When shopping for new windows or exterior doors, it’s easy to focus on appearance, colour, style and price.
But if energy efficiency is one of your priorities, there’s another set of numbers worth understanding.
U-factor. ER. SHGC. Air leakage
These terms appear on energy-performance labels and product specifications, but what do they actually mean?
More importantly, which numbers should you pay attention to when choosing windows and doors for your home?
Understanding these ratings can help you compare products more confidently and choose options that are better suited to your home’s climate, orientation and comfort needs.
For Canadian homeowners, this is especially important. Natural Resources Canada notes that windows, doors and skylights can account for up to 25% of a home’s energy loss. ENERGY STAR-certified windows are about 20% more energy efficient than the average window, while certified doors are about 15% more efficient.
Let’s break down the numbers in simple terms.
Why Energy Ratings Matter
Windows and exterior doors form part of your home’s building envelope – the boundary that separates your conditioned indoor space from the outdoors.
Every opening in that envelope can affect how heat moves through your home.
In winter, you want to keep heated indoor air inside and cold outdoor conditions outside.
In summer, you want to prevent excessive outdoor heat from entering while keeping your cooled indoor air where it belongs.
A high-performance window or door can help by controlling:
- Heat transfer
- Solar heat gain
- Air leakage
- Surface temperatures
- Indoor comfort
However, no single number tells you everything.
That’s why understanding the different performance measurements is important.
1. What Is U-Factor?
U-factor measures how quickly heat transfers through a window or door assembly.
This is one of the most important numbers to understand when comparing energy-efficient products.
The basic rule is simple:
Lower U-factor = better resistance to heat transfer.
In other words, a product with a lower U-factor generally allows less heat to pass through it.
Natural Resources Canada explains that U-factor measures the rate of heat loss through the window assembly and that a lower value indicates better resistance to heat loss.
Why does U-factor matter in winter?
Imagine your home is warm and comfortable inside while the outdoor temperature is well below freezing.
Heat naturally wants to move from the warmer side to the colder side.
If your windows have poor thermal performance, more heat can pass through the glass and frame.
A lower U-factor means the window is better at resisting that transfer.
This can help reduce cold areas around windows and improve overall indoor comfort.
What about doors?
The same principle applies to exterior doors.
A well-insulated exterior door with good thermal performance can help reduce heat transfer between your home and the outdoors.
However, when evaluating a door, you should consider more than the slab itself. The frame, glazing, weatherstripping and overall assembly can also influence performance.
2. What Is Energy Rating (ER)?
Energy Rating, or ER, is an overall performance measurement used in Canada for qualifying fenestration products.
Unlike U-factor, which focuses primarily on heat transfer, ER takes multiple factors into account.
The Canadian ER calculation considers:
- Heat loss through the product
- Air leakage
- Potential passive solar heat gain
That means ER gives you a broader picture of a product’s energy performance.
The rule is:
Higher ER = better overall energy performance.
Natural Resources Canada defines ER as a value that balances U-factor, air leakage and potential passive solar gain.
The City of Calgary also identifies ER as an important measure for energy-efficient windows and notes that a higher ER indicates a more energy-efficient product.
Why is ER useful?
Imagine you’re comparing two windows.
One may have a strong U-factor but allow more air leakage.
Another may balance heat transfer, solar gain and air leakage more effectively.
Looking at ER can help provide additional context instead of judging the product based on one measurement alone.
For homeowners, this makes ER a useful number to look for when comparing Canadian-certified products.
3. What Is Solar Heat Gain Coefficient (SHGC)?
Solar Heat Gain Coefficient, or SHGC, measures how much solar radiation passes through a window or glazed door.
The value ranges between 0 and 1.
A lower SHGC means less solar heat enters through the glass.
A higher SHGC means more solar heat enters.
That doesn’t automatically mean that a lower SHGC is always better.
This is where things get interesting.
Is a lower SHGC always better?
Not necessarily.
Your home’s location, window orientation, climate and shading all matter.
For example, a window receiving strong afternoon sunlight may allow a significant amount of solar heat into the room.
In summer, that may contribute to overheating.
But during colder months, some solar heat gain can actually be beneficial because it can contribute to passive heating.
Natural Resources Canada explains that the appropriate SHGC depends on factors including climate, window orientation, the home’s ability to regulate temperature and available shading.
So instead of thinking:
“Lower SHGC is always better.”
Think:
“The right SHGC depends on where and how the window is used.”
Window orientation matters
A south-, east- or west-facing window can experience very different solar conditions.
A large window receiving strong afternoon sun may have different performance requirements from a shaded window on another side of the house.
This is one reason energy-efficient window selection isn’t simply about choosing the lowest number available.
The goal is to choose a product that works well with the home.
4. What Is Air Leakage?
Air leakage measures how much air passes through gaps, joints and seals in a window or door assembly.
Unlike heat transfer through the glass or frame, air leakage involves actual movement of air.
And drafts are something homeowners can often feel.
You may notice:
- Cold air around a window
- A draft near an exterior door
- Curtains moving near a closed window
- A cold area around a door frame
- Temperature differences near openings
These can all be signs that air is moving where it shouldn’t.
Natural Resources Canada identifies air leakage as one of the key performance measurements for windows and doors.
Lower air leakage is generally better
When comparing products, lower air leakage generally indicates a tighter assembly.
This can help reduce unwanted outdoor air entering the home and conditioned indoor air escaping.
However, the quality of the surrounding installation and sealing also matters.
Even a high-performing product can be undermined by significant air leakage around the opening.
5. How Do U-Factor, ER, SHGC and Air Leakage Work Together?
This is where many homeowners get confused.
You might find one product with an excellent U-factor and another with a higher ER.
Which one is better?
The answer isn’t always as simple as choosing the product with the best-looking individual number.
That’s because the measurements describe different aspects of performance.
Think of them this way:
| Rating | What it tells you | General direction to look for |
| U-Factor | How readily heat transfers through the product | Lower is generally better |
| ER | Overall balance of heat loss, air leakage and solar gain | Higher is generally better |
| SHGC | How much solar heat enters through the glass | Depends on orientation and climate |
| Air Leakage | How much air passes through the product | Lower is generally better |
The best product is not necessarily the one with the most impressive single number.
Instead, you want a combination of performance characteristics appropriate for the home.
6. What About Low-E Glass?
You may also see Low-E mentioned when researching energy-efficient windows.
Low-E stands for low emissivity.
A Low-E coating is a very thin coating applied to glass that helps control radiant heat transfer.
Depending on the coating and application, Low-E glass can help:
- Reduce heat loss during colder weather
- Control solar heat gain
- Improve indoor comfort
- Improve the overall thermal performance of the window
Low-E technology is one component of a high-performance window system.
It works together with the glazing, frame, spacer system, gas fill and other design features.
That’s why simply seeing “Low-E glass” doesn’t tell you everything about how the finished window will perform.
7. Does Argon Gas Make Windows More Energy Efficient?
Another term you’ll frequently encounter is argon gas.
Modern insulated glass units often have sealed spaces between panes that can contain insulating gas such as argon.
The purpose is to reduce heat transfer through the space between the glass panes.
Calgary notes that the spaces between panes are commonly filled with gases such as argon to reduce heat gain and loss through the window.
Argon is therefore one part of the overall insulation strategy.
But again, it shouldn’t be evaluated in isolation.
A window’s overall performance depends on the complete assembly.
8. Double-Pane vs. Triple-Pane: Which Is More Energy Efficient?
The number of glass panes is another factor homeowners commonly consider.
A double-pane window has two panes of glass with an insulating space between them.
A triple-pane window has three panes and two insulating spaces.
Generally, adding another pane can improve insulation and reduce heat transfer.
However, more panes don’t automatically mean that every triple-pane window is better than every double-pane window.
The quality of the complete window matters.
Frame design, glass coatings, gas fills, spacer systems, seals and overall construction all contribute to performance.
For a deeper comparison of the two options, see our guide:
Double-Pane vs. Triple-Pane Windows: Which Is Better for Calgary?
9. Does Energy Efficiency Depend on the Window’s Location?
Yes.
The same window specification may not be ideal for every location in a home.
Consider:
North-facing windows
These typically receive less direct solar heat, making heat-loss performance particularly important.
South-facing windows
These may receive significant sunlight, so solar heat gain can play a larger role.
East-facing windows
Morning sunlight can affect heat gain and comfort.
West-facing windows
Afternoon and evening sunlight can produce significant solar heat gain, particularly during summer.
This is why window selection should consider orientation, glazing area, shading and the home’s overall energy performance.
10. What Should Calgary Homeowners Look For?
Calgary’s climate makes thermal performance particularly important.
Long periods of cold weather can place significant demands on a home’s heating system, while sunny summer days can create unwanted heat gain through large areas of glass.
The City of Calgary recommends paying attention to U-factor, SHGC and ER when selecting energy-efficient windows.
For homeowners comparing products, a practical checklist is:
Look for a low U-factor
This helps reduce heat transfer through the window.
Compare ER
A higher ER indicates better overall performance according to the Canadian rating system.
Consider SHGC
Think about where the window is located and how much direct sunlight it receives.
Pay attention to air leakage
A well-sealed product can help reduce drafts and unwanted air movement.
Look beyond the glass
The frame, spacer, seals and complete assembly all contribute to performance.
Consider ENERGY STAR certification
ENERGY STAR-certified products must meet established technical performance requirements. Current Canadian criteria for residential windows and doors include maximum U-factor or minimum ER requirements, as well as air-leakage requirements.
What Is the Most Important Energy Rating?
There isn’t one number that tells the entire story.
If you’re focused on reducing heat transfer, U-factor is an important number to understand.
If you’re comparing overall Canadian energy performance, ER provides a broader measurement because it incorporates heat loss, air leakage and potential solar gain.
If solar exposure is a concern, SHGC becomes especially important.
And if you’re worried about drafts, air leakage deserves attention.
The best approach is to consider these measurements together rather than choosing a window or door based on one specification alone.
Don’t Choose a Window or Door Based on One Number
Energy efficiency isn’t about finding the lowest number on a specification sheet.
It’s about finding the right combination of performance characteristics for your home.
A window facing strong afternoon sunlight may need different solar-gain characteristics from a shaded window.
A home in a cold climate may place greater emphasis on minimizing heat loss.
A drafty older home may benefit significantly from improving air sealing around windows and doors.
And a homeowner replacing older products should consider the performance of the complete assembly — not simply the glass.
The right choice depends on the home, the climate, the orientation of the opening, the product design and the homeowner’s priorities.
The Bottom Line
Understanding energy ratings makes it easier to make an informed decision when replacing windows and exterior doors.
Remember the basics:
U-factor: Lower generally means less heat transfer.
ER: Higher generally means better overall energy performance.
SHGC: Tells you how much solar heat enters through the glass.
Air leakage: Tells you how much air can pass through the product.
Once you understand these four measurements, comparing energy-efficient windows and doors becomes much less confusing.
And instead of choosing a product simply because it is marketed as “energy efficient,” you can look at the actual performance characteristics and determine whether they make sense for your home.
Looking for Energy-Efficient Windows and doors?
At NORC Windows & Doors, we believe choosing the right products should be straightforward.
Whether you’re replacing aging windows, upgrading exterior doors or improving the overall efficiency and comfort of your home, understanding product performance is an important first step.
Explore our range of windows and doors and consider the performance characteristics that matter most for your home.
Better performance starts with better information.
