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Window Condensation, A/C & Mold??

My raised ranch is 21 years old as are the Andersen windows, almost all are double hung. I've been battling window condensation for years and am at the end of my rope! Virtually all the upstairs windows get about an inch or so of condensation on the glass along the bottoms of the both interior panes (top & bottom windows) from the late fall through the early spring. I do not experience any ice formation though.

After years of battling, the windows' interior pine has taken a beating and has become black with water and mold staining as the heat, moisture and sunlight appear to be a really good environment for it. Obviously, I'm concerned about how to correct the condensation issue - and am concerned about a potential mold problem. I purchased a hygrometer in the hopes of identifying a high humidity issue but it's been averaging 50% which seems reasonable to me.

Most condensation seems to develop overnight. The house has three zones with forced hot water baseboard heating. Temps at night are 68 in the bedroom zone, 65 in the remainder of the upstairs (overall temps range from 65-68 degrees). Because we primarily face westward, temps in the afternoon can get higher duing the mid-afternoon due to the sun. Two adults, two children 5 and under, two dogs and a turle (water evaporation?) live upstairs. One adult lives downstairs and temps in that single zone are usually 62 but may go as high as 66 on occasion but it's not common. There appear to be no condensation issues downstairs though.

One possible related note. We had central A/C installed upstairs shortly after we moved in 15 years ago and have noticed that we feel cold air coming from the registers during the off-season, although it doesn't seem to impact the room temperature. We were told not to worry about it becuase the contractor said it was a "closed system" and that our warm air was only cooling slightly and reciculating but I wonder if this somehow contributes to the condensation problem. Should I be the covering the registers and/or return in the offseason? Might I have a leak? Is it even related?

I'm lost, out of ideas, and feel like I am watching my windows deteroriate around me.

Am I in need of new windows, is my central A/C to blame, is there something I'm missing?

Please help!! :(

Thanks in advance.

Re: Window Condensation, A/C & Mold??

Mike .... Is this happening to all the windows and do you have any fixed pane windows that also do this?

It may be you have leaking windows or it may be simply a case of stagnate moist air at night .
You could try a simple experiment with one window and a small fan. At night pick a window that you can leave the window coverings open and with a fan set at a low speed have it circulate the air around the window to see if the condensation disappears there.... which would likely indicate there is moist air accumlating at the windows overnight.

You mention the A/C vents are drafty if they are above the windows it may be they are contributing to the condensation with the cold air settling down and mixing with the heated air on the window area. You might try sealing the vents preventing cold air infiltrating the living space ... if anything this will help with the comfort.

Just a guess at this point.:)

Re: Window Condensation, A/C & Mold??

Do you have curtains?
heavy cutains down to the floor can help insulate the window area,but can also cause convection ,which can help move air across the window.You can as -canuk- said block off the AC return with some cardboard over the filter.either way warm air is rising and sweating on the high point(windows).

Re: Window Condensation, A/C & Mold??

All of the windows upstairs expereice condensation, some rooms worse than others. Again, we're talking about an 1" or so on average so 'better' may be 1/2" and 'worse' may be 1 1/2". The majority of the windows in the house are double-hung, but we do have a bay window (fixed pane) in the living room and sliding glass doors in the dining room.

Is it possible that my 20+ year old Andersen windows are leaky? Do windows experience this problem as they age? :(

I can give the fan experiment a try; it seems simple enough. Do you have a recommendation as to how far away to place fan from the window? Assuming it works, what does it indicate? Is my house is sealed too tight? Do I need to crack a window every night from fall to spring? :confused:

As for the A/C vents, they are all mounted in the ceiling. The registers in the bedrooms are located mid-way between the exterior and the interior wall, just a few inches from where the ceiling meets the wall. The remaining ones - living, dining room & kitchen - are all in the celing along along the 'center spine' of the house, furthest from the exterior walls and windows. None of them are directly over or near any of the windows.

Anyone have experience in covering the registers and/or returns and their results (very helpful with the drafts, not helpful at all?, etc.) Also, I only mention the A/C because I wanted to provide as much information about the situation or possiblies for this post. I'm not certain if it does or doesn't have an impact on my condensation problem. Maybe someone else could say for sure.

We do have one set of curtains for the bay window in the living room. The kitchen only has a valance curtain and dining room vertical blinds. The two bedrooms facing west have room-darkening Roman shades and the master has mini-blinds. I've been looking for commonalities myslef but haven't come up with anything worthwhile.

Please keep the suggestions coming as I'm all out of my own

Thanks again in advance.

Re: Window Condensation, A/C & Mold??

Do you have a recommendation as to how far away to place fan from the window? Assuming it works, what does it indicate? Is my house is sealed too tight?

As for the fan place it within 3 feet or so to move air across the window.
As for being too tight ... might be tight enough to allow the humidity to raise just enough to create the condensation on the colder window surface.

Does the bay window in the living room have condensation issues?

Re: Window Condensation, A/C & Mold??

Occasionally we also have a problem with nighttime condensation on our Andersen double hung windows. Some windows are about 15 years old and some are about 5 years old. It seems to occur when the humidity in our house reaches about 50% and goes away when it drops to about 40%. It will even occur at the 40% condition when the outside temp drops real low (for us, not Canuk) to around 5*F.

I noticed it will be worse when the micro blinds in the bedroom are closed at night which I guess is due to the lack of air flow. Our heat is wood stove / hot air furnace, temp in the house ranges from 65 - 75 upstairs. It is around 85 in the basement where the wood stove is and there is no condensation on the single pane basement windows!

If I were you, I would try to get the humidity level down to about the 40% range and see if that helps.

I had very bad condensation on the two skylights in the kitchen no matter what the humidity level was. These were recessed into the cathedral ceiling creating a window well where the warm air would get stuck. I solved this completely by placing a layer of 1/2" foam insulation behind the drywall in the window well and making a "storm window" that I installed even with the ceiling slope. This storm window keeps the warm air from getting up into the window well and the skylights have been completely dry even on the coldest nights. The foam may not have been necessary, but I didn't want to take the chance. I mention this in case someone may be browsing with the same issue on skylights.

Hope this helps.

Re: Window Condensation, A/C & Mold??

canuk - I'll give the fan a try over the weekend and post the results, although I beleive it's supposed to warm up here in the next few days. And, yes, the bay window in the living room has condensation issues too. Not as much as the double-hung though.

Last night, coincidentally, the temps dropped into the teens (about 14 degress F) and the condensation rose half to three-quaters of the way up most of the double-hung windows in the house (both top and bottom panes). The kitchen casement had condensation of about 3" and the sliding glass doors about 2". I never did get to check the bay window beofre I left for work. As the temperatue outside falls, the condensation creeps further up the windows. Does that indicate anything worthwhile?

Even downstairs, which normally has no issues, had slight condensation this morning, but nothing near the upstairs. I'm thinking of moving my hygrometer down there to measue the humidity - I know upstairs averages 50% and has issues, curious what downstatis measues while having minimal to no issues.

bp21901 also mentions he has some issues with his Andersen windows, some within a few years age of mine...

  • Is this possibly an Andersen issue? [*]Is it the obsolete 15-20 year old window technology? [*]Have these windows possibly lost their insulative properties as time has passed, are no longer insulating like the should and this is the result?

bp21901 , how do you keep your humidity at 40%, with a dehumidifier?

Again, please keep the suggestions coming as I'm all out of my own

Thanks again in advance and to everyone for their help so far.

Re: Window Condensation, A/C & Mold??

We were about 10* last night and on a couple windows there was about 1/2" of condensation. The windows in two bathrooms, above the kitchen sink (couple dishes soaking) and the bedroom windows (3 cats, 1 dog, 2 people snoring creating lots of humidity!)were the high humidity areas of the house last night. It was all gone by 11 this morning. The rest of the house windows were dry. All windows are Andersen, bedroom and one bathroom windows are about 5 years old, other bath & kitchen window about 15 years old.

I have a dehumidifier that gets used to lower the humidity levels before it gets cold like it has been over the last week. We live in a rather humid area, between two rivers about a mile from each, so if the temp stays in the 40's there is plenty of moisture in the air. The woodstove and hot air furnace can really dry out the air in the house when we get a cold stretch like this. For the last week I have been using an iron pot filled with water on top of the woodstove to put moisture back into the air. There are more automated ways to control humidity levels which you may want to look into. Your hot water system won't dry out the air like our system does. I think you will see improvement if you get a dehumidifier, don't know if it will solve it completely. Hope this helps.

Re: Window Condensation, A/C & Mold??

MRMusial .... bp21901 has mentioned some good points as well.

The fact the fixed pane window also has condensation issues seems to point to a humidity problem. When the humidity is higher inside the house during winter months this warm moist air will condense on the colder window surface.

If was only the double hung windows it could be considered they were only the problem and possibly from leaking ... though leaking windows will show frost build up when the temps. are below freezing.

I'll share a story about a homeowner with similar issues ....

This homeowner had issues with windows having lots of moisture ( condensation). The main living room window is fixed double pane units the other windows are old (1979) style 3 individual class sliders. The sliders are very leaky and would frost up badly during the winter months and the living room fixed pane unit would have condesation build up.

She had different contractors telling her she needed to replace all of the sliders because they are leaking and that would solve the problem.

I had an oportunity to evaluate the situation for her and while the sliders are leaky this was not the root of the problem. Since the fixed pane had condensation ( moisture ) accumulating indicated there was a humidity issue inside the home. We had monitored the humidity level for a week and found levels to be varying between 55-60% ... because of cooking and showers , etc..

The conclusion was to control the humidity inside the home by installing a Heat Recovery Ventilator (HRV) first and at a later time afford to replace the leaky sliders. The leaky sliders would be covered over with sealed plastic film during the winter months.

After the HRV had been installed and balanced the humidity was lowered a minimum of 8% and the condensation problems were resolved in this case.
Interentingly I had spoke to the homeowner last night to discuss the condition of the home this winter and she had said there has been no issues since the install of the HRV. She does notice that when she cooks the humidity can rise up 6% but soon will go back down instead of staying high.

In this case there is a forced air heating system which makes the install of a HRV simple because of the exsiting duct work.

In your case of having hot water baseboard heat you should consider running bathroom fans longer.
If you have a range hood exhaust that vents outside to use that a lot when cooking. This will help with trying to control the humidity level.


when the outside temp drops real low (for us, not Canuk) to around 5*F.

Oh ... that's chilly in my books as well.;):D

Hope this helps.:)

Re: Window Condensation, A/C & Mold??

bp21901 and canuk ....

Thanks so much for all you help. I think you've convinced me that I don't necessarily have leaky windows but rather a humidity issue.

It was a little comforting to hear stories with situations similar to mine. I do have hot-water forced baseboard heating so I'll have to research some of my alternatives, but it sounds like a bathroom fan is in order (I've been opening the windows to date). I wish I had the option of the HRV based upon your success but I'll have to try something else, likely a standalone dehumidifier -- but I know those tend to be pretty noisy and throw off some heat. I guess a little extra heat from the dehumidifier during the winter might not be a bad thing.

Does anyone have a recommendation for a decent standalone dehumidifier? Something that works well, but is relatively quiet for a bedroom? :confused:

All suggestions are very much appreciated.

Thanks again.

Re: Window Condensation, A/C & Mold??

You're welcome Mike.:)

Some additional information :

In an average home up to 20 gallons of moisture per week can be generated during the winter months.

A second problem is the possibility of high humidity in the winter with the temperature set too far down at night.

Cool air can hold less moisture than warm air, so the relative humidity (RH) rises as the air cools.
For instance, house air at a reasonable 35 % RH at 70 F will see an increase to 50 % RH when the same air is allowed to cool to 61 F.

This can lead to condensation on windows ,ceilings and walls (for instance, in closets or behind furniture). The warm moist air will condense on a cold surface.

So if the humidity inside the home is at 50% with the temp at 70 degrees then it will be considerably higher if you lower the temp down at night for example.

Basically, you are creating a more humid environment, all things considered, when you allow the house temperature to drop significantly. This may not be a problem in a dry house or one where you can modify the humidity, for instance by turning off a humidifier.

The house humidity should be monitored, especially in winter.


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