Glass:
Sealed units air gaps

Glass Sealed Units
As the glass of single glazing is a good conductor of heat, the single panes will not insulate well, and if it is as warm as toast indoors and freezing cold outdoors, the temperature of glass in single glazing will be somewhere between the two, i.e. still very cold.

Enter 'double glazing'.

Unlike glass, air is a poor conductor of heat, and so double glazing works by having two panes of glass held apart by a 'spacer bar' with an 'air gap' between, and then sealed up around the perimeter, hence the name 'sealed unit'.

Unfortunately, air is a good convector, which means air circulates inside the unit if the air gap becomes too large. That is why we cannot make sealed units even thicker than today's and then get even better insulation. The widest optimum air gap has been achieved and even exceeded in some ways.

An air gap of between 12mm and 20 mm is fine, but the difference in insulation between the common 16mm air gap and the 20mm air gap is minimal. Some manufacturers have tried to fix this problem by filling the air gap with a 'thicker' gas (commonly argon), but this is not without its own problems.

Because this is a pretty big subject I have split it into a number of sub-categories so that you can easily find whatever specific and detailed information you want. Fact – interesting or boring depending on your point of view:

In 1983 some 6 million square metres of double-glazed sealed units were made for the UK. In 2002 that figure was up to 25 million square metres.

All about 'misted-up' sealed units
Do sealed units have a vacuum?
Air gaps and insulation
Types of glass
About safety glass
Sealed units - manufacturing techniques
Why has my Georgian grille between the glass gone 'yellow'?
Why did my sealed unit explode?
Brewster's Fringes

Drool over the fancy coloured and bevel designs, so many to choose from:
View the coloured window fanlights
View the bevelled window fanlights
View the coloured door tops
View the bevelled door tops