What Is Low-E Glass and When Should It Be Used in Curtain Walls?
A microscopically thin layer of silver, invisible to the eye, doing most of the thermal work in a modern facade.
That is low-emissivity glass in one sentence. The coating reflects long-wave infrared radiation while letting visible light through, which means a building can keep its daylight and lose far less heat through its windows than the physics of plain glass would allow.
Emissivity is the property being modified. Uncoated glass has an emissivity around 0.84, meaning it radiates heat efficiently. A good low-E coating brings that below 0.04. The glass still looks like glass, and it behaves thermally like a different material entirely.
Where it gets interesting, and where specifications go wrong, is in which coating and which surface. Those two decisions can reverse the benefit entirely. PRANCE supplies glass curtain wall systems with project-specific glazing specifications.
How Low Emissivity Glazing Works？
Heat moves through a glazed unit three ways. Conduction through the solid materials, convection in the cavity, and radiation across it.
Radiation is the largest of the three in a sealed unit, and it is the one a coating addresses. Argon fill deals with convection. Spacer design and frame construction deal with conduction.
The coating reflects infrared back toward its source. In winter that means interior heat bouncing back into the room rather than radiating out through the glass. In summer, with the coating positioned differently, it means solar infrared reflected outward before it enters the building.
Same technology, opposite jobs, decided by which surface carries the coating.
Soft Coat and Hard Coat
Two manufacturing routes producing genuinely different products.
Soft coat, also called sputtered or MSVD, is applied in a vacuum chamber after the glass is made. Multiple layers including one, two, or three layers of silver. Emissivity below 0.04 and excellent solar control. The coating is fragile, oxidises in air, and must sit inside a sealed cavity.
Hard coat, also called pyrolytic, is applied while the glass is still hot during manufacture, fusing into the surface. Emissivity typically 0.15 to 0.20, which is considerably worse than soft coat. It is durable, can be exposed to air, can be handled and cut like ordinary glass, and can be used in single glazing.
For curtain walls, soft coat dominates almost completely. The units are sealed anyway, so the fragility is irrelevant, and the performance gap is too large to ignore.
Silver Layers
The number of silver layers in a soft coat determines how selective it is.
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Single silver: basic low-E, good U-value, moderate solar control
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Double silver: the commercial standard, strong solar control with good light transmission
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Triple silver: highest selectivity, used where solar load is extreme and daylight is still wanted
Each additional layer improves the ratio of light admitted to heat admitted, and adds cost. Double silver covers most commercial work. Triple silver earns its premium in regions where coating selection in very hot regions is the governing concern.
Low-E Glass VLT and SHGC Selection
Three numbers decide whether a glazing specification suits a building, and they pull against each other.
VLT, Visible Light Transmittance, is the proportion of visible light passing through, expressed from 0 to 1. A clear double-glazed unit might sit around 0.78. A heavily tinted solar control unit might be 0.20.
SHGC, Solar Heat Gain Coefficient, is the proportion of solar energy admitted, also 0 to 1. Lower means less heat entering. European practice uses the g-value, which is the same concept.
U-value, covered under the whole-system view of heat transfer, measures conductive loss rather than solar gain.
The Light to Solar Gain Ratio
LSG is VLT divided by SHGC, and it is the most useful single figure for comparing coatings.
A ratio above 1.25 indicates a spectrally selective coating that admits daylight while rejecting heat. Below 1.0 means the glass is blocking light at least as aggressively as it blocks heat, which usually means tinting is doing the work rather than a coating.
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Clear double glazing: VLT around 0.78, SHGC around 0.70, LSG approximately 1.1
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Single silver low-E: VLT 0.70, SHGC 0.55, LSG approximately 1.3
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Double silver low-E: VLT 0.65, SHGC 0.35, LSG approximately 1.9
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Triple silver low-E: VLT 0.60, SHGC 0.27, LSG approximately 2.2
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Tinted with low-E: VLT 0.35, SHGC 0.25, LSG approximately 1.4
Indicative figures for comparison. Actual values vary by manufacturer and product and must come from the glass supplier's data.
High LSG matters most in daylit commercial buildings in hot climates, where the design wants bright interiors and low cooling load at the same time.
Which Surface the Coating Goes On？
Surfaces are numbered from outside to inside. In a double-glazed unit, surface 1 faces the weather, surface 2 is the inner face of the outer pane, surface 3 is the outer face of the inner pane, and surface 4 faces the room.
This decision reverses the function of the coating, and getting it wrong is a genuine specification error rather than a preference.
| Climate | Coating surface | What it does |
|---|---|---|
| Cooling-dominated, hot | Surface 2 | Reflects solar infrared before it enters the cavity |
| Heating-dominated, cold | Surface 3 | Reflects interior heat back into the room |
| Mixed climate | Surface 2, usually | Cooling load generally governs in commercial buildings |
| Triple glazing | Surfaces 2 and 5 | Two coatings, one per cavity |
In a cooling climate a coating on surface 3 still improves U-value, but it lets solar energy into the cavity first, where it heats the inner pane and radiates into the room. The building gets a better U-value number and a worse cooling load.
Commercial buildings generate significant internal heat from people, lighting, and equipment, which is why cooling frequently governs even in temperate regions.
When Low-E Should Be Specified？
Nearly always, in any commercial curtain wall. The cases where it adds little are narrow.
Specify it where:
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The building is conditioned, heated or cooled
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Occupant comfort near the facade matters
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An energy code sets U-value or SHGC limits, which most now do
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Condensation risk exists in winter
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Glare and solar overheating affect usable floor area near the perimeter
It adds less where:
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The space is unconditioned, such as a parking structure or warehouse
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The glazing is a small proportion of a largely opaque facade
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The facade is permanently shaded by adjacent structures, which reduces but does not eliminate the benefit
Spandrel zones are a separate case. They are opaque, so the coating serves no daylight function, though low-E on spandrel glass still contributes to the thermal performance of that panel.
Low-E and Other Glazing Requirements
Coatings have to coexist with everything else the glass is doing.
Safety. Low-E coatings are compatible with both tempering and lamination, though soft coats applied before tempering require a heat-treatable coating formulation. Safety glazing and how each type breaks covers the selection.
Acoustics. Coatings have essentially no effect on sound reduction, which depends on mass, asymmetric pane thicknesses, cavity width, and interlayer type. Glazing build-ups that also block noise are designed around different variables.
Photovoltaics. Where a facade incorporates photovoltaic glazing, coating selection interacts with the cell technology and needs coordinating with the PV supplier.
Appearance. Coatings carry a colour cast and a reflectance level. Triple silver products in particular can read noticeably more reflective from outside, which is an architectural decision as much as a technical one.
Verifying What You Are Being Supplied
Four things to confirm on any glazing submittal.
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The specific product name and manufacturer, since performance varies considerably between nominally similar coatings
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VLT, SHGC, and U-value for the complete unit, not centre-of-glass alone where the code calls for assembly values
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The coating surface, stated explicitly in the submittal
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The data source, ideally manufacturer-published or NFRC-certified figures rather than a general catalogue range
Exterior reflectance is worth confirming too. A facade that reads as a mirror from the street can become a planning issue in some jurisdictions, and it is easier to resolve at submittal than after installation.
Frequently Asked Questions
1. Is low-E glass worth the cost in a commercial curtain wall?
Yes in almost every conditioned building. It is the single highest-value glazing upgrade available, improving both U-value and solar control for a modest premium over clear glass, and most energy codes now effectively require it through their U-value and SHGC limits.
2. Which surface should the low-E coating be on?
Surface 2 in cooling-dominated climates, so solar infrared is reflected before entering the cavity, and surface 3 in heating-dominated climates, so interior heat is reflected back into the room. Commercial buildings often favor surface 2 even in temperate regions, since internal heat gains make cooling the governing load.
3. What is the difference between soft coat and hard coat low-E?
Soft coat is applied in a vacuum after manufacture, reaching emissivity below 0.04 with excellent solar control, but it is fragile and must sit inside a sealed cavity. Hard coat is fused into the glass during manufacture, is durable enough for exposed use, and performs considerably less well at emissivity around 0.15 to 0.20.
4. Does low-E glass make interiors darker?
Not necessarily. A spectrally selective double silver coating typically retains VLT around 0.65 while cutting SHGC to approximately 0.35. Darkening comes mainly from tinting rather than from the coating, which is why the light to solar gain ratio is a more useful comparison figure than SHGC alone.
5. Can low-E glass be tempered or laminated?
Yes, with the right product. Soft coats intended for heat treatment are formulated as heat-treatable, and standard soft coats that are not must be coated after tempering. Lamination is compatible in both cases, and the glass supplier should confirm the processing route at submittal.
Specifying Coated Glass Properly
Decide whether the building is cooling-dominated or heating-dominated, because that sets the coating surface. Choose the silver count against the solar load and the daylight the design wants. Compare products on light to solar gain rather than on SHGC alone. Then confirm the coating surface explicitly on the submittal, because it is the detail most often left unstated and most costly to get wrong.
PRANCE supplies curtain wall systems with coordinated glazing specifications covering coating type, surface position, and certified performance data. Send your climate zone, energy code requirements, and daylight targets and request a glazing recommendation.