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I have an old house that has both wooden and aluminum storm windows. Many of the windows develop condensation and frost on the storm windows when it's cold outside. I made sure weep holes were clear, and I've tried various approaches to sealing the interior windows to prevent condensation on the storms. While sealing has reduced the condensation on the storms, it hasn't completely eliminated it, even when following typical winter humidity guidelines.

Is there a certain amount of unavoidable exterior storm window condensation? Is some condensation okay? Is there a point at which storm window condensation would become a problem? Would it indicate that there would be condensation developing in other invisible places in the house, such as wall interiors or attic space?

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How much moisture is ok depends on the window frame material and the material that condensation may drip or seep onto. The primary issue is rot. If your storm windows are aluminum or vinyl, some condensation will not likely cause damage. Wood windows will see degradation of their finish and ultimately wood decay.

The only way to completely prevent condensation may be to increase ventilation between the storm windows and the inner windows, but that obviously decreases their thermal effectiveness.

Your best bet, if decay is a concern, might be to forego the old-fashioned storm windows and use plastic sheeting on the inside of your windows. In my experience, you may still see some condensation on your windows in this case, but only as ice when it's extremely cold.

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  • Are you suggesting I allow more inside air to contact the storm windows? My understanding is that I need the opposite, that humid indoor air reaching the storm interior is what causes the condensation.
    – glenviewjeff
    Dec 18, 2015 at 17:54
  • No, outside air, which will be much drier. But I wasn't actually suggesting that. :)
    – isherwood
    Dec 18, 2015 at 17:55
  • Yes, that makes more sense. I did drill weep holes where none were present. I'm mainly trying to find a reasonable guideline for a tolerable level of moisture. I've seen some really bad cases in Google searches, which would be of obvious concern.
    – glenviewjeff
    Dec 18, 2015 at 17:57
  • As much as you may not want to deal with it, interior plastic film, applied carefully, will probably solve most of your moisture issue and net you much better energy efficiency.
    – isherwood
    Dec 18, 2015 at 17:58
  • I don't mind the hassle, but hadn't figured out how to work around the window treatments, blinds, shades, etc.
    – glenviewjeff
    Dec 18, 2015 at 18:00
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Non-hermetically sealed storm windows with weep holes are always going to have some moisture issues, as, on humid days, the air between the windows will have a lot of humidity.

On cold mornings, this moisture will either freeze or condense on the inside of the outer pane of glass. This is normal, and unless you can hermetically seal the entire outer window, you can't stop it. The weep holes help ensure that when this moisture drips down, it will run off the sill and out, and not be retained on the sill, where it will rot it and create mold.

Basically, as long as your storm windows are open to the outside, you'll always have condensation.

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  • Thanks gbronner; the question is really about what level of condensation is ok--not about how to prevent it. And anyhow, my understanding from research is that exterior air coming into the cavity between the panes actually reduces the condensation. It's the moist interior air leaking into the space that causes it. In my case it's impossible to avoid because it's coming from our unsealed wall cavities.
    – glenviewjeff
    Jan 12, 2016 at 15:56
  • That could be it, but remember that due to daily temperature cycles, the humidity of normal outside air changes enough to cause dew or hoarfrost. In general, as long as the window isn't in a terribly shady spot and/or the sills aren't particularly rot-prone wood, this is ok, even if the dew/frost covers about half the window each day.
    – gbronner
    Jan 12, 2016 at 16:15

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