When selecting a window, look at its R-value, infiltration ratings and construction. Cheap windows are not just a source of heat loss, they can also be the source of condensation problems.

R-value is the measurement of a window's ability to prevent heat loss through the solid portions of the window. The higher the R-value, the less heat will be lost. Look for windows with high R-value ratings. R-values are measured at the center of the glass area. R-values at the edge will be lower than if measured at the center of the glass.

In most new construction and replacement windows, sealed insulated glass has replaced the old single-glazed prime window/storm window combination. Sealed, insulated glass is coated (low-E or low emissivity glass) or filled with argon or other gases to further reduce heat loss and raise the window's R-value. Most sealed insulated glass has metal spacers at the edges.

Infiltration rating or U-factor, measures the window's ability to reduce air leakage through the gaps between the sash and the frame. Most ratings note the air movement when the wind is blowing at 15 MPH. Compare windows and look for ones with low U-factor infiltration numbers less than .35. To prevent condensation, look for windows that have warm air technology. It helps keep the window edges warm.

Generally, casement and awning windows are tighter than double-hung or horizontal slide-by windows. That is because casement and awning windows seal by compressing the sash against the weather-strip. When wind blows against the window, it will seal tighter. The sashes of double-hung and slide-by windows require seals, which can tolerate the abrasive action of sliding sashes. This kind of weather-strip is more prone to leakage.

Finally, high quality materials and proper installation are critical to the window's longevity and for energy conservation. In summary, look for high R-value and low-e and U-factor values.

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