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Humidity – Air moving from cold outside to warm inside ?

tundracycle | Posted in General Questions on

We are having a few issues in our new house including windows with very significant air leaks and extreme differences in humidity throughout the house.

Indoor humidity ranges from about 15% in the scullery hall next to the kitchen to 35% in the loft. Current values are:

Loft = 35%
UL Bedrooms = 21%, UL Master Suite = 29%
ML Living Rm = 24%, ML Kitchen/Scullery Hall = 15%
LL = 26%

The loft is basically a finished attic so we have 4 levels total. Temps range from 66°f – 69°f. I have verified that these are measuring humidity accurately w/ a Gaslab 501. They don’t appear to ever be off by any more than 1%.

We have two air handlers and fans run continuously so we should be getting good air circulation through the house.

Outside is -17°f and 55% RH.

It seems that two things are happening with RH. Thermostats with more window exposure and particularly more western window exposure seem to have much higher humidity. The Kitchen/Scullery Hall thermostat is the most protected from windows and consistently has the lowest humidity. Living Rm thermostat is exposed to a lot of west facing windows.

Just as hot air rises, so does water vapor?  The 4th floor loft consistently has the highest humidity. Stack Effect? BTW, stack effect is very noticeable as LL windows have greatest air leaking through to inside, ML is next. UL you can’t feel much of anything and in the loft you can feel air moving outward. I’m guessing the neutral pressure plane is just above the ML windows.

HOWEVER, would air infiltrating through the windows (and you can feel a lot of it and in some cases see daylight between sash & frame) raise or lower the humidity in these rooms? My limited understanding is that air w/ 55% RH @ -15°f will have a lot less water in it than air of perhaps 20% humidity @ +7-°f.  Correct? Or could this outside air be increasing the humidity? 


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  1. user-5946022 | | #1

    Can't answer your specific question. However, regardless of the response, if you can see daylight between a sash and frame on a closed window, the first order of business is to resolve that.

    Is this a newly built house, or an old house that is new to you?

    If this is a newly built house, what was the leakage in your blower door test?

  2. tundracycle | | #2

    New house completed Jun 2020. Yes, there are a lot of issues w/ the windows (Loewen) leaking air that we are working w/ the local supplier and mfr on. These were supposed to be a step up from Marvin but are proving to be much worse.

    CFM50 = 1980
    Resnet ACH50 = 1.42
    EU ACH50 = 2.01

    We had wanted to be close to 1.0 and our builder expected to be close so these numbers were quite disappointing. They were fairly diligent during framing and other construction steps to seal up as they went. Then the walls and roof (hot roof) are all closed cell foam plus the wall cavities were filled out w/ blown fiberglass and then all ceiling/floor spaces are filled w/ blown fiberglass. The ceiling/floor spaces more for sound transmission but should help somewhat from an energy standpoint.

  3. Expert Member
    Michael Maines | | #3

    W Ramsay, I don't think I have a comprehensive answer for you but a few comments:
    The Gaslab CM501 manual says it measures RH at 25°C (about 77°F) from 20% to 80% with ±3% accuracy, or ±5% accuracy at "other." I'm not sure if "other" means temperature or RH range, but I do know that most humidity sensors are less accurate the farther the temperature is from the calibrated temperature.

    Moisture makes air more buoyant, just as temperature does. So it's not surprising that the air high in your house is warmer and with higher RH than air lower in your house. Interesting that your neutral pressure plane is that obvious. I'm surprised you were able to get a 1.4 ACH50 blower door if you can see daylight through your window sashes.

    There are probably websites or charts that show the relative difference between -15° air at 55% RH and 7° air at 20%, but I use a psychrometric chart and there is so little moisture in either case that you can hardly see the difference between the two. Both are roughly equivalent to 2% RH at 70° so any air leaks will lower your indoor RH.

  4. charlie_sullivan | | #4

    I agree with Michael that for all practical purposes you can consider the moisture content of the outdoor air to be nearly zero. A psychrometric chart and/or calculator can help you understand how that all works. Here's a very simple calculator, and here's a nice chart

    I recommend converting all of your relative humidity readings into dew point numbers or humidity ratio numbers to understand what is going on better.

    The loft could perhaps be explained either by a lower temperature (?) or by less air circulation there. I'm not sure about the kitchen--would be interested to see temperatures by room and maybe a layout of the space.

    I'm surprised you have trouble with Loewen windows--ours are not leaky at all. Maybe the frames got warped in the install? Or might take some adjustment? Are they casement windows? I think you should be able to get that resolved with Loewen.

  5. tundracycle | | #5

    Thank you guys.

    Good point on the 501. The published spec is kind of worst case. Generally about 80% of devices like this are much tighter tolerance (though I believe RH is kind of tough to be very accurate on in any case). We have a number of devices that include hygrometers including from Hannah, TSI and Thermoworks. They are all, including the 501, within 1% of each other so either all of these and the thermostats are off by the same amount or they are somewhat sort of accurate. :-)

    Air circulation in the loft s/b somewhat OK. All zones are set to have the fan on all of the time so except when one of the other two zones on this system is calling for heat then this zone should at least be circulating. That said, it is set to 62°f by default (sets back to that at 8a, 12p, 6p and 10p - though actual temp rarely goes below 67°f) and gets turned up to 69-71°f for maybe 4-6 hrs each day when I'm working up there.

    @Charlie, very heartening to hear of your experience w/ Loewen. Given what we're seeing they appear to have extremely poor quality control in their design & manufacturing. Hopefully they'll be able to fix the problems and we'll be happy. Most of ours are double-hung which is not nearly as good as casement. My studio (on the same property) has Marvin Integrity DH and they are much much tighter than the Loewen both by feel of air movement, feeling of coldness on the back of my hand and based on thermographs w/ a Flir One Pro.

  6. Expert Member
    Dana Dorsett | | #6

    Without knowing the exact temperatures where those RH numbers were taken they are meaningless. The "relative" in "relative humidity" is relative to temperature. A difference of 5F room to room can mean a lot.

    That said those numbers are all pretty low, indicating a moderate ventilation rate. Almost no location in my house is running 15% RH @68F even when it's 0F outside. At 0F temperatures or lower at ANY %RH there just isn't much moisture in the air, and the difference between ventilating with 0F air vs. -20F air isn't going to change the indoor RH by a very big amount. The same can NOT be said for +20F air vs. 0F air.

    Room occupancy makes a difference- living breathing humans put out quite a bit of moisture, so the master bedroom might be running a bit higher than rooms rarely occupied.

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