Neither one is automatically better. The cavity does the insulating, not the overall thickness, and argon stops paying you back once that cavity passes roughly 20mm. A 32mm unit can measure no better than a well built 24mm unit, and in some build-ups slightly worse.
The figure quoted on most glazing enquiries in the UAE is the overall thickness of the sealed unit. It is a useful shorthand for whether a unit will fit a given frame. It is close to useless as a measure of performance, because two units of the same overall thickness can be built completely differently and behave completely differently.
What 24mm and 32mm Actually Describe
A sealed double glazed unit is two panes of glass separated by a spacer bar, with the perimeter sealed and the cavity filled with dry air or an inert gas. The overall thickness is simply the sum of those three parts. Written properly, a specification names all three.
| Build-up | Outer pane | Cavity | Inner pane | What it is for |
|---|---|---|---|---|
| 4 / 16 / 4 | 4mm | 16mm | 4mm | The long-standing 24mm standard. The cavity sits at the efficient width for argon |
| 6 / 12 / 6 | 6mm | 12mm | 6mm | Also 24mm overall, but thicker glass and a narrower cavity. Better acoustically, weaker thermally |
| 4 / 24 / 4 | 4mm | 24mm | 4mm | 32mm overall. The extra 8mm all went into cavity, past the point where it helps |
| 6 / 20 / 6 | 6mm | 20mm | 6mm | 32mm overall, and a genuinely useful one. Thicker glass for span and acoustics, cavity still efficient |
| 6 / 20 / 6.76 | 6mm | 20mm | 6.76 laminated | 32mm overall with a laminated inner pane. Safety, acoustics and security in one unit |
Read those five rows again and the point becomes obvious. Two of them are 24mm and three are 32mm, but the meaningful differences run across that divide rather than along it. A 4 / 16 / 4 unit will out-insulate a 6 / 12 / 6 unit despite being identical in overall thickness, and it will hold its own against a 4 / 24 / 4 unit that is 8mm thicker.
Why a Wider Cavity Stops Helping
The cavity resists heat two ways. It interrupts conduction, because gas conducts far less readily than glass, and it is narrow enough to discourage convection, because the gas has nowhere to circulate.
Widen the cavity and the first effect improves while the second gets worse. Past a certain width the gas has enough room to set up a convection loop: warm gas rises on the hot side, falls on the cold side, and carries heat across the gap that a narrower cavity would have blocked. The two effects cancel out, and then the second one starts to win.
For argon, published guidance places the efficient range at roughly 16mm to 20mm, with convection becoming a measurable factor above about 20mm. The optimum shifts slightly with climate, and warmer conditions push it a little wider, which marginally favours the UAE.
The practical consequence: a 4 / 24 / 4 unit is paying for 8mm of extra sightline and frame depth to land on a U-value no better than 4 / 16 / 4, and possibly a fraction worse.
When 32mm Genuinely Earns Its Place
None of the above is an argument against 32mm units. It is an argument against choosing one because the number is bigger. There are four reasons to specify 32mm, and all four are about what fills the extra space.
Thicker glass for larger panes
Big glass needs to be thicker to handle wind load and to limit deflection. Once you move from 4mm to 6mm on both faces you have added 4mm before touching the cavity. A 6 / 20 / 6 unit is the natural result, and it is a properly specified 32mm unit rather than an inflated 24mm one.
Acoustic performance
Sound and heat respond to different things. Mass blocks sound, so thicker glass helps, and asymmetric panes help more, because two panes of different thickness resonate at different frequencies instead of reinforcing each other. A 6 / 20 / 4 build-up is deliberately mismatched for that reason. Add an acoustic PVB interlayer and the improvement is larger again.
A laminated pane
Replacing the inner pane with a laminated build-up such as 6.76 adds safety, security and acoustic damping, and it adds millimetres. Overhead glazing and anything at height should be laminated regardless of what that does to the overall thickness.
Triple glazing
Three panes and two cavities will not fit in 24mm. This is rare in the UAE, where the problem is keeping solar heat out rather than keeping warmth in, and the money is almost always better spent on coatings.
What Matters More Than Either Number in the UAE
In a Gulf climate the dominant problem is solar radiation coming straight through the glass, not conduction across it. That reorders the priorities completely compared with a European specification.
| Assembly | U-value (W/m²K) | SHGC | Comment |
|---|---|---|---|
| Single glazing | 5.0 to 5.8 | 0.7 to 0.8 | Will not pass a green building submission |
| Standard double glazed unit | 2.7 to 3.0 | 0.60 to 0.70 | The cavity alone does most of this |
| Double glazed, Low-E and argon | 1.6 to 1.8 | 0.25 to 0.40 | The coating, not the extra millimetres, moves SHGC |
Look at which column moves. Going from a standard sealed unit to a coated, argon filled one roughly halves the U-value, but it also cuts the solar heat gain coefficient by more than half, and SHGC is the number that decides how hot a west facing room gets in August. No amount of extra cavity width does that. A solar control Low-E coating does.
The same logic applies to the frame. Aluminium conducts heat extremely well, so a carefully specified unit set into a frame with no thermal break will underperform badly. That is the most common way a UAE glazing job fails to deliver what the glass spec promised.
Dubai's Al Sa'fat green building system sets glazing U-value and shading coefficient limits banded by how much of the external wall is glazed, and Abu Dhabi uses Estidama Pearl instead. These requirements are revised periodically. Confirm the current edition with the relevant authority or your consultant before relying on any specific figure in a submission.
The Frame Has to Accept It
A sealed unit has to sit in a glazing rebate deep enough to hold it with proper edge cover. Systems are designed around a glass thickness range, and 24mm and 32mm are not interchangeable in the same profile. Specifying a 32mm unit into a system designed for 24mm means a different profile, different beads and sometimes different hardware, because the sash is heavier.
This matters most on retrofit work. Upgrading the glass in existing frames sounds like the economical route, and occasionally it is, but the existing rebate sets a hard ceiling on what can go in. That is a measurement to take before the conversation about build-ups, not after.
What to Ask a Supplier
If a quotation says only "32mm double glazed", it has not told you anything you can compare. Three questions resolve it:
- What is the full build-up? Glass, cavity, glass, written in order, with laminated panes given in full notation.
- What coating, and on which surface? A solar control Low-E belongs on surface 2, the inner face of the outer pane. The same coating on the wrong face performs differently.
- What are the U-value and SHGC for the assembly? Not for the glass alone. The frame is part of the result.
A supplier who can answer those three without hesitation is quoting a specification. One who can only repeat the overall thickness is quoting a number.
