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Insulating glass
IGU made of glass is called insulated glass, which refers to heat insulation, not sound[1] or electricity. A less accurate term is "insulating glass", since the glass itself has no insulative properties. It is the air space between the glass layers (lites) that provides the insulation.
It is important that the air remains as immobile as possible to prevent convection currents transferring heat across the insulating gap. This limits the thickness of the air gap used and is the reason for triple glazing.
The space between the lites may be filled with air or an inert gas like argon or krypton which would provide better insulating performance. (Argon has a thermal conductivity 67% that of air.[2]) Typically the spacer is filled with desiccant to prevent condensation and improve insulating performance. Less commonly, most of the air is removed, leaving a partial vacuum, which drastically reduces heat transfer through convection and conduction.[3] This is called evacuated glazing. Similar techniques are also used in insulation products called vacuum insulated panels.
Often the insulating quality is used in reference to heat flow where the gap is the insulating medium. The gap is usually 12mm to 20mm thick. Within this range, the thickness does impact the insulating properties substantially, but smaller gaps have greater heat conduction through the air or other gas, and larger gaps allow more convection within the space leading to higher convective heat loss. A 16mm air gap is often considered the optimum thickness for air although this depends on many factors such as the size of the window, the temperature difference between the two panes and whether it is vertical.
In general, the more effective a fill gas is at its optimum thickness, the thinner the optimum thickness is. For example, the optimum thickness for krypton is lower than for argon, and lower for argon than for air.[4] However, since it is difficult to determine whether the gas in an IGU has become mixed with air at manufacture time (or becomes mixed with air once installed), many designers prefer to use thicker gaps than would be optimum for the fill gas if it were pure. In some situations the insulation is in reference to noise mitigation. In these circumstances a large gap improves the noise insulation quality or Sound transmission class. Asymmetric double glazing, using different thickness of glass rather than the conventional 4-12-4 symmetrical systems, is more important than air gap thickness in improving the phonic insulation properties. This is often overlooked.
As of 2007, argon is commonly used in insulated glazing as it is affordable. Krypton, which is considerably more expensive, is not generally used except to produce very thin double glazing units or relatively thin, or extremely high performance triple glazed units.
In principle, xenon would be even more effective than krypton.
Insulated glass assemblies cannot be cut to size in the field like plate glass but must be manufactured to the proper size in a shop equipped with special equipment.
The effectiveness of insulated glass can be expressed as an R-value. The higher the R-value, the greater is its resistance to heat transfer.
Condensation inside units
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The life of an IGU varies depending on the quality of materials and manufacture. Units typically last from 10 to 25 years, with windows facing south rarely lasting more than 12 years. IGU's typically carry a warranty for 10 years. Condensation collects between the layers of glass when the perimeter seal has failed, and can only be eliminated by replacing the IGU. Companies that market insulated glass rarely mention this when touting the overall efficiency of insulated glass, although it is a significant factor in the overall cost of owning such windows. The standard practice of the glazing industry is that the glass from failed IGU's is not recycled. In Canada, since beginning of 1990, there are some companies offering restoration of these units. They provide ventilation by drilling holes in the glass or spacer. This solution is permanent and they offer warranty from 5 to 20 years. This solution lowers the value of the glass a bit, but it can be a "green" solution when the window is still in good conditions.
Glass coatings
The heat and sound insulation of glazing may also be improved by the use of a film or coating applied to its surface. This film is typically made of polyester or metal, and may give the window a reflective appearance and one-way mirror effect. It may be used on single-glazed windows as an alternative to insulated glazing, or on the outside layer of insulated glazing to further improve its effectiveness.[1] Such coatings may reduce fading of fabric and improve safety in case the glass breaks.[2]. The Solar Heat Gain Co-efficient is a measure which expresses the proportion of incident solar thermal radiation that is transmitted by a window. Visible Transmittance describes the amount of visible light that can pass. Both of these can be independently altered by different coatings.[5]
Tinted glass
Many of the films impart a color to the glass, typically blue, brown or gold, which may be used to enhance a building's architectural style[3]. In automotive applications, where non-reflective and neutral-color tinted glazing is required, heat reflective properties are created using a nano-metre thick silver oxide layer within the windscreen[4]. Insulating automotive glass is also called "athermic glass", "heat reflective glass" or "comfort glass".
Secondary glazing is sometimes used as a cheaper alternative. It consists of a layer of glazing placed retrofitted inside the window, to provide sound and heat insulation. Plastic sheet may be used for heat insulation, but may only last for one season.[5]
Low-emissivity coating
Low-emissivity (Low-E) glass has a thin coating, often of metal, on the glass within its airspace that reflects thermal radiation or inhibits its emission reducing heat transfer through the glass. A basic low-e coating allows solar radiation to pass through into a room. Thus, the coating helps to reduce heat loss but allows the room to be warmed by any sunshine. The low-e coating is usually on the inside pane of glass; if solar control is required then the outside pane of glass would have either a film or a body tint to reflect or absorb solar radiation. The principle of operation is similar to the greenhouse effect in which short wavelength radiation is transmitted through the pane, but longer wavelength radiation is absorbed. However, low-e glass reflects the radiation rather than absorbing it, improving performance compared to the glass in a simple greenhouse.
Estimating Heat loss from Double Glazed Window
Estimating the rate of heat loss is essential in choosing which type of double glazed window to be used in a building to maintain desired thermal comfort. Relevant data and calculation from different calculations are listed below:
Required data:
- Thermal resistance of the glass used
- Physical properties of the gas used in between the gap (such as density, heat capacity and k value)
- Dimension of the double glazed glass
Table 1.1 Relevant correlations to calculate natural convection inside the window gap[6]
| Condition | Correlation |
|---|---|
 |
 |
|
 |
|
 |
By using natural convection correlations that fulfill the given criteria; the overall heat transfer coefficient can be calculated by using equation (1). Hence, to estimate the overall heat loss, equation (2) will be used
_______(1)
_______(2)
Example Calculation of Heat Loss from Double Glazed Window
Q. Estimate the heat transfer coefficient and thermal resistance of the air gap in a double glazed window. Assume that the outer surface of the inner glass pane is at 10 oC and that the inner surface of the outer glass pane is at -10 oC. Assume that the window is 2m tall (H) and 2 m wide, and that the gap between the glass panes is 1.5cm (L). How does the thermal resistance compare to glass 0.003 m2K/W and curtains ~ 0.5 m2K/W? If air is a good insulator, why is such a small air gap used?
Physical Property of air
| μ | 1.65 x 10-5 | Pa s |
| Cp | 1050 | J/kg K |
| k | 0.0242 | W/m K |
Solution
First Calculate 
, 
, 
and 
to determine which natural convection correlation to use.
|
7.5 x -3 |
|
6.67 x 104 |
|
1.10 x 104 |
|
6.94 x 10-1 |
Therefore, the best correlation that fulfill the calculated condition is We then proceed to calculate the heat transfer coefficient of the gas in between the glass. Be reminded that the 
is the width of the gap and the 
is the height of the window. Calculating the Nusselt number will yield 1.11 and further calculating will yield 'h' value of 1.80 W/m^2 K.
Since we've known the thermal resistance value of each glass, we can proceed to calculate the overall heat transfer coefficient of the double glazed window by using equation 2.
To calculate the overall heat loss, we will use equation 2.
References
- ^ GlassMagazine.net - Articles
- ^ Thermal Conductivity of some common Materials
- ^ Development and quality control of vacuum glazing by N. Ng and L. So; University of Sydney
- ^ ASHRAE Handbook, Volume 1, Fundamentals, 1993
- ^ Windows Energy Ratings Scheme - WERS
- ^ Kavanagh.J, 'Heat Transfer, Natural Convection' March 2008, Department Of Chemical Engineering, University of Sydney.
See also
External links
- Energy Efficient Windows and damage to trees / landscaping
- Double Glazing
- GlassMagazine.net - Articles - insulating glass may not help with sound.
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