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Thread: indoor humidity level

  1. #1
    DIY Junior Member
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    Default indoor humidity level

    I have read a lot about indoor humidity level. Some say it should be between 30~40%, others say 40~55%. I also understand it depends on the outside weather temp. For instance, if outside is 0 degrees, one should try to keep the indoor humidity level 30% at most. It is 10 degrees outside, then 35%, 20 degrees 40% and so on so forth.

    I have a whole house humidifier (by honeywell) that looks like this
    Honeywell Whole House Humidifier

    it is fan based, will kick on automatically when furnace supply duct starts supply heat into the house. It is mounted on the supply line and works pretty well.

    I recently got a Hygrometer which can tell the humidity level besides the temp. I noticed my 2nd floor's humidity level is at 47 ~ 50% while the main floor is at 38~40%. The furnace and the whole house humidifer is in the basement. The humidity level there is 35% and almost the same as the main floor. Is this normal? I set the humidity level at 30% on the unit but the house seems to be a bit more humid than the set temp (perhaps this may be due to cooking, people and etc.,)

    My questions are

    1. Is it normal to have the second floor to be more humid than the first? Also the 2nd floor seems a degree or two colder than the main floor (if temp is set at 70 degrees, 2nd floor would be 68~69ish. while humidity level is 7% higher.)

    2. I live in New Jersey (south) what is the desire indoor humidity level during the winter when temp is around 20 to 30 degrees?

    thanks!

  2. #2
    Retired prof. engr. gator37's Avatar
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    Normally in design you shoot for 50% RH, plus or minus 5%.
    Ideally, in reference to your change in RH, if you look at a psychrometric chart you can see that if you heat or cool sensibly (no moisture added or taken out just a change in dry bulb temperature (dbt)) you will see that the RH changes, so for example if you are at 75 degrees F and the temperature drops to 70 degrees F (everything else being constant) your RH will increase OR if you are 75 dbt. and raise the temp to 80 degrees dbt your RH will decrease. I think the term RH is really used for more or a comfort level discussion but if you really get into it, the discussion should be around lbs of moisture per lb of dry air which is also shown on the psy chart which does not change with increase or decrease in dbt (again everything else being constant).

  3. #3
    DIY Senior Member SteveW's Avatar
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    NEG,
    For those of us with colder winters, you are absolutely correct that it outside air temp is really important - and in fact becomes the main factor in determining inside RH. How is your humidifier controlled? Some installers put the humidistat on the return air plenum of the furnace, where it samples the cold air returning to the furnace. An alternative is to have the 'stat in the living space. The best alternative is to have an integrated humidistat/thermostat, like the Honeywell IAQ stat, which is what I have. It has an outside air temp sensor which provides feedback to the thermostat/humidistat, and like now, where it is 5 F here in Omaha, RH in my house is 21% (being limited by the 'stat). When outside air temp is 25, interior RH runs closer to 35%. It never gets much higher than 35%, in fact.

    Too high RH in a cold climate leads to condensation on windows, which can over time lead to wood rot. I have heard also of cases of overly high RH causing rot inside the walls, if they are not adequately insulated or don't have a decent vapor barrier.

    Not sure why your 2nd level would have higher RH, unless you measured right after using the shower. I would be concerned about RH in the high 40% in the winter.
    Last edited by SteveW; 01-11-2011 at 09:04 PM.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Colder air with the same water content will have a higher rh.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    DIY Senior Member SteveW's Avatar
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    Good point, Jim.

  6. #6
    General Engineering Contractor ballvalve's Avatar
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    Humidistats, Hygrometers are notoriously innacurate and fickle. I have 4 in my wine cellar from 5$ to 60$ and they all read different.

    Look on line for a procedure to test them with a saturated salt solution inside a zip lock bag. It will give you the real number. I had one, the 7$ wallymart one that was right on. 3 others needed +10, +23 and +36 to match the good unit. After a year the bad units are still perfectly off by that amount!

    35% sounds awful dry. But then I am hanging Salami and capicola at @ 55F -60% for their aging pleasure.

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    Depends on you're insulation and outside temps the max you cam get safely. You don't want condensation. Above 30% should be ok.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  8. #8
    In the trades Dana's Avatar
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    For comfort and health humans do well with 30-50% RH independent of season. (ASHRAE sez 25-65%, but MDs and allergist say 30-50%).

    In heating dominated climates that experience extended periods where it stays below freezing outdoors, intentionally increasing it above 30-35% range puts the building materials at signficantly higher risk of rot & mold inside of wall & ceiling cavities. In fact, unless you live in a VERY cold climate (8000 heating degree-days or higher), if you need to use a humidifier to keep it UP to 30-35% means you have a leaky, and more susceptible house- mold & rot inducing levels condensation along the ex-filtration path is LIKELY. As much to-do is made in the building trades about the importance of vapor retarders, almost all condensation-related moisture problems in homes are from AIR LEAKS.

    The solution to a too-dry-in-winter home in NJ isn't a humidifier- tightening up the air leaks is a far-better approach, protecting both the building and the humans, while reducing heating-energy use. Humidifier use in winter typically results in greater indoor air quality mold-spore issues come spring.

    The fact that the RH is higher in a cooler room is not an issue- it's a simply a function of the vapor-pressure of water at different temps, as modeled by the psychrometric chart:



    Take the same body of air, moving it horizontally on the chart (up/down in temperature), it will cross different relative humidity curves. The critical curve is the dew point- the temp at which going any lower will cause liquid (or frost) moisture to form on any available surface, such as the exterior sheathing or framing. Note, the dew point of 30-35%RH 68-70F air is between 35-40F, so interior air leaking out through a wall cavity where the sheathing is 33F is going to see significant moisture issues.

    But if you crank up the humidifier to 45-50% you're looking at dew points in the 45F+ range, putting much more of the wood at risk, even in milder climates.

    In someplace like NJ you should NEVER need a humidifier in even a modestly tight house (not a supertight house)- low interior RH (read "lower than 30%) is a symptom of too high a ventilation rate. Tightening it up to where it stays in the low 30s without adding water presents effectively NO risk of poor interior air quality unless you smoke, or use unvented combustion equipment indoors. When it's tight enough that it stays 40%RH or higher even when it's in the 20s or lower outside you might consider some sort of active ventilation. (Heat recovery/energy recovery ventilation preferred, or continous 20-30 cfm exhaust in the bathrooms.)

    My house is by no means the tightest on the block, but it only drops below 30% when the highs for the day are in the low teens or for a few consecutive days (which isn't very often.) YMMV.

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