Storm Window Condensation


Old 12-16-09, 05:29 AM
Thread Starter
Join Date: Jul 2007
Location: USA
Posts: 22
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.
Sponsored Links
Old 12-16-09, 05:39 AM
Thread Starter
Join Date: Jul 2007
Location: USA
Posts: 22
Additional information

A couple of other details I forgot to include. The window glass is LowE with Argon in between. Also, these were complete window replacements, down to the rough opening (not just an insert). Thanks.
Old 12-16-09, 07:40 AM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 20,524
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.
Old 12-16-09, 07:46 AM
airman.1994's Avatar
Join Date: Dec 2005
Location: VA
Posts: 5,632
Yep sounds like high RH to me.
Old 12-16-09, 09:40 AM
Join Date: Oct 2008
Location: New England
Posts: 9,995
Just one added detail concerning the difference between upstairs and down. The natural pressure caused by warm air rising creates a higher pressure upstairs and a lower pressure downstairs, actually all the way to the basement. During an infrared scan we will see cold air leaking in throughout the lower half of the house and warm air leaking out on the upper half. Somewhere in the middle is a neutral boundary where air only moves when the wind blows or temperatures change. So even identical windows may perform differently depending upon their location within your home.

All DH windows will have some leakage and in your case you would probably never have noticed it except for the extra storms that are now trapping the air that is leaking and you see the results.

I would bring this to the attention of the window mfg since you indicated that the storms came with the windows possibly from the same mfg. I have read that some mfg's have upgrade kits and perhaps there is something THEY can do to help.

Bummer when you spend the extra money to try to go the extra mile and still get problems. Let us know what they say.

Old 07-17-12, 08:55 AM
Join Date: Jul 2012
Posts: 4
Sorry to drag up an old thread - but I have the same thing happening to me. We've lived in this house for 5 years and the original owners swear they didn't have the same problem. We are also in WI and every winter I have to keep the upstairs storm windows cracked just a little bit to let the moisture escape.

We had a remodeler come out for some work and we were looking at some of the sills on the upper floor starting to rot. He didn't see any weep holes on the storm windows and said to just drill a few holes from the inside out at the base of the storms to let the moisture escape.

So I'm just curious if you solved this issue and how.
Old 11-03-13, 06:47 AM
Join Date: Nov 2013
Location: USA
Posts: 1
Lightbulb This type of condensation is very normal

Condensation between panes of glass is very normal. The traditional method of addressing it is to drill several small holes through the bottom rail of the OUTSIDE pane of glass. Problem solved. Very little heat is lost; it rises to the top.

Note: Thermopane glass was invented partially to address condensation - if the panes are sealed, water can't get in. It works fine until the seal cracks, causing the permanent fog between the panes that you have seen around town.
Old 11-03-13, 07:06 AM
Banned. Rule And/Or Policy Violation
Join Date: Apr 2013
Location: USA
Posts: 4,297
At the bottom of those storms at one time there was two hump out areas that were not suppose to be caulking over.
Just getting rid if the old caulking in that area should take care of it.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes