Low-e coating is a microscopically thin metallic or metal oxide layer applied to window glass that reflects infrared radiation while allowing visible light to pass through. For Canadian homeowners, it means your windows keep heat inside during winter and block solar heat gain in summer — all without darkening the glass or reducing your view.
What “Low-E” Actually Means
Low-e stands for low emissivity. Emissivity is a material’s ability to emit thermal radiation. Standard uncoated glass has an emissivity of around 0.84, meaning it radiates roughly 84% of the thermal energy it absorbs back into the room — or out of it, depending on the season. A quality low-e coating drops that figure to 0.02–0.04. That’s a dramatic reduction in radiant heat loss.
The coating is invisible to the naked eye. It doesn’t change the look of your window from the street or from inside. What it does change is the U-factor — the rate at which a window conducts non-solar heat flow. A lower U-factor means a better-insulating window. Low-e glass routinely brings double-glazed units down from a U-factor of around 2.8 W/m²K (uncoated) to 1.6–1.9 W/m²K, and triple-glazed units with low-e to 1.0–1.3 W/m²K.
How the Coating Is Applied
There are two main manufacturing methods:
- Hard-coat (pyrolytic): Applied during float glass production while the glass is still hot. It bonds directly to the surface, making it more durable and scratch-resistant. Suitable for single-glazed applications and exposed surfaces.
- Soft-coat (sputtered/MSVD): Applied in a vacuum chamber after the glass cools. This method produces a much lower emissivity rating but must be sealed inside an insulating glass unit (IGU) because it degrades when exposed to air and moisture.
- Surface position matters: In a double IGU, soft-coat low-e is typically placed on surface 2 (inner face of the outer pane) or surface 3 (outer face of the inner pane), depending on the climate and desired balance between solar heat gain and heat retention.
- Triple-glazed units add a second IGU cavity, allowing coatings on surfaces 2 and 5 for maximum thermal performance.
- Seal integrity is critical: A failed perimeter seal exposes the soft-coat to humid air, which causes it to oxidise and lose effectiveness — a common cause of fogging inside the glass.
Most high-performance residential windows today use soft-coat low-e sealed within the IGU cavity. This is why energy-efficient window replacement involves evaluating the entire glazing system, not just the glass.
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Low-E 272 vs Low-E 366: What’s the Difference?
Two coatings come up constantly in residential window specs: Low-E 272 and Low-E 366. Both are Cardinal Glass products widely used across the Canadian window industry.
| Property | Low-E 272 | Low-E 366 |
| Solar Heat Gain Coefficient (SHGC) | ~0.40 | ~0.27 |
| Visible Light Transmittance (VT) | ~72% | ~66% |
| U-factor (double IGU, argon) | ~1.65 W/m²K | ~1.53 W/m²K |
| Best application | Cold climates, south-facing | Mixed climates, west/east-facing |
Low-E 272 lets in more solar heat gain, which is useful on south-facing windows in Ontario winters where passive solar warmth is welcome. Low-E 366 blocks more solar radiation — better for west-facing windows that get intense afternoon sun in summer. Neither coating is universally superior; placement and orientation both matter.
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The i89 Interior Coating: An Additional Layer
Some window manufacturers offer an optional interior coating called i89 (sometimes referred to as ClimaGuard i89 or an equivalent product). This is a separate hard-coat low-e applied to the interior surface of the inner glass pane (surface 4 in a double IGU). It adds another layer of infrared reflection without meaningfully reducing visible light transmission.
In practical terms, the i89 coating can reduce radiant heat loss by an estimated 10–15% on top of what the primary soft-coat low-e already does. For a Toronto home with large north-facing windows, that additional thermal resistance adds up across a full heating season. TotalSeal offers this as an upgrade option in their window configurations for homeowners who want every fraction of a U-value improvement.
“In my experience doing energy audits across the GTA, the biggest thermal improvement most homeowners can make isn’t adding insulation to the attic — it’s replacing their old single-pane or uncoated double-pane windows with modern low-e units. The emissivity difference is measurable with a thermal camera on even a mild winter day. I’ve recorded interior glass surface temperatures 4–6°C warmer on a low-e unit versus an uncoated double pane in the same wall.”
— Marcus Oyelaran, P.Eng., Certified Energy Auditor, Ontario Building Science Group, Toronto
How Low-E Coating Interacts with Gas Fills
Low-e coating doesn’t work in isolation. It’s part of a system alongside the gas fill and the spacer bar inside the IGU. Argon gas — with a thermal conductivity of about 0.016 W/mK versus air at 0.024 W/mK — reduces convective heat transfer across the cavity. Krypton gas performs even better (0.009 W/mK) in the narrower cavities typical of triple-glazed units.
Together, low-e coating and argon gas fill can reduce overall window heat loss by 25–30% compared with an air-filled, uncoated IGU of the same construction. The warm-edge spacer bar — hybrid or non-metallic, versus the older aluminium bar — then reduces edge-of-glass condensation and further lowers the overall U-factor by minimising thermal bridging at the perimeter of the IGU. You can read more about this system in our guide on insulated window frames.
What Low-E Coating Does for Condensation
One of the less-discussed benefits of low-e glass is its effect on interior glass surface temperature. Because the coating reflects heat back into the room, the interior glass pane stays warmer in cold weather. Condensation forms when a surface drops below the dew point of the interior air. A warmer interior glass surface means the dew point is reached less often.
If you’re currently seeing condensation forming between glass panes, that’s a sign the IGU seal has failed and the coating has likely degraded as well — both issues that require full unit replacement, not repair.
Low-E and ENERGY STAR™ Certification
In Canada, ENERGY STAR™ certification for windows is managed by Natural Resources Canada (NRCan) and uses a climate zone system — Zones 1 through 3, with Zone 3 covering most of Ontario and Quebec where winters are most severe. To qualify for ENERGY STAR™ certification in Zone 3, a window must meet a U-factor of ≤ 1.22 W/m²K and an Energy Rating (ER) of ≥ 34.
Achieving those numbers without low-e coating is essentially impossible in a standard double-glazed vinyl unit. Low-e is effectively a prerequisite for ENERGY STAR™ certified windows in Canadian climate zones. When reviewing quotes, ask specifically which low-e coating is specified, on which surface, and whether the IGU is argon-filled. Vague answers are a red flag.
| ENERGY STAR™ Zone | U-factor Max (W/m²K) | Min ER | Typical Geography |
| Zone 1 | ≤ 2.00 | ≥ 25 | BC Coast, Southern Ontario cities |
| Zone 2 | ≤ 1.40 | ≥ 29 | Most of Ontario, Quebec |
| Zone 3 | ≤ 1.22 | ≥ 34 | Northern Ontario, prairies |
If you’ve been putting off window replacement because of cost, check the Canadian grant options for window replacement — federal rebate programmes through the Canada Greener Homes initiative have made ENERGY STAR™ certified low-e windows significantly more accessible for Ontario homeowners.
“When homeowners ask me which specification matters most on a quote, I tell them to look at the coating and the spacer before they look at the frame. You can have a premium uPVC profile, but if the IGU is built with an aluminium spacer and no soft-coat low-e on surface 2, you’re leaving a meaningful amount of thermal performance on the table. The spacer-to-glass junction accounts for roughly 10–15% of total window heat loss in a standard residential unit.”
— Sandra Kowalczyk, RCABC-certified fenestration consultant and building envelope specialist, Mississauga
Common Window Types and How Low-E Applies to Each
Low-e coating is available across virtually all residential window configurations, but it matters more in some than others. Large fixed glazing areas — like picture window replacements or bay window installations — have the most to gain because there’s more glass area to radiate heat.
Operable windows with smaller glazing areas, like awning window replacements or basement hopper windows, still benefit from low-e, but the thermal impact per unit is proportionally smaller. The coating cost is baked into the IGU regardless of sash size, so it’s always worth specifying.
For tilt-and-turn windows, which tend to be larger and are often used on principal facades, pairing low-e 366 with triple glazing and argon/krypton fill is a common spec for new builds and deep retrofits in the GTA.
Key Specifications to Ask for in Writing
Before signing any window contract, confirm these points in writing:
- Low-e coating type (272, 366, or equivalent) and surface position (surface 2 or 3)
- Whether an interior i89 or equivalent secondary coating is included
- Gas fill type (argon or krypton) and fill percentage (target: ≥ 90%)
- Spacer bar material (warm-edge hybrid vs aluminium)
- Declared U-factor and SHGC for the whole window — not just centre-of-glass values
- ENERGY STAR™ certification zone compliance
If you’re comparing bids, this list levels the playing field. A window quoted without specifying coating surface and gas fill percentage isn’t a complete specification. Our guide on how to get the best deal on replacement windows covers this further.
Does Low-E Coating Affect UV Protection?
Yes — and it’s worth mentioning separately. Low-e coatings block a significant portion of ultraviolet (UV) radiation, typically reducing UV transmittance to 1–3% compared with 75% through uncoated clear glass. This matters for interior finishes: flooring, fabrics, artwork, and furniture all fade with cumulative UV exposure. A low-e window effectively acts as a UV filter without any additional film or tint.
This is one reason homeowners with large glazed areas facing south or west notice noticeably less furniture fade after upgrading to low-e units. Pair that with our tips on reducing heat loss in winter and limiting heat gain in summer for a full seasonal strategy.
Low-E Coating Lifespan and Maintenance
Soft-coat low-e sealed inside a properly manufactured IGU will last the life of the sealed unit — typically 20–25 years before seal degradation becomes an issue under normal conditions. The coating itself doesn’t wear out; it’s the perimeter seal that eventually fails, letting moisture into the cavity.
There’s nothing to clean or maintain regarding the coating itself. Standard glass cleaning applies to the exterior and interior surfaces. Avoid abrasive cleaners on any interior surface with an exposed hard-coat low-e — rare in residential units, but possible in some budget products.
If you notice fogging, milky residue, or iridescent hazing inside the glass, that’s a degraded IGU seal. At that point, the low-e coating is compromised too. Review our article on common old window issues to identify whether it’s time to replace rather than repair.
Frequently Asked Questions
What is low-e coating on windows?
Low-e (low emissivity) coating is a microscopically thin metallic layer on glass that reflects infrared heat while transmitting visible light. It reduces the U-factor, limits heat loss in winter, and blocks solar heat gain in summer.
Is low-e coating worth it for Canadian homes?
Yes. In Ontario’s climate, low-e coating is essential for meeting ENERGY STAR™ certification thresholds and meaningfully reducing heating costs. Most high-performance vinyl windows include it as standard in their IGU specification.
Does low-e coating make windows darker?
Not significantly. Low-E 272 transmits about 72% of visible light; Low-E 366 transmits around 66%. Both are comparable to standard clear glass and produce no noticeable tint indoors under typical lighting conditions.
Can low-e coating be added to existing windows?
No. Soft-coat low-e must be sealed inside a factory-manufactured IGU. It cannot be applied to existing glass. If your current windows lack it, full IGU or window replacement is the only option.
Which is better — low-e 272 or low-e 366?
It depends on window orientation. Low-E 272 suits south-facing windows where winter solar gain is useful. Low-E 366 is better for west- or east-facing exposures where summer solar heat needs to be managed.
