| Composition | SiO₂ 72% + Na₂O + CaO + MgO + Al₂O₃ |
|---|---|
| Density | 2,500 kg/m³ |
| Elastic modulus (E) | 70,000 N/mm² |
| Tensile strength (float) | ~45 N/mm² (characteristic) |
| Tensile strength (tempered) | ~120 N/mm² |
| Thermal transmittance (single glazing 6mm) | U = 5.7 W/m²K |
| Thermal transmittance (Low-E DGU) | U = 1.0–1.4 W/m²K |
| Thermal transmittance (triple TGU) | U = 0.5–0.7 W/m²K |
| Reference EU standard | EN 572, EN 1279, EN 12150 · EN 13022 |
In 1989, I.M. Pei unveils the Louvre Pyramid: 603 panes of glass, 675 m² of surface, 21 metres tall. To get it as transparent as Pei wants — not the blue-green tint typical of standard float glass — a purpose-built, extra-low-iron glass (Diamant, Saint-Gobain) is developed, almost colourless. The client is President Mitterrand. The glass has to be the right one, not the available one.
The Louvre Pyramid is the perfect example of how glass in architecture is a system of decisions, not a passive material. Every property — colour, light transmission, solar shading coefficient, post-breakage safety, noise — depends on choices the designer makes (or doesn't make) in the product specification. This guide is here to help you understand what there is to choose.
Composition and basic physics
Glass is an amorphous solid: silicon and oxygen atoms form an irregular lattice, without the crystalline structure of minerals. It's optically transparent because visible-light frequencies don't excite electronic transitions in the material. It's brittle because it has no plasticity mechanisms: cracks propagate without any preliminary plastic deformation to stop them. This brittleness is the central design parameter in structural glass.
Float glass production (the Pilkington process, 1959) consists of pouring molten glass onto a bath of molten tin: the glass floats, spreads out under gravity and solidifies into perfectly flat sheets of uniform thickness (2–19 mm in standard production). The process turned architectural glass into an industrial commodity.
The variants
Float glass
Basic float glass is the starting point for every product. Available in thicknesses from 2 to 19 mm (standard: 4, 5, 6, 8, 10, 12 mm), with maximum pane sizes up to 3.21 × 6.00 m. The standard colour is blue-green, from iron oxides present in the mix (FeO, Fe₂O₃). Extra-clear glass (Optiwhite, Diamant, Starphire) reduces the iron content below 0.01%, practically eliminating the greenish tint — essential when colour perception is critical (museums, historic building facades). Float glass isn't a safety product: it breaks into sharp shards. It's not acceptable for uses at risk of fall or impact without further treatment.
Tempered glass
Tempering (EN 12150) is a heat treatment: the glass is heated to 620–640°C and then rapidly cooled with air jets. Differential cooling creates compressive stresses at the surface and tensile stresses at the core. The result: flexural strength ~120 N/mm² (roughly three times float glass) and, on breakage, fragmentation into small, rounded granules instead of sharp shards. Tempered glass is a "safety glass" (EN 12600, class C). Limit: it can't be cut or drilled after tempering — all processing has to happen beforehand. It's sensitive to nickel sulphide (NiS) inclusions that can cause spontaneous breakage: the Heat Soak treatment (EN 14179-1) in an oven at 290°C for 8 hours reduces this risk by eliminating defective panes before installation.
PVB and SGP laminated glass
Laminated glass (EN ISO 12543) is made by bonding two or more panes of glass with a polymeric interlayer: PVB (polyvinyl butyral) or SGP (Kuraray's SentryGlas Plus). If it breaks, the glass stays bonded to the interlayer — no fragments fall. It's the only product acceptable for overhead applications (glazed roofs, skylights, pedestrian canopies) and for parapets without a handrail. The difference between PVB and SGP is stiffness: PVB is a soft elastomer, SGP is 5 times stiffer. In structural applications (vertical fins, glass beams), SGP transfers loads far more efficiently. Laminated glass can be float-laminated (low strength), tempered-laminated (high strength plus post-breakage safety), or heat-strengthened-laminated (HST, intermediate strength, breaking into large fragments — better for laminated glass because large fragments hold onto the interlayer better).
Double and triple glazing (DGU / TGU)
The double glazed unit (DGU) is made of two panes separated by a sealed 12–20 mm cavity, filled with air or inert gases (argon, krypton). Argon (λ = 0.017 W/m·K) reduces convection in the cavity compared with air (λ = 0.024 W/m·K), lowering the transmittance by 10–15%. The triple glazed unit (TGU) adds a third pane and two cavities: U = 0.5–0.7 W/m²K, but weight and thickness increase (30–50 mm total against 20–28 mm for a DGU). In continental climates with cold winters (northern Europe, northern Italy in climate zone E), triple glazing is the right choice for low-transmittance window components.
Low-E, low-emissivity coating
The Low-E (low-emissivity) coating is a thin film of metal oxides (typically silver, Ag, with layers of TiO₂ and SnO₂) deposited on the DGU's inner surface via magnetron sputtering (hard coat) or pyrolysis (soft coat). It reduces the surface emissivity from ~0.89 to ~0.03, limiting heat re-radiation to the outside in winter. A 4-16-4 DGU with Low-E and argon has U ≈ 1.1 W/m²K against U ≈ 1.8 W/m²K for the same glass without Low-E. The solar factor g (the amount of solar energy that passes through) can be tuned: selective, high-transmission Low-E (g ≈ 0.60) for cold climates where solar gain is needed; Low-E with selective shading (g ≈ 0.30–0.45) for facades with intense solar radiation.
Structural glass: fins and point-fixed
Structural glass uses the material not just as a transparent enclosure but as a load-bearing element. In fin systems, vertical laminated-glass panes (typically 3×10 mm SGP or 3×12 mm) act as vertical bracing for the facade: they transmit horizontal wind forces to the perimeter frames. In point-fixed systems, panes are anchored to the structure with point fixings (spider bolts) through holes at the four corners: the glass works in biaxial bending and the forces concentrate in the metal fittings. The Louvre Pyramid uses a cable and point-fixing system. The Grand Palais's verrière in Paris uses metal trusses with strip glazing. Structural glass design follows EN 13022 and the ECCS guidelines.
ETFE — pneumatic cushions
L'ETFE (etilene-tetrafluoroetilene) non è vetro: è una pellicola polimerica fluorurata. Si usa in architettura come sistema di copertura a cuscini pneumatici (EN 13501-5): due o più strati di ETFE saldati perimetralmente e gonfiati con aria a pressione lievemente superiore all'atmosferico (250–600 Pa). Il risultato: coperture trasparenti di grande luce con peso 1–3 kg/m² contro 25–35 kg/m² del vetro, trasmissione luminosa fino al 95%, durata dichiarata >50 anni (ETFE non si degrada agli UV). L'Eden Project in Cornovaglia (2001, Grimshaw), l'Allianz Arena di Monaco (2005, Herzog & de Meuron) e il Beijing National Aquatics Center (2008) sono i casi più noti. Limite: non è trasparente quanto il vetro otticamente (diffonde la luce), non è un prodotto di sicurezza equivalente per applicazioni a bassa altezza.
Screen-printed glass
Ceramic screen printing (fritting) applies a layer of ceramic enamel to the glass surface before tempering: the particles fuse to the glass and become permanent. The result is partial coverage of the pane that reduces light transmission and the solar factor, and creates decorative effects. Glazed facades with dot-pattern printing are common on office buildings: a pattern of dots that grow larger toward the edge of the pane shields the edge structure and reduces luminance contrast. Screen printing doesn't replace movable shading devices for summer solar control.
Behaviour over time
Float glass is chemically inert under normal conditions. It doesn't corrode, doesn't degrade under UV. The main problem in glazed facades is the seal of the DGU's perimeter sealant: silicone sealant lasts 20–25 years under intense thermal cycling. Condensation in the cavity (fogging) indicates a seal failure and requires replacing the unit. The second problem is differential movement in point-fixed systems: the pane has to be able to expand thermally (glass's expansion coefficient: 9 × 10⁻⁶ /°C) without the fixing fittings transmitting excessive localised stresses.
| Dimension | Score | Assessment |
|---|---|---|
| 🌿 Environmental | 5/10 | Energy-intensive production: 1 kg of float glass requires ~15–20 MJ and generates ~0.9 kg CO₂. Glass is technically 100% recyclable, but separate collection from demolition is low (<30% in Italy). A DGU isn't recyclable as a unit: it has to be disassembled, losing the sealant. Large glazed surfaces increase summer cooling demand if not properly designed. |
| ⚖️ Ethics | 7/10 | Concentrated European production (Saint-Gobain, AGC, Guardian) with high standards. No critical extractive supply-chain element (silica sand is abundant). Silicosis is a historical risk in artisanal glassworking, not relevant to modern industry. Concerns over bird mortality at glazed facades: anti-collision screen printing (5 cm stripes) is an effective response. |
| 🏘️ Social | 7/10 | The transparency of glass has social value: it allows visibility, natural surveillance, a sense of safety in public spaces. Abundant natural light indoors has documented benefits for wellbeing and productivity. Against: fully glazed facades create urban heat-island effects and visual discomfort. |
| 💶 Economic | 6/10 | Cost of a standard DGU (4-16-4 Low-E): €60–120/m². Point-fixed systems and structural roofing: €500–1,500/m². ETFE cushions: €150–400/m² (less than structural glass, more than a standard DGU). Maintenance: a DGU needs replacing after 20–25 years due to seal failure. Double-skin facades raise initial costs but reduce operating heating and cooling costs. |
| Average score | 6.25/10 |
Uses through history
The Middle Ages and Gothic cathedrals (1100–1500): The leaded-glass windows of Gothic cathedrals aren't ornament: they're the light-control system in buildings that can't open large openings without compromising the structure. Medieval glass is blown (cut and flattened cylinders) or cast, irregular, with non-uniform thickness — which is what creates its characteristic iridescent effect. The Sainte-Chapelle in Paris (1248) has walls almost entirely of glass: the structural design is conceived to support the glazing, not the other way round.
The 19th century — the Crystal Palace and iron-and-glass (1840–1900): Joseph Paxton's Crystal Palace (London, 1851) is the manifesto of iron-and-glass architecture: 84,000 m² of surface with 300,000 panes of blown glass, prefabricated and assembled in under 9 months. It's the first time glass is used as a large-scale envelope system, rather than as a window in a solid wall. The prefabricated cast-iron structure is designed for exactly one purpose — to hold up the glass. It's the direct ancestor of the modern curtain wall.
Modernism and the curtain wall (1920–today): Mies van der Rohe experiments with glass towers as early as 1921, with his unbuilt projects for Friedrichstrasse in Berlin. Realisation arrives with the Seagram Building (1958): Mies's bronze-and-glass curtain wall becomes the reference model for fifty years of skyscrapers. In the 1990s, point-fixed structural glass (Dewhurst Macfarlane, Peter Rice) opens the era of facades without visible frames. Today, smart glass (electrochromic, PDLC) changes its optical transmission on electrical command — the roof of Apple Park in California has 740 panels of curved glass, each 3 × 14 m.
Reference numbers for design
| Float 6 mm — LT / U | 89% / 5.7 W/m²K |
|---|---|
| DGU 4-16-4 air — U | 2.7 W/m²K |
| DGU 4-16-4 argon Low-E — U | 1.1 W/m²K |
| TGU Low-E argon — U | 0.5–0.7 W/m²K |
| Solar factor g, shading DGU | 0.30–0.45 |
| Float flexural strength | ~45 N/mm² |
| Tempered flexural strength | ~120 N/mm² |
| ETFE light transmission | up to 95% |
| CO₂, float glass production | ~0.9 kg CO₂/kg |
Research and the contemporary frontier
Vacuum glazing (VIG, Vacuum Insulating Glass) is the most interesting frontier for energy efficiency. Vacuum glass has a 0.1 mm cavity held under high vacuum: U = 0.3–0.5 W/m²K with just 8 mm of total thickness, against 30–50 mm for triple glazing. NSG (Nippon Sheet Glass) has had its Spacia product in production since 2003; the challenge is scale and cost. Photovoltaic glass (BIPV) integrates semi-transparent solar cells into the pane: light transmission 10–40%, power generation 60–130 Wp/m². The Cité du Vin in Bordeaux (XTU Architects, 2016) uses BIPV panels on its facade. 3D-printed glass with complex geometries (MIT Media Lab, 2015 and 2022) allows precision optical objects — not yet structural at large scale, but indicative of where things are heading.