ER rating and U-factor, explained

U-factor measures how fast heat escapes through the window — lower is better. Energy Rating (ER) combines heat loss, air leakage and passive solar gain into a single number — higher is better.

For ENERGY STAR certification in Canada, a window must meet either a maximum U-factor of 1.22 W/m²·K (0.21 Btu/h·ft²·°F) or a minimum ER of 34. It qualifies on one or the other. It does not need both.

That either/or matters when you are comparing two quotes whose labels emphasise different numbers.

U-factor

U-factor is the rate of heat transfer. Lower means less heat escaping. It is measured across the whole window — glass, frame and spacer together — not just the centre of the glass.

That whole-window qualifier is worth knowing, because centre-of-glass numbers look considerably better and occasionally appear in marketing. A triple-glazed unit with a poor frame and an aluminium spacer can have an excellent centre-of-glass figure and a mediocre whole-window U-factor.

Energy Rating

ER is a Canadian measure and it exists because U-factor alone tells you only half the story in a cold climate. A window loses heat, but it also gains heat from the sun. ER balances the two, and subtracts air leakage:

ER = (57.76 × SHGC) − (21.90 × U) − (1.97 × L75) + 40

Where SHGC is solar heat gain coefficient, U is the whole-window U-factor, and L75 is air leakage at 75 pascals. The result is unitless. Higher is better.

The practical consequence: a south-facing window with a higher solar heat gain can score better on ER than a technically tighter window that blocks more sun — because in an Ontario winter that free solar gain is worth real money.

Ontario is not “Zone 2”

A great deal of Canadian window marketing still refers to climate zones — “Zone 2 rated for southern Ontario”, or similar.

There are no ENERGY STAR climate zones for windows in Canada any more. Version 5.0 of the technical specification, in effect since 1 January 2020, replaced the old multi-zone map with a single national criterion. Any quote or brochure telling you that you need Zone 2 or Zone B windows is citing a specification retired six years ago.

It is a useful tell. It usually indicates marketing material that has not been reviewed in a long time.

How to verify a specific window

Rebate eligibility hinges on the exact model being certified, so it is worth checking rather than assuming. Natural Resources Canada publishes a searchable product list where you can confirm the precise model named on your quote before you sign.

One practical warning: leave the ENERGY STAR labels on the glass until after your post-installation energy assessment. Peeling them off on install day because they look untidy is the most common way an otherwise valid rebate claim fails.

What actually matters in practice

Once a window clears the ENERGY STAR threshold, the difference between a good unit and a slightly better one is usually smaller than the difference a competent installation makes. A high-performing window installed with gaps in the insulation, or sealed to a frame that is already failing, will underperform a modest window installed properly.

Ontario requires window installation to conform to CAN/CSA-A440.4 — a national standard, not manufacturer’s instructions. It is a fair question to ask of anyone quoting your job.