Standard Double Glazed Sealed Units
High-performance insulating glass units manufactured at our High Wycombe facility. U-values from 0.9 W/m²K with our standard 28mm specification. All glass brands, all spacer types, optional argon fill.
CSU Recommended Standard
28mm unit (4-20-4) with Pilkington Optitherm S3 soft coat Low-E, 20mm argon-filled cavity, Swisspacer Ultimate warm edge spacer. Centre-pane U-value: 0.9 W/m²K. Exceeds Building Regulations Part L. Achieves WER A+ rating.
Standard DGU — Technical Data
A double glazed sealed unit (DGU) comprises two panes of glass separated by a hermetically sealed spacer bar, with the cavity filled with argon gas for enhanced thermal insulation. CSU Ltd uses a dual-seal system — primary polyisobutylene (PIB) moisture barrier and secondary polysulphide or silicone structural sealant — ensuring long-term durability against moisture ingress and gas leakage. Argon filling, when specified, is automated at minimum 90% concentration.
We supply units incorporating glass from all major manufacturers: Pilkington (Optitherm S3, K Glass S, Suncool, Activ, Optiwhite, Optiphon), Saint-Gobain (Planitherm Total+, Ultra N, Cool-Lite, Bioclean, Stadip, Diamant, Parsol), Guardian (ClimaGuard Premium2, SunGuard) and AGC (iplus, Lacobel, Stopray, Clearvision). Any combination of glass type, coating, spacer and gas fill is available to meet any project specification.
How a DGU Works
The sealed gas cavity between the two glass panes creates a thermal barrier. Argon gas (34% lower conductivity than air) fills this cavity, reducing convective and conductive heat transfer. A Low-E (low-emissivity) coating on the inner pane cavity face (surface 3) reflects long-wave infrared radiation back into the room — effectively acting as a radiant heat mirror. The warm edge spacer bar reduces the thermal bridge at the unit perimeter, eliminating edge condensation. Together, these three technologies transform a basic glass panel (U-value 5.8) into an insulating unit achieving U-values as low as 0.9 W/m²K.
Building Regulations Compliance
Part L of the Building Regulations (England 2021) requires maximum window U-values of 1.4 W/m²K for new build and replacement windows. Our standard 28mm configuration achieves 0.9 W/m²K — exceeding requirements by 36%. Part O (2022) addresses overheating — for south/west-facing glazing, specify solar control coatings (Pilkington Suncool, g-value 0.33 or lower). Part K/N requires safety glass in critical locations — toughened or laminated as appropriate.
Technical Specifications
Performance Data — All Common Configurations
The table below shows thermal, acoustic, solar and weight data for all commonly ordered DGU configurations. Our 28mm standard is marked with ★.
| Configuration | Thickness | U-Value (W/m²K) | Rw (dB) | Light Trans. | g-value | Weight/m² |
|---|---|---|---|---|---|---|
| 4-6-4 Clear Air | 14mm | 3.3 | 27 | 82% | 0.76 | 20 kg |
| 4-12-4 Clear Air | 20mm | 2.9 | 28 | 82% | 0.76 | 20 kg |
| 4-16-4 Clear Air | 24mm | 2.8 | 29 | 82% | 0.76 | 20 kg |
| 4-16-4 Low-E Air | 24mm | 1.4 | 29 | 80% | 0.63 | 20 kg |
| 4-16-4 Low-E Argon | 24mm | 1.1 | 29 | 80% | 0.63 | 20 kg |
| 4-16-4 Low-E WE Argon | 24mm | 1.0 | 29 | 80% | 0.63 | 20 kg |
| ★ 4-20-4 Low-E WE Argon | 28mm | 0.9 | 30 | 80% | 0.63 | 20 kg |
| 6-16-4 Low-E Argon | 26mm | 1.1 | 32 | 79% | 0.62 | 25 kg |
| 6-16-6 Low-E Argon | 28mm | 1.1 | 33 | 78% | 0.61 | 30 kg |
| 6-20-6 Low-E WE Argon | 32mm | 0.9 | 34 | 78% | 0.61 | 30 kg |
| 6.8L Acoustic-16-4 Argon | 26.8mm | 1.1 | 36 | 78% | 0.60 | 26 kg |
| 8.8L Acoustic-16-4 Argon | 28.8mm | 1.1 | 38 | 76% | 0.58 | 31 kg |
| 10.8L Acoustic-20-6 Argon | 36.8mm | 1.0 | 42 | 74% | 0.55 | 36 kg |
| 6 Suncool 66/33-16-4 | 26mm | 1.1 | 32 | 66% | 0.33 | 25 kg |
| 6 Suncool 50/25-16-4 | 26mm | 1.1 | 32 | 50% | 0.25 | 25 kg |
WE=Warm Edge. L=Laminated acoustic PVB. Low-E = Pilkington Optitherm S3 or equivalent. All with argon unless stated.
Thermal Upgrade Path
Each technology delivers a measurable improvement. Combined, they transform basic glass into a high-performance insulating unit:
| Upgrade Step | U-Value | Improvement | Cumulative |
|---|---|---|---|
| Clear 4-16-4 air, aluminium spacer | 2.8 W/m²K | Baseline | — |
| + Low-E soft coat (Optitherm/Planitherm) | 1.4 | 50% | 50% |
| + Argon gas fill (90%) | 1.1 | 21% | 61% |
| + Warm Edge spacer (Swisspacer Ultimate) | 1.0 | 9% | 64% |
| + Increase cavity to 20mm | 0.9 | 10% | 68% |
Applications
- Residential windows and doors: Casement, tilt-turn, sliding sash, bifold — new build and replacement. 28mm (4-20-4) is the standard for all domestic projects.
- Commercial glazing: Offices, retail, shopfronts, curtain walling. Solar control coatings for south/west facades.
- Conservatories and orangeries: Solar control Low-E essential for south-facing roofs. Self-cleaning recommended for overhead.
- Schools, hospitals, public buildings: Acoustic laminated plies for noise reduction (36-50+ dB). Toughened/laminated for safety.
- Extensions and conversions: Garden rooms, loft conversions, basement windows. Part L compliance easily achieved.
- Social housing: Cost-effective glazing meeting Decent Homes Standard and Part L requirements.
Common Mistakes to Avoid
- Specifying air-filled units when argon is available at no extra cost from CSU
- Using aluminium spacers when warm edge eliminates condensation for minimal additional cost
- Not specifying toughened glass for safety-critical locations (doors, low-level, bathrooms)
- Ordering 24mm units when 28mm (4-20-4) gives better U-value at the same glass cost
Gas Fill — Air as Standard, Argon Optional
CSU sealed units are air-filled as standard. Argon gas fill (minimum 90% concentration) is available as an optional upgrade. Argon has 34% lower thermal conductivity than air (0.018 vs 0.026 W/mK), improving thermal insulation by approximately 15%. Argon is an inert, non-toxic, odourless gas that makes up 0.93% of Earth’s atmosphere — it is inert and environmentally neutral. Krypton gas is available by special order for slim-cavity heritage applications where maximum performance is needed in a narrow gap.
| Property | Dry Air | Argon (Optional) | Krypton (Special) |
|---|---|---|---|
| Thermal Conductivity | 0.026 W/mK | 0.018 W/mK | 0.010 W/mK |
| U-Value Benefit | Reference | ~15% better | ~25% better |
| Optimal Cavity | 12-16mm | 16-20mm | 10-12mm |
| Depletion Rate | N/A | <1% per year | <1% per year |
| Relative Cost | 1× | ~1.2× (included free) | ~50× (premium) |
Spacer Bar Options — Warm Edge vs Aluminium
The spacer bar separates glass panes and creates the sealed cavity. Traditional aluminium spacers conduct heat rapidly (160 W/mK), creating a cold thermal bridge at the unit edge that causes condensation and heat loss. Warm edge spacers use low-conductivity materials, reducing this bridge by up to 94%. CSU recommends Swisspacer Ultimate or Super Spacer TriSeal for all projects.
| Spacer | Material | Conductivity | Psi Value | Edge Temp vs Alu | Colours |
|---|---|---|---|---|---|
| Aluminium | Aluminium alloy | 160.0 W/mK | 0.080 W/mK | Baseline (cold) | Silver, White, Black, Brown, Grey |
| Swisspacer Ultimate | Composite polymer | 0.19 W/mK | 0.030 W/mK | +3-4°C warmer | Black, White, Brown, Grey, Oak +15 |
| Super Spacer TriSeal | Silicone foam | 0.17 W/mK | 0.028 W/mK | +3-4°C warmer | Black, Grey |
| Thermix TX.N Plus | Stainless + plastic | 0.60 W/mK | 0.040 W/mK | +2-3°C warmer | Silver / Black |
| TGI Spacer | Stainless + polymer | 0.90 W/mK | 0.050 W/mK | +1-2°C warmer | Silver |
Warm edge improves Uw by 0.1-0.2 W/m²K, eliminates edge condensation, and can improve WER by one full band. Available in widths: 6, 8, 10, 12, 14, 16, 18, 20, 22, 24mm.
Glass Weight Reference
Glass weighs 2.5 kg per m² per mm of thickness (density 2,500 kg/m³). Sealed unit weight = sum of glass panes.
| Glass / Unit | Weight/m² | 600×900 | 1000×1200 | 1500×2000 |
|---|---|---|---|---|
| 4mm single | 10 kg/m² | 5.4 kg | 12 kg | 30 kg |
| 6mm single | 15 kg/m² | 8.1 kg | 18 kg | 45 kg |
| 4-16-4 DGU | 20 kg/m² | 10.8 kg | 24 kg | 60 kg |
| 4-20-4 DGU (28mm) ★ | 20 kg/m² | 10.8 kg | 24 kg | 60 kg |
| 6-16-6 DGU | 30 kg/m² | 16.2 kg | 36 kg | 90 kg |
| 6-20-6 DGU | 30 kg/m² | 16.2 kg | 36 kg | 90 kg |
| 4-16-4-16-4 TGU | 30 kg/m² | 16.2 kg | 36 kg | 90 kg |
| 6.8mm Laminated | 16 kg/m² | 8.6 kg | 19 kg | 48 kg |
| 10.8mm Laminated | 26 kg/m² | 14.0 kg | 31 kg | 78 kg |
★ CSU standard recommendation. Formula: Weight (kg) = Thickness (mm) × 2.5 × Area (m²).
Handling, Storage & Lead Times
- Storage: Store vertically on A-frames under cover. NEVER store flat.
- Handling: Suction cups for units over 25 kg. Mechanical handling over 60 kg.
- Edge protection: Protect edges during transport. Avoid glass-to-glass contact.
- Installation: Install within 28 days of delivery — do not leave units in long-term storage.
- Temperature: Do not install below 5°C. Allow units to acclimatise first.
| Product | Standard | Express | Notes |
|---|---|---|---|
| Standard DGU | 3-5 days | 1-2 days | Most configs |
| Triple Glazed | 5-7 days | 3-4 days | Warm edge std |
| Integral Blinds | 10-15 days | — | Made to order |
| Jumbo Units | 5-10 days | — | Size dependent |
| Shaped Units | 5-7 days | — | Template req. |
| Fire-Rated | 7-14 days | — | Availability |
| Walk-On Glass | 7-10 days | — | SGP lamination |
| Smart Glass | 10-15 days | — | PDLC film |
Frequently Asked Questions
Georgian Bar Options
| Type | Location | Widths | Finishes | Authenticity |
|---|---|---|---|---|
| Internal (Cavity) | Between panes | 8, 18, 25mm | White, Gold, Chrome, Brown, Black | Good — easy clean |
| External (Surface) | Bonded to glass | 18, 25, 45mm | White, Rosewood, Oak, RAL | Very good — 3D look |
| Duplex | Internal + External | 18, 25mm | Matched colours | Best — most authentic |
Related Products & Glass Types
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