Windows General

Residential FAQs - Windows General

1 I'm told that my windows should have a safety stamp on can you tell me more about this?

Building regulations demand that all glass which is low level, close to doors or greater than a certain size should be toughened or laminated to demonstrate that has impact resistance to the required standard.

The stamp confirms the number of the standard it is tested against, the level it achieved and some sort of identification to identify who supplied it. If you have any concerns about whether glass has been installed to the correct standard refer first to your window supplier and if you are not satisfied with their response to the Glass and Glazing Federation at or your local Trading Standards Office.

For more details on where safety glass should be installed refer to Specifier FAQs.

2 What are Window Energy Ratings?

Window Energy Ratings were introduced in 2005 as a means for window companies to demonstrate compliance with Building Regulations concerning energy efficiency of their windows. The Window Energy Rating level is shown on a label attached to the window which looks very much like the kind of label you will have seen in the past on domestic appliances such as fridges and washing machines.

For further information relating to Window Energy Ratings please click here

3 Can Pilkington K Glass™ be used in single glazing or secondary glazing?


Single Glazing

The coating on Pilkington K Glass™ is very hard and is difficult to scratch.  Unfortunately, it is slightly rough (although not in a visible sense), so it may collect marks and dirt in a way which is difficult to clean off.  It is inadvisable to use it as single glazing for this reason with the Pilkington K Glass™ coating to the exterior*.

If Pilkington K Glass™ is used in single glazing with the coating facing the room, it may also promote the formation of condensation.  This is because the coating acts as a barrier to heat, keeping the glass colder than ordinary single glazing.  Because the surface is colder, condensation will form more readily and in larger quantities than on ordinary single glazing. 

The only circumstances where Pilkington K Glass™ would not generally promote condensation on the room surface is when the external temperature is always higher then the room temperature.  This can occur if the room is a cold store, or if the building is in the tropics.

*Note.  Some manufacturers of proprietary systems are using Pilkington K Glass™ in situations where the coating is on the external surface, apparently without significant problems of dirt and marks over several years of use.  However, NSG Group do not and will not take any responsibility for the durability and performance of Pilkington K Glass™ when used in this manner.

Secondary Glazing

Pilkington K Glass™ can be used in secondary glazing, with the Pilkington K Glass™ as the inner pane with the coating facing the air gap between the two panes. Although the coated surface will have to be cleaned occasionally, it is generally protected by being in the cavity and washing with clean soapy water or a clear window cleaning solution should maintain it in good condition. 

There is a risk of the coating being marked by the sliding sashes when used in secondary sliding glazing systems.

As with any secondary glazing, there is a risk of condensation on the inner surface of the outer pane, unless the air gap has some ventilation to the outside, possibly just through gaps in the existing window.

4 What is the U value of my windows?

The U value is one measure of how well your windows are keeping your heat in. So called Low Emissivity coatings which offer heat insulation normally define their efficiency using the U value. The lower the U value the better the insulation. There is now however a new measure of the overall energy efficiency of your windows called the Window Energy Rating (WER). 

Measured on an A to G scale and using the labelling system we are all used to seeing on fridges and washing machines, the WER takes account of how well the window utilises free heat energy from the sun as well. 

A window with a rating from E up to the top level which is A, meets the regulations concerning the energy efficiency of buildings. Ask your window installer for more details.

5 Where should the Pilkington K Glass™ pane be positioned in my double glazing containing patterned glass?

Normally Pilkington K Glass™ would be on surface 3 of the double glazed unit counting from the outside ie. the surface on the inside of the unit on the pane which faces the interior of the house.

However, patterned glass such as Pilkington Texture Glass is normally positioned on the inside since the surface is not flat.

In this case Pilkington K Glass™ can be positioned on surface 2 ie. the inner surface of the external pane where thermal efficiency is effected to a negligible degree.

6 Where should the Pilkington K Glass™ pane be positioned in my double glazing?

Pilkington K Glass™ is a low emissivity glass, specifically designed to enhance the thermal insulation of insulating units. It achieves this by being positioned in the insulating unit with the Pilkington K Glass™ coating on a surface facing the cavity.

In a double glazing unit, counting the surfaces from the exterior of the building, The Pilkington K Glass™ coating should be on surface 2 (the inner surface of the outer pane) or surface 3 (the cavity surface of the inner pane).

Pilkington K Glass™ performs the function of enhancing the thermal insulation (reducing the U value) equally well whether it is positioned on surface 2 or surface 3. However, with any coated glass, there may be slight optical effects when the coating is present. These optical effects are reduced when Pilkington K Glass™ is used with the coating on surface 3, rather than surface 2.

General advice is that the preferred location of the Pilkington K Glass™ coating is on surface 3 of the unit, but if the glass types used in the unit make it difficult to assemble the unit with the Pilkington K Glass™ coating on surface 3, then no loss of thermal performance is obtained by using it on surface 2. In particular, when Pilkington K Glass™ is combined with Pilkington Texture Glass, where the Pilkington Texture glass is normally installed as the inner pane of a double glazing unit, the Pilkington K Glass™ should be installed with its coating on surface 2. Any slight optical effects will be masked by the presence of the Pilkington Texture Glass. 

7 How can I tell whether I have got Pilkington K Glass™ in my windows?

Your window company should demonstrate this to you when he has completed the installation by using a detector which will light up in the presence of Pilkington K Glass™ within the double glazed unit.

8 How can my windows help to reduce the noise level in my home?

There are a number of ways that this can be achieved including using double glazed units which have uneven thicknesses of glass in the front and back panes, using regular laminated glass in one of the panes or for the best reduction, use of special laminated glass such as Pilkington Optiphon™. Ask your window company for advice.

9 Can you provide advice on how to clean my windows?

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.


Alternatively, have Pilkington Activ™ installed in your replacement windows or conservatory. This revolutionary new coated glass which goes on the external surface of windows or conservatories has a unique dual action. When exposed to daylight the coating breaks down any organic dirt deposits such as bird droppings and tree sap and then when it rains the rain water washes the loosened dirt away leaving the windows streak free. It is available from window companies nationwide for all external glass areas of your home but is especially beneficial in hard to reach areas such as conservatory roofs, rooflights, inaccessible windows or just to save time and keep the large expanses of glass we love in our homes looking clean. For more information, click through to the Pilkington Activ™ product page. 

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