Double-Glazed Units (DGU / IGU) — Complete Guide
Double-Glazed Unit (DGU) — also called Insulated Glass Unit (IGU) or sealed unit — is a glazing assembly of two (or three for TGU) glass panes hermetically sealed with a spacer at the edge, creating an insulating air or gas-filled cavity between them. The cavity dramatically reduces thermal & acoustic transmission compared to single glazing.
A typical DGU configuration written as "6 + 12A + 6" means: 6 mm outer glass + 12 mm air cavity + 6 mm inner glass = 24 mm total. Substituting "A" with "Ar" indicates argon-filled cavity, and "Kr" indicates krypton.
DGU performance is governed by IS 7392 (Part 1) — Double Glazed Hermetically Sealed Units and EN 1279 (1 to 6) — Insulating Glass Units. Major Indian processors: Saint-Gobain Climalit / Climaplus, AIS Stronglas DGU, Sejal Double Glazed, Gold Plus IGU, Glass Wall Systems, FG Glass, Glasstech India, Tata Pravesh IGU, Excel Glasstech.
DGU / IGU Construction
A standard DGU has 5 critical components:
- Outer Glass Pane: 4–12 mm, often toughened + solar control / low-E coated (coating typically on Surface #2)
- Spacer: Aluminium / stainless steel / warm-edge (TGI / Swisspacer) hollow profile around the perimeter; holds the two panes apart and contains desiccant
- Cavity: 6–20 mm filled with dry air, argon, krypton or SF6 gas
- Inner Glass Pane: 4–12 mm; can be laminated for acoustic / safety
- Edge Seal (Double Seal):
- Primary Seal: Polyisobutylene (PIB) — very low gas/moisture transmission
- Secondary Seal: Silicone / polysulphide / polyurethane — structural strength
- Desiccant: Molecular sieve (zeolite) inside spacer absorbs residual moisture — prevents internal fogging
Why two seals? PIB has excellent gas / moisture barrier but no structural strength. Silicone / polysulphide has strength but higher moisture transmission. Using both creates the most reliable hermetic seal — specified by IS 7392 & EN 1279.
How DGUs Reduce Heat & Sound
Thermal Insulation
Heat transfers through glass by three modes:
- Conduction — through glass + spacer (small fraction in well-designed DGU)
- Convection — air currents in the cavity (suppressed when cavity is 12–16 mm)
- Radiation — long-wave IR between glass surfaces (largest fraction — reduced by Low-E coating)
DGU reduces overall U-value from ~5.8 (single 6 mm) to:
- ~2.7 W/m²K (plain glass + 12 mm air)
- ~1.4–1.7 W/m²K (with Low-E coating)
- ~1.0–1.3 W/m²K (Low-E + argon fill + warm-edge spacer)
- ~0.5–0.7 W/m²K (triple glazed with two Low-E + krypton)
Acoustic Insulation
DGUs reduce sound through:
- Mass: 2 glass plies vs 1
- Decoupling: cavity isolates panes
- Asymmetry: different glass thicknesses break resonance
- Laminated acoustic interlayer: PVB Acoustic dampens vibrations
Typical Rw values:
- Single 6 mm: 30 dB Rw
- 6 + 12 + 6 mm DGU: 34 dB Rw
- 10 + 16 + 6 mm DGU (asymmetric): 38–40 dB Rw
- 8.76 (Acoustic Laminated) + 16 + 6 mm DGU: 45 dB Rw
- 10.76 (Acoustic Lam) + 20 + 8.76 (Acoustic Lam): 50–52 dB Rw (premium)
Common DGU Configurations
| Config | Total Thickness | U-Value | SHGC | Rw (dB) | Use |
|---|
| 4 + 12A + 4 (clear) | 20 mm | 2.7 | 0.76 | 29 | Residential entry-level |
| 5 + 12A + 5 (clear) | 22 mm | 2.7 | 0.75 | 31 | Residential standard |
| 6 + 12A + 6 (clear) | 24 mm | 2.7 | 0.74 | 33 | General DGU |
| 6 + 12A + 6 (Low-E) | 24 mm | 1.6 | 0.40 | 33 | Premium offices / homes |
| 6 + 12Ar + 6 (Low-E) | 24 mm | 1.4 | 0.39 | 33 | High-performance |
| 6 + 16Ar + 6 (Low-E) | 28 mm | 1.2 | 0.39 | 34 | Best-in-class DGU |
| 10 + 16Ar + 6 (asymmetric Low-E) | 32 mm | 1.3 | 0.40 | 40 | Acoustic + thermal |
| 8.76 Lam + 16Ar + 6 Low-E | 30.76 mm | 1.3 | 0.35 | 44 | Safety + acoustic + thermal |
| 10.76 Lam Acoustic + 20Ar + 8.76 Lam Acoustic Low-E | 39.52 mm | 1.1 | 0.30 | 50 | Premium hotel / airport |
| Triple: 4 + 12Ar + 4 + 12Ar + 4 (2x Low-E) | 36 mm | 0.6 | 0.40 | 36 | Cold climate / passive house |
| Triple: 6 + 14Kr + 4 + 14Kr + 6 (2x Low-E) | 44 mm | 0.5 | 0.38 | 40 | Passive house (premium) |
Spacer Types
| Spacer | Material | Thermal Conductivity | Notes |
|---|
| Aluminium (standard) | Mill-finish aluminium | ~160 W/m.K (highly conductive) | Cheap; creates "cold edge" & thermal bridge |
| Stainless Steel | SS 304 | ~15 W/m.K | Less thermal bridging than aluminium |
| Warm-Edge (TGI) | SS foil + plastic | ~0.9 W/m.K | Reduces edge condensation, +ΔU 0.1–0.2 W/m²K |
| Warm-Edge (Swisspacer) | Fibre-reinforced plastic + SS foil | ~0.14 W/m.K | Best warm-edge; premium |
| Warm-Edge (Super Spacer) | EPDM foam + acrylic adhesive | ~0.20 W/m.K | Flexible foam spacer |
| Thermix | Plastic + SS foil | ~0.17 W/m.K | Used in passive house |
For ECBC / GRIHA / IGBC compliant projects, warm-edge spacer is strongly recommended — reduces condensation & improves overall window U-value.
Cavity Gas Fill
| Gas | Thermal Conductivity (W/m.K) | Cost | Notes |
|---|
| Dry Air | 0.025 | Lowest | Default; basic DGU |
| Argon (Ar) | 0.017 | Low | 30% better than air; standard for low-E |
| Krypton (Kr) | 0.009 | High | Best for thin cavities (8–10 mm) |
| Xenon (Xe) | 0.005 | Very high | Specialised aerospace / lab |
| SF6 (Sulphur Hexafluoride) | 0.013 | High | Acoustic (high density); banned in EU (GHG) |
Argon is the practical choice: ~30% better insulation than air, only ~5% extra cost. Krypton is for ultra-thin units (window mass / weight constraints).
Manufacturing Process
- Glass Preparation: Cut, edge-polish, drill, wash and dry the two glass panes
- Coating Verification: If low-E coated, verify coating orientation; edge-delete coating from perimeter (8–10 mm) if needed for sealant compatibility
- Spacer Bending: Aluminium / steel spacer is bent to the required shape; ends joined with corner key / butt joint sealed with PIB
- Desiccant Loading: Molecular sieve (3A) is filled into the spacer hollow before final assembly
- PIB Application (Primary Seal): Continuous PIB bead applied to spacer sides
- Assembly: Spacer + first glass pane joined under pressure; if gas-filled, place in gas filling chamber
- Gas Filling (optional): Argon / krypton purged into cavity (90% concentration target) via small hole or gas press
- Second Pane: Second glass pressed into PIB; unit slightly compressed in press
- Secondary Seal: Silicone / polysulphide / polyurethane is extruded into the perimeter channel; cured for 24 hours
- Inspection: Visual + dew-point + edge seal width measurement; argon concentration tested by sample
- Labelling: Each DGU labelled with date, manufacturer, performance class
- Packing & Despatch: Vertical stacking with corner protection
Physical Properties (Standard 6+12Ar+6 Low-E)
| Property | Value |
|---|
| Total Thickness | 24 mm |
| Weight | ~30 kg/m² |
| U-Value (centre of glass) | 1.4–1.6 W/m²K |
| U-Value (whole window, with warm edge & UPVC frame) | 1.6–1.9 W/m²K |
| SHGC | 0.35–0.45 |
| Light Transmission (VLT) | 45–75% (depending on low-E) |
| Sound Reduction (Rw) | 32–36 dB |
| Edge Seal Life | 20–30 years (premium); 10–15 yr (basic) |
| Argon Retention Rate | < 1% per year |
| Cavity Pressure Variation | ±5% with altitude / temp |
| UV Cut | 40–99% (depending on configuration) |
| Maximum Size | 3000 x 5000 mm (manufacturer-limited) |
| Operating Temp | −40 to +80 °C |
Top Indian Brands
| Brand | DGU Products |
|---|
| Saint-Gobain Glass India | SGG CLIMALIT, CLIMAPLUS, CLIMATOP (triple), SGG CONFORT (acoustic) |
| Asahi India Glass (AIS) | AIS Stronglas DGU, AIS Ecosense DGU, AIS Acousticglas DGU |
| Gold Plus Glass Industry | Gold Plus IGU, GP Plus DGU |
| Sejal Glass | Sejal DGU, Acoustic DGU, Triple Glazed |
| Modiguard / Sisecam | Modi DGU |
| Glass Wall Systems | GWS DGU (large facade) |
| FG Glass / FuYao | FG DGU |
| Glasstech India | Glasstech IGU |
| Tata Pravesh | Pravesh DGU Windows |
| Pilkington (NSG) | Pilkington Insulight (Sun / Phon / Thermo) |
| Guardian Industries | Guardian ClimaGuard / SunGuard DGU |
| Excel Glasstech | Excel DGU & structural glazing |
DGU vs Single Glazing vs Triple Glazing
| Parameter | Single 6 mm Clear | DGU 6+12+6 Low-E Ar | Triple 4+12+4+12+4 2xLow-E Kr |
|---|
| U-Value (W/m²K) | 5.8 | 1.4 | 0.5 |
| SHGC | 0.82 | 0.40 | 0.40 |
| VLT | 89% | 65% | 72% |
| Rw (dB) | 30 | 33–36 | 40–42 |
| Weight (kg/m²) | 15 | 30 | 30 |
| Thickness | 6 mm | 24 mm | 36–44 mm |
| AC Saving | baseline | 30–45% | 50–60% |
| Condensation Risk | High (single) | Low | Very low |
| Cost (Rs/sqft) | 85–120 | 650–1200 | 1500–3000 |
| Suitable for India | Old construction | Premium homes / offices | Hill stations / passive houses |
Where to Use DGU / IGU
- Premium residential homes & villas
- High-rise apartments (above 5th floor — noise)
- Office buildings & corporate towers
- Hotels & resorts (acoustic + thermal)
- Hospitals (acoustic + thermal + safety)
- Airport terminals (45+ dB Rw acoustic DGU)
- Auditoria & recording studios
- Conference centres
- Schools & universities (along busy roads)
- Heritage building retrofits (preserves facade with discrete DGU)
- Cold climate regions (Himalaya, Kashmir, Ladakh)
- Coastal high-wind locations (cyclone-rated DGU)
- Banks, jewellery, ATMs (laminated security DGU)
- Industrial control rooms (acoustic isolation)
- Data centres & server rooms (thermal control)
- Cold storage / freezer doors (specialised DGU)
- Greenhouses (Low-E for heat retention)
- Solar PV BIPV modules (custom DGU)
Advantages
- Excellent thermal insulation — 50–80% lower heat loss / gain vs single glazing
- Significant AC energy saving — 30–55% reduction in cooling cost
- Acoustic comfort — 5–20 dB reduction over single glazing
- Eliminates condensation on inside glass surface
- Improved comfort — no cold spots near windows in winter, no hot spots in summer
- UV protection (with Low-E coating)
- Compatible with all framing systems (UPVC, aluminium, timber)
- Mandatory for ECBC, GRIHA, IGBC LEED compliance
- Configurable: thermal-only, acoustic-only, safety, security, all-in-one
- Long lifespan (20–30 years for premium DGU)
- Wide range of sizes & configurations
- Compatible with motorised blinds inside cavity
- Solar Low-E configurations cut heat without sacrificing light
- Reduces HVAC sizing — capex saving on AC equipment
- Carbon footprint reduction (life-cycle energy) — LEED points
- Available in toughened / laminated / coated / patterned combinations
Disadvantages
- 5–10x more expensive than single glazing (initial cost)
- Heavier — requires stronger frame & hardware
- Thicker — needs deeper window pocket (28–45 mm)
- If seal fails, internal fogging / moisture / argon loss (irreparable)
- Cannot be field-modified (must be factory-made to exact size)
- Long lead time (15–30 days)
- Altitude / cavity pressure issues for delivery between cities (de-aerate vent caps used)
- Sealant degradation over time (esp. with UV exposure if non-silicone)
- Argon escapes ~1% / year — performance slowly degrades over 20 yr
- Cavity desiccant saturates over time if seal poor
- Spontaneous breakage risk (if toughened plies w/o HST)
- Edge condensation possible with aluminium spacer in humid / cold
- Triple glazing very heavy — significant hardware upgrade needed
- Repair = full unit replacement
- Local seismic / wind stress can stress seal over time
Common Defects in DGU
- Internal Fogging: Seal failed; moisture in cavity reaches dew point
- Edge Seal Failure: PIB / silicone breakdown
- Desiccant Saturation: Dust / debris signature inside cavity
- Argon Loss: Performance degradation over time
- Internal Spider / Crack Pattern: Spontaneous breakage (NiS in toughened)
- Spacer Bowing: Pressure differential (altitude / temperature)
- Surface Coating Defects: Pinholes / scratches in low-E coating
- Insect / Dust Inside: Manufacturing contamination
- Pillowing: Centre panes bow inward (cold) or outward (hot)
- Sealant Discoloration: UV degradation of polysulphide seal
Tests on DGU / IGU
- Dew Point Test — EN 1279-2 (must be < −40 °C)
- Edge Seal Width Test (min 8–10 mm)
- Climate & UV Cycle Test — EN 1279-2 (56 days)
- High Humidity Test — 47 days at 55°C/95% RH
- Gas Concentration Test — EN 1279-3 (argon > 90%)
- Gas Leakage Rate — < 1% per year
- Mechanical Shear Test — secondary seal strength
- Adhesion Test — PIB & secondary sealant
- U-Value Calculation — EN 673
- SHGC & VLT Calculation — EN 410 / ISO 9050
- Acoustic Test — ISO 10140 (Rw, C, Ctr)
- Visible Defects Inspection
- Dimension & Squareness
- Bow / Warp / Pillowing
- Wind Load Capacity (per IS 875 Part 3)
Cost (Approximate, Indian Market, 2025-26)
| DGU Configuration | Cost (Rs/sqft) |
|---|
| 4 + 12A + 4 Clear (residential entry) | 320 – 450 |
| 5 + 12A + 5 Clear | 380 – 520 |
| 6 + 12A + 6 Clear | 450 – 620 |
| 6 + 12A + 6 Low-E (basic) | 650 – 850 |
| 6 + 12Ar + 6 Low-E | 720 – 950 |
| 6 + 16Ar + 6 Low-E Premium (Cool-Lite SKN) | 900 – 1250 |
| 10 + 16Ar + 6 Low-E (acoustic) | 1050 – 1450 |
| 8.76 Lam + 16Ar + 6 Low-E | 1250 – 1750 |
| 10.76 Lam Acoustic + 20Ar + 8.76 Lam Acoustic Low-E | 2000 – 2800 |
| Triple Glazed (4+12+4+12+4 2xLow-E Ar) | 1500 – 2200 |
| Triple Glazed Premium (Krypton, 2x Low-E) | 2400 – 3500 |
| Warm Edge Spacer Upgrade (vs Aluminium) | + Rs 80 – 150/sqft |
| Argon Fill Upgrade (vs Air) | + Rs 30 – 70/sqft |
| Toughened Both Panes Upgrade | + Rs 250 – 400/sqft |
Installation: Rs 100–250/sqft for UPVC / aluminium framed assembly.
Best Practices
- Specify warm-edge spacer for premium & ECBC-compliant projects — eliminates edge condensation
- Use argon-filled cavity for Low-E DGUs — small cost, 25% performance gain
- Place low-E coating on Surface #2 for tropical India
- Avoid colourful tints + low-E together unless thermal stress analysed
- For acoustic application, use asymmetric DGU (different glass thicknesses) and PVB acoustic laminate
- For safety + thermal, laminate the inner pane
- For overhead glazing, mandatory inner laminate
- Cavity 12–16 mm optimal — less = poor insulation, more = convection
- For high-altitude delivery, use breather tubes / capillary vents
- For coastal & cyclone zones, edge seal width ≥ 12 mm
- Use silicone (not polysulphide) for structural glazing or UV-exposed edges
- Always heat-soak the toughened plies of safety DGU
- Ensure spacer corners are perfectly mitred / corner-keyed
- Inspect dew point & argon fill on delivery samples
- Demand IS 7392 / EN 1279 certification — required for ECBC compliance
- For long DGUs (> 2.5 m) include intermediate setting blocks
- Maintain edge clearance 3 mm + glass tolerance in the frame
- For projects with motorised blinds inside the cavity, plan the spacer profile & access early
- Use silicone-only secondary seal if facing direct UV (structural glazing)
- Replace whole unit if seal failure / fog — field repair not viable
Applicable Standards
| Standard | Description |
|---|
| IS 7392 (Part 1) | Double Glazed Hermetically Sealed Units — Specification |
| IS 14900 | Float Glass — Substrate |
| IS 2553 | Safety Glass (when toughened / laminated) |
| IS 3548 | Glazing Code of Practice |
| EN 1279-1 to 6 | Insulating Glass Units — Generalities, Tests, Production |
| EN 410 | Determination of Luminous & Solar Properties |
| EN 673 | Determination of U-Value (Thermal Transmittance) |
| EN 12898 | Determination of Emissivity |
| EN ISO 10140 | Acoustic Insulation Measurement |
| ISO 9050 | Solar & Light Properties of Glass |
| ASTM E2188 / E2189 / E2190 | USA IGU Performance Tests |
| ASHRAE 90.1 | USA Energy Standard for Buildings |
| ECBC 2017 | India Energy Conservation Building Code — Glazing |
| GRIHA / IGBC LEED | India Green Building Rating — Glazing Performance |
| NBC 2016 Part 11 | National Building Code — Sustainability |
Conclusion
Double-glazed units are the standard glazing for modern energy-efficient buildings in India. For new construction targeting ECBC, GRIHA or IGBC LEED ratings, DGU with Low-E coating + argon fill + warm-edge spacer is essential. The recommended hierarchy:
- Entry residential: 5+12A+5 clear DGU (Rs 380–520/sqft)
- Standard premium home: 6+12Ar+6 Low-E DGU (Rs 720–950/sqft)
- Office / hotel: 6+16Ar+6 Solar Low-E DGU (Rs 900–1250/sqft)
- Acoustic-critical (airport / hospital): 10/8.76 Lam + 20Ar + 6/8.76 Lam Low-E (Rs 2000–2800/sqft)
- Cold climate / passive house: Triple glazed with Krypton & warm edge (Rs 2400–3500/sqft)
Base components: float glass substrate; toughened for safety; laminated for acoustic / fall protection; tinted / reflective / low-E for solar control; frosted / etched for privacy panels.
Hold the DGU assembly in UPVC windows (best thermal continuity) or aluminium sliding windows with thermal-break profile (best for large openings).