Almost every conversation about windows starts in the wrong place: the glass. It is the visible part, it is what the salesperson has a sample of, and it is where the marketing money goes. But a window is an assembly — glazing, gas fill, spacer, frame, and the joint between the frame and the wall — and in a cold climate the weakest of those five decides the outcome.
This site is a reference for that whole assembly. It is written for Canadian conditions, which matters more than it sounds: the ratings used here are not the ones used in the United States, the climate zones are defined by heating degree days rather than by state, and a specification that is generous in Vancouver is thin in Edmonton.
Start here
Reading the ratings
U-factor, Energy Rating (ER), solar heat gain and air leakage — what each number measures, which direction is better, and where each one misleads.
02Glazing & coatings
Double versus triple, low-emissivity coatings, argon and krypton fills, and warm-edge spacers — what each layer buys and what it costs.
03The envelope around it
Air leakage and thermal bridging at the rough opening. Where good units are routinely undone by ordinary installation.
04Climate zones
Canada's heating-degree-day zones, and how the sensible specification shifts as you move from Zone 4 to Zone 8.
05Planning a retrofit
Insert versus full-frame replacement, what each approach can and cannot fix, and how to sequence a whole-house project.
Three things worth knowing before you spend anything
The rating on the sticker is for the whole unit, not the glass
Glass-only performance figures look dramatically better than whole-unit figures, because the frame and the edge of the glazing are where most of the heat leaves. When two quotes disagree wildly, check whether one is quoting centre-of-glass and the other is quoting the tested assembly. Only the second one is comparable.
Air leakage is not the same problem as heat loss
A window can have an excellent U-factor and still make a room uncomfortable, because the discomfort is coming from air moving through the joint between the frame and the wall rather than through the window. That joint is site work. No product specification protects you from it.
Orientation changes the right answer
Solar heat gain is a liability on a west elevation in summer and an asset on a south elevation in winter. Specifying one glazing package for every opening in the house is simple, and it is usually slightly wrong somewhere.
On sources. Where a number here is set by a standard or a program — an ENERGY STAR threshold, a climate zone boundary, a test method — this site points at the body that maintains it rather than restating it, because those thresholds are revised. Natural Resources Canada publishes the current ENERGY STAR criteria for Canada, and the National Research Council maintains the model codes the zones come from.