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Insulated glazing
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When multiple glass panes or "lites" are assembled into units, they are commonly referred to as "insulated glass", "Double glazing/ Double Glazed Units" (UK and Europe) or Insulating Glass Units (IGU). The proper technical term for the assembly is hermetically sealed units, meaning that the environment inside of the unit is isolated from the external environment.
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When multiple glass panes or "lites" are assembled into units, they are commonly referred to as "insulated glass", "Double glazing/ Double Glazed Units" (UK and Europe) or Insulating Glass Units (IGU). The proper technical term for the assembly is hermetically sealed units, meaning that the environment inside of the unit is isolated from the external environment. These units are produced with the intention of maximizing the thermal insulating properties of a gas contained in the space formed by the unit while at the same time providing clear vision through the unit at low temperatures.
Most IGUs are double glazed, but IGUs with three sheets or more, i.e. "triple glazing" are becoming more common due to higher energy costs. Insulated glazing may be framed in a sash,frame or in a curtain wall. IGUs are also commonly used for replacement windows.
Other applications of IGUs include acoustic attenuation.
Comparison of IGU characteristics
IGUs are manufactured to varying degress of performance. Typical IGU units supplied for high performance construction in Europe are over 4.5 times more efficient than standard IGU available in North America.
| Glazing assembly | U-factor | R-value | SHGC | VT | acoustic attenuation |
|---|
| unit -> | W/m2/K | BTU/(h*ft2*°F) | m2K/W | h*ft2*°F/BTU | % | % | dbRW+ctr | | standard IGU (in the US) | 2.84 | .5 | .35 | 2.0 | 76 | 81 | | | Medium-SHGC, low-e | 1.48 | 0.26 | 0.68 | 3.8 | 58 | 78 | | | European triple glazed | 1.00 | 0.18 | 1.00 | 5.7 | | | 45
| | German passive house standard window | 0.60 | 0.11 | 1.67 | 9.5 | | | | | For comparison: | | single glass | 6.25 | 1.10 | 0.16 | 0.90 | 87 | 90 | | | standard wood wall, 2x6, R-19 fiberglass | 0.49 | 0.09 | 2.06 | 11.7 | | | |
Components
Glass
Glass is used to provide light and allow vision through otherwise opaque surfaces. While the composition and manufacturing of glass is covered elsewhere, for the purposes of this article, its importance to the construction is its dimensional stability over a wide temperature range. IGUs are manufactured with glass in range of thickness from 3 mm to 10 mm or more in special applications. Laminated or tempered glass may also be used as part of the construction. Most units are manufactured with the same thickness of glass used on both panes but special applications such as acoustic attenuation or security may require wide ranges of thicknesses to be incorporated in the same unit.
To reduce shear effects on the sealed unit (a major cause of premature failure), manufacturers use a rule of thumb that permits a difference of 1 mm between panes of glass used in the unit and still maintain the warranty for the unit. For example, a unit may be ordered with a 4 mm pane on the exterior and a 3 mm pane on the interior. These variations are allowed for architectural and cost reasons. Other combinations can be specified and produced but the manufacturer may reserve the right to limit the term of the warranty or refuse to warranty the unit altogether.
The performance of glass can be modified through the use of the following:
Tinted Glass
While clear glass is the most common glass component of IGUs, tinted glass is used as both an architectural feature. The primary colors available include bronze, gray and green . The degree of tint depends on both the composition of the glass and the thickness of the lite. Tinted glass is usually placed on the exterior of the IGU.
Coated Glass
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. Such coatings may reduce fading of fabric and improve safety in case the glass breaks.. 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.
Low-Emissivity Glass
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 direct sunshine. The low-e coating is usually on the inside pane of glass on the surface facing the air space; if solar control is required then the coated surface is moved to the inside face of the outside pane of glass to reflect or absorb solar radiation. The change in location of the coating does not affect the insulating properties of the IGU, only the percentage of solar heat gain. Further solar radiation control can be added through the use of tinted glass and/or metallic coatings . 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. Low-e glass reflects the radiation rather than absorbing it improving performance compared to the glass in a simple greenhouse. Its effect can be noticed by an increase in temperature of the inside glass surface and the reduction of condensation that would normally form on the unit because of a change in the dew point.
There are two types of low-e coatings available, "hard-coat" and "soft-coat". Hard-coat glass is manufactured by applying molten tin to the glass surface as the glass sheets are being manufactured. The tin bonds to the surface of the glass and forms a relatively thick coating. Hard-coat glass is considered a medium performance coating since the emissivity is greater compared to the soft-coat product. The advantage of hard-coat glass is that it does not require special handling in the IGU assembly process to maintain the surface's coating integrity and does not scratch easily. It does require that the glass surface in contact with the spacer be abraded to improve adhesion of the sealant. Soft-coat glass uses vacuum deposition to apply a metallic coating to the glass surface as an additional manufacturing step. While the metallic film is very thin compared to the hard coat it does require special handling and storage as the surface is easily damaged. Choosing a soft-coat glass over a hard-coat glass improves thermal performance of the IGU by about 13% . Most low-emissivity glass sold for IGU manufacturing is of the hard-coat type.
Spacer
The glass panes are separated by a "spacer". Most spacers are constructed of either thin gauge steel or aluminium for thermal expansion stability or cost reasons. In cold climates, this may result in water or ice forming at the bottom of the sealed unit because of the heat loss in those areas. To reduce heat transfer through the spacer and increase overall thermal performance, the spacer may be constructed of fiberglass or use a hybrid design of metal and plastic.
Typically, the spacer is filled with desiccant to prevent condensation and improve insulating performance.
Construction
IGUs are manufactured on a made to order basis on factory production lines. The width and height dimensions, the thickness of the glass panes and the type of glass for each pane as well as the overall thickness of the unit must be supplied to the manufacturer. On the assembly line, spacers of specific thicknesses are cut and assembled into the required overall width and height dimensions and filled with desiccant. On a parallel line, glass panes are cut to size and washed to be optically clear. An adhesive sealant is applied to the face of the spacer on each side and the panes pressed against the spacer. If the unit is gas filled, two holes are drilled into the spacer of the assembled unit, lines are attached to draw out the air out of the space and replaced with the desired gas. The lines are then removed and holes sealed to contain the gas. The units are then sealed on the edge side using either polysulphide or silicone sealant or similar material to prevent humid outside air from entering the unit. The desiccant will remove traces of humidity from the air space so that no water appears on the inside faces of the glass panes facing the air space during cold weather. Some manufacturers have developed specific processes that combine the spacer and desiccant into a single step application system .
Thermal Performance
The maximum insulating efficiency of a standard IGU is determined by the thickness of the space containing the gas. Too little space between the panes of glass results in radiant heat loss between the panes (the inside surface of one pane cools the surface of the other pane) while too wide a gap results in convection current losses (gas begins to circulate because of temperature differences and transfers heat between the panes). For further information, see the article heat flow. Typically, most sealed units achieve maximum insulating values using a gas space of between 5/8 to 3/4” (16-19 mm) when measured at the centre of the IGU. When combined with the thickness of the glass panes being used, this can result in an overall thickness of the IGU of between 7/8 and 1” for 3 mm glass (22-25 mm) to 1 1/2” (28-31 mm ) for 1/4” plate glass.
IGU thickness is a compromise between maximizing insulating value and the ability of the framing system used to carry the unit. Some residential and most commercial glazing systems can accommodate the ideal thickness of a double paned unit. Issues arise with the use of triple glazing to further reduce heat loss in an IGU. The combination of thickness and weight results in units that are too unwieldy for most residential or commercial glazing systems, particularly if these panes are contained in moving frames or sashes.
These issues can be solved in various ways. Ideally, a perfect vacuum provides the most thermal insulation value. Less commonly, most of the air is removed, leaving a partial vacuum, which drastically reduces heat transfer through convection and conduction. This is called evacuated glazing. Similar techniques are also used in insulation products called vacuum insulated panels. In practice, however, these types of systems are not used commercially as the panel strength must increase to counteract the effects of atmospheric pressure so the actual viewing area through the glass is quite small.
The alternative is to replace air in the space with inert gases such as argon (argon has a thermal conductivity 67 % that of air), , krypton (krypton has about half the conductivity of argon) or xenon to increase the insulating performance because of the higher mass (density) of these gasses compared to air but at costs that increase dramatically with the type of gas used, xenon being the most expensive. 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. However, since it is difficult to determine whether the gas in an IGU has become mixed with air at time of manufacture (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. Argon is commonly used in insulated glazing as it is the most 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 unitsXenon has found very little application in IGUs because of cost.
Insulating Properties
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. A standard IGU consisting of regular panes of glass and air has an R- value of 2.
A rule of thumb in standard IGU construction is that each change in the component of the IGU results in an increase of 1 R-value to the efficiency of the unit. Adding Argon gas increases the efficiency to about R-3. Using Low emissivity glass on the exterior pane will add another R-value. Properly designed triple glazed IGUs with low emissivity panes on the middle and exterior positions and filled with argon gas in the spaces result in units with R-values as high as R-5. Some multi-chambered IGUs result in R-values as high as R-12.5
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 thicknesses of glass rather than the conventional 4-12-4 symmetrical systems will improve the acoustic attenuation properties of the IGU at the cost of longevity if the unit is used to separate exterior and interior environments. This is due to the differing thermal expansion rates of the glass being used and the shear stress placed on the edge and spacer sealants. If standard air spaces are used, sulfur hexafluoride is used to replace or augment an inert gas and improve acoustical attenuation performance.
Longevity
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 (Northern Hemisphere) or the north (Southern Hemisphere) rarely lasting more than 12 years. IGU's typically carry a warranty for 10 years. The double glazing in windows was invented in 1930s, was commonly available in USA in the 1950s as Thermopane, so after almost 79 years, the manufacturing process is well known. The brandname Thermopane has entered the vocabulary of the glazing industry as the equivalent name for an IGU.
Condensation collects between the layers of glass when the perimeter seal has failed and when the dessicant has become saturated, and can only be eliminated by replacing the IGU. Seal failure and subsequent replacement results in a significant factor in the overall cost of owning IGUs.
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 condition.
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
To properly estimate the heat loss through any window, one needs to take into account not only the pane and gap, but also the thermal properties of sash, frame and sill. Thermal bridging through any of these can lead to huge energy losses. Also, better is to use the overall window performance values, rather than just that at the glass center.
- 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
Calculation
Table 1.1 Relevant correlations to calculate natural convection inside the window gap
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 1.5 cm (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.
See also
External links
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