Signs your double glazing has failed in Hobart.
A double-glazed unit does not fail loudly. It fogs quietly between the panes one winter and keeps looking almost normal while its insulating value drains away. Here is how to read the early signs, how to tell a genuine failure from ordinary room condensation, and why the fix is usually far cheaper than the quote you are dreading.
Fog between the panes means the unit is blown.
Fog between the two panes is a blown unit, and it is the clearest sign a double-glazed unit has failed. A sealed unit has a spacer bar around the edge filled with desiccant that absorbs the small amount of moisture trapped inside at manufacture. When the perimeter seal degrades, outside air carrying moisture leaks in, the desiccant eventually saturates, and from then on water condenses between the panes every time the temperature swings. You cannot clean it because the surface is sealed inside, and once it fogs the insulating value is already falling.
People often ask whether it can be drilled, dried and resealed. There are outfits that sell that. We do not, because the desiccant is spent, the gas fill is long gone, and you are left with a vented cavity that will simply fog again in a different way. The honest answer is that a blown unit is a replaced unit.
There is a standard behind all of this, and it is useful to know its name when you are arguing a warranty. Insulating glass units are made to AS/NZS 4666, the Australian and New Zealand Standard for insulating glass units, which covers how a unit is assembled and how its perimeter seal is required to hold up under accelerated weathering. A unit that fogs early has failed against that standard, not merely disappointed you, and that is the language a manufacturer responds to. Separately, where the failed unit sits in a door, a side panel or a window within 500mm of the floor, the replacement unit must be built with Grade A safety glass under AS 1288, so a like-for-like swap is not always quite like for like.
The distinction that matters most.
Before you assume the worst: condensation on the room side of your double glazing is not a failure. Water on the inside face, the surface you can touch and wipe, is a room humidity issue rather than a glass fault. It means the air in the room is holding more moisture than the glass surface temperature can sustain, which in a Hobart winter usually points to drying clothes indoors, unflued heating or poor ventilation. Water sealed between the panes, which you cannot wipe, is the failure. If you can wipe it away, the unit is fine.
This one distinction saves a lot of unnecessary quotes. In a Hobart July, with the heating running and washing on the airer, plenty of perfectly good double glazing runs a little condensation in the corners on the worst mornings. That is the room, not the glass. Reach out and touch it. If your finger comes away wet, nothing is broken.
The early warnings, before it fogs for good.
Watch for four things. A faint haze or oily film on the inside faces of the glass that does not wipe off. A slight distortion or waviness in the reflection, which means the gas gap pressure has changed. Condensation appearing between the panes only on the coldest mornings and clearing by lunchtime, which is the stage before permanent fogging. And perimeter sealant that has gone hard, chalky or cracked where you can see it. Any of those means the unit is on the way out rather than already gone.
The intermittent-fogging stage is the one worth catching. A unit that mists between the panes on a frosty morning in Kingston and is clear again by noon has a seal that is leaking but a desiccant that is still doing its job. That unit will typically go permanently cloudy within a season or two. Catching it there does not save the unit, but it lets you plan the replacement into a summer booking rather than discovering it mid-winter when lead times are longest. The timing guide explains why that matters so much in this city.
What kills units early in Hobart.
- Thermal cycling on west-facing glass. A cold Hobart night then hard afternoon sun off the mountain side works the seal every day of the year.
- Water sitting on the bottom edge. Blocked or missing drainage in the sash lets water pool against the seal. This is the number one killer and it is an install fault, not a glass fault.
- Salt air. Units close to the Derwent, through Bellerive, Howrah and the Sandy Bay waterfront, carry salt-laden air that is hard on frames and seal edges.
- Poor original install. Units packed hard against the frame with no setting blocks, or bedded in the wrong sealant, fail years early.
- Frame movement. Timber sashes on older homes move, and a unit that gets twisted loses its seal.
How long they should actually last.
A well-made insulated glass unit should give 20 years or more, and most carry a manufacturer warranty against seal failure in the range of 10 to 15 years. What shortens that in Hobart is thermal cycling and water. West-facing units take a cold night then hard afternoon sun, which works the seal every single day, and any sash that lets water pool along the bottom edge of the unit will kill the seal early. Units near the Derwent also carry salt-laden air, which is hard on the frame and the seal edge alike.
If a unit has fogged inside ten years, it is worth checking the warranty before you pay for anything. Keep the invoice from the original install. Manufacturers generally want the unit identified and the failure photographed, and a glazier who fitted it properly will help you chase that rather than quietly quoting you a new one.
Worked example: one fogged unit in Moonah.
A 1,200 by 900 lounge unit in a Moonah home, in the Glenorchy City Council area, fogged permanently after about twelve years, aluminium frame in sound condition. The owner assumed a whole new window and had braced for something near 2,000 dollars. What it actually needed: we measure the failed unit and the frame rebate, order a matching low-E unit, pull the beads, lift the blown unit out and set the new one on fresh setting blocks. Six hundred and eighty dollars on 2026 rates, inside the 450 to 900 dollar per-unit band, about an hour on site, frame untouched. No permit and no council involvement either, because the opening never changed.
Usually not, is the answer to whether you have to replace the whole window, and this is the good news that saves people thousands. The sealed unit is a separate component sitting inside your frame, so in most cases we measure the failed unit, order a replacement to match, and swap it into the existing sash. That runs 450 to 900 dollars per unit in Hobart, against 700 to 1,600 dollars per window for a full retrofit. Replacing the whole window only becomes necessary when the frame itself has rotted, corroded or moved enough that it will not hold a new unit properly. Full numbers are on the pricing page.
Frequently asked questions.
Why is there fog between the two panes of my double glazing?
That is a blown unit, and it is the clearest sign a double-glazed unit has failed. A sealed unit has a spacer bar around the edge filled with desiccant that absorbs the small amount of moisture trapped inside at manufacture. When the perimeter seal degrades, outside air carrying moisture leaks in, the desiccant eventually saturates, and from then on water condenses between the panes every time the temperature swings. You cannot clean it because the surface is sealed inside, and once it fogs the insulating value is already falling.
What are the early warning signs before a double-glazed unit blows?
Watch for four things. A faint haze or oily film on the inside faces of the glass that does not wipe off. A slight distortion or waviness in the reflection, which means the gas gap pressure has changed. Condensation appearing between the panes only on the coldest mornings and clearing by lunchtime, which is the stage before permanent fogging. And perimeter sealant that has gone hard, chalky or cracked where you can see it. Any of those means the unit is on the way out rather than already gone.
Do I have to replace the whole window when a double-glazed unit fails?
Usually not, and this is the good news that saves people thousands. The sealed unit is a separate component sitting inside your frame, so in most cases we measure the failed unit, order a replacement to match, and swap it into the existing sash. That runs 450 to 900 dollars per unit in Hobart, against 700 to 1,600 dollars per window for a full retrofit. Replacing the whole window only becomes necessary when the frame itself has rotted, corroded or moved enough that it will not hold a new unit properly.
How long should double glazing last in Hobart?
A well-made insulated glass unit should give 20 years or more, and most carry a manufacturer warranty against seal failure in the range of 10 to 15 years. What shortens that in Hobart is thermal cycling and water. West-facing units take a cold night then hard afternoon sun, which works the seal every single day, and any sash that lets water pool along the bottom edge of the unit will kill the seal early. Units near the Derwent also carry salt-laden air, which is hard on the frame and the seal edge alike.
Is condensation on the room side of my double glazing a failure?
No, and this is the distinction that matters most. Water on the inside face, the surface you can touch and wipe, is a room humidity issue rather than a glass fault. It means the air in the room is holding more moisture than the glass surface temperature can sustain, which in a Hobart winter usually points to drying clothes indoors, unflued heating or poor ventilation. Water sealed between the panes, which you cannot wipe, is the failure. If you can wipe it away, the unit is fine.
Keep reading.
More plain guides for Hobart homeowners:
Think you have a blown unit?
Send the quote form with a photo taken on a cold morning. If it is a failed seal we will price the unit swap, and if it is just room condensation we will tell you that instead.