Low-E Glass
Low-E glass looks exactly like ordinary glass, but a coating you cannot even see is quietly bouncing heat back where it came from. Here is what "low emissivity" actually means, how the coating controls heat flow, the crucial split between passive and solar-control Low-E, and how to land the coating on the right surface for your climate.
What is Low-E glass?
Low-E is short for low emissivity. Emissivity just measures how readily a surface radiates heat back out as infrared. Plain clear glass has an emissivity around 0.84, so it happily re-radiates 84% of the heat it soaks up. A good Low-E coating drags that down to somewhere between 0.02 and 0.20, a huge cut in how much heat leaks through the window.
The coating pulling this off is a microscopically thin film of metal or metal oxide, so thin it is completely invisible and does nothing you would notice to the clarity. Look at a coated pane straight on and you will never spot it. And yet it quietly rewrites how the whole window handles heat.
How emissivity controls heat flow
Heat sneaks through a window three ways: conduction (straight through the glass and frame), convection (air currents washing over the surface), and radiation (infrared energy beaming across the gap or through the glass).
In a double-pane unit, radiation is the big one: it moves roughly 60–70% of the heat crossing the air gap. Low-E goes straight for that path. By turning the glass surface into a poor emitter and a strong reflector of infrared, the coating chokes off the dominant heat route while leaving conduction and convection alone.
It is also why Low-E does its best work tucked inside an IGU, facing the gas gap. Coat the outside of a single pane and you get some benefit, but seal it inside the unit facing the argon or krypton and it can go to work on the biggest source of loss.
The two types of Low-E: passive vs. solar control
Not all Low-E behaves the same way, and the difference comes down to solar heat gain, the free warmth that rides in on sunlight, as opposed to the heat that conducts and radiates simply because it is cold outside and warm inside.
Passive Low-E (high solar gain)
Passive Low-E is tuned to let the sun's heat in while still trapping your heating so it cannot escape. That gives it a high solar heat gain coefficient (SHGC), usually 0.40 or above. In a cold climate that free solar gain is a gift, it shaves the heating bill. A south-facing window in Minnesota can pull real warmth off the low winter sun, and passive Low-E is built to let it.
Passive Low-E is the right choice when:
- You are in a heating-dominated climate (ASHRAE climate zones 5, 6, 7)
- Windows face south or southwest with good winter sun exposure
- Reducing heating cost is the primary goal
Solar control Low-E (low solar gain)
Solar control Low-E does the opposite, turning away a big chunk of the sun's energy, mostly the infrared but a little of the visible light too. Its SHGC usually lands between 0.20 and 0.35. In hot climates, or on any west-facing glass, that unwanted solar gain is what runs your AC ragged, and solar control Low-E keeps the heat out while still letting the daylight through.
Solar control Low-E is the right choice when:
- You are in a cooling-dominated climate (climate zones 1, 2, 3)
- Windows face west or east, where afternoon sun is intense
- Reducing air conditioning load is the priority
- The building has large floor-to-ceiling glazing areas
Soft coat vs. hard coat
There are two completely different ways to get a Low-E coating onto glass, and the one you pick decides where the glass can actually be used.
Soft coat (MSVD, magnetron sputtered vacuum deposition)
Soft coat goes on after the glass is already made, in a separate vacuum chamber where silver layers are sputtered onto the surface under exquisite control. It is the high performer, emissivities as low as 0.02 are on the table, but there is a catch: it is chemically reactive. Leave soft coat exposed to open air and it oxidizes and degrades fast.
So soft coat always has to live sealed inside an IGU, on one of the protected interior surfaces, away from air and moisture. You cannot use it as single-pane glazing, and it has to be built into a unit soon after coating, most makers give you a handling window of six months or less.
Hard coat (pyrolytic)
Hard coat (also called pyrolytic Low-E) goes on while the glass is still glowing hot on the float line, so the chemistry fuses right into the surface. The payoff is toughness: hard coat is genuinely part of the glass, not a film sitting on top of it.
Because it is bonded in, hard coat can face open air and get handled without a second thought, which lets it work as single-pane glass or as the outer lite of a unit. The trade-off is performance: it only reaches emissivities around 0.15–0.20, so it never matches soft coat. And it looks a touch different, a faint haze or iridescence you catch at an angle.
| Property | Soft coat (MSVD) | Hard coat (pyrolytic) |
|---|---|---|
| Emissivity | 0.02 – 0.10 | 0.15 – 0.20 |
| Durability | Reactive, must be sealed in IGU | Extremely durable, exposable |
| Fabrication deadline | 6 months (typical) | No limit |
| Visual appearance | Very neutral | Slight haze/iridescence |
| Solar control options | Wide range (SHGC 0.20–0.60+) | Limited |
| Single-pane use | No | Yes (limited applications) |
Surface placement: why it matters
Here is a detail that trips people up: which surface the coating lands on changes what it does. In a double-pane unit the four glass faces are numbered 1 to 4, outside to inside. Surface 1 is the outermost face of the exterior pane; surface 4 is the room-side face of the interior pane.
Surface 2 (the inside face of the outer pane) is home base for solar control Low-E. Sitting there facing the gap, it turns solar heat away before it can even cross the airspace, exactly what you want in a hot or mixed climate. It is the default for most residential and commercial solar-control units in the US.
Surface 3 (the inside face of the inner pane) is where passive Low-E belongs. It lets the sun's heat in through the clear outer pane, then catches the long-wave heat trying to re-radiate back out and holds it on the room side, ideal in cold climates where every bit of solar gain offsets the furnace. On triple-pane units you will often see extra coatings on surface 2 and/or surface 5.
Hot climate: Solar control Low-E on surface 2, blocks solar heat before it enters the gap.
Mixed climate: Solar control Low-E on surface 2 with a moderate SHGC (0.25–0.40), a balance of solar control and heating-season benefit.
Performance numbers to know
Whenever you spec or compare Low-E, these are the numbers staring back at you from every NFRC label and manufacturer data sheet:
| Metric | What it measures | Typical range for Low-E IGU |
|---|---|---|
| U-factor | Overall heat loss rate (lower = better insulation) | 0.20 – 0.35 (double pane) |
| SHGC | Fraction of solar heat that enters (lower = less solar gain) | 0.20 – 0.60 depending on type |
| VT (visible transmittance) | Fraction of visible light that passes through (higher = brighter interior) | 0.40 – 0.70 |
| LSG (light-to-solar gain) | VT ÷ SHGC, measures how well a glazing transmits light while blocking heat | 1.0 – 2.0 (higher is better for hot climates) |
| Emissivity | How well the surface radiates heat (lower = better Low-E performance) | 0.02 – 0.20 |
A high LSG just means the coating is good at telling apart the visible light you want and the infrared heat you don't. The best modern solar-control coatings clear an LSG of 1.6, letting in far more light than heat.
Major Low-E product families in the US market
A handful of manufacturers make the Low-E glass you will see named on specs and shop drawings. Each brands its own version of the same idea:
- Cardinal Glass, LoĒ-272, LoĒ-i89, LoĒ-366 (triple silver)
- Guardian Industries, SunGuard SNX, ClimaGuard
- AGC (formerly Asahi Glass), Sunergy, Stopray
- Vitro Architectural Glass (formerly PPG), Solarban series
- Pilkington (NSG Group), Pilkington Activ, Energy Advantage
Product lines change often, so always confirm the current numbers against the manufacturer's own data sheet. Our Brand & Product Database (Pro feature) keeps that performance data organized by product line.
Common questions
Can you see which side the coating is on?
Yes, and there is a classic field trick for it. Hold a small flame (a lighter or match) near the edge and you will see a little row of reflections, one per glass surface. The one bouncing off the Low-E coating shows up in a slightly different color, usually a bit more orange or yellow, than the rest. Glaziers and inspectors use this all the time to find the coated side.
Does Low-E glass affect plant growth?
Barely. Solar control Low-E trims some of the near-infrared, but the visible band plants actually photosynthesize with (roughly 400–700 nm) sails through almost untouched. For most houseplants on a Low-E windowsill you will not notice a thing. Only the very aggressive solar-control coatings, with VT under 0.40, dim the light enough to bother the real sun-lovers.
Can Low-E glass be broken and re-glazed?
If a lite in a Low-E IGU breaks, the whole sealed unit has to go, not just the one pane. The seal is compromised, the gas fill is gone, and you cannot swap a single lite. With soft coat, the replacement unit has to be built from fresh coated glass in current stock; there is no re-coating a pane out in the field.
Does Low-E work in single-pane windows?
Hard coat Low-E can; soft coat cannot (it degrades the moment it is exposed). But be realistic about the payoff: a single pane of Low-E gives you only a sliver of what a double-pane IGU does, because the gas gap is where most of the insulation actually lives. Single-pane Low-E really only shows up in niche cases, interior partitions or secondary glazing in heritage buildings.