FENSA-registered glazing experts. Made-to-measure sealed units, shipped UK-wide, backed by a 10-year warranty.
28mm triple glazing — thinner-than-standard TG for slim-rebate frames
Triple-glazed thermal performance where your frame rebate can't accept the standard 36mm build. Most TG sealed units sit at 36mm or 44mm overall — too thick for the great majority of UK frames designed for double glazing. The 28mm TG build (typically 4mm-8mm-4mm-8mm-4mm with argon fill) gives you three panes and two low-E coatings in the same depth as a standard 28mm DG unit — meaning it drops into existing 22mm-rebate UPVC, timber, and aluminium frames without requiring a full window replacement. Expect a U-value around 1.0-1.1 W/m²K, around 30% better than standard low-E DG, with a small daylight transmittance trade-off (roughly 68-72% versus 76-80% on coated DG).
Triple-glazed without changing the windows
Most UK homes built post-1990 have window frames designed for a 28mm DG cassette. The 28mm TG drops straight in — no frame swap, no new sashes, no FENSA replacement notification. Useful for upgrading thermal performance on a passive-house-aspirations retrofit, on north-facing exposed walls, or where the existing DG has failed and you want to upgrade rather than like-for-like replace.
Configure 28mm TGArchitect, retrofit specialist, glazier
UKCA-marked with BS EN 1279 compliance and Declaration of Performance on request. Specify 28mm TG when the client wants the thermal step-up of TG but a full-frame retrofit is out of scope or unaffordable. Trade-tier pricing on 5+/20+/50+ unit orders, Net 30 accounts, priority dispatch for retrofit specialists.
Trade pricing & bulk ordersWhen is 28mm TG the right call?
Six scenarios where the thinner triple build wins versus standard DG or full 36mm TG.
Frame accepts 32mm — consider 32mm TG
If your rebate accepts 32mm, the 32mm build is better balanced thermally — see the 32mm TG range.
UPVC retrofit (90s-00s frames)
Standard 28mm-rebate UPVC casement frames. 28mm TG drops in with no frame mod required.
Timber casement retrofit
Period-look timber casements designed for 28mm DG. TG upgrade preserves the external sightline.
Aluminium frame retrofit
Slim-line aluminium frames with 22-24mm rebate depth. 28mm TG is the upper limit before a frame swap.
Passive House aspirations
Not a full PassivHaus build, but pushing U-values down where possible. 28mm TG is the cheapest step-up.
Custom 28mm TG
Shaped, oversized, or non-standard rectangles. Configure live.
Compatibility check before you order
28mm TG is heavier than 28mm DG (around 50% heavier — three panes instead of two). Before specifying, check the hardware can carry the load.
3-point pre-order check
- Rebate depth. Same as a standard 28mm DG — needs 22mm rebate depth minimum. Easy check: pop a glazing bead, measure with a steel rule.
- Hinge / hardware load rating. 28mm TG weighs roughly 27 kg/m² (versus 17 kg/m² for DG). Standard friction-stay hinges are rated to about 25 kg; large casement lights over 1m² with 28mm TG may need heavy-duty hinges. Check the existing hardware — if the hinges are tired, factor in a hinge upgrade.
- Sealed-unit measure. Same as standard DG — rebate-width plus 6mm and rebate-height plus 6mm. The 6mm is the 3mm-each-side overlap.
Pricing transparency
28mm TG is typically 40-55% more expensive than equivalent 28mm DG per square metre — the third pane, second coating, second cavity gas charge, and additional handling all add cost. Energy savings versus low-E DG typically pay back in 7-10 years on a south or west-exposed wall, 12-15 years on a sheltered north elevation. Trade tier pricing on volume.
28mm triple glazing — common questions answered
Q: How does 28mm TG compare thermally to standard low-E DG?
A standard low-E argon-filled 28mm DG (4mm-20mm-4mm) gets you to around 1.2-1.4 W/m²K. The 28mm TG build (4-8-4-8-4 with argon and two low-E coatings) drops that to around 1.0-1.1 W/m²K — roughly a 25-30% improvement. That's the thermal gap. The light-transmittance gap goes the other way: coated DG sits at 76-80% visible transmittance; the 28mm TG with two coatings is around 68-72%. Worth the trade in cold-climate, exposed, or thermal-priority rooms; less worth it in mild-microclimate sheltered south-facing lights.
Q: Why pick 28mm TG over 36mm or 44mm TG?
Compatibility, not performance. 36mm TG and 44mm TG outperform 28mm TG thermally (the wider cavities are thermally more efficient — typical U-values around 0.7-0.9 W/m²K). But neither fits into a standard 22mm rebate. If your frame is designed for DG (which is the case for virtually all UK residential frames pre-2010), 28mm TG is the only TG option short of a full window replacement. New-build or full-frame-swap installations should specify 36mm or 44mm; retrofit-only situations specify 28mm.
Q: Is 28mm TG noticeably heavier? Will my hinges cope?
Yes — about 50% heavier per square metre than equivalent 28mm DG (27 kg/m² versus 17 kg/m²). For most casement lights under 1m², standard hardware copes fine. For larger lights — picture windows, tilt-and-turn lights over 1m², French casement leaves — you may need heavy-duty friction stays or upgrade the hinges at the same time. The right check: open the existing light and feel for hinge wear or droop. If the hinges are tired now, replace them when you fit the TG.
Q: Does 28mm TG use argon, krypton, or air fill?
Standard build is argon fill in both cavities. Argon is the cost-effective default — denser than air, gives a meaningful thermal step-up over air-fill, and is what the energy ratings cited above assume. Krypton fill is available on request and improves U-value by a further 0.05-0.10 W/m²K (useful when the narrow 8mm cavities limit argon's efficiency); the price premium is significant (krypton is roughly 10x more expensive than argon by weight) so it's specified for Passive House-grade builds rather than general residential retrofit.
Q: Can 28mm TG be made in safety glass (toughened or laminated)?
Yes — safety-grade variants are available. Toughened (BS EN 12150) on the inner and/or outer panes, or laminated (BS EN 14449) on the inner pane (the most common safety spec for residential retrofit). Safety glass is legally required in critical locations: any pane below 800mm AFFL, within 300mm of a door edge, or in floor-to-ceiling primary glazing. The configurator flags critical-location warnings automatically. Note: laminated inner pane adds 2-3mm to the overall build — verify the rebate will still accept the unit before ordering.
Q: How long is the payback period?
Depends on orientation, exposure, and current heating bills. Typical UK semi-detached retrofitting 28mm TG onto a south or west elevation (high heat loss, high cost-of-heat capture): 7-10 year payback versus low-E DG. North or sheltered elevations: 12-15 years. If you're retrofitting because the existing unit has failed (misted, blown seal) and the cost comparison is "new DG versus new TG" rather than "do nothing versus new TG", the payback gap closes substantially — the marginal cost of the TG upgrade often pays back in under 5 years.
Q: Does 28mm TG affect acoustic performance?
Slightly better than equivalent 28mm DG, but TG isn't an acoustic solution by itself. A standard 28mm DG gives you around 28-30 dB Rw acoustic insulation; 28mm TG with two cavities buys you maybe 2-3 dB of marginal improvement — not a meaningful step. For acoustic priority (busy road, flightpath, railway corridor), specify a laminated acoustic interlayer (PVB acoustic) on the inner pane — that jumps insulation to 38-44 dB Rw. See the acoustic glass guide for the full breakdown.
Q: Lead time and warranty for 28mm TG?
10-year manufacturer warranty on edge-seal integrity (BS EN 1279-2 weathered durability) and 5-year on low-E coatings — same warranty as our DG range. Lead time: 7-10 working days for rectangular stock-spec, 10-14 days for shaped or oversized (any dimension over 1500mm), 12-16 days for shaped TG with safety-glass variants. The TG production line runs slightly behind DG because of the extra coating and second-cavity steps.
Related guides and specifications
Frame won't accept 36mm? 28mm TG drops in.
Call the helpline on 0117 330 3057 (08:00-18:00 Mon-Fri), or request a quote. Free rebate-compatibility check before order.