BS EN 1279 (Parts 1–6)Approved Document LBS EN 673 (U-value calculation)BS EN 16612 (Isochoric pressure)CIBSE Guide AReviewed Technical Specification
Insulating Glass Units (Double & Triple IGUs)
Hermetically sealed multi-pane glazing assemblies engineered for thermal transmittance control, solar shading, and condensation prevention.
Engineering Definition: An assembly consisting of at least two panes of glass separated by one or more hermetically sealed cavities containing argon or krypton gas and dehydrated with molecular sieve desiccant (BS EN 1279 Parts 1–6).
Precision Specification
Composite Architectural Glass Assembly
Multi-Ply Safety Specification
Accessible Static View
Schematic exploded horizontal cross-section showing sequential structural placement.
01
Outer Structural Ply(Thermally Toughened Glass)
10.0mmExternal impact and wind barrier
02
Engineered Interlayer Core(Structural SentryGlas / Acoustic PVB)
1.52mmViscoelastic damping & residual load support
03
Inner Structural Ply(Thermally Toughened Glass)
10.0mmInterior barrier and post-breakage integrity
BS EN 12150 / BS EN 14449 Architectural ReferenceIllustrative diagram — project structural verification required
Physical & Mechanical Performance Characteristics
Safety Class:Determined by individual glass plies (inner/outer toughened or laminated to BS EN 12600)
Bending Strength:Shares wind load between plies via cavity pressure transfer; calculated per EN 16612 isochoric pressure formulas
Thermal Shock Resistance:Low-E soft coats absorb solar energy; thermal stress calculation required to determine if toughening is mandatory (delta T > 40K)
Fracture Behaviour:Depends on selected plies; safety glass required on inner or outer depending on impact zone and fall risk
Typical U-Value (Ug):1.0 to 1.2 W/m²K (Double Glazed 28mm Argon + Soft Coat Low-E); 0.5 to 0.7 W/m²K (Triple Glazed 44mm Krypton/Argon)
Acoustic Reduction (Rw):Rw 31 (-2; -5) dB (standard 4-16-4) up to Rw 46 (-2; -6) dB (dissimilar thicknesses + acoustic laminated pane)
Max Sheet Dimension:Up to 3210mm × 6000mm (stepped or structural silicone glazed)
Density & Mass:2.50 kg/m² per total combined mm glass thickness + ~1.5 kg/m perimeter spacer and secondary seal
Key Architectural Advantages
- •Regulatory compliance: meets and exceeds Approved Document L commercial non-domestic U-value targets (Uw <= 1.6 W/m²K)
- •Substantial reduction in commercial HVAC heating and cooling operating expenditure
- •Elimination of internal cold downdraughts and perimeter surface condensation
Critical Engineering Constraints
- •Seals will fail prematurely if drainage paths in commercial window rebate are blocked (standing water degrades secondary polyurethane/polysulphide seals)
- •Altitude barometric changes between factory and installation site can cause pillowing or dishing unless capillary breather tubes are fitted and crimped on site
- •Stepped units for structural silicone facades require UV-resistant secondary silicone seal (polyurethane cannot be exposed to direct sunlight)
Fabrication & Edge Processing Tolerances
Cut After Toughening:PROHIBITED (Catastrophic Fracture)
Heat Soak Testing (HST):Recommended to BS EN 14179
Dimensional Tolerance:Panes <= 2000mm: ±2.0mm; panes > 2000mm: ±3.0mm; overall thickness tolerance ±1.0mm (BS EN 1279-1)
Thickness Tolerance:Cavity width tolerance ±1.0mm
Flatness & Roller Wave:Overall bow max 3.0mm/m (measured on individual tempered plies before assembly)
Hole & Cutout Geometry Rules:Gas-filled IGUs cannot have through-holes in the sealed cavity; ventilator cutouts must use insulated edge cassettes
Units cannot be modified after perimeter sealant cure. Any breather tubes must be pinched, sealed, and soldered at final elevation.
Structural Design Basis (Eurocodes / BS EN 16612)CEN / BSI Harmonised
Characteristic Strength:Constituent plies: fg,k = 45 N/mm² (annealed), 70 N/mm² (heat-strengthened), 120 N/mm² (toughened)
Elastic Modulus (E):70,000 MPa (glass plies)
Poisson's Ratio (ν):0.22
Linear Expansion (α):9.0 × 10⁻⁶ K⁻¹
Design Bending Stress Formula (BS EN 16612):Cavity isochore factors per BS EN 16612: Wind pressure divided between panes based on relative flexural stiffness and cavity air stiffness
Architectural Specifier Checklist
Verify these technical criteria prior to contract documentation:
1Specify spacer type: warm-edge composite spacer bar (psi value ~0.032 W/mK) over standard aluminium spacer (psi ~0.075 W/mK).
2Identify coating surface location: Surface #2 (inner face of outer pane) for solar control; Surface #3 (outer face of inner pane) for passive heat retention.
3Check if structural glazing (SSG) is utilised: requires silicone secondary seal compliant with ETAG 002.
4For projects located > 800m above sea level or shipped across mountain passes, mandate factory breather tubes.
Frequent Commercial Applications
Network Procurement Routing
BuyGlass.co
Commercial cut-to-size glass manufacturing, direct supply, and transactional quoting platform.
Recommended Action: Calculate whole-window U-value and obtain commercial sealed unit supply quotations.
Proceed to BuyGlass.coReviewed by: Facade Thermal Specialist, BuyGlass Editorial Panel
Last Standard Verification: 2026-10-30