Low-e glass: coating types and where they sit

What are the main types of low-e glass?

The split comes from how the coating is made. A hard coat is applied while the glass is still hot during manufacture, so the layer fuses to the surface. It is durable and can be handled and stored with few precautions, but it usually reflects somewhat less heat than the alternative. A soft coat is applied afterwards in a vacuum chamber, in thin layers that often include silver. It performs better and is the usual pick for high-performance units, yet it is more delicate and has to sit inside the sealed cavity, away from air and moisture.

Neither type is better in every case. For the basics of how a coating reduces radiant heat flow, start with our guide to low-e windows.

How many layers does a coating have?

Soft coats are often described as single, double or triple silver, meaning the number of silver layers in the stack. More layers allow a more selective coating: one that passes visible light, holds back heat, and can be tuned to let more or less of the sun's heat through. A manufacturer's spec sheet shows the outcome rather than the recipe, in the form of the solar heat gain coefficient, visible transmittance and U-factor for the full unit. Those figures are explained in the efficient windows guide.

Which glass surface gets the coating?

Glass surfaces are numbered from the outside in. In a double-pane window, surface one is the outside face of the outer pane, two is the inside face of the outer pane, three is the outside face of the inner pane and four is the room side.

• Surface two. A common spot for coatings that limit solar heat, which suits hot or sunny climates.
• Surface three. Often chosen where winter heating matters, since it lets more sun in while keeping heat inside.
• Two coatings in a triple unit. Triple glazing can carry coatings on more than one surface, which is one reason it reaches lower U-factors.

These positions are typical, not fixed rules. Window makers combine coating position, gas fill and spacer to reach a target, so compare the finished unit instead of the position alone. The next step up, with triple panes and insulated frames, is in high performance glazing.

How do you match glass to a Canadian climate and house?

Think about what the window must do most of the year. A house on the prairies with long, cold, sunny winters often benefits from a coating that admits winter sun, particularly on south-facing glass. A west-facing room that overheats in summer may need a solar-control coating, even at the cost of a little free winter heat. The passive solar design guide explains how glazing, orientation and shading work together.

It also helps to keep things in proportion. Glass is one part of the envelope, and an uninsulated attic or an air-leaky basement can lose more heat than a good window ever saves. Our upgrade priorities guide sets out the order that usually pays back first.

What should you ask a window seller?

Request the glass package in writing, with the coating type, surface position, number of panes, gas fill and spacer type. Ask for the U-factor, solar heat gain and energy rating for the whole window, since centre-of-glass figures look better than the full product. Check whether the model appears on the certified list, as described in ENERGY STAR certified windows in Canada. Warranties on sealed units are a useful test of confidence, so read the term and what it covers.

Quick answers