Understanding Double Glazing Energy Efficiency

26 June 2014

How Double Glazing Works

Energy efficiency and energy efficient products are the raison d’etre of double glazing manufacturers.  Quite simply, energy efficient windows are better for the environment and save money on heating bills.  Here is a useful guide to understanding the terminology, ratings and values applied to double glazing, how it works and how to choose a reputable installer.
How double glazing works 

What are U Values?

U-Values measure how well heat is transferred by the entire window/door - the frame, sash and glass - either into or out of the building.  U Values are measured in W/m2K, the lower the U-Value number, the better the window or door will keep heat inside a building on a cold day.
Two different U-Value figures are used in the glazing industry:
Ug = thermal transmittance of the glazing itself (or the ‘centre-pane’ U-Value)
Uw value = thermal transmittance of the whole window - including glazing and frame.
Uw value is most commonly used as Building Regulations have requirements for the whole window U values of windows and doors. 

What are Window Energy Ratings?


BFRC Energy Rating

The British Fenestration Rating Council (BFRC) scheme is the UK’s national system for rating the energy efficiency of windows and is recognised within the Building Regulations as a method to show compliance. BFRC window energy ratings use a traffic-light style A-E ratings guide similar to that used on ‘white’ goods (such as fridges, freezers, washing machines etc). As well as taking U values into consideration, the BFRC energy ratings also take into account the nominal solar heat gain provided by windows.  
Each window rated by the BFRC has a unique label, which displays the following information:
1. The rating level – A, B, C etc..
2. The energy rating e.g. -3kWh/(m2K) in this example the product will lose 3 kilowatt hours per square metre per year.
3. The window U value e.g. 1.4 W/(m2K)
4. The effective heat loss due to air penetration as L e.g. 0.01 W/(m2K)
5. The solar heat gain e.g. g = 0.43
Simply put this will determine how well a product will perform the functions of helping you contain and conserve heat within your building in the winter.
Solar Gain free heat 

What is Solar Heat Gain?

Solar heat gain refers to the increase in temperature in a space, object or structure that results from solar radiation. The amount of solar gain increases with the strength of the sun, and with the ability of any intervening material to transmit or resist the radiation.
Objects struck by sunlight absorb the short-wave radiation from the light and reradiate the heat at longer infrared wavelengths. Where there is a material or substance (such as glass) between the sun and the objects struck that is more transparent to the shorter wavelengths than the longer, then when the sun is shining the net result is an increase in temperature — solar gain. This effect, the greenhouse effect, so called due to the solar gain that is experienced behind the glass of a greenhouse, has since become well known in the context of global warming.
When discussing the properties of windows and doors shading coefficients are commonly mentioned properties. Shading coefficients measure the solar energy transmittance through windows.
g-value is the coefficient commonly used in Europe to measure the solar energy transmittance of glass - called a Solar Factor on some window literature (%) i.e. 53% = 0.53;
Solar Heat Gain Coefficient (SHGC) is used in the United States and most commonly refers to the solar energy transmittance of a window or door as a whole, factoring in the glass, frame material (wood, aluminum, etc.), sash (if present), divided light bars (if present) and screens (if present). SHGC may also refer to the solar energy transmittance of the glass alone (sometimes more specifically termed centre-of-glass SHGC), in which case it is analogous to g-value.
Primary transmittance is the fraction of solar radiation that directly enters a building through a window compared to the total solar insulation, the amount of radiation that the window receives. The secondary transmittance is the fraction of inwardly flowing solar energy absorbed in the window (or shading device) again compared to the total solar insulation.

How does Double Glazing work?

Windows are designed to let in light, while blocking air and objects from entering the house. In addition, windows provide some insulation, but not much. Windows are only a few millimetres thick, and heat can easily flow through them, making it more expensive to heat and cool your house. Double glazed windows are designed to let light through while blocking the movement of heat, turning your windows into good insulators.
Double glazed windows have two panes of glass separated by a layer of trapped air. The space between the windows is very well insulated so that little or no air can leak between the panes of glass. In addition, there is often a desiccant between the panes to absorb any stray moisture and stop fog from forming between them. The windows can be clear or covered with a tinted or reflective coating.
Double glazed windows insulate in a few different ways. Having two panes of glass doubles the amount of glass that heat has to get through, slowing its movement and aiding insulation. The real insulating power, however, comes from the trapped air. Heat is motion in molecules--the hotter a material is, the faster its molecules move. When excited molecules bang into nearby molecules, they spread the heat. This is called conduction. Because air molecules are spread out, they do not conduct nearly as well as a solid like gas. As a result, the trapped air acts as an insulator. Some heat can still flow through the air, but it moves much more slowly than it does through glass.
Double glazed windows can also be improved by adding coatings. Different coatings are used for different purposes. For example, a reflective metallic coating on the outside of the outer pane will stop infrared radiation from passing through the glass from the outside, preventing the sun from heating the room. The same coating on the outside of the inner pane, however, will stop infrared from leaving the room, keeping it warmer. Double glazed windows can also be tinted to reduce the light coming in, just like normal windows.

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