Solar-control coatings & switchable glass
The most powerful layer on a glass facade is invisible - a stack of metal films thinner than a wavelength of light that lets the view and the daylight through while turning the heat back. And the newest glass changes its mind on demand.
One coating decides whether a glass tower in Chennai is a daylit office or an oven with a view. It is a few atoms of silver.
Stand inside a modern glass office and notice that it is bright but not blazing, and that the glass feels cool even in the afternoon sun. That is not the glass - clear glass lets in roughly seven-tenths of the sun's heat. It is a **solar-control coating**: an invisibly thin stack of metal-oxide and silver films sputtered onto the glass that lets daylight and the view through while reflecting away the infrared heat. The holy grail is **selectivity** - lots of light, little heat - and the best coatings get remarkably close. Newer still is **switchable glass**, which changes its own tint at the flick of a switch or as it warms, turning the facade into a dynamic, controllable filter. In a high-radiation country like India, this thin layer is the single biggest lever on the cooling bill. This lesson is how it works and how you choose it.
The invisible layer that controls the sun
Sputtered silver films reflect heat while passing light
A low-emissivity (low-E) / solar-control coating is a microscopically thin multilayer stack deposited on glass, usually by magnetron sputtering in a vacuum - layers of transparent metal oxides sandwiching one or more ultra-thin films of silver, the active layer. Silver is nearly transparent to visible light but highly reflective to infrared, so the coating passes the daylight and the view while turning back the sun's heat (and, on the warm side, re-radiated room heat).
The number of silver layers sets the performance. A single-silver coating gives moderate solar control. A double-silver coating is the modern workhorse for hot climates - strong solar rejection with good light transmission. A triple-silver coating is the high-performance tier, pushing selectivity higher still (very low heat gain at a usable light level), at higher cost and with tighter handling needs.
Two practical points. First, these soft (sputtered) coatings are delicate and must live on a protected surface inside an IGU - typically surface #2 (the inside face of the outer pane) in a cooling climate, where they intercept the sun before it enters the cavity. (Hard pyrolytic coatings are tougher and can face weather, but are less selective.) Second, the coating is also a thermal (low-E) layer: it lowers the IGU's U-value as well as its SHGC - one stack, two performance jobs.
Silver passes light and blocks heat. One layer is good, two is great for India, three is for when you must have both a lot of light and almost no heat.
VLT, SHGC and the ratio that measures a good coating
Two numbers describe what a glass does with the sun. Visible Light Transmittance (VLT) is the fraction of daylight that gets through - high VLT means a bright interior and good views. Solar Heat Gain Coefficient (SHGC) is the fraction of total solar heat that gets through - low SHGC means less cooling load. In a hot climate you want high VLT and low SHGC at the same time - and those two pull against each other, because the simplest way to cut heat (a dark tint) also cuts the light.
The figure of merit that captures this is selectivity, or the Light-to-Solar-Gain ratio (LSG = VLT / SHGC). A plain bronze tint might give VLT 0.50 and SHGC 0.55 - LSG below 1, poor: you lose light and keep heat. A good double-silver spectrally-selective coating might give VLT 0.50 and SHGC 0.27 - LSG close to 1.9: the same daylight for half the heat. The whole point of a sputtered coating is to push selectivity well above what a tint can do, because it works on the spectrum - passing the visible band and rejecting the near-infrared - rather than just dimming everything.
This is the number that matters for an Indian facade. ECBC and Eco-Niwas Samhita cap SHGC (and, through RETV/U-value, the heat); the design skill is hitting that SHGC cap while keeping VLT high enough for daylight and views - which means choosing for selectivity, not just for a low SHGC.
Glass that changes its own tint - electrochromic and thermochromic
A fixed coating is a single compromise for every hour of every day. Switchable (dynamic) glass breaks that compromise by changing its tint. Electrochromic (EC) glass has a coating that darkens when a small DC voltage drives ions into an electrochromic layer; switch it and the glass goes from clear (high VLT, higher SHGC) to deeply tinted (low VLT, low SHGC) over a few minutes, controllable by zone, schedule or a building-management system. It lets a west facade run clear for the daylit morning and tint down against the brutal afternoon sun - one glass, many states.
Thermochromic (TC) glass is the passive cousin: it darkens automatically as it heats up, with no wiring - cheaper and self-regulating, but uncontrollable (it responds to glass temperature, not to your needs, so it may tint on a bright cold morning when you wanted the warmth). Suspended-particle (SPD) and liquid-crystal (PDLC) devices switch mainly privacy/glare fast, but are weaker on solar heat.
The trade-offs are real. Dynamic glass cuts peak cooling load and glare and can shrink mechanical shading, but it costs several times more than coated glass, needs wiring and controls (for EC), tints over minutes rather than instantly, and at its clearest still admits more heat than a good fixed solar-control unit. As of 2026 it is a premium tool for high-glare, high-radiation facades - not yet the default - but it is the clearest direction of travel for the glass facade.
The coating is how you keep the glass _clear-looking_ while meeting the energy code - a good spectrally-selective coating gives you a neutral, high-VLT facade that still has a low SHGC, instead of the dark, mirrored tints clients dislike. Specify by _selectivity_ (LSG), not by tint colour, and remember the coating sits inside the IGU so it does not change the visible glass colour much. If you want a facade that adapts - clear morning views, tinted afternoon glare control on a west elevation - switchable glass delivers it, but budget for several times the cost and the controls, and bring it in at concept because it changes the shading strategy.
Drive the glass selection from the ECBC/ENS SHGC cap and the daylight/VLT target together, choosing the coating with the best LSG that meets both - single-silver for modest needs, double-silver as the hot-climate default, triple-silver where you must have high light and very low heat. Confirm the coating surface (#2 in a cooling climate), that it is a soft sputtered coating inside the IGU, and that its low-E function is also doing your U-value (don't double-count or omit it). For switchable glass, model the cleared-state SHGC for the worst case, specify the control logic, and check the wiring, framing and BMS interfaces early - the dynamic state range is the spec, not a single number.
Soft sputtered coatings are fragile - they live on an inside IGU surface for a reason, and a coated pane handled coating-side-out, or cleaned with an abrasive, can be scratched or hazed before it is even assembled. Check the coating is on the specified surface (suppliers mark it; an IGU built coating-on-the-wrong-surface fails the energy spec silently). For electrochromic glass, the wiring tails and connectors at the pane edge are delicate and must be protected during glazing, and the control gear commissioned with the facade. The performance you can't see - the coating - is exactly the one most easily damaged before anyone notices.
Eco-Niwas Samhita 2018 (India)
Residential SHGC / envelope (RETV)
Caps the glazing's effective SHGC and folds it into the RETV envelope limit for residential buildings. It sets the SHGC ceiling the coating must meet; it does not reward high VLT directly, so daylight remains the designer's call.
ECBC 2017 (India)
Commercial SHGC, U-value, VLT
Sets maximum SHGC and U-value (and minimum VLT-linked daylight provisions) for commercial envelopes by climate zone. It is the SHGC cap your coating selection is built around - selectivity is how you also keep the daylight.
ASHRAE 90.1 / NFRC 100-200
Glazing performance ratings (global)
ASHRAE 90.1 sets envelope SHGC/U limits; NFRC defines how VLT, SHGC and U-value are measured and rated, so quoted numbers are comparable. They standardise the metrics - the centre-of-glass vs whole-window distinction still matters for real performance.
EN 1096
Coated glass
Specifies and classifies coated glass (including sputtered/soft coatings) and its durability classes. It governs the coating's durability and handling class - not the energy target, which the codes above set.
“To cut solar heat you just use darker, more tinted glass - the darker it looks, the cooler the building.”
A dark tint cuts heat by cutting _everything_, including the daylight and the view, so it has poor selectivity - you end up with a gloomy interior that still needs artificial light (which adds its own cooling load) and a SHGC that a good clear-looking coating beats. A spectrally-selective sputtered coating works on the spectrum, passing the visible band and rejecting the near-infrared, so it delivers a far better light-to-heat ratio than any tint. Choose for selectivity (VLT/SHGC), not for how dark the glass looks.
Worked example - choose glass on selectivity for an ECBC SHGC cap
Pick the vision glass for a Chennai (warm-humid, ECBC composite) office where the SHGC must be <= 0.27 and the architect wants a bright, clear-looking interior (target VLT >= 0.45). Compare three options by selectivity and choose.
The project ECBC/ENS SHGC cap and daylight target, two or three glass-coating datasheets (VLT, SHGC, U-value), and the IGU make-up from Lesson 6.1.
GIVEN
ECBC SHGC cap : SHGC <= 0.27
Daylight target : VLT >= 0.45
Selectivity : LSG = VLT / SHGC (higher = better)
OPTIONS (centre-of-glass, IGU make-up)
A bronze tint, no coating : VLT 0.50 , SHGC 0.55
B single-silver low-E : VLT 0.55 , SHGC 0.34
C double-silver low-E : VLT 0.50 , SHGC 0.27- 1Compute selectivity (LSG = VLT/SHGC) for each. A: 0.50 / 0.55 = 0.91. B: 0.55 / 0.34 = 1.62. C: 0.50 / 0.27 = 1.85. Higher LSG means more daylight per unit of heat.
- 2Screen against the SHGC cap (<= 0.27). A (0.55) fails badly - the tint keeps over half the heat. B (0.34) fails the cap too. Only C (0.27) meets the ECBC SHGC ceiling. The dark tint, despite 'looking' protective, is the worst performer.
- 3Screen against the VLT target (>= 0.45). A: 0.50 ok but already eliminated on SHGC. B: 0.55 ok but eliminated on SHGC. C: 0.50 - meets the >= 0.45 daylight target. C passes both gates.
- 4Choose C - the double-silver coating - because it is the only option that meets the SHGC cap and keeps VLT high, and it has the best selectivity (1.85). Note that a triple-silver coating could push SHGC lower still at similar VLT if the cap were tighter or the cooling target more aggressive.
- 5Confirm the build detail. Place the soft sputtered coating on surface #2 of the IGU, verify the supplier's centre-of-glass numbers (VLT 0.50 / SHGC 0.27), and remember the same coating is lowering the U-value (its low-E function) - so it serves both the SHGC and U checks.
- 6Consider the dynamic alternative. If the west facade has severe afternoon glare, note that electrochromic glass could run clearer than VLT 0.50 most of the day and tint to a very low SHGC only when needed - better comfort, at several times the cost. For this brief, fixed double-silver (C) is the right, economical answer.
You’ll walk away with
A glass choice justified by numbers: option C (double-silver low-E, VLT 0.50 / SHGC 0.27, LSG 1.85) is the only one meeting the ECBC SHGC cap while keeping the interior bright - and the selectivity ratio, not the tint, is what proved it. The exact comparison a facade engineer presents to the energy consultant.
Two quick ways to read solar control in the field.
- 01Compare two glass buildings: a dark, mirrored older tower and a newer clear-looking one. The mirrored glass is usually a reflective/tinted product with poor selectivity; the clear-looking modern one almost certainly carries a spectrally-selective sputtered coating doing the same heat rejection while staying transparent. The look tells you the generation of coating.
- 02Find any electrochromic or switchable glass (some airports, boardrooms and premium offices have it) and watch it tint. You are seeing a facade change its own SHGC and VLT in real time - the single-compromise fixed coating, made dynamic.
The decisive layer on a glass facade is an invisible sputtered coating of silver films that passes daylight and rejects infrared heat - and the way to judge it is selectivity, the VLT/SHGC ratio, not the tint. Double-silver coatings are the hot-climate default, triple-silver the high-performance tier, and switchable glass turns the single fixed compromise into a dynamic, controllable one. In high-radiation India this thin layer is the biggest single lever on the cooling load.
Solar-control / low-E coatings are sputtered silver-film stacks that pass visible light and reflect infrared heat, also lowering U-value; single-silver is moderate, double-silver the hot-climate workhorse, triple-silver the high-performance tier (soft coatings live on IGU surface #2). Selectivity = LSG = VLT/SHGC measures a good coating; you want high VLT and low SHGC together. Switchable glass (electrochromic, thermochromic) changes tint dynamically at a premium cost.
What is the difference between VLT and SHGC?
VLT (Visible Light Transmittance) is the fraction of daylight that passes through the glass - higher means a brighter interior and clearer views. SHGC (Solar Heat Gain Coefficient) is the fraction of total solar heat that passes through - lower means less cooling load. In a hot climate you want high VLT and low SHGC at once, and the ratio of the two (VLT/SHGC, the Light-to-Solar-Gain ratio or selectivity) measures how well a glass achieves that; a good spectrally-selective coating gives a high ratio.
What is a low-E coating and which surface does it go on?
A low-emissivity (low-E) / solar-control coating is a microscopically thin sputtered stack of metal oxides and silver that passes visible light while reflecting infrared heat, lowering both the SHGC and the U-value of the glass. Soft sputtered coatings are delicate and must sit on a protected surface inside the insulating glass unit - typically surface #2 (the inside face of the outer pane) in a cooling-dominated climate like India's, so it intercepts solar heat before it enters the cavity.
Is electrochromic switchable glass worth it for Indian facades?
It can be, on high-glare, high-radiation facades (west and south-west elevations, atria) where its ability to tint dynamically cuts peak cooling load and glare and can reduce or remove mechanical shading. But as of 2026 it costs several times more than fixed coated glass, needs wiring and controls, tints over minutes rather than instantly, and at its clearest still admits more heat than a good fixed double-silver unit. It is a premium tool, not yet a default - justified case by case against a high-selectivity fixed coating.
Peer-reviewed journals & authoritative standards
- 01Li, X. & Wu, Y. A review of complex window-glazing systems for building energy saving and daylight comfort. — Journal of Building Physics (SAGE), 2025.
- 02An approach to calculate the equivalent solar heat gain coefficient of glass windows with fixed and dynamic shading in tropical climates. Journal of Building Engineering. — Journal of Building Engineering (Elsevier), 2018.
- 03Multi-objective optimization of glazing and shading configurations for visual, thermal, and energy performance of cooling-dominant climatic regions of India. — (peer-reviewed; via ResearchGate), 2024.
- 04Eco-Niwas Samhita 2018 (ECBC for Residential Buildings), Part I: Building Envelope. — Bureau of Energy Efficiency, Govt. of India, 2018.
_That closes Module 6: glass as an engineered material - processed, sealed into IGUs, structurally fixed, made to fail safely, and coated to control the sun. Next, Module 7 takes the engineered components and asks where they all meet - the detailing and interfaces where facades succeed or fail._