A patch of fog in the middle of the glass that will not wipe off, inside or out. It is not dirt and it is not poor ventilation: the glass unit has lost its seal. This guide explains what happened — and why the fix is simpler than it looks.
Touch the fog with a finger, from inside the house. If your fingerprint stays on the glass and the mist wipes off with a cloth, it is surface condensation: household moisture settling on cold glass. That is solved with ventilation, or with glass that insulates better — and our signs guide covers that case in detail.
If the glass is dry to the touch and the haze stays put, on both sides, however much you clean — it is between the two panes, in a space no cloth can reach. That is the case this guide is about. And it has one consequence worth stating up front: no product, trick or ventilation routine will fix it, because the problem is not in the room's air. It is inside the glass.
Double glazing is not “two panes of glass”: it is a factory-sealed unit. The two panes are held apart by a spacer bar, the cavity is filled with dry air or a more insulating gas (typically argon), and the whole perimeter carries a double barrier of sealant. Inside the spacer there is also a desiccant — a material that absorbs whatever traces of moisture remain in the cavity. This assembly, manufactured to the EN 1279 standard, is what keeps the glass clear and insulating for decades.
The mortal link is the sealant. It ages with solar radiation and with thermal cycling — the frame expands and contracts every day, and every cycle works the bond. Once the sealant cracks, humid outside air starts entering the cavity with every pressure change. For a while the desiccant covers it up, absorbing what comes in. The day it saturates, moisture starts condensing on the glass — and that is when the fog appears.
That is why fog between the panes is not a cosmetic issue you can ignore: it is the certificate that the seal is gone. The insulating gas has left, humid air has moved in, and the process only runs one way.
The failure does not arrive overnight. Knowing the stage helps you gauge the urgency — and avoid paying for “solutions” aimed at the wrong stage.
Early in the morning, or on cold days, a patch of fog appears in the middle of the glass and fades within hours. It looks harmless — and it is the first warning: moisture is already getting into the cavity, but the desiccant in the spacer bar can still absorb it as the day warms up.
The patch stops disappearing. You clean the glass inside and out and it is still there, because it sits in the one place no cloth can reach: between the two panes. Many people live with this stage for months convinced it is grime — it is the desiccant, saturated, absorbing nothing more.
Water starts condensing into droplets, running down the inner face of the cavity and leaving whitish mineral streaks that bond to the glass. By this stage you see the traces even on dry days. The unit has been dead for a long time — it just had not said so this clearly.
At any of the three stages the cause and the fix are the same. The only difference is how long the unit has been dead — and the mineral streaks of stage three will not come off even after a postponed replacement.
The good news about this problem: in most cases you replace the glass unit, not the window. The beads holding the glass come off, the dead unit comes out whole, a new made-to-measure one goes in, and the frame, the hardware and the sealing to the wall stay exactly as they were. It is a job of hours.
About the alternative you will find if you search: “defogging” by drilling — piercing the glass, drying the cavity, fitting valves. We would rather be direct: it treats the haze, which is the visible part, but it does not restore the insulating gas, replace the broken seal or regenerate the desiccant. The unit ends up dry and still dead. As a temporary cosmetic measure it may have its place; as a repair, it is not one.
When does the right answer become a complete window? When the dead unit is the symptom rather than the disease: a warped frame, leaky sashes, hardware at the end of its life. If the window has two or three complaints at once, it is worth putting the full diagnosis next to quotes for both options before deciding.
Fog that will not wipe off?
At the survey we confirm whether the glass unit is enough or the frame needs attention too — and quote both in writing.
No, and that is exactly what distinguishes it from normal condensation. The fog sits inside a factory-sealed cavity with no access of any kind: no cloth, product or trick reaches it. If the mist wipes off, it is on the surface and the glass is fine — the 30-second test above shows how to tell the difference.
Not in any real sense. There is a “defogging” service that drills the glass, dries the cavity and fits valves — it treats the visible part, but returns none of what the unit lost: the seal is still broken, the insulating gas is gone and the desiccant is saturated. It is a cosmetic, temporary fix, not a repair. The effective solution is replacing the glass unit.
In most cases, no. The glass unit comes out of the frame and a new one goes in, with the frame, hardware and perimeter sealing untouched — a job of hours, not a building project. Replacing the complete window only makes sense when the frame is also at the end of its life: warped, leaky, or single-glazed. Our signs guide helps you assess the rest of the window.
The cause is always the same — the perimeter seal stopped being airtight — but the timeline varies a lot. The seal ages with sunlight and with daily expansion cycles; a quality unit, properly installed, lasts decades. Early failures point to a cheaper seal or spacer, or to poor installation: badly placed packers and blocked drainage leave the base of the unit sitting in water, and the seal is not made to live wet.
Worse than it did when it was sealed. The cavity of these units leaves the factory filled with dry air or with a more insulating gas such as argon; once the seal is broken, that barrier degrades and humid air moves in. The unit is still better than single glazing, but it is no longer the double glazing you paid for — and the haze takes away the clarity, which is the part you look at every day.
It depends on age and on what was bought. Insulating glass units are manufactured to EN 1279, which imposes tightness and seal-durability tests, and manufacturers warrant them against internal condensation — terms vary by brand, so it is worth digging out the paperwork before paying for a new unit. On top of that, consumers in Portugal have the 3-year legal guarantee of Decree-Law 84/2021: a unit that fogs internally within that period is a lack of conformity, with the right to free repair or replacement from the seller. That is also the regime for the windows we install.
It depends on the size, the glass build-up (Low-E, acoustic, laminated) and access to the opening, so we do not quote a number without measuring — an invented figure here would be worth less than none. What we can say for certain: it is substantially cheaper than a complete window, and the quote is itemised in writing, with the new glass composition identified.
Almost always — it is the right question to ask at this moment. If the unit is being replaced anyway, the price difference for better glass is just the difference between units, with no extra labour. It is the chance to move to <a href="/en/guides/low-e-glass/">Low-E</a> in a cold room or to <a href="/en/guides/acoustic-glass/">asymmetric acoustic glass</a> in a bedroom facing the street. The limit is the frame: the rebate has to accept the new thickness, and that is confirmed at the survey.
We assess the unit and the frame in the same visit, and tell you which replacement makes sense — even when it is the cheaper one.
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