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Insulating glass speaks to what is insulating glass, why use insulating glass.
This glass helps to protect from building heat loss as window glass is a poor thermal insulator. A single sheet of glass, known as single glazing, conducts heat about 5 times as fast as a one inch (25 mm) of polystyrene foam insulation and 20 times as fast as a well insulated wall.
A second sheet of glass applied to a window with an airspace between the two sheets, known as double glazing, cuts this rate of heat loss in half while a third sheet with its additional airspace, known as triple glazing, reduces the rate of heat loss to about one third of the single glaze window. However, the triple glaze still loses heat about six times as fast as the wall in which it is placed.
Continuing to add additional sheets of glass and airspaces adds weight, bulk and expense to the glazing unit and the frame that holds it, making double and triple glazing the practical maximums for normal building glazing applications.
To prevent moisture condensation within the airspace of double or triple glazing, which is also referred to as insulating glass units or IGUs, the units are usually hermetically sealed at the time of manufacture with dry air inserted in the space between the glass panes. Originally for small lights of double glass, the edges of the two sheets were simply fused together.
However this detail is seldom used now because the fused glass edge is highly conductive of heat. Instead, a hollow metal edge spacer, which is called a spline, is inserted between the edges of the sheets of glass and the edges are closed with an organic sealant compound. A small amount of a chemical drying agent known as a dessicant, is left inside the spacer to remove any residual moisture from the trapped air.
The air is always inserted at atmospheric pressure to avoid structural pressures on the glass. Note, that when insulating glass units do exhibit internal condensation, it is a sign of failure of the edge seal and the unit must be replaced.
The thickness of the airspace in insulating glass units is less critical to the units insulating value than the mere presence of airspace. From 3/8 inch (9 mm) up to about 1 inch (25.4 mm) which results in an air space ½ inch (13 mm) thick if ¼ inch (6 mm) glass is used.
For slightly improved thermal performance, stainless steel, which is less conductive of heat, may be used instead of aluminum for the spacer and a sealant material may be placed between the glass and the spacer for a thermal break. For even better thermal performance, warm edge spacers made of thermally broken aluminum or extruded rubber may be used.
The thermal performance of insulated glazing units can also be improved by introducing gases with greater density and lower thermal conductivity than that of ordinary air between the sheets of glass. Depending on the gas used and the thickness of the space between the glass sheets, improvements in thermal performance of 12 to 18 percent are possible. Argon and Krypton are the gases commonly used.
The performance of glazing as a thermal insulator is quantified as its U factor. U factor is expressed as BTUs per square foot hour degree Fahrenheit. (BTU/sq ft-hr-deg F) or in metric units (W/sq m- deg K). U factor is the mathematical reciprocal of R and as such, lower values represent improved thermal performance. For more detailed information, glass manufacturers product literature should be consulted.
Insulated glazing products that rely on the evacuation of most of the air from the space between the glass sheets are also under development. When combined with with low emissivity coatings such vacuumed insulated glazing units are predicted to achieve U factors as low as 0.080 BTU/sq ft-hr deg F (0.45 W/sq m – deg K) in units not more than ½ inch (12 mm) in total thickness.