Residential FAQs - Windows - Technical

1 Why have I got external condensation on my windows?

We receive a lot of enquiries about the appearance of external condensation particularly in the spring and autumn. Whilst we state in our literature that fitting modern low glazing increases the chances of external condensation, it does seem to surprise many customers. Firstly we need to say that the appearance of external condensation is not a fault in the glass or the windows. The phenomenon is a natural and predictable event caused by the outer pane of the glazing being colder that the glass that it replaced. With single glazing and older style double glazing a larger proportion of heat was lost to the outside through the glass. With modern low e glass products more of the heat is kept inside and the outer pane is not heated as much. Moisture condenses out of the air onto a cold surface that is said to be below the dew point. The dew point varies with the air temperature and the amount of moisture it contains. In spring and autumn in particular the glass temperature can fall to a low level during the night and the dew point can be comparatively high in these seasons. The glass is more often likely to be below the dew point in these conditions and the moisture condenses onto the surface. 

We are all obliged to fit more thermally efficient windows in our homes to comply with the building regulations. There are only a few exceptions to the regulations and they tend to apply to unheated spaces that would suffer external condensation to the same extent anyway. The trend is to use glass that has lower Ug values over time and the lower the Ug value the lower the outer pane temperature is likely to be and the bigger the risk of condensation on the external surface.  In northern European countries where they use triple glazing with very low Ug values the phenomenon is understood and accepted. The householders are focused on saving energy and maintaining a comfortable internal environment.


There is not much that can be done to avoid the risk of condensation to the outside. Heating the room more would have an effect but this understandably is not a good option. In many cases the condensation does not last long. A little heat from the sun warms the outer glass enough to evaporate the moisture and a gentle breeze or wind will do the same job. Those people who have fitted Pilkington Activ™ may also notice that they get fewer occurrences. To reduce the risk of condensation on the outside glass surface we recommend our Pilkington Anti-condensation Glass


Pilkington Activ™ self cleaning glass is as prone to condensation as any other glass but the properties of this product means it doesn’t allow the water to form beads on the surface of the glass and so you don’t see the effect to the same degree.
You may notice that not all of the panes are affected by early morning condensation even in the same window. Even subtle differences in orientation and the position of objects outside the window can change the surface temperature of the glass to the point that one pane suffers and another does not. Any object (be it an overhang, canopy, tree etc.) blocking off the window to a clear night sky may also have the effect reducing the occurences.


A plus point is the knowledge that your windows are keeping the heat in as they are designed to do thus proving that you have a superior insulating glass product. The presence of external condensation in a particular season does not mean that the glass will suffer the same throughout the year. Any occurrence is beyond the control of the window supplier and is a natural result of the environmental conditions. 


If you are experiencing condensation to the inside of the room or there is condensation between the panes of glass then that is a different problem. Misting between the panes indicates a seal failure and the glass should be replaced. Misting inside the room may be as a result of a failed unit seal but is more likely to be the humidity or moisture content of the air in the room being very high, e.g. from a bathroom or kitchen. Bottled gas and paraffin heaters produce a lot of moisture and even breathing expels enough moisture in an unventilated space to cause the formation of condensation. Increasing the ventilation to such spaces helps control the problem.

2 Why have I got a haze/milky appearance on my windows?

Haze is an optical phenomenon which makes the glass look like it is covered in a very fine,  uniform, layer of dust when viewed from an oblique angle or viewed under strong light incident on the glass at an oblique angle.

Pilkington K Glass™ can, under certain lighting conditions, display this phenomenon to a limited extent.

The reason for this is that the Pilkington K Glass™ coating is not as smooth as the glass surface.  While this is not obvious to the eye when examining the glass, some people who regularly handle Pilkington K Glass™ can tell which side the coating is on by the feel of it.

The optical effect of the slightly rougher surface is to scatter a small proportion of the light incident on it (in exactly the same way as a thin layer of dust would, which is why it looks similar).  With Pilkington K Glass™, the amount of scattered light is generally less than half of one percent of the light coming through the window, so under most viewing conditions it is not obvious.  However, when incident sunlight is at an oblique angle and the view through the glass is of a shaded area, then the scattered light can become more visible, giving rise to the appearance of haze.

Most coated glasses are susceptible to the phenomenon of haze, to a greater or lesser extent.  The amount of haze on Pilkington K Glass™ is limited as far as practicable, and is generally better controlled than on its direct competitors in the field of low emissivity coatings.

3 Why do my net curtains look grey/dirty?

The benefits of reduced condensation and high thermal insulation obtained from using Pilkington K Glass™ are due to the presence of a special transparent metallic type coating on one of the surfaces of the double glazing unit.  This ultra-thin coating is transparent but has a very small effect on white light transmission.

Pilkington K Glass™ has high light transmission and often appears indistinguishable from clear float glass.  For this reason it is recommended that the presence of the coating is confirmed by the use of a Pilkington K Glass™ detector after installation.  However, the coating does have a pale straw coloured tint and when a light coloured object or material is viewed through the glazing, depending on local circumstances and conditions, occasionally a slight darkening effect may be seen.

This may make net curtains or Georgian Bars look grubby or discoloured in certain situations.

Unfortunately, there is no way of overcoming this characteristic, although it does confirm the presence of Pilkington K Glass™ in the windows.

4 My double glazing units have failed, who will repair them?

You need to refer in the first instance to your window supplier who will inspect the units and then take up the matter with the unit supplier if appropriate. This is not something that NSG Group is able to assist with directly since your contract is with you installer. If Pilkington units are installed and considered to be at fault your window installer will take up the matter with the supplying NSG Group company directly.

5 My window supplier says he has installed a Pilkington patterned glass but it doesn't look right, can you help?

You do need to take this up with your installer. In order to help please browse through our range of Pilkington Texture Glass and the Pilkington Oriel Collection patterns.

6 The labels that were on my windows have left marks can you advise on cleaning them off?

Cleaning of Glass
Glass should normally be cleaned with a clean cloth and a clear non-abrasive cleaner, such as clean soapy water or a proprietary window cleaner.  For glass with a patterned, etched or sandblasted surface, a stiff bristle or nylon brush can be used to remove dirt from the recesses in the surface.
If the glass does not become clean using the above methods the following can be tried:-
Use a mild acid (e.g. vinegar) or a mild solvent (e.g. methylated spirit), taking care not to spill any of these on other surfaces or materials.
If neither treatment works then the glass will need to be abraded and re-polished.  this is likely to be expensive and should be left to experts.  The process may result in some optical distortion where the glass surface has been removed.
Special Notes on Cleaning
Organic solvent cleaners should be avoided on mirrors since they may attack the backing paint.
Leaded Glass should only be cleaned with a clear non-abrasive cleaner.  The cleaner should be of a type that does not remove the patina (aged surface finish) of the lead.
Patterned glass, acid etched glass and sandblasted glass should only be cleaned with a clear non-abrasive cleaner.  Any insoluble material in the cleaner may deposit in the recesses of the etching or sandblasting and may be very difficult to remove.