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I mainly quote 'air gaps' on this page - referring to the actual gap between the panes of glass.
The overall sealed unit thickness will therefore be the air gap plus the thickness of the two panes of glass. Therefore when I say 20mm air gap, (add the 4mm+4mm glass) this would be for a sealed unit with an overall thickness of 28mm, etc.
In my view, quite an over hyped subject, and let me explain why I say this.
In the first place we had a 6mm air gap, and that was double glazing in it's early days, and thought of as marvelous compared to single glazing. As things advanced a few manufacturers moved up to an 8mm air gap (and the salesman could say ours is better because...). A 12mm air gap followed, and really became the norm, with scientifically measured fact sheets to prove it performed a lot better than the early 6mm air gap, as it most certainly did.
In my opinion what happened next was a scramble by double glazing salesmen to push their companies for bigger air gaps to impress the public into believing that their product was better that their competitors. So, next came the 16mm air gap, and yes, JUST measurably better insulation that the 12mm air gap, but ONLY JUST!, nothing like the big difference between single glazing and 6mm, and between the 6mm and 12 mm! Then what happened?, you've probably guessed - yes the 20mm air gap, got to be the best thing since sliced bread, hasn't it?, the bigger the better?. Well, yes again from the salesman's point of view of course.
The fact is that an air gap of 6mm will loose heat through the two panes of glass 'feeling' each other's temperature (radiated heat loss). As the gap gets wider it was found that the insulating properties did not increase exponentially (i.e. double the gap was not double the insulation), because a critical thickness of gap was reached where air would circulate inside the sealed unit, as hot air rises from the inner (hotter) pane, cold air falls from the outer pane (convected heat loss). So we got to 20mm air gap and enough was enough, the salesman could not cajole his boss to keep increasing the gap so that it sounded better than his competitors.
Because a 20mm air gap sealed unit had the inherent problem of increased air circulation inside, the manufacturers 'fixed' this by substituting an inert i.e. 'thicker' gas for the dried air that was normally sealed inside the unit, and this seemed to sort out the problem of the air circulation. I say 'seemed to' because at least it does work when the sealed unit is brand new, because if I have my facts right, and as I understand it (someone correct me if I am wrong): sometimes up to 100% of the argon in this type of unit will have seeped out by the time it is three years old! I would add that not all sealed units lose all of the Argon in such a short time. It is true that some will only lose 1% a year, BUT the biggest issue is how full of Argon they are in the first place. Often the method of filling the units with Argon can be very hit and miss and dependant on the skill of the operator, not necessarily a metering machine. And in the case of Georgian grille inside, this will more than likely contained normal air and not Argon. Argon is most commonly used because it is relatively cheap, with Krypton and Xenon having a place where high performance in narrow cavities is required, but unfortunately the high cost of these gases makes them prohibitive for normal domestic applications. There is a gas called SF6 which can be used to increase sound reduction by some 2-3 dB but this can also have an adverse effect on thermal performance.
I say Argon would probably be better that non Argon, but I would not really reckon to pay much extra at all for it, and I would not choose who I decided to order my windows from on the basis of if it did or did not have Argon filled units. If anyone wants Argon gas in their sealed units from me in their new windows then I just add the cost as about a square metre extra, and if a customer thinks it may be worth it to them then that's fine, but I do not do Argon filled as 'standard'.
I personally prefer sealed units with a 16mm air gap with it's very adequate all round insulating properties, combined with it's expected long life span. Having said that, almost all of the replacement work that I fit is in 28mm glazing, by popular demand, and I am happy with that.