Glass processing & the IGU
Float glass leaves the line as a soft, breakable sheet. By the time it reaches the facade it has been cut, ground, toughened, laminated and sealed into a multi-pane cassette — and every one of those steps is engineering.
The glass in a facade is not a material you buy. It is a process you specify, step by step.
Most people think of glass as a thing: a clear sheet you order by the square metre. A facade engineer thinks of it as a sequence. A ribbon of molten glass floats off a bath of tin, cools to a perfect flat sheet, gets cut, ground, heated to the edge of softening and quenched, maybe bonded to a second sheet with a tough interlayer, then sealed into a sandwich with a hollow spacer and a dry gas fill. Each step adds strength, safety or performance — and each adds a way to fail. In Mumbai and Bengaluru, a sealed unit that fogs internally after three monsoons is almost always an edge-seal that was specified or built wrong. To engineer glass you must first understand how it is made.
From float ribbon to sealed cassette
Float glass is born flat, then cut, ground, toughened and laminated
Almost all architectural glass starts as float glass. Molten glass at around 1100 C is poured onto a bath of molten tin; because glass floats on tin and both surfaces seek a level, the ribbon emerges optically flat. It is annealed (cooled slowly to relieve stress) and cut into stock sheets. As float, it is only annealed glass - it breaks into large dangerous shards and has modest strength.
Processing turns that raw float into a facade product. First cutting to the panel size, then edgework - grinding and arrissing the cut edge, because a sharp or chipped edge is a crack waiting to grow under stress. Then comes heat treatment: the sheet is heated to about 620 C and rapidly quenched with air jets, locking the surfaces in compression. This is toughened (fully tempered) glass - roughly four to five times the strength of annealed, and when it does break it dices into small blunt fragments.
The alternative safety route is lamination: two (or more) glass plies are bonded under heat and pressure with a tough plastic interlayer (PVB or a stiffer ionoplast). Laminated glass that breaks stays in the frame, held by the interlayer - the right choice for overhead glazing, balustrades and any pane where falling glass is the hazard.
Annealed breaks into daggers. Toughened dices into gravel. Laminated cracks but stays put. Pick the failure mode you can live with.
A sealed unit is two panes, a spacer, a desiccant and two seals around a dry gas cavity
A facade rarely uses a single pane. It uses an insulating glass unit (IGU), also called a double-glazed unit (DGU): two glass panes separated by a hollow spacer bar around the perimeter, enclosing a sealed cavity. The point of the cavity is thermal: a layer of still gas is a far better insulator than glass, so a 6-12-6 IGU has a centre-of-glass U-value around 2.7-2.8 W/m2K against roughly 5.7 for single glass.
The spacer sets the cavity width (commonly 12-16 mm) and is hollow, filled with desiccant - a drying agent that absorbs the small amount of moisture trapped at assembly so the cavity never fogs. The seal is dual: a primary seal of polyisobutylene (PIB) is the real moisture and gas barrier - a thin continuous bead between spacer and glass; the secondary seal (silicone or polysulphide) is the structural bond that holds the unit together and gives it durability. The cavity is often filled with argon (or krypton) instead of air, because the heavier gas conducts and convects less, dropping the U-value further.
A modern refinement is the warm-edge spacer - a low-conductivity spacer (structural foam or stainless steel) replacing the old aluminium box, which was a thermal short-circuit around the entire pane edge. The edge is where IGUs both insulate worst and fail first.
Two panes, one cavity, two seals. The primary seal keeps the gas in and the water out; the secondary seal keeps the unit together.
An IGU lives or dies at its perimeter seal
The glass faces of an IGU almost never fail. The edge seal does. If the primary PIB seal is breached - by UV ageing, by the secondary seal pulling under thermal cycling, by poor factory workmanship - moisture creeps into the cavity, the desiccant saturates, and the unit fogs internally with condensation that no amount of cleaning will remove. The argon, if present, slowly leaks out and the U-value drifts back toward single-glass performance.
This is why an IGU is specified to a standard (EN 1279 in most premium Indian specs), which sets the durability and gas-loss tests the unit must pass. It is also why the secondary seal depth matters structurally: too shallow and the unit delaminates; in structural silicone glazing (Lesson 6.2) the secondary seal also has to carry the pane's wind load to the frame, so its width is a real calculation. Detailing matters too - an IGU sitting in standing water in a poorly drained rebate will fail its edge seal years early, which is why glazing pockets are drained and the unit is set on setting blocks, never directly on the frame.
When you draw a glass facade you are choosing a process, not a colour. 'Clear glass' could be annealed, toughened or laminated, single or double, with or without a coating - and each choice changes the price, the safety classification, the thickness and the look (toughened glass can show faint roller-wave distortion; laminated can have a green or warm interlayer tint). Specify the _make-up_ - e.g. '6 mm toughened, low-E #2 / 16 argon / 6.38 laminated' - not just 'double glazing'. And give your IGUs somewhere to drain: a flush, watertight-looking rebate that ponds water is how you fog a thousand units.
Own the full glass make-up and the edge condition. Decide annealed vs heat-strengthened vs toughened per pane (driven by thermal stress and safety - Lesson 6.3), interlayer type and thickness for laminated panes, cavity width and gas fill for the target U-value, and spacer type for the edge thermal bridge and condensation risk. Specify to EN 1279 with a gas-fill and a durability requirement, and call up the secondary-seal depth - especially for SSG, where it is structural. Detail the glazing pocket to drain and set the unit on EPDM setting blocks at quarter points. The unit you draw lasts thirty years; the one you forget to drain lasts five.
An IGU is a sealed instrument - treat it like one. Never nip a corner, never store units flat in standing water, never let the edge seal sit in sunlight for weeks before glazing (UV ages PIB). Check setting blocks are present and at the right positions before the unit is captured - a unit bearing directly on a hard frame edge concentrates stress and can spontaneously crack. If you see a unit fogged from inside on a building, that is a dead edge seal, and the only repair is replacement. The whole reason the unit is built in a factory and not on site is to keep that seal perfect.
EN 1279 (Parts 1-6)
Insulating glass units
The European IGU standard - durability, moisture penetration, gas-loss and edge-seal tests. Widely written into premium Indian specs, but it is a performance/durability standard, not a thermal target; you still set U-value separately.
EN 12150 / IS 2553 (Part 1)
Toughened (tempered) glass
Define the heat-treatment, strength and fragmentation requirements for safety toughened glass. IS 2553 Part 1 is the Indian flat-glass safety standard; it sets fragmentation counts but does not, by itself, require heat-soak testing.
EN ISO 12543 / IS 2553 (Part 2)
Laminated glass
Specify the build-up and performance of laminated glass and its interlayer. They classify safety and impact performance but leave interlayer _stiffness_ (PVB vs ionoplast) - which governs structural laminated design - to the engineer.
ECBC 2017 / Eco-Niwas Samhita 2018 (India)
Glazing thermal limits
Set the U-value and SHGC the IGU must achieve in the envelope. They give the target the make-up must hit - not how to build the unit to hit it durably.
“Double glazing is double glazing - any sealed unit gives you the energy saving the brochure quotes.”
The brochure U-value is the _centre of glass_ on the day it was made, with a perfect argon fill and a warm-edge spacer. A real unit with an aluminium box spacer has a much colder edge (and condensation risk), and a unit whose edge seal leaks loses its argon over years and drifts back toward single-glass performance. The performance you get is the performance of the edge seal and the spacer over time - not the centre-pane number on the datasheet.
Worked example - specify and check an IGU make-up
You are specifying the vision glass for a commercial office in Hyderabad (ECBC composite climate). The energy consultant needs centre-of-glass U <= 3.0 W/m2K and SHGC <= 0.27. Build the IGU make-up and check the thermal cavity logic.
The ECBC/ENS glazing targets for the project, a glass-coating datasheet, and a glazing-system section showing the pocket and bite dimensions.
GIVEN
Target (ECBC) : U_cog <= 3.0 W/m2K , SHGC <= 0.27
Outer pane : 6 mm toughened, soft-coat low-E on surface #2
Cavity : ? mm , gas = air or argon
Inner pane : 6.38 mm laminated (6 + 0.38 PVB)
Approx U_cog (W/m2K), 6/cav/6 unit, low-E e~0.04:
air 12 mm ~ 1.8 argon 12 mm ~ 1.5
air 16 mm ~ 1.7 argon 16 mm ~ 1.4
(single glass U_cog ~ 5.7 for reference)- 1Start from the target. ECBC wants U_cog <= 3.0 and SHGC <= 0.27. A plain air-filled double unit (no coating) sits near 2.7 - it would pass U but never reach SHGC 0.27. The SHGC target forces a low-E / solar-control coating, which we place on surface #2 (inside face of the outer pane) - the standard position for a cooling climate.
- 2Pick the panes for safety and strength. Outer = 6 mm toughened (resists thermal stress under the coating and wind - Lesson 6.3). Inner = 6.38 laminated, so if the inner pane breaks it stays in place over the occupied floor. Note both choices are made before the thermal check.
- 3Choose the cavity. From the table, a 16 mm argon cavity with the low-E coating gives U_cog ~ 1.4 W/m2K. That is comfortably under the 3.0 target - the coating and gas fill do the heavy lifting, not the cavity alone.
- 4Sanity-check the make-up notation: 6T(low-E#2) / 16Ar / 6.38L. Total nominal thickness 6 + 16 + 6.38 = 28.38 mm. Confirm this fits the framing system's glazing pocket and the structural silicone bite (Lesson 6.2) before freezing it.
- 5Add the durability requirement. Specify the unit to EN 1279 with an argon-retention requirement and a warm-edge spacer, and require heat-soak testing on the toughened pane (Lesson 6.3). Without these, the 1.4 number is true only on day one.
- 6Read the result back. U_cog ~ 1.4 (target 3.0 - pass with margin), SHGC set by the chosen coating to <= 0.27 (confirm from the coating datasheet, Lesson 6.4). The make-up is 6T low-E#2 / 16 argon / 6.38 laminated, EN 1279, warm-edge, heat-soaked.
You’ll walk away with
A complete, defensible IGU specification - 6T(low-E#2)/16Ar/6.38L, EN 1279, warm-edge, heat-soaked - that meets the ECBC U-value with margin and is ready to be checked for SHGC and structural bite. The exact line a facade engineer hands to the energy consultant and the glass processor.
Two quick checks to make the IGU real.
- 01Find a fogged double-glazed window near you (offices and older malls have them). The mist is _inside_ a sealed unit you cannot clean - trace it and you have found a dead edge seal, the most common IGU failure.
- 02Read the spacer of any double-glazed unit edge-on: a silver metal box is an aluminium spacer (cold edge); a grey or black low-conductivity bar is a warm-edge spacer. The colour tells you the edge thermal bridge.
Facade glass is a process, not a material: float, cut, edgework, heat-treat or laminate, then seal into an IGU of two panes, a desiccant-filled spacer, a primary moisture seal and a structural secondary seal around a dry gas cavity. The faces insulate; the edge seal decides the lifespan. Specify the whole make-up - panes, coating surface, cavity, gas, spacer and durability standard - not just 'double glazing'.
Float glass is annealed (weak, dangerous shards) until processed: cut, edged, then toughened (strong, dices) or laminated (stays in frame). An IGU = two panes + hollow desiccant spacer + primary PIB seal (moisture/gas barrier) + secondary structural seal, around an argon cavity. Warm-edge spacers cut the edge thermal bridge. The edge seal, not the glass, decides how long the unit lasts.
What is the difference between an IGU and a DGU?
They are the same thing. IGU (insulating glass unit) is the international term; DGU (double-glazed unit) is the common Indian and UK usage for a two-pane sealed unit. A triple-glazed unit (TGU) is three panes and two cavities. All are sealed cassettes of glass panes separated by spacers around a dry, often gas-filled, cavity.
Why do double-glazed units fog up from the inside?
Internal fogging means the edge seal has failed. Once the primary polyisobutylene seal is breached, humid air enters the sealed cavity, the desiccant in the spacer saturates, and moisture condenses on the inner glass faces where it cannot be cleaned. It is a permanent failure - the only fix is to replace the unit - and it is usually caused by edge-seal ageing, poor factory sealing, or a glazing pocket that let the unit sit in standing water.
Does argon gas fill really make a difference, and does it leak out?
Yes to both. Argon is denser than air and conducts and convects heat less, so it lowers the centre-of-glass U-value of a low-E unit noticeably (for example from about 1.7 to 1.4 W/m2K in a 16 mm cavity). It does leak slowly - a few percent a year through a good seal - which is why EN 1279 sets a gas-loss limit and why a leaky edge seal that loses its argon is a real, gradual loss of performance.
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.
- 02Bedon, C. et al. Performance of structural glass facades under extreme loads - design methods, existing research, current issues and trends. Construction and Building Materials, 163. — Construction and Building Materials (Elsevier), 2018.
- 03Eco-Niwas Samhita 2018 (ECBC for Residential Buildings), Part I: Building Envelope. — Bureau of Energy Efficiency, Govt. of India, 2018.
_An IGU is just a pane until you have to hold it on the building - and on modern facades that is done not with metal but with a bead of silicone or a handful of bolts. That is structural glazing, next._