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.
Convective Loop
All windows have what is called a convective loop that happens on the interior of the window. This is caused by the outside temperature meeting with the inside temperature and causing a consistent air movement in front of the window. This only happens when the temperature differential is substantial, but can easily be mistaken for a draft.
The Addition of Storm Windows With Double Pane Windows
When storm windows are added to double pane windows, a gap is created between the storm window and the double pane window. In this gap, the temperature differential between the interior and exterior of a home collide more violently forming condensation that builds up on the exterior surface of the double pane window. This moisture will also build up on the bottom edge of the interior panes of glass and can start to cause moisture damage to the window.
The amount of moisture and ice build up will vary depending on how well the storm window is sealed and how much the temperature inside the home differs from the exterior of the home. There is also the possibility that nothing at all will happen. The issue with storms over double pane windows is that you may never have an issue and then one day these problems just start occurring. If you have a system like this that is causing problems, simply remove the storm window. Since double pane windows are designed to stand alone, you should not see any drop in energy efficiency.
One issue that is also common is window warpage in warm weather climates. The space between the storm windows and the double pane window becomes a super-heated cavity. This is especially true with low-E windows that reflect UV. This causes temperatures well above normal exterior temperatures that tend to damage the frame of the window and can negate the window’s air sealing ability.
Double pane windows are not designed to be installed in conjunction with storm windows. It is a practice that should be avoided, as you should always operate windows in the application they are designed for.
Adam Yeomans - Adam Yeomans has been writing Internet content for just over a year for the purpose of informing people on how to properly complete home ...
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