Argon filled double glazing is one of the few pieces of glazing specification that has entered general conversation, which means it is also one of the most oversold. The gas does a real job and it is worth having, but it is one of three things inside a sealed unit that decide how the unit performs, and it is not the most important of them. Understanding what it contributes, and what it does not, makes it much easier to read a quote and to tell a specification from a slogan.
What the gas is actually doing
Heat crosses the cavity of a sealed unit partly by conduction through whatever fills it and partly by convection as that filling circulates. Argon is denser and less mobile than air, so it conducts less readily and it circulates less willingly in the gap. Replacing the air with argon therefore cuts the heat crossing the cavity without changing anything you can see. It is inert, so it does not react with the glass, the coating or the seals, which is why it is used rather than something more exotic.
The coating usually matters more
The low emissivity coating on the inner pane is the largest single contributor to a modern unit's thermal performance, because it reflects long wave heat back into the room instead of letting it radiate across the cavity. Radiation is a bigger share of the loss than conduction and convection are, so a unit with a good coating and plain air outperforms a unit with argon and no coating. When a salesperson leads with the gas and says nothing about the coating, that ordering tells you something.
The spacer bar is the quiet third component
The spacer holds the panes apart and seals the cavity, and it runs right round the edge of the unit where warm interior glass meets cold exterior glass. A traditional aluminium spacer conducts heat straight across that junction, which is why edges are the coldest part of an old unit and where condensation appears first. Warm edge spacers in less conductive materials reduce that path. It is worth asking which spacer is being supplied, because it affects both performance and how the unit ages.
Argon leaks slowly, and that is normal
A sealed unit is not perfectly sealed forever. Gas escapes very gradually through the edge seal over the life of the unit and air moves in the other way, so the benefit reduces slowly over many years. This is expected behaviour rather than a fault, and it is one reason the seal quality and the spacer matter as much as the fill. A unit whose seal has failed outright announces itself differently, by misting between the panes, and at that point the unit is replaced rather than refilled.
What to ask for in writing
Ask the supplier to state the make up of the unit: the thickness of each pane, the width of the cavity, which pane carries the coating, whether the cavity is argon filled, what the spacer is, and whether any pane is toughened or laminated. That is a single line of specification and any competent supplier can produce it. A quote that describes the glass only as high performance or energy efficient has told you nothing you can compare with another quote.
Questions people ask about argon filled double glazing
Can I have argon added to my existing windows?
No. The fill is put in when the sealed unit is made and the unit is then closed. Changing it means new units, which is a glazier's job into your existing frames if those frames are sound.
Is krypton better than argon?
It performs better in a narrower cavity, which is why it appears in slim units and some triple glazing, and it is considerably more expensive. In a normal cavity width, argon is the sensible choice and the difference is small.
How can I tell if a unit is really argon filled?
Not by looking at it. This is a matter of the supplier's specification and paperwork, which is exactly why the unit make up is worth having in writing before you order rather than after.