FENSA-registered glazing experts. Made-to-measure sealed units, shipped UK-wide, backed by a 10-year warranty.
Double-glazed sealed units — replacement and upgrade
Single-pane upgrade, like-for-like replacement, or a thermal upgrade on an existing double-glazed unit? Modern double-glazing is the UK default, and for good reason: a 24mm or 28mm argon-filled low-E sealed unit hits Building Regs Part L (U-value 1.4 W/m²K or better), cuts heat loss against single glazing by around 70%, takes the worst of road noise off the room, and pays back in 8-12 years on energy bills alone. Pane Relief manufactures double-glazed sealed units in every standard UK cavity build — 14mm slim, 20mm, 24mm, 28mm — with the right coating for the use case. See the spec finder below, or use the energy-rated guide if you're comparing A/B/C ratings.
Upgrading or replacing your own DG
Free UK mainland delivery, 10-year manufacturer warranty on edge-seal integrity (BS EN 1279-2), FENSA-registered installer recommendations in your postcode, and a phone helpline (08:00-18:00 Mon-Fri) to sanity-check spec against your existing rebate before you order. Most stock double-glazed units ship in 5-7 working days.
Configure my unitInstaller, glazier, energy-retrofit specialist
Net 30 trade accounts, tiered volume discount on 5+/20+/50+ unit orders, scheduled site delivery for retrofit projects, priority dispatch for FENSA installers. UKCA-marked, BS EN 1279 compliance, BFRC-rated low-E units; Declaration of Performance and U-value calculations on request for Part L sign-off.
Trade pricing & bulk ordersWhich double-glazed build do you need?
Six common double-glazed sealed-unit specifications. Pick by cavity depth, coating, or application — each links to the relevant product or filter.
24mm argon-filled
The UK workhorse. 4mm-16mm-4mm with argon gas fill and low-E coating. U-value 1.4 W/m²K. Fits most modern UPVC and aluminium rebates.
28mm argon low-E
4mm-20mm-4mm — deeper cavity, slightly better thermal performance. U-value 1.2 W/m²K. Standard for modern UPVC tilt-and-turn frames.
Low-E A-rated
Soft-coat low-E (Pilkington K Glass, Saint-Gobain Planitherm). BFRC A-rated as standard, A+/A++ with krypton. Hits and exceeds Part L.
Safety (toughened/laminated)
BS EN 12150 toughened or BS EN 14449 laminated double-glazed units. Required below 800mm or beside doors.
Acoustic DG
Asymmetric build (6mm-16mm-4mm + PVB acoustic interlayer). Cuts road, rail, and flightpath noise by up to 39dB.
Slim 14mm / 16mm
Conservation, listed-building, and Victorian sash retrofit. Fits original timber rebates built for single glazing. U-value 1.6.
Measure twice, order once
Double-glazed units come in fixed cavity builds (14, 20, 24, 28mm) — you can't fit a 28mm unit into a 24mm rebate. Confirm both dimensions and thickness before ordering.
The 5-step measure
- Measure the visible glass width at top, middle, and bottom — use the smallest.
- Measure the visible glass height at left, middle, and right — use the smallest.
- Add 6mm to both dimensions for the sealed-unit size.
- Measure the unit thickness edge-on with vernier callipers — this is the make-or-break dimension. 24mm and 28mm look very similar to the eye but won't interchange in the rebate.
- Photograph the kitemark or BS EN reference printed on the edge of the existing unit. The numbers tell us the original coating spec — we'll match or upgrade as you choose.
Pricing transparency
Prices on this collection are "From £X" — the smallest stock 24mm low-E argon-filled unit at base spec. Larger sizes, A+/A++ coatings, krypton fill, safety glass (toughened or laminated), acoustic interlayers, slim-profile and oversized panels are priced live in the configurator.
For full thermal-performance comparison see energy-rated glass units.
Double glazing — common questions answered
Q: What U-value should I be targeting for replacement double glazing?
Building Regulations Part L (England & Wales) requires U-value of 1.4 W/m²K or better for replacement glazing in existing dwellings. A standard 24mm argon-filled low-E sealed unit comfortably meets this (typically 1.3-1.4). Upgrading to A+ rated (krypton fill or warm-edge spacer) takes you to around 1.0-1.1 — useful if you're going for SAP improvements on EPC, or if you're insulating an extension. Below 1.0 you're into triple-glazing territory.
Q: Argon, krypton, or air — does the gas fill actually matter?
Yes — but with diminishing returns. Argon is the standard fill (cheap, denser than air, cuts convection across the cavity, drops U-value by around 0.2-0.3). Krypton is denser still, fits narrower cavities efficiently, and drops U-value by another 0.1-0.2 — but costs significantly more. Air-filled units are obsolete except in budget commercial spec. For 24mm/28mm cavities, argon is the right answer for 95% of cases; krypton makes sense only in narrow conservation builds or where you're chasing every fraction of a U-value.
Q: My existing rebate is 20mm — can I upgrade to 24mm?
Generally no — the rebate is part of the frame and is built around a fixed unit thickness. A 24mm unit won't seat properly in a 20mm rebate. You have two routes: (1) stay with a 20mm replacement (we stock these — slightly thinner cavity but still argon-fillable, U-value around 1.6-1.8) or (2) replace the whole window with a frame built for 24mm or 28mm. If the existing frame is sound, route 1 is dramatically cheaper.
Q: How much noise reduction can I expect from standard double glazing?
A standard 24mm double-glazed unit (4-16-4 symmetric) gives around 27-30 dB reduction versus an open window — useful for general suburban noise but not transformative against road, rail, or flightpath. For real acoustic improvement you want an asymmetric build (6mm outer + acoustic PVB laminated inner pane) or a dedicated acoustic sealed unit (rated 36-39 dB Rw). See the acoustic glass guide if noise is the primary driver.
Q: When does triple-glazing make sense over double?
Triple-glazing makes sense when (a) you're going for Passive House or near-Passive standard on a new-build or major retrofit, (b) you're installing a heat pump and want to keep flow temperatures low, (c) you're north-facing with no solar gain to lose, or (d) you're targeting EPC A. For a standard UK home with cavity insulation and a gas boiler, the marginal U-value gain (1.2 down to 0.6) usually doesn't justify the cost or the extra frame weight. See our triple-glazing collection.
Q: How long do double-glazed units last?
Industry-typical edge-seal life is 20-25 years, varying with exposure and installation quality. South-facing units in coastal locations fail earliest (UV plus salt-spray accelerate seal degradation); sheltered north-facing units routinely last 30+ years. Our 10-year manufacturer warranty (BS EN 1279-2 weathered durability) is the floor expectation. Misting between the panes is the universal end-of-life signal.
Q: Is double-glazing the right answer for a single-glazed Victorian property?
Usually yes, but the wrong-shape units cause problems. Original Victorian timber sash and casement rebates were built for 4mm or 6mm single glazing and won't accept a standard 24mm sealed unit. The right answer is a 14mm or 16mm slim-profile double-glazed unit — argon-filled with low-E, U-value around 1.4-1.6, fits the original rebate without modification, and satisfies most conservation-area planning constraints. See slim-profile units.
Q: What safety glass spec is required for double-glazed units near doors or floor-level?
Approved Document N and BS 6262-4 mandate toughened (BS EN 12150) or laminated (BS EN 14449) for any pane below 800mm above floor, or within 300mm of a door edge if below 1500mm. In a double-glazed sealed unit the spec applies to either or both panes depending on the use case — toughened-toughened is standard for floor-level windows; laminated outer + toughened inner is preferred for security and acoustic combo. The configurator highlights legal requirements based on dimensions and cill height.
Q: What's the lead time on double-glazed units?
Stock 24mm or 28mm argon low-E rectangular units: 5-7 working days. Custom-size non-stock rectangular: 7-10 days. Toughened or laminated safety: +2-3 days for the processing cycle. Slim-profile 14mm/16mm: 7-10 days. Acoustic laminated: 10-14 days. Shaped, arched, or oversized (any dimension over 1500mm): 10-14 days. Free phone sanity-check on spec before you order — 0117 330 3057.
Q: How do I know if my existing unit is double or triple glazed?
Look at the edge — a sealed unit has visible spacer bars between the panes. Two panes = one spacer = double-glazed. Three panes = two spacers = triple-glazed. Thickness is also a clue: 14-28mm is universally double; 36-44mm is triple. The candlelight test (hold a candle next to the glass at night and count the reflections — two for double, three for triple) is the classic non-disassembly trick.
Related guides, collections, and specifications
Need help speccing the right double-glazed unit?
Call the helpline on 0117 330 3057 (08:00-18:00 Mon-Fri), or request a quote. Trade accounts unlock live tier pricing and Net 30 terms.