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Community and Q&A

Condensation on windows

user-6665481 | Posted in Energy Efficiency and Durability on

I am a HERS rater, back in the summer, i tested a house for a builder.
built about a year ago and lived in.
​they​ had a problem with condensation ​collecting​ on windows
windows face north, and air changes in house were 2.4ACH
they had one continuous fan that was not working
so they replaced this fan which is guest bath, off kitchen, near laundry.
this now is supposed to run at 60 cfm continuous.
They replaced a second fan in the master bath to run continuous at 50,
occupied at 100cfm. ​both on the main floor, there is no second floor,
but a finished walk out basement.

exhaust rate now meets ashrae standard of 110 cfm,

so, thought problem was fixed, just got an email,
they want me out again to diagnoses because
windows are still condensating.
​any ideas that can help ?

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  1. Expert Member
    Dana Dorsett | | #1

    Did you actually measure the cfm, or are you counting on it meeting the manufacturers' specs? Duct impedances and house depressurization can sometimes defeat that. A large fraction of bath fans "as installed" don't meet the spec in-situ, primarily due to the ducts.

  2. user-2310254 | | #2


    Have you measured the indoor humidity level? Where is the home located?

  3. user-6665481 | | #3

    Not yet, I will be measuring the fans on wednesday. Indoor humitidy level was 53% in the summer with AC running. Have not been back to house since. Ill measure humidity again with furnace running.

  4. slow2decide | | #4

    We had condensation on our windows of a newly constructed home. It was due to a broken solenoid in the whole house humidifier. The humidifier switch said "off" yet it was running unbeknownst to us. Our humidity levels were high throughout and it caused mold on the basement rafters and mold spores throughout the air in the house(which our builder paid to remediate). Not suggesting that is what it is, I am just saying I witnessed an unlikely cause can lead to this issue and it resulted in mold- FAST. We now turn the water source to the whole house humidifier off for the summer season. We had moved-in in July, so I have NO IDEA why the default was to leave the water source on and just turn the unit to off. Good luck finding the source.

  5. Expert Member
    Dana Dorsett | | #5

    A humidifier is almost always a bad idea. More often than not it's just band-aid for low-humidity comfort in an air leaky or over ventilated house, when the better solution would be to tighten up the house and control the ventilation rates.

    A house that tests at 2.4ACH/50 would have no reason to install a humidifier, unless the place is being over-ventilated (which it might arguably be if it really hits 110 cfm, ASHRAE 62.2 notwithstanding.)

  6. Expert Member
    ARMANDO COBO | | #6

    My first question would be if the HVAC system was designed, installed and commissioned properly... have all the supplies been tested for correct cfm delivery. Second, does the system has an IAQ thermostat capable of managing humidity. Third, is there supply mechanical ventilation for the house controlled by thermostat or an ERV/HRV?. Fouth, what type of windows and how well were installed.

  7. user-6665481 | | #7

    There is no humdifier on the system. Supplies have all been tested, but I have not compared to manual J yet (someone else did manual J) System does not have an IAQ thermostat. Unfortunatley, no ERV, but this would take care of the problem and may be the right answer. Windows appear to be installed well as they did not produce any excessive air infiltration; however, they are the next likely suspect. THank you all for your insight and responses.

  8. GBA Editor
    Martin Holladay | | #8

    It's impossible to provide much advice unless we know more. The most important missing information is the interior relative humidity (RH) during the winter.

    Excessive air infiltration around windows is almost never the cause of window condensation problems in winter.

    You haven't told us anything about the windows. I'm assuming that they are at least double-pane windows that comply with code requirements -- but you didn't tell us. Some owner-builders install single-pane windows (for example, an artistic window with stained-glass or something like that). So tell us about the windows.

    There are only two ways to solve this problem: lower the indoor RH, or raise the temperature of the window glass (for example, by adding a storm window). Ventilation will usually lower the indoor RH during the winter, when outdoor air is cold, but you don't know the airflow rate through the bath fans unless you measure the airflow rate.

  9. ohioandy | | #9

    Dwayne, I had the same problem, had it discussed on a Q&A Spotlight here. Need to know where the humidity is coming from. In conjunction with the blower door test, it's useful to do an infrared camera study, which reveals where exactly outside air is sneaking through the walls. (Running two fans on an exhaust-only basis means that makeup air has to come from somewhere.) How many people live in the house? Any obvious sources of humidity like excessive showering, hot tub, lots of houseplants, cooking and baking? Is the sump well covered with a sealed lid? Was a sheet of plastic installed under the basement slab? Does the basement backfill drain freely? Is the dryer vent leaky? Are rain gutters dumping water at least ten feet from the house?

    A balanced HRV or ERV system--in conjunction with careful air sealing--is the best way to handle this situation. However, it's best practice to minimize humidity sources, too, so the equipment doesn't have to work so hard.

  10. user-6665481 | | #10

    windows are typical newer construction. double pane, low E. I didn't see window sticker, but builder normally uses U-value 34, SHGC 27. I think Andersen. ill be there on wednesday again to gather winter humitidy level. There is no excessive behavior. no fish tanks, I dont remember any house plants. its a very ordered, organized upper income household. 2 adults, 2 tennage children. typical showers with exhaust fans used. Kitchen had a range hood which is use, and kitchen is not even used every night for cooking. ill keep you all informed after more testing. thank you for the help.

  11. christopherw | | #11

    Do the windows have an i89 LowE interior coating, what Andersen calls "Heatlock"? That reduces condensation resistance pretty significantly. I don't know if Andersen publishes Condensation Resistance numbers, but you can get an idea of the impact of from Cardinal's Technical Glass Guide. It seems to show "LoE-272® / LoE-i89 (#4)" with condensation at 36% relative humidity versus 53% relative humidity for the same window without the i89 coating. In my house, that would make the difference between lots of condensation and no condensation.

  12. Expert Member
    Dana Dorsett | | #12

    Christofer Welles writes:

    "Do the windows have an i89 LowE interior coating, what Andersen calls "Heatlock"? That reduces condensation resistance pretty significantly."

    Indium tin oxide on surface #4 reduces the U-factor to the mid- U0.20s or lower, depending on what's on surface #2, glass spacing, and the gas used between the panes. That's a fairly high performance window (for a double-pane), and not a "...U-value 34, SHGC 27 ..." window, described as the builder's typical window.

    But even is it is a double-low-E window it'll only be a significant contributor to this problem in the cooler half of US zone 5 and colder.

    The outdoor temperature and indoor temperature are both relevant factors. Simply stating an indoor RH at which condensation occurs has no meaning without both the outdoor and indoor temperatures at which the #4 surface temp at the dew point. See Cardianals chart (predicated on a 70F indoor temperature) on p22 (p25 in PDF pagination) here which lists three different outdoor temperatures (-20F, 0F, and +20F):

    Dwayne still hasn't clued us in to where this house located, which would give us some idea as to what the average winter outdoor temperatures are like.

  13. christopherw | | #13

    Dana is absolutely correct. The values I chose to pull were for a 0F outdoor temperature (which we're hitting in my neck of the wood at this time). I didn't notice that the numbers specified as likely don't line up with any surface #4 coated windows.

    The evidence indicates my idea is unlikely in this particular case.

  14. RMaglad | | #14

    throwing this out there...does the occupant have honeycomb style blinds, that may restrict air movement?
    Our house, RH 35% with builder grade dual paners, gets significant overnight condensation with our blackout blinds down. If we open them up, no condensation, but then of course our bedrooms are flooded with light pollution making it less comfortable to sleep. (yes, the room is literally black, no LEDS, no alarm clock)

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