What is the proper construction of a storm window?

Asked by Dan
Scandinavia, WI

'I have condensation between my storm window and interior window. What is the proper construction of a storm window? Does it require holes to let the moisture out? If so, my windows don't have any means to release the moisture. What do you suggest I do to correct this? They are aluminum clad windows, not sure if just drilling a hole at the bottom will solve my problem. But if I don't do something, I risk my windows rotting from the moisture. Thanks..'



Condensation is the result of warm moist air coming into contact with a colder surface with a temperature at or below dew point. On older windows with storm window attachments the condensation can form in several surfaces depending on how the windows/storm windows are performing.

Condensation on the interior of the window

If the condensation is on the interior side of the window you may want to reduce the amount of interior moisture. Since the windows (in winter) are going to be colder than the surrounding wall moisture may condense on the inside glass if the window is otherwise well sealed.

Condensation on the interior of the storm window

If the condensation is on the interior surface of the storm window it means that the warm, moisture laden air in the house is passing through the primary window or rough opening assembly. Once this warm, moisture laden air reaches the inside glass surface of the storm window it gives up its moisture in the form of condensation.

This is the most common condition. As a result, weep holes are critical to give the buildup of condensation a place to drain.

Make sure weep holes are clear

On aluminum storm window assemblies the manufacturer typically puts dimples in the bottom on the frame assembly to allow for moisture to drain.

What often happens is these holes get sealed when the installer caulks the windows during installation or they get painted over after a few years. The weep holes might also get clogged from dirt and debris buildup on the interior sill.

The reason they dimple the bottom frame rather than drill a hole in the face of the frame is to allow the water to drain from the lowest possible location to prevent water from building up and rotting the sash. If you have aluminum storm windows check the bottom of the window closely and I suspect you will see two small dimples that make the weep holes. You may have to clean any caulk or sealant from the bottom of the window to locate the drainage sites.


For more information:

Read "How do you stop windows from getting condensation?" a Q&A answered by Michael Holcomb.

For additional information on storm windows check out this EPA link.

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