You have two window quotes in front of you, and the only figure that differs is the U-factor: 1.4 on one, 1.2 on the other. Does that gap show up on a −30°C night in Edmonton, or does it round away?
That answer starts with the U-factor. What is a U-factor, you might ask? It tells you how fast heat passes through a window, written in watts per square metre for each degree of temperature difference (W/m²·K).
Lower is better, because less of your heat leaves the house. ENERGY STAR Canada’s U-factor limit is 1.22. Below, you will see how the number is measured, how much of your heating bill it moves, and what to ask before you sign.
What is a U-Factor on a Window?
In more detail, a U-factor is the rate at which heat flows through your window from the warm side to the cold side, for each square metre of window and each degree of temperature difference.
Canadian ratings use watts per square metre per kelvin (W/m²·K), and one kelvin of difference equals one degree Celsius. You will also see the same figure called the U-value.
Here is what that means on an Edmonton winter night. With your house at 21°C and the air outside at −30°C, the 51-degree gap pushes about 62 W through every square metre of a window rated 1.22 (1.22 × 51). A window rated 2.0 passes about 102 W under the same conditions.
You may also run into R-value, which runs the other way. R-value (RSI in metric) measures resistance to heat flow, so the higher it is, the better. U-factor measures how much heat gets through, so the lower it is, the better.
To compare the two, divide 1 by the U-factor to get RSI: 1.22 becomes RSI 0.82, or about R-4.65 in imperial. Natural Resources Canada notes that windows are not normally given an R-value, so expect a U-factor on the label.
How is U-Factor Measured?
Once you know what the number means, you may wonder who measures it. Accredited labs do, and the result belongs to a specific window design rather than a brand name.
In Canada, the method comes from CSA A440.2, the standard for the energy performance of windows, doors, and skylights. The American counterparts, which you may see on US labels, are ANSI/NFRC 100 and 200. Certification bodies such as CSA Group and Intertek verify the results and list the product.
The number you want on a spec sheet is the whole-window figure. It averages three zones, weighted by area: the centre of the glass, the band of glass at the edge where the spacer sits, and the frame.
Centre-of-glass figures look better because they leave out the zones that lose more heat. If a brochure quotes only a glass number, you are reading about the glass, not the finished window.
Labs rate a tested sample size. If your window is cut to a different size, it has a different share of frame and edge, so its rating can shift. That matters if your openings are not standard, and it is one more point to weigh when you choose between custom and standard windows.
Where Does U-Factor Fit in an Energy-Efficient Window?
That number does more than describe one window. U-factor is the most direct way to see how well a window holds your heat in, and it anchors ENERGY STAR Canada’s window rules.
Natural Resources Canada’s “Keeping the Heat In” guide says windows, doors, and skylights can account for up to 35% of a house’s total heat loss.
Older NRCan material gave up to 25%, so treat 35% as an upper bound. Where your own house lands depends on how much glass it has and how well that glass performs.
If you shop for ENERGY STAR windows, the ENERGY STAR Canada specification (Version 5.0, in effect since January 1, 2020) gives them two ways to qualify: a U-factor of 1.22 W/m²·K or lower, or an Energy Rating (ER) of 34 or higher. The same limits apply across the country, regardless of climate zone.
If you want to go further, NRCan’s ENERGY STAR Most Efficient 2024 criteria ask for a U-factor of 1.05 or lower or an ER of 40 or higher.
The ER path trades U-factor against sun and air leakage. In the specification’s formula, each 0.1 drop in U-factor adds about 2.2 points, and a higher solar heat gain coefficient (SHGC) adds points too.
So a window with a high SHGC can reach ER 34 with a U-factor above 1.22. That credit assumes the sun reaches the glass. If your window faces north or sits in shade, it rarely gets that help, so the U-factor is the safer number to read.
What a Lower U-Factor Does to Your Glass and Your Comfort
So what does a lower number actually buy you? A lower U-factor does three things for you: it slows heat loss, keeps the inside glass surface warmer, and raises the humidity at which condensation starts. The first shows on your heating bill, and the other two show at the window itself.
The table below is a worked example, not a lab test. It assumes 21°C indoors, −30°C outdoors, and an inside-surface resistance of 0.13 m²·K/W from ISO 6946, the standard for calculating building-component U-values.
It estimates the middle of the glass, the warmest part of the window. The edges and frame, where you will often see fog first, run colder.
| Window U-factor (W/m²·K) | Inside the glass surface at −30°C outside | Indoor humidity where fog starts at 21°C |
| 2.0 | 7.7°C | about 42% |
| 1.61 (maximum on the City of Edmonton checklist) | 10.3°C | about 51% |
| 1.22 (ENERGY STAR Canada) | 12.9°C | about 60% |
| 0.80 | 15.7°C | about 72% |
With a window rated 2.0, condensation starts at the centre of the glass once your indoor humidity passes about 42%. At 1.22, you can hold out to about 60%.
Real windows tend to fog first at the edges, where the surface is colder than these figures. Read the table for the direction and rough size of the effect, not for exact readings.
What Brings a U-Factor Down?
Several design choices work together to bring the number down: low-E coatings on the glass, argon or a similar gas between the panes, warm-edge spacers in place of metal ones, insulated frames, and a third pane.
No single feature sets the number, and only the whole-window rating shows you the combined result. Low-E coatings come in two broad kinds, passive and reflective coatings, which treat winter sun differently.
Why Does U-Factor Matter for Alberta Homes?
Those numbers carry extra weight where you live. Alberta winters keep a large temperature gap across your windows for months, so the rating counts for more here than in a mild climate.
The province’s building code also sets a floor for new construction. The National Building Code – 2023 Alberta Edition, known as NBC(AE) 2023, has been applied since May 1, 2024.
The City of Edmonton’s energy code checklist for the prescriptive path lists a maximum U-value of 1.61 W/m²·K for windows and for exterior doors. ENERGY STAR Canada’s 1.22 sits 0.39 below it.
That figure is a minimum for projects the code covers. Requirements for your own project, including a window replacement, depend on the scope of the work, so confirm them with the City of Edmonton or a qualified professional before relying on any number in this article.
A cold climate also changes how a window behaves day to day. The 2.0 and 1.22 rows in the table above differ by 5.2°C of glass-surface temperature (7.7°C against 12.9°C). You notice that difference in how soon fog forms and how cold the window feels from across the room.
Does U-Factor Matter More in Alberta Than in Other Provinces?
Against Vancouver, Toronto, and Montreal, yes. Against Winnipeg, no. The measure that decides it is heating degree-days, which add up how far and how long the outdoor temperature sits below 18°C across a year.
The table converts each city’s degree-days into the heat a window conducts per square metre per year at a U-factor of 1.0 (degree-days × 24 ÷ 1,000). The degree-day figures come from Natural Resources Canada’s CanmetENERGY (2026).
| City | Heating degree-days (base 18°C) | Heat conducted per m² of window per year at U 1.0 | Compared with Edmonton |
| Vancouver | 2,825 | 68 kWh (0.24 GJ) | 55% |
| Toronto | 3,520 | 85 kWh (0.30 GJ) | 69% |
| Montreal | 4,200 | 101 kWh (0.36 GJ) | 82% |
| Edmonton | 5,120 | 123 kWh (0.44 GJ) | 100% |
| Winnipeg | 5,670 | 136 kWh (0.49 GJ) | 111% |
At the same U-factor, a square metre of window in Edmonton conducts about 1.8 times the heat it would in Vancouver, 1.45 times in Toronto and 1.2 times in Montreal. Winnipeg runs about 11% higher than Edmonton.
These figures count conduction only and ignore the sun and internal heat. They also shift with the weather station and years, a source averages, so use them to compare places, not to predict your own bill.
How Much Does U-Factor Actually Affect Your Heating Bill?
Enough that you will notice, but not enough to carry the bill on its own. In the example below, swapping windows rated 2.0 for ones rated 1.22 saves about 6.2 GJ of heat a year.
That is roughly 6% of the 100.3 GJ a year that the Canada Energy Regulator reports for the average Alberta household using natural gas (2015 data).
The example assumes you have 18 m² of windows (about 194 sq ft), a round number picked for illustration rather than an Edmonton average, and Edmonton’s 5,120 degree-days. At that size, every 1.0 of U-factor costs you about 2,200 kWh, or 8 GJ, a year.
| Change in rated U-factor | Heat saved per year (18 m²) | Share of 100.3 GJ |
| 2.0 to 1.22 | 6.2 GJ | 6.2% |
| 1.61 to 1.22 | 3.1 GJ | 3.1% |
| 1.22 to 1.05 | 1.4 GJ | 1.3% |
| 1.22 to 0.80 | 3.3 GJ | 3.3% |
To turn gigajoules into dollars, multiply the savings by the delivered rate on your gas bill, then divide by your furnace’s efficiency.
The example counts conduction only, so winter sun through south-facing glass and air leaking around the frame will move your real result in either direction.
Where Do the Savings Start to Shrink?
Savings scale with the size of the drop, not the starting number. Each 0.1 off the U-factor saves you about 0.8 GJ a year across 18 m².
The jump from 2.0 to 1.22 takes 0.78 off; the next step, from 1.22 to 1.05, takes 0.17. Whether a lower rating justifies a higher price is a question for your two quotes and this arithmetic, not a rule.
How Do You Check a Window’s U-Factor Before You Buy?
By now, you know which number to look for, so the last step is checking it on the quote in front of you. Ask for the whole-window U-factor, in W/m²·K, for the size and glass package you are buying, and ask where the rating is listed.
A quote that answers these five questions is easy for you to compare:
- Whole window or centre of glass? Martin Holladay, then editor of Green Building Advisor, pointed out in a 2017 forum thread that some manufacturers quote glass-only figures, which cannot be compared with whole-window ratings.
- Which size? The rating should match your opening or the nearest tested size. If only a standard size is rated, compare that size across quotes.
- Which units? A US label in Btu/h·ft²·°F reads about 5.7 times smaller than the Canadian figure. A US 0.30 equals 1.70 W/m²·K, which misses the 1.22 limit. Multiply by 5.678 to convert.
- Who certifies it? Ask for the certification body and listing. Natural Resources Canada keeps a list of ENERGY STAR certified windows, doors and skylights.
- What are the ER, SHGC, and air-leakage ratings beside it? U-factor alone says nothing about sun gain or drafts.
Where Can You See Custom Windows in Edmonton?
If you would rather see how a window is built before you compare numbers, Supreme’s showroom in Edmonton is set up for that. You can look at the products in person and get measuring guidance before you order.
Supreme has made vinyl and aluminum windows in Edmonton since 1970, and every product is Alberta-made. Each window is custom-made to your openings and designed for Alberta’s long, cold winters.
Bring your opening sizes and the questions above, and ask what rating applies to each size and glass package. Because the windows are made to order, the figure that counts is the one for your size.
Once you settle on a window, Supreme builds it in Edmonton and refers you to approved, experienced installers for the installation. Support and follow-up come after.
What Are the Common Myths About U-Factor?
Most mix-ups come from treating one number as the whole story. Three are worth clearing up before you decide.
Does a Lower U-Factor Always Mean a Better Window?
Mostly, but a rating compares only one thing. A window with a very low U-factor and a low SHGC admits little winter sun, which can cost your south-facing rooms some free heat. That trade-off is what the Energy Rating weighs.
Does the Label Number Match the Window in Your Wall?
Not exactly. The label describes the product as tested. It says nothing about gaps around the frame, missing insulation in the rough opening, or air moving through the joint. Those losses add to what the U-factor predicts, which is why the details of professional window installation matter alongside the number you read on the label.
Does Condensation Mean the Window Is Faulty?
Not by itself. Fog on the room side of the glass depends on the glass temperature and the indoor humidity together. In the table above, even a 1.22 window reaches its fogging point at about 60% humidity on a −30°C night, so a humid house can fog a well-rated window. Moisture between the panes of a sealed unit is different and usually points to a failed seal.
A Number Worth Pinning Down on Every Quote
Now that you know what a U-factor is, the recap fits in a line: a lower whole-window number, rated for your actual size, means less heat leaving on an Edmonton winter night. Read it beside the Energy Rating and air-leakage values rather than on its own.
Supreme builds custom vinyl and aluminum windows in Edmonton, so the size you order is the size that gets made. The company has been doing it since 1970, and its windows are Alberta-made and designed for local winters.
To talk through window ratings for your home, call Supreme at 780-447-1070 or reach the team through the contact page.
Frequently Asked Questions:
Short answers to the follow-up questions that come up most.
What is a good U-factor for windows in Edmonton?
ENERGY STAR Canada certification generally requires a U-factor of 1.22 W/m²·K or lower, or an Energy Rating of 34 or higher. Natural Resources Canada’s Most Efficient criteria go to 1.05 or lower. Edmonton’s energy code checklist lists a maximum of 1.61 for windows. Lower numbers hold more heat in.
Is U-factor the same as U-value?
In everyday use, yes. Both describe how much heat passes through a window, in W/m²·K, and labels and sales material use either term.
How do you convert U-factor to R-value?
Divide 1 by the U-factor to get RSI, the metric measure, then multiply by 5.678 for imperial R. A U-factor of 1.22 is RSI 0.82, or about R-4.65. Windows are rarely sold with an R-value.
Does U-factor apply to doors and skylights?
Yes. Exterior doors, patio doors, and skylights are rated the same way. The City of Edmonton’s checklist lists a maximum of 1.61 W/m²·K for exterior doors alongside windows.
Can a window with a low U-factor still be drafty?
Yes. U-factor measures heat moving through the product, not air leaking through gaps. Air leakage has its own rating, and installation quality affects it.