How the zones are defined
Heating degree days (HDD, base 18 °C) accumulate the amount and duration by which the outdoor temperature sits below a comfort baseline over a year. A place with 3,000 HDD has a mild, short heating season; a place with 8,000 has a long, severe one. Canada's model energy codes group locations into zones on that basis, which is why two cities in the same province can sit in different zones and two cities two thousand kilometres apart can sit in the same one.
| Zone | Heating degree days (18 °C) | Roughly |
|---|---|---|
| 4 | under 3,000 | Vancouver, Victoria, the mildest coastal edge |
| 5 | 3,000 – 3,999 | Toronto, Hamilton, Windsor, much of southern Ontario |
| 6 | 4,000 – 4,999 | Ottawa, Montréal, Halifax, London |
| 7A | 5,000 – 5,999 | Calgary, Edmonton, Winnipeg, Québec City |
| 7B | 6,000 – 6,999 | Northern prairie and near-north communities |
| 8 | 7,000 and above | Whitehorse, Yellowknife, the territories and far north |
Bands and city placements above are approximate and given for orientation only. Zone boundaries and the degree-day figures assigned to individual locations are set in the model codes and revised periodically — confirm the value for a specific address against the current edition of the applicable code before designing to it.
What actually changes as you move up
The specification does not change category by category so much as it changes in emphasis. The same five variables matter everywhere; their relative weight shifts.
Zone 4 — mild, coastal
Heat loss matters least here, and the long shoulder seasons make solar control and ventilation comparatively more important. Good double glazing with a well-chosen coating is a defensible specification. Rain management is the dominant failure mode on the west coast, so flashing detail deserves more attention than the third pane would.
Zones 5 and 6 — the populated middle
This is where the double-versus-triple argument is genuinely open, and where the answer depends on the house rather than the climate. If the walls are being upgraded anyway, or the project has an energy target, triple glazing stops being a luxury. If the windows are being replaced in isolation in an otherwise leaky house, the money often does more elsewhere. Per-elevation solar tuning starts paying real dividends in this band, because both the heating and cooling seasons are substantial.
Zone 7 — the prairie band
Long heating seasons and cold, clear winters change the calculation in two directions at once. Heat loss is now the dominant term, which argues for triple glazing and warm-edge spacers. But clear winter skies also mean meaningful solar gain on south elevations, which argues against reflexively specifying the lowest-SHGC product in the catalogue for every opening. Interior surface temperature — and therefore condensation — becomes a design consideration rather than an afterthought.
Zone 8 — the north
Here the envelope leads and everything else follows. Triple glazing is a baseline rather than an upgrade, air-tightness detailing is not optional, and the interaction between indoor humidity and cold interior surfaces governs a great deal of the practical outcome. Logistics also start to matter: sealed units are heavy and fragile, and replacement lead times are long enough to belong in the plan.
A caution about national averages. Advice written for "Canada" is usually written for Zones 5 and 6, because that is where most of the population is. If you are in Zone 7 or 8, a national rule of thumb will tend to under-specify. Natural Resources Canada's home energy efficiency resources are organised with that variation in mind, and Efficiency Canada tracks how the provinces differ in what they actually require.