If you have double glazing, eventually the units will "fail" which is what it's called when the unit starts to mist up and condensation starts to appear inside the unit, a process that eventually results in stains appearing on the inside of the glass and, in extreme cases, may actually cause water to collect at the bottom. Some companies may inform you that the entire window needs replacing, but this is not true. The double glazed unit itself can be replaced for a lot less than an entire window.

Aluminium Space BarCredit: eGDC, LtdHow Units are Made

Double glazed units are made from two panes of glass. These pieces of glass are held apart by, usually, hollow bars of aluminium (spacer bar), the two longer ones of which are filled with desiccant. The edges of the unit are then sealed with sealant. The alternative to aluminium bars is a bar that has desiccant properties built in.

The desiccant in the bars absorbs moisture that gets into the unit. The units are not airtight, so moisture can get in. Those units made with bars that have built in desiccant properties work in exactly the same way.

How Units Fail

As mentioned, units are not airtight, so moisture gets in to them. If the unit has been poorly fitted (it isn't resting on spacers, so the bottom of the unit will be sitting in water when it rains), more moisture will enter the unit.

Misted Double Glazed UnitCredit: eGDC, LtdThe desiccant inside the spacer bars will absorb any moisture that enters the unit. This can only absorb a finite amount of moisture though. Once the desiccant has absorbed as much moisture as it can, the surplus moisture will start appearing inside the unit.

At first, this will be patches of condensation that appear on the inside of the unit, which will often vanish if the window is in direct sunlight, only to reappear once the sun leaves the unit. More and more water will appear inside the unit, and it has been know for, rarely, units to actually have water collecting at the bottom of the double glazed unit, sometimes to a depth of several inches.

The Unit Has Failed

Once moisture starts to appear inside a unit, the unit has "failed" and needs replacing. The length of time it takes for this to happen varies a lot, and errors in fitting and manufacture can speed up the process.

It is rare for a double glazed unit to be guaranteed for longer than five years - although the frames themselves are usually guaranteed for ten. If a unit fails before the end of the guaranteed period, whether that is five or ten years (or occasionally higher), it should be covered by a guarantee, and be replaced free of charge. If the company that fitted the double glazing is no longer in existence, then it will be difficult to claim on the guarantee, unless the company provided an insurance backed guarantee, where the guarantee is provided by an insurance company, not the company that fitted the windows.

Replacing the Unit

If the unit is out of guarantee, or the company that fitted the windows has disappeared and didn't provide an insurance backed guarantee, the unit will need replacing. Unless you have any plans to fit the unit yourself, you will need a company to come and quote for a new unit.

Here, you need to be careful. Large double glazing companies do not specialise in this type of work, as it isn't really worth their while. It is not unknown for a representative from such a company to tell you that you actually need a new window, not a new unit, which is substantially more expensive.

This is not really the fault of these people; they are sales reps, not fitters or manufacturers, and are often lacking in technical knowledge. Their job is to sell double glazing, not make or install it.

Even if a company of this type gives you a quote, given their overheads, it could still be much dearer than it should be. Smaller, specialist companies are what you are looking for here.

Finding a Company

In your Yellow Pages (or similar), in a local newspaper's classifieds or by using a search engine to find a local company, you want a company that does double glazing repairs. Such companies are often very small proprietorships, and will have experience with replacing failed double glazed units. Quotes should be free, and will be much cheaper than the window replacement that larger companies may recommend.

Glazing ToolsCredit: eGDC, LtdFitting a New Unit Yourself

It is possible to fit a new unit yourself, but it may not be worth the effort.

Glazing PackersCredit: eGDC, LtdYou will need tools to do the job; a glazing mallet, a glazing shovel and some glazing packers. These can be bought from local glazing supply companies or online. The glazing packers come in a range of sizes and keep the unit out of the water that collects inside a window frame when it rains.

You will need a local double glazed unit manufacturer to buy a new unit from, and you are going to need the correct size, width and composition of the unit.

Externally Beaded WindowCredit: eGDC, LtdJudging these without experience is not the easiest thing to do. Glass type and thickness can vary greatly, as can the actual overall thickness of the unit. Additionally, windows may be internally or externally beaded, the difference being whether the bead is fitted on the inside of the window or the outside, and externally beaded windows may have the units fastened in with security tape or rubber gasket. Security tape is strong, compressible double sided adhesive tape that sticks the double glazed unit to the window frame, and will need slicing through with a knife so that the unit can be removed.Internally Beaded WindowCredit: eGDC, Ltd This is just the two main types of uPVC window available; there are also aluminium frames, which may require the window frame to be partially dismantled, and wooden windows, which could have the units fastened in with beads, putty, sealant or a combination of the three. Although hardwood windows have advantages, ease of replacement of failed units is not one of them.

Bead ChannelsCredit: eGDC, LtdTo measure the width and height of a double glazed unit in a uPVC window frame, you would measure from the channel of bead on one side, which is the join where the window bead meets the window frame, to the same place on the other. With most uPVC windows, 10mm would then be deducted from this. This would be done for both width and height, and is done the same way whether the window is internally or externally beaded.

The overall thickness of the unit is much harder to gauge. It can be done by the naked eye with experience, but otherwise is most easily done using a pair of callipers or an electronic detector to judge the thickness. This will only work if both sides of the window can be accessed. Common thicknesses for uPVC windows are 28mm and 20mm, although many other sizes are possible, especially for wooden and aluminium frames. The spacer bars that partly determine the overall thickness of the unit are also available in different colours.

Finally, there is the type of glass in the window. It may be patterned, requiring the same pattern to be matched. More recent units, especially in the UK, may be made with the inside pane made from low E energy saving glass.[1] Low E glass can be detected using an electronic device, but it can be possible to tell from what tint is added to something white viewed from the other side of the unit. Dirty yellow and purple tints will reveal the most common low E glasses.

The glass may also be toughened safety glass. This is required in doors, units in windows close to doors and in units that are below a certain height above the floor. If only one side of a unit falls below this height, then it would only need to have one side made in toughened glass.

Should You Fit Your Own?

If you know what you are doing, and have experience at this, this is a simple job. It isn't difficult to learn either. The biggest problem is judging the size, thickness and composition of the unit that you are going to replace. It may not be worth trying though. Large units can be too expensive to risk making an error with, and the cost saving on changing one small unit can be less than the cost of the tools you will need to buy.

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