Selecting Replacement Windows for Your Home
Compared to a well-insulated wall, a double glazed window can lose nearly 10 times as much heat when comparing the same square footage. Cheap windows are not just a source of heat loss, they also can be the source of condensation problems. When selecting a window, you should look at both the R-value and the infiltration ratings.
R-value is the measurement of a window’s ability to prevent heat loss through the solid portion 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.
In most new construction and now available in replacement windows, you will find sealed insulated glass has replaced the old single-glazed prime window/storm window combination. Sealed insulated glass can be coated with 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. R-values at the edge will be lower than if measured at the center of the glass. That is why you are prone to get condensation forming at the edges of the window glass.
Infiltration ratings measure 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 infiltration numbers.
As a general rule, 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-stripping. When the wind blows against the window, it will seal tighter. The sashes of double-hung and slide-by windows require seals that can tolerate the abrasive action of sliding sashes. This kind of weather-stripping is more prone to leakage.
Proper installation of any windows is critical if the window is to function the way it has been designed.
In review, check for the highest R-value rating possible and the lowest infiltration number rating possible.
Source: Selecting Replacement Windows for Your Home, #684; Univ. of Minnesota Extension Service.