It is a rare sort of ‘oily’ looking effect on a double glazed sealed unit, and it does not mean that the perimeter edge seal has broken down, or that the sealed unit is faulty in any way.

The appearance of the optical phenomenon known as Brewster’s Fringes is not a defect of the glass and can occur with any glass of high optical and surface quality. This phenomenon is a result of the high quality now being achieved world wide by modern methods of glass manufacture.

Brewster’s Fringes occurrence is wavelengths of light meeting up with each other when they are exactly 180 degrees out of phase – an example of this phenomenon is known to physicists as the ‘interference’ of light. The effect is similar to, although usually much smaller than, the interference fringes which can sometimes be seen on toughened glass windscreens.

In the case of insulation glass installations, Brewster’s Fringes only occur when the surfaces of the glass in the double glazing sealed unit are flat and the two panes of glass are parallel to each other, i.e. when the light transmission properties of the installation are of a very high order. What happens is that some of the incident light from the Sun meets light reflected from one of the surfaces of the insulating glass in such a way that they are 180 degrees out of phase and cancel each other out, thereby giving rise to a fringe (sort of oily looking) effect, small in area on the glass when viewed from a particular angle. Alternatively, different parts of the incident solar radiation may be refracted through the glass by different amounts and end up by being 180 degrees out of phase. This phenomenon is not a defect of the product, being dependant on the laws of physics and not the quality of the insulating glass sealed unit. In fact it arises because modern glass made by the float process is flat and, therefore, free of the distortion inherent in sheet glass.

The occurrence of Brewster’s Fringes is in it’s nature rather like (though very much rarer than) the fact that under certain conditions, you could see a reflection of yourself in and window or door, when it is supposed to be clear glass that you can see through, and no-one could claim that this is a defect of the glass.

Special Note:

With any patterned double glazed sealed units, the above phenomenon does not occur, simply because of the very nature of the diffuse qualities of an obscured glass.
Source: www.thewindowman.co.uk

No, condensation on windows is not the fault of the window, unless the window seal has failed and the moisture is in between the two sheets of glass. However, by replacing drafty windows or even installing a new roof, you are reducing air flow in your home and making it tighter. Tighter homes retain more humidity cutting down on the ventilation

Windows present a difficult problem and years ago there used to be only one practical means of dealing with the traditional single glazed domestic window. This usually took the form of collecting the water forming in a channel at the bottom of the window and leading it via ‘weep holes’ to the external face. These ‘weep-holes’ or condensation outlets are small holes about 1/4″ in diameter drilled at an angle through the frame so that water will pass by normal gravitation to the outside. It was preferable to have a small copper tube passing though a timber frame so as to prevent a tendency for saturation to take place.

The easy answer these days to reduce window condensation is with double-glazing, and normally with PVC-U frames. The insulation properties of both framework and glass is improving all the time as technology moves on, and as measures are introduced by government to improve energy efficiency in line with the Kyoto Protocol. However, glass is still glass, and no matter how well insulated by double-glazing, the interior pane will always be susceptible to the formation of condensation in a situation where the relative humidity reaches impossible limits. With double glazing, houses have become more effectively sealed, keeping any moisture produced within the house and providing better conditions for condensation to occur, therefore ventilation has become highly important.

It is unlikely that a home in the UK can be condensation free, however by keeping your property properly maintained and thinking about your lifestyle and decoration, you should be able to live with condensation without it ruining your life

Walls and other surfaces can be wiped down regularly.

While drying clothes indoors, ventilate the room.

When people come in with wet coats, hang them outside the living area to dry. A good reason for a porch.

Keep the kitchen door closed when cooking, and open the windows.

After a bath or shower, try to ventilate the room to the outside, not to the rest of the house – just opening a window (and closing the door) will help.

Open all windows at least once a day. Change all of the air in the house.

If security permits, leave top windows in the bedrooms open, especially at night, when the outside temperature drops and the indoor temperature rises as the heating kicks in. Breathing is a major contributor to condensation!

Reduce the number of indoor house plants, as plants increase humidity levels

If condensation is excessive, try a dehumidifier to remove excess humidity from the air

Condensation is an extremely underestimated cause of damage to our homes. It is responsible for rot in rafters, joists and wooden window frames; it can cause mould and fungal growth on walls and ceilings and if not dealt with, can damage our health.
Air in the home is actually a mixture of air and invisible water vapour. The average family produces 10 to 50 litres of moisture a day from activities like cooking, bathing, washing dishes, and doing laundry. Condensation is the conversion of the water vapour into a liquid and usually happens when the vapour cools. All air contains water vapour. The quantity contained depends on the temperature of the air. This ratio is called the relative humidity. Hot air is able to carry much more moisture than cold air, so as the temperature of air rises, in broad terms, it expands and attracts a greater volume of vapour. The temperature at which the vapour begins to condense is called the dew point. Condensation occurs whenever warm, moist air comes in contact with a surface or object cold enough to chill the moisture in the air below its dew point.
If this happens near the ground, to a small layer of air, dew or frost will be formed, if a larger amount of air is involved, mist or fog will arise. If this happens to air that is rising in the atmosphere and expanding. Clouds will form
If it happens in the home….It’s called condensation
As soon as warm air, containing vapour, hits a cooler surface, it will condense. This is most obvious on windows and wall tiles, but it is happening on the walls and ceilings a lot of the time. The common assumption is that if we insulate ourselves and warm the house thoroughly, then this should not happen…But it will, the air temperature will rise until it finds a surface cooler unless we let that air out and some cooler air in, which brings us on to ventilation.
Ventilation is the answer to condensation. There are many ways we can deal with symptoms or effects of condensation, but only one way to deal with the cause. We must ensure the home is adequately ventilated.

The double glazed unit consists of 2 pieces of glass held apart around the edge by a (usually silver coloured) spacer bar. This bar is filled with a desiccant, tiny silica balls similar to those found in parcels to reduce the moisture levels. In the case of double glazed units, the silica soaks up any residual moisture held in the space within the unit when it is first made. Around the outside of this metal spacer bar, sealant is pumped to seal the unit and complete the process.

Misty Window Example

After a period of time the seal ‘breaks down’, causing a hairline fracture in the seal, thereby letting air containing moisture in. Various factors can accelerate this breakdown, such as wood frames and putty, poor frame drainage, poor installations not leaving sufficient gap between the glass edge and the frame,  locations in extreme sunny positions or where excessive vibrations occur.

Once air can get into the unit, the silica originally used becomes fully saturated and condensation starts to form as temperatures rise and fall against the side of the glass.

If your units were installed during the last 10 years, check your guarantee and see if you are still covered.

Despite rumours to the contrary, it is not possible to repair the units by splitting them and re-sealing them!

As with all lead products i.e. lead sheet, pipes, flashings, etc., the adhesive lead strips, which we use for our sealed units, are subject to oxidation. This ‘weathering’ process will form a naturally protective patina when exposed to the atmosphere.

This patina is a layer of highly insoluble lead salts, which gradually form on the lead surface to eventually give the familiar grey lead appearance. Examples of this effect can be seen on old leaded lights, such as in church windows.

During the initial stages of the oxidation process, lead can display various colours including blue, bronze and green. It is probable that such films are extremely thin and in fact have no intrinsic colour but appear coloured due to an interference effect Similar interference colours may be observed when oil is present on a wet road. Lead is most likely to appear coloured when placed indoors or in protected locations.

When lead comes into contact with moisture, rainwater, condensation etc. at early stage, discoloration, spotting and white powdery deposits (usually basic lead carbonate) can form and may ‘run-off’. The degree to which all these occur is governed by the environmental conditions, but with longer term weathering, the lead will take on its’ familiar appearance.

Investigations have shown that the patina formation follows the route: lead, lead oxide, basic lead carbonate, normal lead sulphite, and normal lead sulphate. The ‘run-off’ stage occurs when non-adherent basic lead carbonate is formed, usually through contact with moisture. Each of the stages in patina formation is adherent, highly insoluble lead salts and in practice, as these salts develop with weathering, they stifle the basic lead carbonate release. The final patina being approximately 30% normal lead sulphite, 60% normal lead sulphate and 10% normal lead carbonate. However, this can vary dependant upon location, time and airborne impurities.

Suggested after care

In the event that you experience an accelerated rate of oxidation directly after installation, you should make every effort to clean the ‘run-off’ deposits, as soon as possible. Failure to do so could result in the deposits drying onto the glass and becoming more difficult to clean. This effect can be compared to lime scale on a shower screen.
To clean the glass, simply use a water-based glass cleaner with a paper or cloth towel, whilst removing any stubborn stains with a household non-abrasive cream cleaner. Avoid agitating the surface of the lead, as much as possible, as this will slow down the weathering process.