When choosing new windows, most homeowners focus on the obvious details: frame colour, design, and glass performance. While these features shape the look and efficiency of your home, there is another number that can make or break how your windows actually perform – the Air Leakage (AL) rating.
Even a high-efficiency window can underperform if air is sneaking around its edges. Drafts are not just a comfort issue; they also drive up energy bills and undermine insulation. Understanding Air Leakage ratings helps you make smarter, longer-lasting choices for your home, especially in a cold Canadian climate. According to research published in the Canadian Building Digest by Natural Resources Canada, uncontrolled air infiltration remains one of the top causes of residential heat loss in northern climates. This guide explains how AL ratings work, what they mean, and why they deserve as much attention as the U-factor on your window label.
Understanding Air Leakage (AL) Ratings
Air Leakage ratings measure how much air can pass through a window assembly under specific test conditions. The test doesn’t measure heat transfer through the glass (that’s the U-factor) but rather the amount of air that leaks through joints, seals, and moving parts.
In Canada, this rate is measured in litres per second per square metre (L/s·m²). In the United States, the same concept is shown on energy labels as cubic feet per minute per square foot (cfm/ft²). The rule is simple: the lower the number, the tighter and more airtight the window.
Windows are tested in controlled lab conditions, with air pressure applied to simulate wind loads. The results help you compare one product against another – giving a real-world indication of how draft-free each window will be after installation.
Why Air Leakage Ratings Matter
A window’s Air Leakage rating directly affects daily comfort, heating efficiency, and overall indoor air quality. Low-AL windows perform better in every measurable way.
Reduces Drafts and Cold Spots: Low air leakage means less cold air slipping through gaps around the sash or frame. In practical terms, that’s fewer chilly corners and a steadier indoor temperature throughout the winter. A well-sealed window allows you to sit comfortably by the glass without feeling a breeze.
- Lowers Energy Bills: According to Natural Resources Canada, uncontrolled drafts can account for up to one-quarter of a home’s total heat loss. Even small leaks force your furnace or heat pump to work harder. Choosing windows with tight seals and low AL ratings directly translates to lower heating costs.
- Improves Noise Control: The same openings that let air leak also allow sound to travel. Airtight windows provide better sound insulation, cutting down on outdoor noise from traffic or nearby activity. A lower AL rating is an easy way to improve both peace and quiet indoors.
Comparing Air Leakage by Window Style
Not all windows seal equally well. Their design – specifically how they open and close – determines how much air can escape. Windows that rely on compression seals, where the sash presses tightly against the frame, tend to perform best.
| Window Style | Typical Air Leakage | How It Works |
| Fixed (Picture) Window | Lowest | No moving parts – completely sealed unit. |
| Casement or Awning Window | Very Low | Operates with a crank that presses the sash tightly into the frame. |
| Sliding Window | Moderate | Relies on weather stripping; sliding motion makes airtightness harder to achieve. |
| Double-Hung Window | High | Two sliding sashes create multiple potential leak points. |
Casement and awning windows are often the preferred choice in cold regions because their locking mechanism naturally tightens the seal when closed.
How to Choose Windows for Low Air Leakage
When evaluating products, check the certification labels displayed on the frame or in the manufacturer’s documentation.
- ENERGY STAR Certification (Canada) – ENERGY STAR-rated windows in Canada must meet both insulation and air leakage standards. The air leakage rate must be 1.5 L/s·m² or less to qualify. This ensures that certified products meet the minimum airtightness required for energy efficiency in all Canadian climate zones. Educational resources such as Rutgers University’s Green Building Manual explain that air leakage performance is one of the strongest predictors of long-term comfort and durability.
NFRC Label (North America) – On labels used in both Canada and the U.S., look for an “Air Leakage” value of 0.30 cfm/ft² or lower. The lower the value, the better the seal. Many premium windows achieve AL ratings well below this threshold.
- Ask for the Exact Number – Manufacturers often highlight U-factors but overlook air leakage performance in sales materials. Always request the verified AL rating from your supplier to compare models accurately.
A high-quality window will have its AL rating available from the manufacturer. Always ask for this specific number, not just the U-factor.
Common Mistakes to Avoid
- Focusing Only on the U-Factor: Thermal insulation is important, but it doesn’t stop air movement. A window with an excellent U-factor but poor air sealing will still feel drafty and waste energy.
- Overlooking Installation Quality: Even the most airtight window can leak if installed poorly. Gaps between the frame and the wall’s rough opening can bypass factory seals entirely. Proper flashing, insulation, and sealing during installation are as important as the window rating itself.
- Choosing the Wrong Style: Match the window style to its location. Large sliding or double-hung windows facing prevailing winter winds will allow more air infiltration. Use fixed or casement windows in those areas to maintain comfort and efficiency.
The “Too Airtight” Question
As modern homes become tighter, some homeowners worry about indoor air quality. A well-sealed home can reduce natural ventilation, trapping humidity and pollutants inside. Building science experts recommend a balanced approach: build tight, but ventilate right. A common test for measuring home airtightness is the blower door test, which quantifies air leakage and helps ensure proper ventilation balance.
Canadian building codes now require mechanical ventilation for new homes that achieve very low air exchange rates. Systems such as Heat Recovery Ventilators (HRVs) or Energy Recovery Ventilators (ERVs) exchange stale indoor air with fresh outdoor air while recovering heat from the outgoing stream. This maintains comfort and efficiency while ensuring healthy indoor air.
Frequently Asked Questions (FAQ)
1. What should I look for when checking Air Leakage (AL) ratings?
Check the NFRC or ENERGY STAR label for the air leakage value. In Canada, the standard limit is 1.5 L/s·m², with lower numbers indicating better airtightness. Always aim for the lowest possible value. Window style also matters – casement and fixed windows tend to be more airtight than sliding designs.
2. How do I choose the best window style for low air leakage?
For maximum airtightness, choose a fixed (picture) window. For an operable window, a casement or awning window offers the best performance because its locking mechanism creates a strong compression seal.
3. What are the advantages and disadvantages of a low AL rating?
- Advantages: Dramatically reduced drafts, lower energy bills, a quieter home, and improved overall comfort.
- Disadvantages: There are no disadvantages to the window itself. However, a very airtight house (from windows, doors, and wall sealing) must have a mechanical ventilation system (like an HRV) to maintain fresh, healthy indoor air quality.
Conclusion
Air Leakage ratings are one of the most reliable indicators of real-world window performance. While insulation values measure how a product slows heat transfer, AL ratings reveal how well it resists the movement of air – and that’s what truly determines comfort.
By paying attention to this small but powerful number, you can eliminate drafts, lower heating bills, and enjoy quieter, more consistent indoor temperatures year-round. For homeowners in Canada’s diverse climate zones, choosing windows with verified low AL ratings is one of the simplest and most effective ways to achieve a high-performance home.