Heritage / Slim-Fit Sealed Units
Narrow-profile units using 6-12mm spacers for heritage windows, Crittall steel frames, period timber sash and conservation areas. Heritage glass types available to replicate original handmade glass character.
Heritage Recommendation
14mm (4-6-4) with Pilkington K Glass S Low-E, 6mm argon cavity, warm edge. U-value: 1.8 W/m²K. Thinnest possible thermal upgrade. For premium performance: krypton gas achieves 1.5 W/m²K in the same 14mm profile.Heritage — Conservation Glazing
CSU Ltd supplies slim-profile sealed units using narrow spacer bars (6-12mm) for heritage and conservation applications. These units deliver meaningful thermal improvement while maintaining the slimmest possible profile to fit period frames. Always specify the widest spacer the frame allows — plus Low-E and argon to maximise performance within the narrow cavity.
Slim Configuration Performance
| Configuration | Thickness | U-Value | Rw | Weight/m² | Frame Application |
|---|---|---|---|---|---|
| 3-6-3 Air | 12mm | 3.1 | 27 | 15 kg | Thinnest — very narrow timber |
| 3-6-3 Low-E Argon | 12mm | 2.0 | 27 | 15 kg | Thermal upgrade, narrow rebate |
| ★ 4-6-4 Low-E Argon | 14mm | 1.8 | 28 | 20 kg | Heritage sash — MOST POPULAR |
| 4-6-4 Krypton Low-E | 14mm | 1.5 | 28 | 20 kg | Premium heritage — krypton boost |
| 4-8-4 Low-E Argon | 16mm | 1.5 | 29 | 20 kg | Crittall steel, slim aluminium |
| 4-10-4 Low-E Argon | 18mm | 1.3 | 29 | 20 kg | Best balance slim + performance |
| 4-12-4 Low-E WE Argon | 20mm | 1.1 | 30 | 20 kg | Standard slim where rebate allows |
Slim units keep a noticeably narrower sightline:
Heritage Glass Types
| Glass | Character | Thickness | Best For |
|---|---|---|---|
| Restoration Glass | Slight undulation, seed imperfections | 4mm | Grade I/II listed buildings |
| Cylinder Glass | Linear distortion, handmade look | 4mm | Period properties, churches |
| Pilkington Optiwhite | Ultra-clear, no green tint | 4mm | Maximum clarity |
| Pilkington K Glass S | Clear Low-E, toughenable | 4mm | Max thermal in slim frames |
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.
A Note on Handling Slim Units
Slim-cavity units tolerate less flex than modern equivalents, so they travel and store upright — padded A-frames, under cover, never laid down. The narrow sightline puts the seal close to the edge: keep spacer pads between units and don’t let glass rest on glass.
Weight is rarely a problem at heritage sizes, but check before lifting where leaded or georgian-bar designs add up — suction handling from 25 kg. Glaze within 28 days, above 5°C, after the units have acclimatised indoors. Lead times follow the glass spec and are confirmed with your quote.
Related Products & Glass Types
Common Questions
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