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Window U-Factor, Energy Rating and R-Value: What Each Number Means, What a Good One Is in Ontario, and Why Centre-of-Glass Numbers Are Not the Window
Windows & Doors
U-Factor, Energy Rating, R-Value: The Three Numbers on a Window, and the One That Is Not Really a Number
A window in Canada carries two certified numbers and one that salesmen like to talk about. The two certified ones are measured on the whole window, frame and all, by an accredited lab, and listed where you can check them. The third is the centre of the glass, measured on a square of glass with no frame, and it is always better than the window you are buying. Here is what each means and what a good one is.
The numbers
1.22W/m2K or lower: ENERGY STAR Canada U-factor
34or higher: ENERGY STAR Canada Energy Rating
40ER or higher: ENERGY STAR Most Efficient
1.6 / 25roughly what the Ontario code floor has been for new houses
U-factor: how fast heat leaves
Watts of heat lost per square metre of window for every degree of temperature difference. Lower is better. A single-glazed window is about 5; clear double about 2.8; double Low-E argon in a vinyl frame 1.1 to 1.4; good triple 0.8 to 1.0; the best triples under 0.7. The whole-window U-factor includes the frame, which is worse than the glass, and the edge of the glass at the spacer, which is worse again. That is why the certified number is always higher (worse) than the centre-of-glass number in the brochure, and why the brochure number tells you nothing about what you are buying.
Energy Rating: the Canadian one
The ER is a Canadian index that combines heat loss (U-factor), solar heat gained through the glass over a heating season, and air leakage through the window, into one number where higher is better. It was built for a heating climate: a window that lets winter sun in scores higher than one that blocks it, which is why a high-gain coating usually beats a low-gain one on ER. Fixed windows score higher than openers because they do not leak air. 34 is the ENERGY STAR floor; mid-grade doubles sit at 30 to 38, triples at 36 to 46.
Its weakness: it assumes the solar gain is wanted. On a west wall it is not, so a window with a lower ER and a low-gain coating can be the better window for that wall. Use ER to compare windows for the same wall, not to pick the coating. The coating page.
R-value: the one to translate and forget
R is the reciprocal of U in US units, and it is what insulation is sold by. Window companies quote centre-of-glass R-values (“R-5 glass!”) because they are flattering: R-5 centre-of-glass is about a whole-window U of 1.0 to 1.1, which is a decent double or an ordinary triple, not a wall. If someone gives you an R-value for a window, ask for the certified whole-window U-factor instead and the conversation gets more honest.
How to look a window up
- Get the manufacturer, the series name and the glass option from the quote.
- Natural Resources Canada keeps the ENERGY STAR Canada specification and a searchable product list; the manufacturer’s own site usually links its certified ratings by series.
- Find the line for that series and that glass. Read the U-factor and the ER. Both have to be there; if the series is not listed, the window is not certified, whatever the sticker on the sample says.
- Compare the two or three quotes on the same two numbers. A quote that will not give the series name cannot be looked up and has failed this step.
What the code asks for
Ontario’s energy supplement to the Building Code (SB-12) sets window performance for new houses and additions as part of a package of wall, attic and mechanical requirements. For most of the last decade the floor for windows has been a U-value of about 1.6 W/m2K or an ER of about 25, with the better packages asking for 1.4 or lower, and new-house designers often go to 1.2 or triple to make the whole package work. Replacement windows in an existing house are not inspected for it. Anything ENERGY STAR certified clears it comfortably either way. The new-home window page has the practical side for a build.
Where homeowners go wrong
- Comparing a centre-of-glass number to a whole-window number. It is like comparing the middle of the mattress to the mattress.
- Picking by ER alone. ER rewards solar gain. On a west wall, you may want less.
- Trusting the sticker on the sample. The sample is the best series. The certification is by series and glass; look up yours.
- Chasing 0.1 of U-factor for $200 a window. Below about 1.0, the next step is expensive and the comfort difference is small. Spend it on foam and a better installer.
Questions people ask
What is a good U-factor for windows in Ontario?
1.22 or lower is ENERGY STAR and plenty for a replacement. 1.0 or lower for a new energy-efficient house or a north-facing picture window. Under 0.8 is the top of the market.
What is a good Energy Rating?
34 or more is ENERGY STAR. 40-plus is Most Efficient. Fixed windows score higher than openers by design.
Is a higher Energy Rating always better?
For heating, yes. For a wall that overheats in summer, a lower-ER window with low-gain glass can be the right one. Read both numbers and know the wall.
Put the numbers on the quote
The scope the calculator writes asks every company for the certified U-factor and ER by unit. Then the grid has something real to compare.
Sources and further reading
- ENERGY STAR Canada technical specification for windows, doors and skylights: Natural Resources Canada. The U-factor and Energy Rating thresholds, and the certified product lists.
- Ontario’s Building Code: the official page, with the current regulation. Egress is 9.9.10, glazing and windows 9.6 and 9.7, energy is Supplementary Standard SB-12.
