Low-emissivity Glass

Low-emissivity Glass

What is Low-emissivity glass?

Low-emissivity glass (or low-e glass as it is commonly referred to) is a type of energy-efficient glass designed to prevent heat escaping through your windows to the cold outdoors. Low-e glass such as Pilkington K Glass™ or Pilkington Optitherm™ has an invisible coating which dramatically reduces heat transfer and reflects interior heat back into your room.

Unfortunately, many older double glazed units do not contain low-emissivity glass and are therefore not energy-efficient. By replacing your existing window glass with low-e glazing, you can improve the energy efficiency of your home, reduce your monthly bills and decrease the size of your carbon footprint. Pilkington low-e glazing units can make your home more than twice as energy-efficient in comparison to older double glazing with no low-e coating.

Low-e glass is essential for rooms or buildings with a high proportion of windows or glass doors, such as conservatories and sun rooms. The use of low-e glazing helps to retain heat even in winter, allowing you to comfortably use these rooms for more months of the year. Low-e glass is also recommended for north or east facing windows, where a larger proportion of heat loss would be expected.  For south and west facing glazing where overheating can become a problem in the summer months, your installer may recommend the use of solar control glass.

NSG Group offers a range of single low-emissivity glass products and also double and triple glazed units incorporating low-e glass:

Single low-emissivity glass:

  • Pilkington K Glass™ N
  • Pilkington Energy Advantage™ 
  • Pilkington K Glass™ N OW
  • Pilkington Optitherm™ S3 
  • Pilkington Optitherm™ S1A

Double or triple low-emissivity glazing:

  • Pilkington Insulight™ Therm
  • Pilkington Insulight™ Therm Triple
  • Pilkington Spacia™

Save money with Low-e glazing

It may surprise you to learn that up to 25% of the heat in your home can be lost through your windows. Without energy-efficient glazing, you are effectively throwing your money out of the window. 

Did you know that by installing low-e windows, you can significantly improve your Window Energy Ratings and reduce the amount of energy lost through your windows by up to 75%!  Considering these figures and the cost of fuel today, it is easy to see why more and more people are opting for A-rated windows by incorporating low-e glass into their existing window frames.

Why not use energy-savings calculator to see for yourself just how much you can save by installing a product from our low-e glazing range.

How Low-emissivity Glass works

Heat always flows towards the cold. Therefore, window glass without a low-e coating will absorb the heat from your home and radiate it onto the colder outside surface, where it is lost. Low-e glass has a special coating which is a poor radiator of heat and does not allow heat to be transferred to the outside. Instead, the low-e coating actually reflects the heat back into your room. 

Pilkington K Glass™ or Pilkington Optitherm™ is coated so that it becomes selective for which wavelengths of energy it allows to pass and which it reflects. Most of the sun’s energy arrives at the low-e window as short wave radiation and that is allowed to pass through the low-e coating i.e. it is not very reflective to these wavelengths. Some of the energy is absorbed by the glass as it passes through. The absorbed energy heats the glass. The energy that has entered the living space is also absorbed by the items in the room such as furniture and carpets. These items also get warmed by the energy. Warm items re-radiate the energy and this includes fires and radiators within the room as well as the warmed furniture. This re-radiated energy is no longer short wave radiation.  The energy now travels back towards the low-e window as long wave radiation. The low-e glass does not let the longer wavelength radiation emit from the coated low-e surface and is an effective reflector. The energy that has entered the room and the heat energy that was already present is reflected back from the low-e glass. Even the glass that was heated by the energy passing through it emits long wave energy and it can send it in two directions. Some is sent back out into the atmosphere and some to the inside of the building. The result is a neutral appearance glass that lets energy in and helps prevent heat getting out. 

By placing low-e glass into a double glazed unit, the individual glass pane temperatures change. As more heat is retained, the outer pane of glass is not heated as much by escaping energy and the inner pane is keeping more heat in and becomes warmer. This has two effects. There are less cold draughts from convection near the windows and the risk of condensation on the glass is reduced.  In this way, low-e glazing not only prevents heat loss, but also encourages warmth during the colder months of the year.

The rate at which glass transmits heat out of the building is measured by the Ug value. The units for the Ug value are W/m2K (Watts per square metre per Kelvin temperature difference). The larger the figure, the greater the heat loss.  If you have a house that has 20 mof glass and its Ug value is 5.8 (single glazing) and it is 10 degrees colder outside than inside then you will be losing 1160 Watts (=5.8 X 20 X 10). This is a rate of loss greater than one bar on an electric fire.  By replacing the glass with a low-e product the Ug value might become 1.5 so you are now only losing 300 Watts (almost a quarter of the heat loss).  Improved window seals and draft proofing will also improve the overall insulation properties of the window saving even more energy.

The other aspect of energy efficiency is the ability of the glass to transmit heat. The Ug value property and the rate at which heat is allowed into the building is a more thorough evaluation of the effectiveness of the glass. The total transmittance of the glass or more correctly the total solar heat transmittance or g value is the value given to the glass. The higher the figure the better for gaining heat which is the objective in most domestic situations. Low-e glass types can be compared using the Ug value and the g value. Another measure that is used for comparing low-e glass types is the shading coefficients. Shading coefficients compare the solar heat transmittance properties of the glass by short and long wavelengths. The proportion of energy that each glass allows through is compared back to a standardised thin piece of clear glass with known properties. Shading coefficients are given for the short wave energy, long wave energy and the two combined into the total shading coefficient. A glass with a relatively high long wave shading coefficient will have absorbed a lot of energy and be re-radiating it into the building. It is likely to be a coloured, highly absorbing glass. This information is of particular interest to architects and also heating and ventilation engineers, as they can calculate the heating and cooling loads for a building when they know how much energy will either come through or escape from a building.     

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