Sidebar: High Elevation Problems Jeopardize Gas-Fill Windows
Gas Fills for Better Energy Performance
Since the 1970s, many manufacturers have used low-conductivity gas fills in windows to improve energy performance. Gases used today—argon (inexpensive and widely used) and krypton (expensive and far less frequently used)—boost performance because the conductivity through these gases is lower than it is for air. If kept perfectly still, air insulates to about R-6 per inch, while argon insulates to R-8.9 per inch and krypton to R-16.7 per inch (k values of 0.024, 0.016, and 0.0086 W/m·°K, respectively). Actual improvement in window energy performance is not this great, however, because gases move around. Convective loops depress the R-value, and the amount of convection is governed by the viscosity of the gas. Argon has about the same viscosity as air—beyond about a half-inch (13 mm) spacing, further improvements in energy performance are not achieved with greater interstitial spacing because of convective loops. Krypton is less viscous, though—for that gas, no improvement is achieved beyond about 1⁄4” (6 mm). Interestingly, a low-conductivity gas fill results in a greater improvement in energy performance when it is used in windows with low-emissivity (low-e) coatings. This is because the low-e coating increases the difference in temperature (∆T) across the layer of gas-fill; therefore, reducing the conductivity across that space has a bigger impact. The impact of gas fills on overall energy performance of windows can be very significant. For a typical low-e double-glazed unit, using argon instead of air reduces total heat loss through the window by about 10%. In a triple-glazed window with two low-e coatings, using krypton instead of air reduces total heat loss by 30% to 40%!
