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We recently replaced about 2/3 of our windows with reasonable Anderson ones (vinyl covered wood). We're in Chicago, and I try to keep the humidity around 40-45% during the winter for comfort. I generally get very little / no condensation on the new windows, and a touch on the old ones. With temps dipping into the -20s F this week in Chicago, I'm wondering if I should drop the humidity level in the house to not have the windows freezing over. The new windows are definitely picking up a pretty significant amount of condensation, and I'm concerned about damage.

Question: will a day or two of heavier condensation cause any permanent damage?

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With the extreme cold you are experiencing it may be tough to get the humidity low enough for no ice on the Windows. As far as occasional condensation if you wipe it up there should be no long term damage. When I lived in Ohio we had some -20 temps and even argon filled double Payne Windows were freezing in the kitchen and bath. I just wiped what did not drain through the weep holes and everything was fine.

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The critical point here is that your interior humidity level should be based on the outside temperature. At zero Fahrenheit you should be down around 25% inside, or maybe lower.

That said, it's difficult to eliminate all window condensation while surviving as a human. Weather can change abruptly, and it's difficult to get your house to keep up.

When things get very wet I'll go around and wipe up the liquid water each morning before it starts puddling. Wood damage occurs fairly easily. A bit of fog isn't a big deal.

  • Don't know why there are some down votes with no comments, but I agree, with your answer also, lower humidity really helps. I heard numbers on the way in to work like -27 and -40 & -50 but think the -40 and -50 are probably wind chill numbers that's cold enough to have your tires freeze to the ground and drive right out of recaps happened to me in 81 or 82. Someone posted a chart here on stack exchange in the past that agrees with going lower I will try to find it. – Ed Beal Jan 30 at 19:07
  • -37° F here this morning. It's temporary, so I won't attempt to lower the RH in my home. – isherwood Jan 30 at 19:24
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    Glad I am close to the ocean never been below -23 with wind chill at -80 but that was on the flight line in ohio, maybe -40 & -50 were real numbers that is crazy cold. – Ed Beal Jan 30 at 19:37
  • fwiw i didn't pick this as the answer for two reasons: 1. interior RH should be based on outside temp and the rating of your windows. The number you quote bakes that in, and doesn't account for whatever windows I may have. 2. the actual question wasn't "what should the RH be," it was "am I ok having condensation for a day or two." – kolosy Jan 31 at 16:45
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Where I live, condensation on windows is a direct reason for replacing the windows. It means that the Q-rating (heat transfer) through the window is way over the minimum limit. There should not be condensation on windows that form droplets - maybe if you are cooking something, exiting a sauna or whatever - they can fog over, but they should clear up again.

I suggest contacting the contractor and ask what the Q rating of the windows is supposed to be.

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    With the record lows they are experiencing this is normal. I don't think it has been this cold since I lived there in the 80's we had -23 with wind chill in the -80 range even triple Payne probably won't stop these rare extreme temps from creating freezing condensation. – Ed Beal Jan 30 at 15:12
  • @EdBeal wind chill isn't a factor in that calculation, but ok if it is extremely cold ref usual winter temperature then it might be prudent to not run the humidifier those days. I'd still ask the contractor / supplier what the Q rating is supposed to be. – Stian Yttervik Jan 30 at 15:18
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    Blowing wind actually absorbs more heat than stagnant air so it will affect the structure having lived in those conditions it makes a difference. Wasting time finding what the factor is just that a waste of time the Windows are in and they are experiencing record lows, this is to be expected. – Ed Beal Jan 30 at 15:33
  • @EdBeal Transportation of heat from the outer surface of your window is not, or perhaps better said as should not be the rate limiting step in the heat transfer from inside bulk volume -> outside bulk volume. This is also the reason why a well functioning window will not have condensation forming inside, because that signifies that there is a large temperature gradient between inside surface and inside surface convection layer. If you are at these situations, where temperature gradients is at either boundary, your windows have a total Q rating that is off the mark by at least a factor 10. – Stian Yttervik Jan 30 at 16:04
  • Ed's right. Windy days draw away heat more readily and result in a colder inner surface. This obviously results in more condensation. You can toss around greek symbols and calculations all you want (I do have some physics training), but reality doesn't care. Condensation occurs on even the best windows if conditions are extreme. – isherwood Jan 30 at 17:43

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