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How much heat is lost through glass?

Glass windows are a common source of heat loss in buildings. How much heat is lost through glass in a domestic or commercial property depends on several factors, such as the type of glass and how it is used.
Single-pane glass can lose up to ten times the heat of the same area of an insulated wall. Double glazing can reduce that heat loss by as much as 50%, while triple glazing can cut heat loss even further to around 20%. This is because the space between multiple panes of glass is usually filled with gases, such as argon or krypton, which reduce heat transfer.
Low-E (low emissivity) coatings can also be applied to glass surfaces to reflect radiant heat and improve overall energy efficiency. It's worth noting that windows can both gain and lose heat. In cold weather, glass may allow heat to escape from a warm room to the colder exterior. The reverse can happen in hot weather, and outdoor heat can enter a cool room.
On average, houses lose around 10% of their total heat through windows, most of it through the glass. There's a useful formula that can be used to determine the heat loss through a window, taking into account the window size, the temperature difference and the glass U-value:

Heat Loss = U-value x Area x "T

Where the U-value is the thermal transmittance of the glass, Area is the surface area and "T is the temperature difference between the inside and outside. The area and temperature difference are easily measured, while the manufacturer usually provides the U-value.

Heat loss through glass can depend on the type of glass, its thickness, any additional insulation and the quality of the seal. Here are the factors in more detail:

  Type of glass: Different types of glass offer varying insulating properties. Double or triple-pane windows offer significantly more insulation than single-pane windows, with multiple layers of glass separated by insulating spaces to reduce heat loss by conduction and convection.

  Low-E coatings: Insulation can also be increased by applying low-emissivity coatings, reflecting heat. These are invisibly thin layers of metal or metallic oxide covering the surface of the glass. Low-E coatings can reflect heat back into the room, reducing heat loss to the exterior.

  Glass thickness: Heat loss is significantly impacted by the thickness of the glass. Thicker glass gives better insulation.

  Temperature difference: A greater difference in temperature between the inside and out will create a larger temperature gradient and cause more heat to be lost through the glass.

  Window seals: Windows with high-quality sealing will better insulate your interior, minimising air leaks.

  Gas fills: Double-pane or triple-pane windows may have the spaces between their glass panes filled by gas with insulating properties (such as Argon or Krypton).

  Solar gain: The orientation of your windows relative to the sun can greatly affect the solar heat gain they receive throughout the day. This solar gain can help offset heat loss, especially for south-facing windows in colder climates.

  Insulation: Other ways to add insulation and rescue heat loss include window films, curtains or blinds.

Ensure your property is well insulated

To get specific information about your windows and the level of heat loss through them, you may need to contact your window/glass manufacturer for more information about the insulating properties of their products. Otherwise, you can consult a professional for advice on your situation. Replacing old windows can provide significant insulation benefits for your property.

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