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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.
Gas Fill

Air Space

U-factor as a function of air-space thickness and emittance.