You are standing in front of a new window and there is a sticker on the glass covered in numbers and acronyms that mean nothing to you. If you have ever tried to compare windows for your Ontario home and felt buried in fine print, this guide is for you. We will walk through the window energy ratings that actually matter, explain what each one measures in plain terms, and show you how to use them to choose windows built for our winters and summers.
What are window energy ratings?
Window energy ratings are standardized scores that tell you how well a window controls heat, sunlight, and air movement. In Canada, every window is evaluated to the Canadian Standards Association (CSA) A440.2 standard, so you can compare products fairly no matter who makes them.
The four numbers you will see most often are U-factor, solar heat gain coefficient (SHGC), visible transmittance (VT), and the Energy Rating (ER). Each one answers a different question about how a window behaves once it is in your wall. Once you know what they mean, the sticker stops being a wall of jargon and becomes a useful comparison tool.
What is U-factor and why does it matter?
The U-factor measures how quickly heat escapes through a window. The lower the number, the better the window keeps warmth inside your home.
In Canada, U-factor is shown in watts per square metre Kelvin (W/m2K), and you may also see the imperial version in British thermal units per hour per square foot Fahrenheit. In both cases, a lower number means a more efficient product. For a heating-dominated climate like ours, this is the rating to watch most closely. A low U-factor means less of the warmth you are paying for leaks out through the glass and frame during a long London or Kitchener winter.
To earn ENERGY STAR® certification in Canada, a window needs a U-factor of 1.22 W/m2 or lower, or an Energy Rating of 34 or higher. The top performers go further. To qualify for the ENERGY STAR Most Efficient designation, a window must hit a U-factor of 1.05 W/m2·K or lower, or an Energy Rating of 40 or higher. When you compare two windows, U-factor is usually the first number worth checking.
What is the solar heat gain coefficient (SHGC)?
SHGC measures how much of the sun’s heat passes through the glass, on a scale from 0 to 1. A lower number blocks more solar heat, and a higher number lets more of it in.
This one is about balance rather than chasing the lowest possible figure. In an Ontario home you want some free solar warmth in January and protection from overheating in July. The right SHGC depends on which way a window faces. Caution is advised with windows above an SHGC of 0.45 that face south or west, especially when there is more glass than wall, because this can leave rooms uncomfortably warm and push up cooling costs. Low-E coatings are how we fine-tune this. They let a window manage solar heat without turning your living room into a greenhouse in summer.
What is visible transmittance (VT)?
VT measures how much visible daylight comes through the glass, also on a 0 to 1 scale. A higher VT means a brighter, more naturally lit room.
Visible transmittance is the daylight number. It refers to the amount of visible light allowed through the window, and a lower number means less light enters the room. Here is where good glass earns its keep. A quality Low-E coating can hold solar heat in check while still letting plenty of daylight through, so you are not trading a bright kitchen for a cooler one. If a room feels dark, a higher VT helps. If glare is the problem, a lower VT calms it down.
What is the Energy Rating (ER) number?
The Energy Rating is a single, unitless score used in Canada that combines heat loss, solar gain, and air leakage into one number. The higher the ER, the more efficient the window is overall.
This is the rating that does the balancing for you. Instead of weighing three separate numbers, you get one figure that demonstrates the balance between U-factor, SHGC, and air leakage. For the curious, the formula behind it looks like this.
ER = (57.76 × SHGC) − (21.90 × U-factor) − (1.97 × Air Leakage) + 40
In plain language, the score rewards a window for capturing useful solar heat and penalizes it for losing heat and leaking air. Solar gain carries the most weight in the equation, which is why two windows with similar insulation can still post different ER scores. A higher ER is better, and for most Ontario homes an ER of 34 or more is the certified baseline, with 40 or more marking the most efficient products available.
One thing worth knowing if you are comparing older advice online. The three ENERGY STAR climate zones that existed from 2015 to 2019 were consolidated into a single national zone starting January 1, 2020. That means you no longer need to match a window to a specific zone map. One standard now applies across the country, which makes shopping simpler.
What about air leakage?
Air leakage is the fourth input feeding the ER, and it measures how much outside air sneaks through a window. To earn certification, products must also have an air leakage rate at or below 1.5 litres per second per square metre. A drafty window undoes the work of every other rating, which is why the seal and the install matter as much as the glass.
This is also where the quality of the installation decides everything. Even a window with excellent ratings will underperform if it is fitted poorly. We include installation by our own expertly trained technicians in the price of every replacement window, and we do not hire subcontractors, so the people who build your windows stand behind how they go in. If you want to see your options in person, you can book an in-home consultation and we will walk through the ratings that fit your home.
How do these ratings work together for an Ontario home?
No single number tells the whole story, so the goal is a sensible combination. Our climate runs cold, humid, and hard on materials, with freeze-thaw cycles that test every seal. For most homes across Southern Ontario, that points to a low U-factor for winter warmth, a moderate SHGC tuned to each exposure, and an ER comfortably in the certified range or above.
Think of it this way. The U-factor keeps the heat you paid for inside. SHGC decides how much free warmth and summer heat you let in. VT keeps rooms bright. The ER ties the first three together into one easy comparison. A window that scores well across all of them is one that will keep a home in Hamilton, Barrie, or Windsor comfortable in February and July alike.
What rating should you look for?
Start with the ENERGY STAR® symbol, then read the numbers behind it. A certified window meets a real, tested standard, and ENERGY STAR qualified products reduce overall annual energy costs by about 8 percent compared with a conventional product, while keeping a home more comfortable and reducing condensation in cold weather. Aim for an ER of 34 or higher or a U-factor of 1.22 W/m2 or lower at minimum, and reach for the Most Efficient thresholds where it suits your budget and exposure.
Every Centennial vinyl replacement window is ENERGY STAR® certified and custom manufactured in our London, ON facility. We were the first window company in Canada to be ENERGY STAR® certified, and we have been named ENERGY STAR Manufacturer of the Year five times, so these ratings are not new territory for us. They are the standard we have helped set for decades.
Built for our climate, backed for life
Understanding the numbers puts you in control of the decision. U-factor, SHGC, VT, and the ER each tell you something specific, and together they reveal whether a window can handle an Ontario winter without driving up your bills. When your windows are also custom manufactured, installed by our own team, and covered by our lifetime No Fine Print Warranty, you get performance on paper and accountability in practice. That is The Centennial Difference.
To find the right windows for your home and climate, book an in-home consultation today.