Glass:
Sealed units air gaps
Double glazed sealed units and
air gaps between the glass.
On this page I am mainly 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. For example, when I say ‘20mm air gap’ (add the 4mm + 4mm
glass) this would be for a sealed unit with an overall thickness of
28mm. In my view, this is quite an over-hyped subject. Let me
explain why I say this. In the early days of double glazing we had a
6mm air gap, and thought that was marvellous 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 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.
What happened next was a scramble
by double-glazing salesmen to push their companies into producing
bigger air gaps to impress the public into believing that their
product was better that their competitors’. So, next came the 16mm
air gap with, yes, JUST measurably better insulation that the 12mm
air gap, but ONLY JUST, nothing like the big difference between
single glazing and 6mm or even between the 6mm and 12 mm! Then what
happened? You've probably guessed – yes, the 20mm air gap –
which has 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 lose 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, and as hot air rises from the inner (hotter) pane, cold
air falls from the outer pane (convected heat loss).
The manufacturers ‘fixed’ the
inherent problem of increased air circulation inside the 20mm air
gap sealed unit by substituting an inert i.e. 'thicker' gas such as
argon for the dried air that was normally sealed inside the unit.
This seemed to sort out the problem of the air circulation. I say
'seemed to' because it works when the sealed unit is brand new, but,
if I have my facts right (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! Not all sealed units
lose all of the argon in such a short time. Some will lose only 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 dependent on the skill of the operator, not
necessarily on a metering machine. And where the window has a
Georgian grille inside, this will more than likely contain normal
air and not argon.
Argon is most commonly used
because it is relatively cheap. Krypton and xenon have a place where
high performance in narrow cavities is required, but unfortunately
these gases are too expensive for normal domestic applications.
There is a gas called SF6, which can be used to increase sound
reduction by some 2-3 decibels, 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 for
it, and I would not choose my supplier on the basis of whether they
do or don’t use argon in their filled units.
If anyone wants argon gas in their
sealed units from me, I add about £12 a square metre extra to the
cost, but I do not do argon-filled units as standard. I personally
prefer sealed units with a 16mm air gap with its very adequate
allround insulating properties, combined with its expected long
lifespan. 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.
Misted-up
double-glazed sealed units
Do sealed units have a vacuum?
Types of glass
About safety glass
Double Glazed Sealed units - manufacturing techniques
Exploding Double Glazed Sealed Units
Brewster's Fringes
