Low-E, Argon or Triple Pane? Choosing Glass for a New Sealed Unit

If you're replacing a sealed unit in an existing frame, the upgrade that usually makes sense is low-E glass with argon and a warm-edge spacer. A low-E coating adds essentially no thickness, and argon simply replaces air in the same gap, so the new unit fits the frame you already have. In the GTA, that upgrade runs roughly 10 to 20% more than plain clear glass.

Triple pane is a different decision. It insulates better, but the unit is thicker and much heavier, and most frames built for double glazing can't carry it. So for a glass repair, the realistic upgrade is low-E and argon; triple pane only comes up when a window is being replaced anyway, which is a different job. Here's how each option works in the GTA.

The short answer#

Low-E coatings: the upgrade that matters most#

Ordinary clear glass has an emissivity of about 0.84, meaning it radiates heat readily. High-performance low-E coatings bring that down to about 0.02, using a layer roughly 500 times thinner than a human hair. Natural Resources Canada says low-E coatings can reduce heat loss in the winter and heat gain in the summer by up to 50%.

There are two families, and the difference matters:

• Passive low-E has a high solar heat gain coefficient (SHGC). It lets winter sun help heat the house, which suits south-facing glass.
• Solar-control low-E has a lower SHGC. It cuts solar heat gain and air-conditioning load, which suits west-facing glass and rooms that overheat.

As a rule of thumb, NRCan suggests an SHGC of 0.3 to 0.4 for new or substantially renovated Canadian homes. Summer is becoming a bigger part of the picture here. Toronto averaged 14 days above 30 °C a year in 1991–2020, up from 9.7 in 1961–1990.

You may also hear about hard-coat and soft-coat low-E. Hard-coat (pyrolytic) is applied while the glass is being made. It's more durable but has slightly higher emissivity. Soft-coat (sputtered) is applied afterwards in a vacuum and reaches very low emissivity. In a sealed unit, the coating sits on an inside face where it's protected, so either can work.

Argon: cheap, invisible and worth having#

Argon conducts about 67% as much heat as air. It's inert, non-toxic and odourless. A typical fill is 90% argon and 10% air.

Argon does leak slowly, at a conservatively estimated 1% a year. A window stays effective while 80% of the gas remains, which is about 20 years at that rate, and a drop from 90% to 80% argon costs only 2 to 3% of the insulating value.

How much does argon add overall? For a standard double-pane low-E unit, going from argon to plain air costs about 10 to 15% of its insulating value. That's modest but real, and argon adds little to the price, so it's standard in most replacement units.

Krypton performs about as well as argon in one-half to two-thirds of the gap, so it's used where gaps are narrow, as in some triple-pane units. It rarely makes sense in a double-pane replacement.

Warm-edge spacers#

The spacer around the edge of the unit used to be aluminum, which conducts heat well. That's why condensation and frost often form around the edges of older units. "Warm-edge" spacers, usually plastic-based or stainless steel, cut heat transfer at the edge and help prevent that edge condensation. If winter condensation is a problem in your home, specify one. Our condensation and humidity guide covers the other half of that fix.

Triple pane: what you get, and why it rarely fits an existing frame#

The performance#

Better double-glazed windows reach U-factors of 1.2 W/m²·K or lower, and triple-glazed windows go as low as 0.8. Here is how the two compare in Toronto:

That warmer inner surface is the real comfort gain. It means less cold radiating off the glass and less condensation. Classic National Research Council data from 1960 shows the same pattern: at −29 °C outside, double windows tolerated about 32–39% indoor humidity before condensing, against 49–53% for triple. Modern low-E units do better than those old figures, but the gap between double and triple remains.

On energy alone, triple pane's simple payback is 15 years or more.

Why it rarely fits an existing frame#

Triple-pane units are thicker. Common overall thicknesses run from about 7/8 in up to 1¼–1⅜ in, and your frame's glazing pocket was sized for the unit it came with. They're also 30 to 50% heavier, and that extra weight can strain older or weaker frames, causing warping, sagging or the breakdown of seals, so hinges and operators need checking.

Glass-only replacement restores a window to its original spec. If you want to beat the original spec, or the frame has failed, that is a full-frame job. In practice, low-E, argon and a warm-edge spacer are the realistic upgrades inside an existing frame, and they get you most of the practical benefit without touching the frame.

Richmond Hill, Newmarket and the colder north#

The GTA isn't one climate. Heating degree-days (HDD), a measure of how much heating a location needs, rise noticeably as you move north from the lake:

Newmarket needs about 20% more heating than downtown Toronto. On the national building code's HDD bands, Toronto falls in Zone 5 (3,000–3,999) while Richmond Hill and Markham sit right at the start of Zone 6 (4,000–4,999). For Ontario's own energy-efficiency rules (SB-12), the whole GTA is in Zone 1 (under 5,000 HDD).

What that means in practice: triple pane pays back somewhat faster in Richmond Hill or Newmarket than downtown, and the comfort gain is greater, but it's still a long payback. For a foggy or broken unit in a good frame, even in a colder York Region home, low-E, argon and a warm-edge spacer are the right call. Triple pane only enters the picture if the frames have failed and the windows are being replaced anyway. For local housing and climate detail, see our Richmond Hill window and glass guide.

Safety and specialty glass#

• Tempered glass is required by the Ontario Building Code in places such as sliding doors, storm doors, some entrance-door glass and some sidelights. A replacement unit must match. Our tempered vs laminated guide explains where.
• Laminated glass bonds glass to a plastic interlayer. It holds together when broken, blocks about 88% of UV in window glass, and improves sound insulation. It's a sensible choice facing a busy road, or for a ground-floor unit where security matters.

What upgrades cost#

Prices vary with size, glass make-up and access.

ENERGY STAR and rebates#

ENERGY STAR Canada certifies complete windows. Under Version 5.0, a window qualifies with a U-factor of 1.22 W/m²·K or less, or an Energy Rating of 34 or more. Because the label applies to the tested window, a new sealed unit in an old frame doesn't carry it.

Ontario's Home Renovation Savings Program pays $100 per rough opening for ENERGY STAR windows and doors, with a home energy assessment and at least two upgrades required. A glass-only replacement generally doesn't qualify. Confirm with the program before you plan around it.

What we'd do#

For a replacement sealed unit in a sound frame, our default is low-E glass (passive or solar-control depending on which way the window faces), argon fill and a warm-edge spacer, at the thickness your frame was built for. We add tempered or laminated glass where code or security calls for it. If you're set on triple pane, we'll check the frame honestly. If the sash can't carry it, that's a window-replacement decision rather than a glass repair, and we'll tell you so instead of forcing a unit that doesn't fit.

Next step#

Choosing glass is part of every sealed-unit job we quote. See our sealed-unit replacement service, where we replace the sealed glass unit and keep your frame, or contact us with the window's size, which way it faces and a photo of the spacer label.