Why Aluminum Windows Fog Between the Panes

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Fog between the panes of an aluminum window is caused by moisture getting inside the sealed cavity between two sheets of glass. This happens when the edge seal or the desiccant that absorbs humidity breaks down, usually after years of temperature swings and sun exposure. Once the seal fails, the trapped air can no longer stay dry and condensation forms on the inner glass surfaces.

What is happening inside a double-glazed unit

A double-glazed unit is two panes of glass separated by a spacer and sealed around the edges. The cavity between them is normally filled with dry air or gas, and a drying agent called desiccant sits inside the spacer to absorb any moisture present during manufacture.

As long as the seal holds, the air inside stays dry and the glass looks clear. Fogging is simply the visible sign that moisture has entered the cavity and is condensing on the glass.

The main causes of seal failure

Several factors can break the seal over time:

Constant expansion and contraction from daily temperature changes
Strong sun on one side of the window, which heats the frame and glass unevenly
Ageing of the sealant, which becomes brittle and loses adhesion
Exhausted desiccant that can no longer absorb humidity
Small defects from the original manufacture or from installation stress

Often it is a combination of these rather than a single cause.

Why aluminum frames can be involved

Aluminum conducts heat quickly, so the frame and the edge of the glass can change temperature faster than the centre of the pane. This creates stress on the sealed edge, especially in hot climates where sun hits the window for many hours each day.

Poor drainage in the frame or a glazing bead that is not seated correctly can also let water collect near the edge of the unit, which speeds up deterioration of the seal.

How to tell fogging from ordinary condensation

Condensation on the outside of the glass or on the room side of the window is normal under certain humidity conditions and is not a seal problem. Fogging between the panes is different: the mist or film is trapped inside the unit and cannot be wiped off.

If you see a haze, streaks, or water droplets between the two layers of glass that do not change when you clean the surface, the seal has most likely failed.

What can be done about it

In most cases the sealed glass unit must be replaced. Cleaning the inside of the glass is not possible because the cavity is closed. The usual approach is to measure the existing unit, confirm the glass thickness and spacer type, and fabricate a replacement that fits the existing aluminum frame where the frame is still sound.

If the frame itself has corrosion, bent sections or poor glazing beads, it may be more economical to address those at the same time. Exact sizing and glazing details are confirmed with the engineer during the on-site measurement.

Preventing fogging in future

You cannot stop all ageing, but you can reduce stress on the seal. Keep the frame and the drainage channels clean so water does not sit against the edge of the glass. Where direct sun is intense, external shading such as awnings or louvres helps reduce temperature swings.

When ordering new aluminum windows, ask about the quality of the sealed unit and the spacer system, and make sure the glazing is installed without twisting the frame.

Frequently asked questions

Can fogged double glazing be repaired?

In most cases the sealed unit cannot be repaired because the cavity is closed. Replacing the glass unit is usually the practical solution.

Does fogging mean the window is dangerous?

Fogging itself does not make the window unsafe, but a failing seal can indicate that the unit is near the end of its life. Have it checked if the glass feels loose or shows cracks.

Is fogging always caused by a bad window?

Not always. Seal failure can come from age, sun exposure, or temperature stress, even on good quality windows.

How do I know if I need a new frame as well?

If the frame is corroded, bent, or has damaged glazing beads, those should be addressed at the same time. An on-site inspection will show whether the existing frame can be reused.

Common mistakes to avoid

Many owners wipe or clean the inside of the fogged glass, which does nothing because the moisture is sealed inside. Waiting too long can also let the frame and surrounding sealant deteriorate further.

Our recommendation

Have the window inspected on site to measure the unit and check the frame condition before ordering a replacement, so the new glass fits correctly the first time.

Summary

Fogging between aluminum window panes means moisture has entered the sealed cavity because the edge seal or desiccant has failed. The sealed unit usually needs replacing, and a site check can show whether the frame can be kept.

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