WINDOW TECHNOLOGIES: Assembly
As higher-performance commercial facades are developed, the use of low-conductance gas fills becomes more common. Gas fills improve the thermal performance of insulating glazing units by reducing the conductance of the air space between the layers. Originally, the space was filled with air or flushed with dry nitrogen just prior to sealing. In a sealed-glass insulating unit, air currents between the two panes of glazing carry heat to the top of the unit along the inner pane and settle down the outer pane into cold pools at the bottom. Filling the space with a less conductive, more viscous, or slow-moving gas minimizes the convection currents within the space, reducing conduction through the gas and the overall heat transfer between the interior and exterior.
Manufacturers generally use argon or krypton gas fills, with measurable improvement in thermal performance. Both gases are inert, nontoxic, nonreactive, clear, and odorless. Krypton has better thermal performance than argon and is more expensive to produce. The optimal spacing for an argon-filled unit is the same as for air, about ½ inch. Krypton is particularly useful when the space between glazings must be thinner than normally desired, for example, ¼ inch. A mixture of krypton and argon gases is also used as a compromise between thermal performance and cost.
Argon and krypton occur naturally in the atmosphere, but maintaining long-term thermal performance is certainly an issue. Studies have shown less than 0.5% leakage per year in a well-designed and well-fabricated unit, or a 10% loss in total gas over a twenty-year period. The effect of a 10% gas loss would only be a few percent change in U-factor on an overall product basis. Keeping the gas within the glazing unit depends largely upon the quality of the design, materials, and, most important, assembly of the glazing unit seals.
The use of low-conductance gas fills is far less common in commercial glazing than it is in residential windows. This results from the fact that solar control technologies are more important in typical commercial buildings than techniques for reducing heat transfer by conduction. However, as higher performance facades are developed, gas fills may become more common in commercial building windows as well.