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Reducing window condensation

milesadams | Posted in General Questions on

I am from Minnesota and had a question about window condensation and ice on our new windows and doors now that is winter.

1. The house was built around 2000, split level, original forced air furnace and ac, no air exchanger, roughly 1300 sq ft down stairs and 1300 sq ft upstairs. We added a door to the lower level for renters, The upstairs is vaulted. 6 people living. 4 bed 3 bath.

2. My understanding is that high humidity levels are contributing to window condensation and ice, and the lower the temp the lower the humidity should be.

3. Regardless of humidity, whenever our windows are closed, is it important to have some sort of fresh air exchange for general health and oxygen levels or is it not that big a deal? I feel like we have a well insulated tight house especially with the new windows.

4. I’ve heard mixed opinions of what to get from hrv/erv, whole house dehumidifier, stand alone dehumidifier, or decentralized hrv/erv. I think most people I talk to have a biased opinion about it.

Thanks for any info,


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  1. mpg9999 | | #1

    What is the relative humidity inside your house? Buy a cheap humidity monitor if you don't have one. If your indoor humidity level is too high in the winter time then increasing your ventilation will lower it. No need for a dehumidifier in the winter. Are you running exhaust fans when showering and cooking? Any unusual sources of indoor humidity like a lot of indoor plants?

  2. GBA Editor
    Martin Holladay | | #2

    Michael is right. It's hard to provide advice unless we have some baseline data. Knowing your indoor relative humidity level is important.

    It would also be a good idea to know the results of a blower door test -- to know whether your house is leaky or tight. Some utilities offer free or subsidized home energy audits that include a blower door test -- you might want to investigate whether there are any programs like that where you live.

    In general, if you have new windows, significant condensation or ice build-up is a sign of high indoor humidity. That said, it's not too unusual to see a little condensation when outdoor temperatures drop below 0F. For more information on this issue, see Rating Windows for Condensation Resistance.

  3. Andrew_C | | #3

    Regarding 3), fresh air: blower door data would be useful. It may also be useful to monitor CO2 for a while. CO2 is a reasonable proxy for air quality. And if you have a number of people/animals in the house, even if you don't have gas appliances that aren't externally vented (like a gas cooktop), CO2 numbers can build fairly quickly to levels that adversely effect us.

    Whether you follow ASHRAE's current ventilation rate guidelines, or something slightly lower as has been proposed more recently by other groups, IMO some controlled ventilation would be good for most homes with reasonable air-sealing.

    A small part of the solution might be to put timer switches on all of your bath fans that automatically shut off. These encourage people to use the fans and let them run long enough to be useful. As an example, Lutron MA-T51-WH Maestro Countdown Timer.

  4. Jon_R | | #4

    > CO2 is a reasonable proxy for air quality.

    Say there is only one person in a house producing little CO2. How is this related to say formaldehyde concentration? Data?

  5. Andrew_C | | #5


    I agree that CO2 is not a conclusive indicator for other types of pollutants. Recent studies do indicate that CO2 may be a direct pollutant and not just an indicator of ventilation.
    Also, it makes sense to try to control the source first; dilution is not the solution. However, almost all of our homes have some pollution, and dilution is PART of the solution.

  6. brp_nh | | #6

    How much condensation are you seeing? Just a small amount on the edges of the glass on cold nights or are you seeing significant condensation/frost day and night?

    With 6 people in a medium size house, I bet your winter humidity levels are too high. You probably need some type of continuous ventilation and short bursts of your bath fans may not be cutting it. Get a few of the cheap monitors to place around the house to get a feel for relative humidity levels. I believe the ideal winter levels in a cold climate are 30-40%, probably less than 40% if comfortable to you.

    A blower door test or home energy audit would certainly be helpful to guide your ventilation strategy. Regardless of that, I would make sure your bath fans are hooked up correctly with well sealed ducts. If they are cheap fans and not performing well, you could upgrade to Panasonic WhisperGreen models.

    If you decide to upgrade your ventilation could be as simple as making sure your bath fans are running properly and putting them on some type of timer to run continuous low or boost a short time each hour.....or you may want to install a ducted HRV.

    For a real world example, I have some friends that have a well built (but not "high performance") house (zone 6, NH) with standard bath fans. They were seeing some consistent window condensation. They self installed a Fantech HRV, result was no more window condensation and fresher air.

  7. emturnerNWO | | #7

    Miles, we live just north of you (in Canada, near the Manitoba/Ontario border)--so our climate is very similar. We don't have a forced air furnace like you do (we have in-floor heat) so it's a different scenario, of course, but this is my personal experience, which you may find helpful... We have lived in a tight house with an HRV for over a year now, and I can't say enough good things about it. The fresh air is really nice. A couple of times, when I've accidentally turned it off, the stuffiness / staleness of the bedroom air has actually woken me up (thinking, "why can't I breathe?!"). With the recent cold snap, we got some condensation (frost) along the windows, so I turned the humidity setting down a little. Problem solved. It's really remarkable. For new builds here, HRVs are mandatory, but even if they weren't, I would still install one--for me it's definitely worth the money just from a comfort standpoint, especially in such a cold climate where you are battened down tight for so many months.

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