A heated house in an Ontario January is a bucket with holes in it. The furnace or heat pump pours thermal energy in; conduction, convection and radiation carry it back out through every square metre of the building envelope. The indoor temperature holds steady only while those two rates match, which means the heating bill is not really a measure of how warm the house is — it is a measure of how fast the house leaks. Everything in this topic is and the three transfer mechanisms, applied to a building instead of a beaker.
Where the heat escapes
| Path out | Rough share of the loss | Dominant mechanism |
|---|---|---|
| Air leakage — gaps around doors, windows, vents and the attic hatch | 25–35% | Convection: warm air simply leaves and cold air replaces it |
| Walls | 20–30% | Conduction through the studs and the cavity |
| Roof and attic | 20–25% | Conduction upward, fed by warm air rising to the ceiling |
| Windows and doors | 10–25% | Conduction through the glass, plus radiation to the cold sky |
| Basement walls and floor slab | 10–20% | Conduction into ground that stays near |
Two things in that table surprise people. The first is that air leakage usually beats every solid surface — a house can be beautifully insulated and still bleed heat through a finger-width gap at the top of the wall. The second is that the roof matters more than its area suggests, because convection inside the house delivers the warmest air straight to the ceiling, so the temperature difference driving conduction is largest exactly where you would least like it to be.
Insulation and R-value
Insulation works by trapping air. Still air is a poor conductor, so a material that holds millions of tiny pockets of it — glass-fibre batt, blown cellulose, rigid foam — blocks conduction, and keeping the pockets small stops convection currents forming inside them. The performance of a layer is quoted as its R-value, which is a measure of resistance to heat flow. A bigger number means heat escapes more slowly, and the values of the layers in a wall add together.
| Building element | RSI (metric, ) | R-value (imperial) | Comment |
|---|---|---|---|
| Single pane of glass | 0.16 | R-0.9 | Barely better than a hole with a pane over it |
| Double glazing, argon filled | 0.5 | R-3 | Roughly three times the resistance of a single pane |
| Triple glazing with a low-emissivity coating | 0.9 | R-5 | Standard in new Ontario construction |
| glass-fibre batt in a wall cavity | 3.5 | R-20 | A common Ontario wall requirement |
| of blown cellulose in an attic | 8.8 | R-50 | Attics get the most, because that is where the loss is |
| of solid brick | 0.08 | R-0.5 | Brick is thermal mass, not insulation |
Why double glazing works
A sealed double-glazed unit is two panes with a gap of roughly between them, and it attacks all three transfer mechanisms at once. The gap is filled with still air or, better, argon, which conducts heat far more poorly than glass — that blocks conduction. The gap is deliberately narrow, so the trapped gas cannot set up a circulating current between the warm pane and the cold one — that blocks convection. And one inner surface carries a microscopically thin low-emissivity coating, a metallic film that is transparent to visible light but reflects infrared back into the room — that blocks radiation. Triple glazing simply repeats the trick with a second gap.
Thermal mass
Walk into an old brick school or a stone church on a hot July afternoon and it feels cool. That is thermal mass, and it is a direct consequence of . Brick, concrete and stone have a middling specific heat capacity of roughly 840\ \text{J/{(kg}\cdot^\circ\text{C)}}, but a masonry wall has an enormous . The product is therefore huge, so an enormous has to flow in before is anything worth noticing. The building absorbs the afternoon's heat with barely any temperature rise and releases it again through the cool of the night.
Heat pumps move heat rather than making it
An electric baseboard heater turns essentially every joule of electricity into a joule of heat in the room, so by the definition of efficiency it scores 100% and cannot be beaten. A heat pump beats it anyway, because it is not trying to make heat at all — it is a refrigerator running backwards. A fluid is expanded until it is colder than the outdoor air, so heat flows into it from outside; it is then pumped indoors, compressed until it is hotter than the room, and gives that heat up. The electricity runs the pump, not a heating element.
A coefficient of performance of means three joules of heat delivered indoors for every joule of electricity bought. No law of conservation is broken: two of those three joules were already present in the outdoor air and were merely carried inside. The number falls as the outdoor temperature drops, because there is less thermal energy available to harvest and the pump has a larger temperature gap to work across — which is why a cold-climate model rated to operate down to about matters in Ontario, and why many installations keep a backup heat source for the coldest nights.
From joules to the bill and the emissions
Household energy is billed in convenient units rather than joules. Electricity is sold by the kilowatt-hour, where ; natural gas is sold by the cubic metre, each of which releases roughly when burnt. The environmental difference between them is decided by what is behind the meter. Burning gas in a furnace emits carbon dioxide at the house, every time. Ontario's electricity grid is largely nuclear and hydroelectric and is among the lowest-emitting in North America, so a joule of heat delivered by a heat pump carries a small fraction of the emissions of the same joule delivered by a gas furnace — and every joule the building never loses in the first place, because someone insulated the attic, carries none at all.