is not hard. Heat loss through a surface is the area times the degree-days divided by the R-value, so doubling the R halves the loss, and the first inches of insulation are worth far more than the last. This page runs the numbers for a 1,200 sq ft attic in Simcoe County going from R-20 to R-60, in gas, propane and electricity, says what the air sealing adds, and gives the payback by fuel without rounding in anyone’s favour. Some of these numbers are smaller than you were told. Some are bigger.
The arithmetic, once
A 1,200 sq ft attic floor is 111.5 square metres. R-20 is RSI 3.52, so the ceiling passes 1 divided by 3.52, or 0.284 watts per square metre for every degree of difference between the house and the attic. Simcoe County runs about 4,500 degree-days a year, which is 108,000 degree-hours. Multiply: 0.284 times 111.5 times 108,000 is about 3,420 kWh of heat through the ceiling every winter at R-20. At R-60, RSI 10.57, the same ceiling passes about 1,140 kWh. The difference, 2,280 kWh, call it 2,300, is what the top-up saves, about 8.2 gigajoules.
Notice the shape of that. Going from R-20 to R-40 saves 1,710 kWh. Going from R-40 to R-60 saves another 570. The second half of the insulation does a third of the work of the first half, and R-60 to R-80 would save 285 kWh for the same money again. That is the diminishing return, and it is why the code stops at R-60 and so do I. On an attic already at R-40, the arithmetic says do the air sealing and spend the rest on the rim joists. The R-value page explains the units; this is what they cost and save.
What 2,300 kWh is worth, by fuel
| Fuel | Arithmetic | Saved per year | Payback on the $5,447 attic |
|---|---|---|---|
| Natural gas, 95% furnace | 8.2 GJ at 37.5 MJ per cubic metre, divided by 0.95, is about 230 m3; at roughly $0.50 a cubic metre all in | $110 to $140 | 40 years and up |
| Propane, 90% furnace | 8,200 MJ at 25.3 MJ per litre, divided by 0.90, is about 360 L; at $0.90 to $1.10 a litre | $320 to $400 | 14 to 17 years |
| Electric baseboard | 2,300 kWh at about $0.15 all in | about $340 | 16 years |
| Cold-climate heat pump, seasonal COP about 2.5 | 2,300 kWh divided by 2.5 is 920 kWh at $0.15 | about $140 | 40 years |
The $5,447 is the whole attic job with HST from the cost page, including $1,200 of air sealing, $300 of baffles and $200 for the hatch, none of which is in the 2,300 kWh. The insulation alone is $3,120 plus HST, $3,526, which shortens every payback above by about a third.
Read that table honestly and two things fall out. On gas, attic insulation does not pay for itself on energy in the life of the furnace, and I will not pretend otherwise; gas is cheap in Ontario and the attic was already at R-20. On propane, oil or baseboard, which is half of the houses north of Barrie, the attic pays back in the middle of its own life and keeps going for forty years after. The fuel decides, not the insulation.
The air sealing is where the rest of the savings hide
The 2,300 kWh is conduction only, heat moving through the insulation. On a 1975 house the bigger loss is the air going up through the ceiling plane: around every top plate, the plumbing stack, the wire holes, the pot lights, the hatch, the bath fan. A house at 10 ACH50 loses roughly as much heat to air leakage as through all its walls together. The $1,200 of attic air sealing, done before the blow, typically takes 5 to 10% off the whole heating bill of a pre-1980 house on its own, and on a $2,000 gas bill that is $100 to $200, in the same range as the insulation and for a third of the money.
Put together, attic air sealing plus R-20 to R-60 on that bungalow comes to $210 to $340 a year on gas and $600 to $900 on propane. That is the honest “up to” number, and it is for a leaky, under-insulated house. A 2005 house at R-40 and 4 ACH50 gets a fraction of it. The air sealing article has the leak list and the order, and if the house has never been tested, the home energy audit page covers the blower door that measures it.
Rule of thumb: on any house older than 1990, the air sealing saves at least as much as the top-up, costs a third as much and has to happen first, because once the cellulose is on the ceiling nobody is finding the top plates again.
The savings that are not on the bill
Ice dams. An attic at R-20 with a leaky ceiling melts the roof from underneath and refreezes it at the eaves; the R-60 attic with sealed penetrations and clear baffles is cold, which is what a roof deck wants to be. One eavestrough and fascia repair is $1,500 and one ceiling stain is a weekend; the ice dams page has the mechanism. Comfort: the bedroom ceiling at R-60 is a few degrees warmer to the skin in January, which is why people turn the thermostat down a degree without noticing, and a degree is 3 to 5% of the bill, which is not in the table either. Equipment: the heat pump sized after the attic is a size smaller, and that one is $4,000 to $7,000 at install, which is more than the insulation cost. Those are real, and they are also exactly the things a salesman reaches for when the energy arithmetic lets him down, so take them for what they are.
Where the savings claims go wrong
- Counting from R-0. “Insulating your attic saves 25%” is true for an attic with nothing in it. From R-20 the conduction saving is about a third of that, and almost no Ontario attic is empty.
- Counting the house, not the attic. The attic is a fifth to a quarter of the conduction loss on a bungalow and less on a two-storey. A saving of two-thirds of the attic’s share is 10 to 15% of the conduction bill, not two-thirds of the bill.
- Pricing it against last winter. A cold winter and a warm one differ by 15% in degree-days. Use a normal year, about 4,500 for Simcoe, and say so.
- Mixing the fuels. A number quoted for a baseboard house in Parry Sound does not transfer to a gas house in Barrie. Gas is a third the price per kWh of heat.
- Burying the air sealing in the insulation line, then crediting the insulation for the savings the sealing produced. Separate lines, separate savings.
Questions people ask
How much does attic insulation save per year in Ontario?
On a 1,200 sq ft attic taken from R-20 to R-60 in Simcoe County, about 2,300 kWh of heat a year: $110 to $140 on natural gas, $320 to $400 on propane, about $340 on electric baseboard. Add $100 to $200 on gas, two to three times that on propane, for the attic air sealing done first on a leaky older house.
Is R-60 worth it over R-50 in the attic?
Barely, on energy. R-50 to R-60 saves about 110 kWh a year on a 1,200 sq ft attic, $6 on gas. The code asks for R-60 on a new house, the rebate asks for R-50 on an old one, and the difference in cost is about $0.65 a square foot. Do R-60 if the truck is already there and the money is not needed for the air sealing.
What is the payback on attic insulation?
By fuel. On gas, 40 years or more for the insulation alone; on propane or baseboard, 14 to 17 years, and shorter once the air sealing is counted. The rebate, if the house and contractor qualify, takes up to $1,000 off the top afterward, but never count it until it is paid.
Does insulation save more than air sealing?
On a house older than about 1990, no. The attic air sealing usually saves as much or more than the top-up, costs about a third as much and has to be done first. On a house that is already tight, the insulation is the bigger piece of what is left.
Why is my bill the same after insulating?
Usually one of four things: the ceiling plane was never sealed; the bath fan still vents into the attic; the house was already at R-40 and the saving is $50 under the noise of a cold winter; or the thermostat went up a degree because the house finally felt comfortable. Compare degree-day-adjusted use, not dollars.
Price the attic, then do the arithmetic for your fuel
The attic insulation cost calculator gives the installed number for your attic area and your starting depth, which is the top half of the payback fraction. Put your own fuel and your own bill under it and you have the honest answer in a minute. If a quote in your hand promises a percentage saving, the $99 review will say what that percentage is of.
Sources and further reading
- Keeping the Heat In, Section 5: Roofs and attics: NRCan on checking existing insulation, fixing moisture first, air sealing before insulating, baffles, hatches, chimneys and pot lights.
- Keeping the Heat In: Natural Resources Canada: the plain-language Canadian guide to air sealing, insulation materials, basements, walls and ventilation.
- OntarioHeatLoss.ca: the stamped heat-loss design that insulation and airtightness change, and where the HRV and equipment sizing come from.
