Double Glazing.
Our standard sealed unit is a 28mm 4-20-4 build: 4mm toughened outer pane, 20mm cavity at 90% argon fill, soft-coat Low-E on the cavity face and a warm-edge spacer bar. Centre-pane Ug from 1.0 W/m²K.
Double Glazing
Sealed Glass Units
Standard Unit
28mm (4-20-4)
Centre-Pane Ug (W/m²K)
From 1.0
Whole-Window Uw (W/m²K)
From 1.2
Acoustic Rw
Up to 40 dB
Sealed Unit Guarantee
15 Year
Install
FENSA Reg 40373
What is actually inside the unit
Most double glazing quotes say “A-rated” and stop there. Here is the full build we fit as standard on every uPVC and aluminium window: a 28mm sealed unit in a 4-20-4 configuration — 4mm toughened float outside, a 20mm cavity, 4mm inside. The cavity carries a 90% argon fill, which conducts heat more slowly than air and improves the U-value by around 15% at no extra cost. There is no air-filled entry spec.
A soft-coat low-emissivity coating sits on the cavity face of the inner pane. It is a microscopically thin metallic layer that reflects room heat back inside in winter and blocks part of the solar load in summer, with no visible tint. The panes are held apart by a warm-edge spacer bar rather than aluminium — worth 0.1–0.2 W/m²K on its own, and the single biggest factor in whether the glass edges run cold and attract condensation.
That build gives a centre-pane Ug of 1.0 W/m²K and a whole-window Uw of about 1.2 W/m²K — comfortably inside the 1.4 W/m²K limit Approved Document L sets for replacement windows in existing dwellings, and equivalent to a Window Energy Rating of A+. Standard acoustic performance is 31 dB Rw. A modern sealed unit typically lasts 20–25 years before the cavity gas dissipates; ours carry a 15-year guarantee against misting and seal failure.
Units are made to size, so shaped, arched and raked openings are no more difficult than rectangles. If you are weighing the third pane, the honest comparison is on the triple glazing page and in our double vs triple guide.
Specifications
Standard build 28mm 4-20-4: 4mm toughened float outer, 20mm cavity, 4mm inner, 90% argon fill, soft-coat low-E on the cavity face of the inner pane, warm-edge spacer bar. Centre-pane Ug 1.0 W/m²K; whole-window Uw approximately 1.2 W/m²K depending on frame, calculated to BS EN ISO 10077-1 — the figure Building Regulations use, and the one that appears on your FENSA certificate. Approved Document L (2022) sets a limit of 1.4 W/m²K for replacement windows in existing dwellings and 1.2 W/m²K for the new-build notional building, so the standard spec clears both. Solar gain (g-value) sits in the 0.50–0.62 range and unit weight is around 20 kg/m². Acoustic performance is 31 dB Rw as standard, rising to 38–40 dB Rw with a 6.8mm acoustic laminated outer pane. Available in toughened, laminated and toughened-laminated combinations. Krypton fill is available where cavity depth is restricted, though on a 20mm cavity argon already performs at its optimum.
Upgrades by problem, not by tier. West-facing rooms that overheat: solar-control coating. Main road, railway or Heathrow approach: 6.8mm acoustic laminated outer pane for 38–40 dB Rw. Accessible ground-floor windows and doors: 6.8mm laminated inner pane, which holds together in the frame rather than breaking out. Bathrooms and overlooked side elevations: obscure levels 3 to 5, reeded, or sandblasted. Conservation and period elevations: slim-line low-E with astragal bars. Passivhaus and deep retrofit: step up to triple glazing. Also available patterned, coloured, and cut to shaped, arched and raked openings. Replacement-only service for failed units in existing frames, without replacing the frame.
Glass Options
Clear is the default and the right answer for around 80% of London homes. Tinted covers the solar-control coatings used on west-facing and heavily glazed elevations, where the aim is to cut summer heat gain rather than daylight. Frosted covers the obscure range, levels 3 to 5, plus reeded and sandblasted patterns for bathrooms, stairwells and overlooked side windows. Every option is made to size in the same 28mm 4-20-4 build, so upgrading one window in a run does not change how the others look.
