Do low-e windows pay off in a cold climate?
Low-e windows have a microscopically thin metallic coating on one or more glass surfaces that reflects radiant heat, so less warmth escapes in winter and less enters in summer. In most Canadian homes the coating is standard on new double and triple glazing and is worth paying for when windows are being replaced.
What does low-e mean on a window?
The "e" stands for emissivity, the tendency of a surface to give off heat as infrared radiation. Ordinary glass has high emissivity, so a cold pane in January pulls heat from everything warm in the room. A low-emissivity coating cuts that radiation sharply, and the glass itself stays closer to room temperature.
The coating is far thinner than a human hair and does not change how the window looks in any obvious way. Some units have a faint tint or a slight reflection at certain angles, nothing more. For a plain-language overview of the other numbers on a window label, see our guide to U-factor, SHGC and ER.
How does a low-e coating work in winter and summer?
Heat moves through a window in three ways: conduction through the frame and glass, convection in the gap between panes, and radiation across the gap. Gas fills such as argon slow convection, and spacers and frames deal with conduction. The coating attacks radiation, which in a double-pane unit is a large share of the loss.
In winter the coating reflects heat from the room back inside. In summer, depending on the type, it can also reflect part of the sun's heat before it enters. That second effect is why different coatings suit different houses, and why a coating chosen for a Calgary living room may not be the right one for a west-facing bedroom in Windsor.
Which low-e coating suits a cold Canadian climate?
Most of the year the heating season dominates in Canada, so a coating that lets in some of the winter sun while holding in room heat is a common choice for south-facing glass. Coatings made to block solar heat are better suited to windows with strong afternoon sun or to houses that overheat. Mixing coatings between orientations is allowed and sometimes smart.
The details of hard and soft coatings, surface positions and multiple layers are covered in low-e glass types and coatings. To see how a coating fits with triple panes, krypton or argon fills and insulated frames, read about high performance glazing.
What does a low-e window cost and save?
The coating adds only a modest amount to the price of a window, and many suppliers fit it as standard, so the real choice is usually between glazing packages rather than a coated or uncoated unit. Prices vary by region, size, frame material and year, and quotes decide. What you pay for in practice is the frame, the glazing count and the installation.
Savings depend on the windows being replaced. Moving from old single or clear double glass to a good low-e unit can lower heat loss and raise comfort noticeably, because there are fewer cold spots and less condensation. Replacing a recent low-e window gains very little. A home energy audit shows where the heat actually leaves your house, and the upgrade priorities guide explains why attics and air leaks often come before glass.
How can you tell if a window is low-e?
Hold a flame or a small light, such as a phone torch, to the glass. In a clear unit you see two reflections for each pane surface. With a coating, one of the reflections has a different colour from the rest. A lab check is not needed for a purchase, because the spec sheet and label list the glazing and the performance values.
Ask the seller for the exact glass package in writing: number of panes, coating type, gas fill and spacer. If the product is certified, the model will appear on the official list described in our guide to ENERGY STAR certified windows. Winter fog on the inside surface is a separate issue, covered in why windows get condensation.
Quick answers
Does low-e glass block daylight?
Only a little. Most coatings let in the bulk of visible light, though some solar-control types are slightly tinted or more reflective.
Can a low-e coating be added to old windows?
Retrofit films exist, but their performance and lifespan vary and they can void a window warranty or stress the glass. Factory-applied coatings inside the sealed unit are more dependable.
Do low-e windows stop condensation?
They help, because the inner pane stays warmer, but indoor humidity and ventilation decide whether moisture forms. A very humid house can still show condensation.
Is low-e the same as double glazing?
No. Double glazing means two panes with a gap. Low-e is a coating that is usually applied to one of those panes to improve the unit's performance.