The glass in a modern window is a specification rather than a material. Two units that look identical can differ in the coating, the gas between the panes, the spacer at the edge and whether either pane is toughened or laminated, and each of those changes what the window does and what it costs.
Coatings do most of the thermal work
A low emissivity coating is a microscopically thin metal oxide layer on one of the internal faces that reflects long wave heat back into the room while letting daylight through. It is the single largest reason a modern unit outperforms an older one, and it is invisible. Solar control coatings work the other way, rejecting some incoming solar energy, which is what you want on a large south or west facing glazed area and what you do not want on a north facing room.
Argon gas windows and what gas fills are worth
Argon is denser than air and conducts less heat across the cavity, giving a measurable improvement for very little cost, which is why it is close to standard. Krypton performs better in a narrow cavity and costs considerably more, so it appears mainly in slim units where the cavity cannot be made wider. Gas fills do slowly permeate out over many years, which is one reason a very old unit underperforms its original rating even before the seal fails.
Tempered glass windows, laminated glass and where each is required
Toughened glass is heat treated so it breaks into small blunt pieces. Laminated glass has a plastic interlayer that holds the fragments together. Safety glass of one kind or the other is required in critical locations: low level glazing near floor level, glazing beside a door, and overhead glazing. Laminated is also what improves security and acoustics, which is why it appears in ground floor and roadside specifications well beyond where it is compulsory.
How long do windows last? What wears out, and when
The frame is usually the long lived part. The sealed unit is the consumable: its edge seal eventually admits moisture and the unit mists, which can happen well before the frame is finished and is replaceable on its own. Hardware, hinges and gaskets are serviceable parts that benefit from occasional adjustment and lubrication. Judging the whole window by the age of the glass, or replacing whole windows because the glass has gone, is the common and expensive mistake.
Questions people ask about types of window glazing
Is argon in double glazing worth paying for?
It is close to standard now and adds a real improvement for very little cost. It is not something to pay a large premium over, and it is not a substitute for a good coating.
When is toughened or laminated glass required?
In critical locations: low level glazing, glazing next to a door, and overhead glazing. Laminated is additionally chosen for security and noise where it is not compulsory.
Can I tell if my units have a coating?
A flame held near the glass shows reflections of different colour where a coated surface is present, which is the simple trade check. A glazier can confirm it properly.
Do sealed units have to be replaced with the whole window?
No. Where the frame and hardware are sound, the unit alone can be replaced, which is far cheaper and is the right answer for a misted window.