Forum Title: Storm Window Condensation
This might be a dead horse subject, but I can't find much information to solve my problem. This past summer I installed new Marvin CUDH clad double-hung windows, with combination storm windows, in my 1908 four-square. As the temperature has dropped here in Wisconsin, there has been condensation forming off and on on the inside of the storm windows on the upstairs windows. When it gets cold enough, it is just a coating of frost on the windows, normally forming on the upper sash of the storm, starting in the middle. The downstairs windows, which are the same clad windows with storms, do not get this condensation or frost. Since these are new windows, I am perplexed as to why there would be air loss enough to cause the condensation. When I crack the storm slightly (put in first hold-up notch, about 1 open) the condensation goes away. Any thoughts? The weep/vent holes all appear to be open. Why only on upstairs windows? And why all of them? I haven't tested, but the inside humidity is low as we rarely get any condensation on the inside of the main windows. We run forced air heat which does a good job of keeping things dry.
Category: Windows & Doors Post By: DAVID MILLER (Yuba City, CA), 01/18/2019

Have you considered sliders instead of DH?

- KATHERINE ROGERS (Ann Arbor, MI), 03/02/2019

Options #2 and #4 are what I see the most often. Having the stool come up to the sash gives you a nicer looking job and helps seal the window a little better.

- CHARLOTTE FRANKLIN (Bossier City, LA), 03/01/2019

The old windows are french pane up and down sliding windows. Those are on the inside with the storm windows on the outside. We use the 3m window film on the inside to prevent the drafts. Ok, I'll check the drain holes at the bottom. Thanks!

- TOMMY FOWLER (Conway, AR), 02/20/2019

As you might have already figured out, the frost likely indicates that warm humid air from inside the home is finding its way out the window to the storm window. This often will mean you have a leaky/drafty window, but there can also be other contributing factors. It sounds like your question is, why all the upstairs ones frost, but not the downstairs ones? Assuming the installation and insulation techniques are the same on both... my first thought would be that warm air rises (warm air has the ability to hold more moisture than cold air). If it is warmer upstairs it may also be more humid upstairs, meaning the dewpoint is higher upstairs than it is downstairs. But the opposite could also be true... if it is colder upstairs, there would be less heat loss out the glass. Downstairs where it may be warmer, heat loss might be such that it keeps the storms from frosting. Less heat loss would mean a colder storm window. But radiant heat loss alone is probably not the answer... since there is frost it likely indicates that humid air is being introduced, so there is also air movement to consider. When the storm window is open, there is more ventilation. Warm humid air does not get trapped, it simply exits the house. Storm windows often frost up when their perimeter and/or weep holes have been caulked shut, sealing them up tight enough that the air between the prime window and the storm window is trapped. It could also have something to do with your HVAC setup. If you have no (or not enough) cold air returns upstairs, everytime the furnace kicks on, it would create positive pressure in the rooms upstairs, which might tend to force air out the windows- for instance, if the bedroom doors are closed, etc. Also bedrooms are typically upstairs and at night we create a lot of air just by breathing, doors are often shut at night, etc. Blinds and/or curtains being shut will make glass colder. These could be factors as well. Ensure that all your windows are locked, as they often seal tighter when locked. I don't know that there is one definite answer to your question, but an energy audit might be an interesting way to help you figure out where the most heat loss is occurring on your home. You could also try putting a 3M window insulator kit on a couple of the windows and see if it helps. If it does, the plastic is working as both an air barrier and a vapor barrier, preventing air loss out the window.

- BERNICE BANKS (Boca Raton, FL), 03/07/2019

I think Joe has the answer. Humid inside air is leaking through to the space between the outside storm and the inside window. Or, there is anouther source of moisture entering that cavity. But those drain holes would typically both drain any water and vent out any other moisture. Bud

- KRISTINA SINGH (St. Petersburg, FL), 03/04/2019

Yep sounds like high RH to me.

- ALVIN LUCAS (Hemet, CA), 02/02/2019

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