Many of the windows that were produced pre-1980’s were single pane windows. These windows tend to be drafty because of the lack of modern day seals on the windows. In order to reduce the drafts, storm windows were installed as a common practice over these windows. With the advancements in technology and the ability to produce windows with multiple panes, windows have become exceptionally more efficient. However, with the efficiency improvements come some issues that happen when storm windows are used in conjunction with double pane windows.

Modern Window Technology

Modern double pane windows are produced with two panes of glass that generally are filled with a combination of Argon gas and nitrogen. The space between the glass can be filled a maximum of 98% full to allow for expansion of the gases during different temperatures. The seals on windows are generally made of synthetic materials such as Kevlar, polyurethane, and vinyl. However, some window manufacturers still use horsehair dipped in silicon to create sash and sill seals.

The windows generally have a low-E coating on one of the panes of glass to reflect heat in the summer and absorb heat in the wintertime. There are multiple variations and upgrades of these windows, but this is a typical make-up of a double pane, Energy Star-compliant window. The average R-value of these windows is R-2.3. Since this is significantly less than the wall cavities of a home, the real energy efficiency is in the air sealing ability of the windows.