How long does double glazing really last?
Honest answer for a South East Melbourne house: the glass never wears out, the frame goes 30-40 years, and the one part that decides everything - the edge seal holding argon in the cavity - carries a 10-year warranty and typically runs 20-25 years before it starts letting go. Here is what ages, in what order, and what it costs when it finally does.
Nothing about double glazing lasts forever, and nothing fails all at once.
The glass does not wear out. The seal does.
An insulating glass unit is two panes separated by a spacer bar, with the cavity filled with argon and closed off by a two-stage edge seal - a primary butyl barrier that stops gas escaping, and a secondary structural seal that holds the sandwich together. AS 4666 is the Australian Standard that governs how those units are made and how long they must survive accelerated weathering. Everything you have ever heard about double glazing "going cloudy" is that seal, and only that seal. The float glass on either side of it is chemically inert and will outlive the house.
This matters because it changes how you should think about the purchase. You are not buying a window with a 20-year clock on it. You are buying a frame that will still be serviceable in 2060 and a glass cartridge inside it that may need swapping once, somewhere around 2050, for a fraction of what the original job cost.
Realistic numbers, component by component.
- IGU edge seal: 10-year warranty, 20-25 years typical service life in Climate Zone 6. West and north-west facing units sit at the short end, south-facing at the long end.
- Argon fill: roughly 0.5-1 percent loss per year through a compliant seal. Down about 10-18 percent at year 20, costing a few percent of U-value. Not detectable in use.
- Thermally-broken aluminium frame: 30-40 years. Powder coat may chalk on the western elevation from year 15 and can be recoated.
- uPVC frame: 25-35 years. Australian-market profile is UV-stabilised for our sun, which the cheap grey-import stuff is not.
- Timber sash carrying a retrofit IGU: as long as you keep painting it. We work on 1890s Old Berwick sashes that are on their fourth century of service.
- Hardware - rollers, hinges, stays, locks: 8-15 years. Consumable. Budget $80-$220 per opening to refresh.
- Low-E coating: soft-coat Low-E lives on surface 2 or 3, sealed inside the cavity, never touched by air or hands. It lasts exactly as long as the seal does.
What kills a unit early around here.
Three things, in order. First, standing water in the sill. If the frame drainage slots are blocked with silt or the installer never cut them, the bottom edge seal sits in water and fails a decade early. That single detail is the difference between a 25-year unit and a 9-year one. Second, thermal cycling on west-facing glass - a Hampton Park or Cranbourne west elevation can swing from 8°C at 6am to 62°C surface temperature on a 41°C February afternoon, and that daily expansion works the seal. Third, frame racking from reactive clay. Much of the flat southern Casey corridor is classed H1 or H2 under AS 2870, and seasonal ground movement twists frames that were not shimmed and packed with a bit of tolerance.
What the end of life actually costs in 2026.
Take a real scenario. A 1998 brick-veneer in Berwick, 18 openings, fully retrofitted with thermally-broken aluminium and argon Low-E IGUs. At year 22, one 1,800mm x 1,200mm unit on the western living room wall starts showing a haze at the bottom corner on cold mornings. Nothing else in the house has moved.
You do not replace the window. You replace the glass cartridge. A like-for-like IGU swap into the existing frame runs $580-$980 for that size in 2026, done in about ninety minutes, with the frame, hardware and paintwork untouched. The equivalent full-window replacement would be $2,400-$3,800 from our pricing guide. So the honest total cost of ownership on that opening is the original install plus roughly $780 once, twenty-two years later. Spread across the whole house you should expect to reglaze two or three units somewhere between year 18 and year 28, not eighteen of them.
That is the number nobody quotes you at the sales table, and it is the one that makes double glazing sensible. If you want to understand the failure mode before it happens, read our guide to replacing a fogged IGU, and the practical steps in double-glazing maintenance that push the seal date out by years for the price of an hour with a hose and a soft brush.
Retrofit vs replacement changes the arithmetic.
If you keep an original timber sash and drop a slim IGU into the rebate - the approach on our retrofit existing frames page - the frame has no expiry date at all. Repaint it every 8-12 years and it simply continues. Compare that to a full unit swap using double-glazed windows in a new frame: better U-value today, but you have introduced a frame with a finite life where there was not one before. Both are right answers. They are right for different houses, which is why the measure happens before the quote and not after. There is more on how the housing stock splits across the corridor on our Berwick page.
Lifespan questions we get every week.
How long does double glazing actually last in a South East Melbourne home?
Plan on 20-25 years of clear service from the insulating glass unit and 30-40 years from a thermally-broken aluminium frame. The glass itself does not wear out at all. What ages is the edge seal holding the argon in the cavity, and the hardware you touch every day. A well-made AS 4666 unit, installed square, drained properly and kept clean, will usually still be clear at year 25. A bargain imported unit sitting in a west-facing Cranbourne wall with a blocked sill drain can fog at year 8.
What fails first - the glass, the seal or the frame?
The hardware fails first, usually between year 8 and year 15 - rollers flatten, hinges drop, locks stiffen. That is a cheap fix and not a reason to replace anything. The edge seal is second, typically year 18 onward, and that one is terminal for the unit because once moisture gets into the cavity the fogging never goes away. The frame is almost always the last thing standing. We regularly pull 30-year-old aluminium frames out of Berwick houses that are still structurally fine and only get replaced because they have no thermal break.
Does the argon really leak out of a double-glazed unit over time?
Yes, slowly, and it is designed to. A compliant AS 4666 unit loses roughly 0.5 to 1 percent of its argon fill per year through the edge seal. After 20 years you are down about 10-18 percent, which costs you a few percent of the U-value - not something you would ever feel. Argon leakage only matters when a seal has genuinely failed, and by then you will see the fogging long before you notice the thermal drop. Anyone telling you the gas needs topping up every few years is selling you something that does not exist.
Do double-glazed windows last as long in Cranbourne and Hampton Park as they do in Berwick?
Slightly less on the flat southern side of the City of Casey, and the reason is soil rather than air. Cranbourne, Hampton Park, Lynbrook and Lyndhurst sit on highly reactive clay classed H1 or H2 under AS 2870. Seasonal ground movement racks the frame a few millimetres each year, which works the corner joints of the IGU. Berwick and Endeavour Hills sit higher on firmer ground with less movement. It is worth 2-4 years of seal life either way, which matters less than whether the installer packed and shimmed the frame properly in the first place.
What does the 10-year IGU warranty actually cover?
It covers edge-seal failure resulting in visible moisture or fogging inside the cavity, and nothing else. It does not cover breakage, scratches, hardware wear, condensation on the room-side surface, or the frame. Read the wording before you sign a quote: some warranties are pro-rata, meaning a year-9 failure gets you 10 percent of the unit cost rather than a free replacement. Ours are non-pro-rata on the IGU seal for the full 10 years, 7 years on the thermally-broken frame and 12 months on workmanship.
Where we measure and reglaze.
Wondering how much life is left in your glazing?
We will tell you honestly at the measure - including when the answer is "leave it alone for another eight years". Free in-home measure across South East Melbourne.