Glass · Buying guide
28mm, 36mm or 44mm? Sealed unit thickness, explained
Read the numbers outside to inside
A sealed unit is described by its build: pane, cavity, pane. Our standard double is written 4-20-4, meaning a 4mm toughened outer pane, a 20mm cavity and a 4mm inner pane, 28mm in all. A triple adds a pane and a cavity, so 4-12-4-12-4 is 36mm and 4-16-4-16-4 is 44mm. The thickness on its own tells you almost nothing; the cavity depth, the gas in it and the coatings on the glass are what set the performance. The frame then decides which of those builds will physically fit.
The three builds we fit
Every figure below is the one published on our own product pages for the unit in question. Centre-pane Ug is the glass alone; whole-window Uw is the figure on your FENSA certificate and the one Building Regulations use.
| Unit | Build | Fill | Centre-pane Ug | Whole-window Uw | Acoustic Rw | Weight |
|---|---|---|---|---|---|---|
| 28mm double | 4-20-4 | 90% argon | from 1.0 | about 1.2 | 31 dB, up to 40 | 20 to 22 kg/m² |
| 36mm triple | 4-12-4-12-4 | Argon, two cavities | 0.6 to 0.7 | 0.8 to 0.9 | 36 to 38 dB, up to 44 | 30 to 33 kg/m² |
| 44mm triple | 4-16-4-16-4 | Krypton, two cavities | 0.5 to 0.6 | 0.76 to 0.9 | 36 to 38 dB, up to 44 | 30 to 33 kg/m² |
Ug and Uw in W/m²K to BS EN ISO 10077-1. Approved Document L (2022) allows up to 1.4 W/m²K for replacement windows in an existing home, so all three clear it.
Which frames take which thickness
The glazing pocket is fixed by the profile. A 28mm unit fits everything we sell. A 36 or 44mm triple has to be matched to a frame that publishes a pocket deep enough, which rules out the slimmest period profiles.
| System | Published glazing range | Takes |
|---|---|---|
| uPVC casement, flush and sash | 28mm as standard | Double; triple depends on the profile, confirmed at survey |
| APX Heritage aluminium | 24 to 36mm | Double, or 36mm triple |
| APX Picture window | Up to 52mm | Double, 36mm or 44mm triple |
| Cortizo Vision sliding | 26 to 30mm | Double only |
| Schüco ASE lift-and-slide | 24 to 40mm | Double, or 36mm triple |
| APX47 sliding | 22 to 65mm | Double, 36mm or 44mm triple |
| X-Fold 120 bifold | 25 to 45mm | Double, 36mm or 44mm triple |
| Roof lantern | 28mm | Double, self-cleaning |
Why not thicker everywhere
Triple glazing is a frame decision. A 36 to 44mm unit will not go into a pocket built for 28mm, and a triple unit in a poorly insulated frame gives back most of the gain through the frame instead. It also weighs roughly half again as much, so hinges and sash sizes change with it, and it lets in less sun, g-value 0.40 to 0.55 against 0.50 to 0.62 for double, which helps a west elevation and penalises a south one. Where the frames are staying, or the profile is slim, the money is better spent on a high-performance 28mm double: an acoustic laminate for a main road, a solar-control coating for an overheating room, a slim-line low-E for a conservation elevation.
Frequently asked
What does 4-20-4 mean?
It is the build of a sealed unit read from outside to inside: a 4mm pane, a 20mm sealed cavity, then a 4mm pane. Add them and you get the unit thickness, 28mm. A triple unit is written the same way, so 4-12-4-12-4 is three 4mm panes with two 12mm cavities, 36mm in total.
Is 28mm double glazing the standard?
Yes. The 28mm 4-20-4 unit is what we fit as standard on every uPVC and aluminium window: 4mm toughened outer pane, 20mm cavity with a 90% argon fill, soft-coat low-E on the cavity face of the inner pane, warm-edge spacer. Centre-pane Ug from 1.0 W/m²K and a whole-window Uw of about 1.2 W/m²K, inside the 1.4 W/m²K that Approved Document L sets for replacement windows.
Can I get 28mm triple glazing?
Only by squeezing three panes into the space made for two, which leaves cavities of around 8mm each. That is too narrow for argon to work at its best, so a 28mm triple unit does not perform much better than a good 28mm double and costs more. If the frame only takes 28mm, the better spend is a high-performance double: acoustic laminate, solar-control coating or a slim-line low-E, depending on the problem.
What is the difference between 36mm and 44mm triple glazing?
Cavity depth and gas. Our 36mm unit is 4-12-4-12-4 with two argon cavities, centre-pane Ug 0.6 to 0.7 W/m²K. The 44mm unit is 4-16-4-16-4 with krypton, 0.5 to 0.6 W/m²K, and is the make-up used for Passivhaus and EnerPHit work. Both need the frame to be specified for them from the start; neither fits a pocket built for 28mm.
What about 32mm units?
32mm sits between the two standards, usually as a 4-24-4 double or a thin triple. A 24mm argon cavity gives little over 20mm, so we do not carry 32mm double as a standard build. If a frame is sized for 32mm we glaze it with the best unit that fits, and say what that is on the quote.
Argon or krypton: does the gas matter?
Argon improves the U-value of a 20mm cavity by around 15% against air at no real extra cost, which is why there is no air-filled option in our range. Krypton conducts less heat still, but it is expensive and only earns its keep in narrow cavities, which is why it appears in the 44mm 4-16-4-16-4 build and not in the 28mm double.
How much heavier is triple glazing?
A 28mm double unit weighs around 20 to 22 kg/m²; a triple unit weighs around 30 to 33 kg/m², roughly 50% more. Hinges, friction stays and sash sizes are chosen for that weight at survey, which is another reason triple is a frame decision rather than a glass swap.
Can shaped or arched windows take these units?
Yes. All of the builds on this page are made to size, including shaped, arched and raked units. The constraint is the frame: a slim heritage profile may take a 24 to 36mm unit, so a shaped triple has to be checked against the profile before it is priced.
Does thicker glass make the window quieter?
Thicker is not what does it; unequal panes and lamination do. A standard 28mm double gives 31 dB Rw. Swap the outer pane for a 6.8mm acoustic laminate and the same frame reaches 38 to 40 dB. Standard triple gives 36 to 38 dB, and up to 44 dB with a laminated outer pane. For a main road or the Heathrow approach, the laminate matters more than the third pane.
