How to diagnose and fix misted double glazing

How do you know if your double glazing has failed?

Misted double glazing

As double glazing experts, this is a question that we’re asked a lot. Double glazing is made by putting two panes of glass together with a thermally efficient spacer bar in between that is slightly smaller than the overall dimensions of the glass to form a gap between them. The glass is then sealed together using a hot melt adhesive to form a double glazed unit. The air trapped inside the two glasses (or gas for improved thermal efficiency) creates an insulating barrier that keeps heat in and the cold out.

It is only when the seal on the double glazed glass breaks down, or fails as is known in the industry, that condensation will get into the inside of the glass unit, resulting in misted double glazing.

Why does double glazing mist up?

Double glazed sealed units have only broken down (failed) if moisture or “misting” appears between the two double glazed glass panes. More importantly, it is clearly no longer a “sealed unit”, so it will not be performing the function of a sealed double glazed unit; it can no longer keep the cold out, or allow energy generated by your heating system in and allow solar gain to be retained within the dwelling. The seal can break for a number of reasons, including the age and condition of the window frame or a problem with the manufacture of the unit. It could also be due to how the window was originally installed or a chemical cleaning material used on the window that over time has damaged the seal.

Is it worth replacing broken down sealed units?

While replacing sealed units that have broken down is not all that important during the warmer summer months, it is vital to have the work done prior to the onset of winter. Broken sealed units are leaking expensive energy out from your home into the environment, making your heating system work harder than it needs to. Failed sealed units can also be very unsightly, so, all in all, they are worth replacing to restore the energy efficiency of the windows and/or doors that the defective sealed units are in.

For UK homeowners with modern energy rated aluminium or uPVC double glazing, replacing broken down sealed units is simple and easy to do. Properties with very old fenestration systems or timber double glazing many find replacement more costly, especially if the units are secured into the frames with putty rather than wooden beads.

CN Glass supply and install double glazed units to the strict British Standard and we will provide you with a 10 year sealed unit guarantee. Investing in home maintenance by replacing any broken down (failed) sealed units will definitely be money well spent.

Do I need to change my window frames?

This is a common misconception. Replacement double glazed units are a much cheaper alternative to installing new windows. As a misted up window is due to the seal on the glass, which has nothing to do with the frame itself. Indeed, as long as the frame is in good condition, you simply just need to replace the glass unit. When performing double glazing repairs, it is a great opportunity to upgrade the glass to A-rated energy efficient to get better insulation and lower your heating bills.A misted double glazing unit from a broken down DGU

Understanding condensation

On the inside of the inside glass pane

Condensation on the inside of double glazing means there is a ventilation problem within that particular room and/or property that needs addressing. You can also get condensation on the inside of the units when clothes are being dried in a room when it is cold and wet outside.

On the outside of the outside glass pane.

Condensation on the exterior means the double glazed unit is performing exactly as it should and you will notice it disappear throughout the day as the sun rises. This phenomenon is a natural occurrence and not a fault with the glass or the window, and is not due to your double glazed window failing as described above. The situation occurs mainly in Spring and Autumn, when the glass temperature falls to a low level at night, with the moisture of a heavy morning dew condensing against the cold surface of the glass.

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