Even in the absence of solar radiation and air infiltration, heat transfer through the windows is more complicated than it appears to be. This is because the structure and properties of the frame are quite different than the glazing. As a result, heat transfer through the frame and the edge section of the glazing adjacent to the frame is two-dimensional.
Therefore, it is customary to consider the window in three regions when analyzing heat transfer through it: 1 the center-of-glass, 2 the edge-of-glass, and 3 the frame regions, as shown in Fig. Then the total rate of heat transfer through the window is determined by adding the heat transfer through each region as. Also, the inverse of the U-factor is the R-value, which is the unit thermal resistance of the window thermal resistance for a unit area.
Consider steady one-dimensional heat transfer through a single-pane glass of thickness L and thermal conductivity k. The thermal resistance network of this problem consists of surface resistances on the inner and outer surfaces and the conduction resistance of the glass in series, as shown in Fig. Using common values of 3 mm for the thickness and 0. That is, the glass layer itself contributes about 2 percent of the total thermal resistance of the window, which is negligible.
The situation would not be much different if we used acrylic, whose thermal conductivity is 0. Therefore, we cannot reduce the heat transfer through the window effectively by simply increasing the thickness of the glass. But we can reduce it by trapping still air between two layers of glass.
The result is a double-pane window, which has become the norm in window construction. Therefore, the thermal resistance of 1-cm-thick still air is equivalent to the thermal resistance of a cm-thick glass layer. Disregarding the thermal resistances of glass layers, the thermal resistance and U-factor of a double-pane window can be expressed as Fig. Roughly half of the heat transfer through the air space of a double-pane window is by radiation and the other half is by conduction or convection, if there is any air motion.
Therefore, there are two ways to minimize h space and thus the rate of heat transfer through a double-pane window:. Minimize radiation heat transfer through the air space. The emissivity of an ordinary glass surface is 0. Therefore, the effective emissivity of two parallel glass surfaces facing each other is 0.
But when the glass surfaces are coated with a film that has an emissivity of 0. Then for the same surface temperatures, radiation heat transfer will also go down by a factor of Even if only one of the surfaces is coated, the overall emissivity reduces to 0.
Thus it is no surprise that about one-fourth of all windows sold for residences have a low-e coating. Therefore, it is usually more economical to coat only one of the facing surfaces. Note from Fig. Minimize conduction heat transfer through air space. The convection effects can be reduced by filling double-glazing with heavier, less mobile gas molecules.
Argon is most commonly used; the heavier gases krypton and xenon give better performance but are more expensive. These gases are all by-products of the industrial liquefaction of air to produce oxygen. Convection can also be reduced by limiting the space available for gas movement. This is the principle used in most insulation materials.
Various forms of transparent insulation have been developed that use a transparent plastic medium containing bubbles of trapped insulating gas. These materials could eventually revolutionise the construction of windows and walls, but at present the materials are expensive, not robust and need protection from the rigours of weather and ultraviolet light.
Alternatively the glazing can be evacuated. Convection currents cannot flow in a vacuum. However, a very high vacuum is required and it needs to last for the whole life of the window, which may be 50 years or more. Also the window will need internal structural spacers to stop it collapsing inwards under the air pressure on the outside. These spacers conduct heat across the gap, slightly reducing the overall performance.
The vacuum gap does not need to be very thick and is only 0. Note that these convection currents are only concerned with what happens inside a double-glazing unit. There are also likely to be movements of air through the seals around the edges of windows, but that is a matter of ventilation and infiltration see Section 2.
A simpler way to reduce the convection effects is to insert extra panes of glass or transparent plastic film between the panes of double-glazing turning it into triple- or quadruple-glazing. Heat energy can be radiated , in the same manner as it is radiated from the sun to the earth. The quantity of radiation is highly dependent on the temperature difference between the radiating body and its surroundings. The roof of a building, for example, will radiate heat away at night to the cold atmosphere.
In a double-glazed window heat will be directly radiated from the inner pane across the gap to the outer one. Low iron glass is commonly used in the outer pane of multiple-pane units, low-e glass being used for the inner pane.
Hardwood frames are preferable and products are available that meet the stringent standards set by the Passivhaus standard. Outside of this margin, extra heat is needed to compensate for the increased feeling of discomfort. GreenSpec accepts no responsibility or liability for any damages or costs of any type arising out of or in any way connected with your use of this web site. Data and information is provided for information purposes only, and is not intended for trading purposes.
Neither GreenSpec nor any of its partners shall be liable for any errors in the content, or for any actions taken in reliance thereon. Heat gain Though the general balance in the UK is for windows to lose heat, they also, to a varying degree absorb heat and in some instances of high performance windows, there can be a net gain. Shading Shading the window areas during the summer months is usually desirable. This crudeness of measurement has led, in the last few years, to a system developed by the British Fenestration Rating Council www.
Most importantly for the window specifier, it encourages manufacturers to publish verifiable data on Ux, g-value and air permeability. Though u-values are still often quoted, the BFRC rating system is gaining popularity through its citing by the Building Regulations in England and Wales Passivhaus certification Another useful standard gaining ground is that provided by certification by the Passivhaus Institute.
Gas filled units Filling the gap between the glass panes with low conductivity gas such as argon or krypton as well as the more expensive xeon at atmospheric pressure improves the window performance by reducing conductive and convective heat transfer.
Insulating spacer bars Multiple glazing units are spaced and sealed by spacer bars around the perimeter of the unit. Although double glazing insulates against differing air temperatures, it does not insulate against radiant heat. This makes a double glazed unit a good choice for permitting the winter sun to warm the inside of the house. An improvement that can be made to the thermal performance of insulating glazing units is to reduce the conductance of the air space between the layers of glass.
In most double glazing units, the space is filled with air or flushed with dry nitrogen just prior to sealing.
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