What Is Insulated Glass?
Insulated Glass is the combination of two or more panes of glass sealed with air, or inert gas between the panes. The construction of an insulated unit is designed to help control energy transfer from outside to inside or vice versa. It accomplishes this by controlling:
- Conduction – Heat flow through a solid material, i.e. window frame, or glass
- Convection – Heat flow by the movement of a fluid or gas
- Radiation – Energy transmitted from heat source (sunlight)
The selection of materials and the design of an insulated glass unit will determine how a unit can save energy. An Insulated unit consists of:
Spacers are the material used to separate the two panes of glass. Spacer can insulate by using materials that reduce the “conduction” from the outside pane of glass to the inside pane. Using metal systems where the metal does not touch the glass and is cushioned by a bed of sealant limits the conduction that can occur. Intercept Spacer is an example of this type of system.
FIGURE A - INTERCEPT Spacer
Some more dense materials that do not include any metal, like siliconized foam spacer, can further improve the inside pane’s temperature. Edgetech’s SuperSpacer is an example of a foam spacer.
FIGURE B - SuperSpacer Unit
Desiccant is a drying agent that is inside the spacer material to absorb all of the initial moisture inside the unit.
The two panes of glass must be hermetically sealed together using a bonding agent like hot-melt butyl, silicone, polysulfide, etc. The sealant bonds the inside pane to the outside pane and creates the sealed airspace which creates the insulation effect. The dead air space further lowers the conduction effect.
As stated above, the dead air improves the conduction effect. However, convection can occur, which is the air turning over inside the unit and raising or lowering the temperature of the inside pane based on the outside conditions. The convection effect is minimized by trying to maintain optimal spacer thicknesses based on the design of the unit. In general, a ½” airspace is proven to minimize the convection effect. To further improve the energy performance, the use of an inert gas like argon, or krypton, which is heavier than air, can greatly limit both the conduction and convection effects. To take it further, a “Triple Glazed” unit can combine three panes of glass and two airspaces for some of the best energy performance.
FIGURE C - Triple Glazed unit with SuperSpacer
Most of the insulated unit is glass. The glass that is selected is the only component that can have an impact on all three types of heat transfer. The use of tinted glass, like bronze or gray, can absorb energy and not allow it to pass on to the inside of the house. More prevalent is the low emissivity glass, or low-e.
Low-E glass is designed to reflect radiant heat and can be used to reflect heat outside in cooling climates or reflect heat inside in heating climates. Low-e is also referred to as spectrally selective, which means it selectively allows only certain waves to penetrate through the pane of glass. By controlling the temperature of the outside pane of glass, it not only reduces radiant heat gain or loss, but also minimizes convection and conduction by helping to maintain the ambient temperature. The images below are thermal photographs of a foam spacer unit with clear glass on the left and a foam spacer unit with low-e glass on the right. The greens and blues show cooler temperatures, while the yellows and reds represent warmer temperatures, demonstrating the ability of low-e glass to control the heat.
|Thermal image of Clear Glass||Thermal image of Low E Glass|