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Community and Q&A

15% indoor humidity?

jackofalltrades777 | Posted in Energy Efficiency and Durability on

I read that an article from  the Minnesota Department of Public Service, with an indoor temperature of 70 degrees. They recommend the following indoor humidity levels:

• If outside temperature is 10 to 20 degrees, humidity indoors should not be more than 35 percent.
• If outside temperature is 0 to 10 degrees, humidity indoors should not be more than 30 percent.
• If outside temperature is 10-below to 0, humidity indoors should not be more than 25 percent.
• If outside temperature is 20-below to 10-below, humidity indoors should not be more than 20 percent.
• If outdoor temperature is lower than 20-below, inside humidity should not be more than 15 percent

My question is how does one even get indoor humidity level to the 15% range? That is bone dry desert type of air.

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  1. Expert Member
    Dana Dorsett | | #1

    The dew point of 15% RH/70F air is +21F.

    Unless you have a LOT of indoor humidity sources that's pretty easy to hit by adjusting the ventilation rate when the "...outdoor temperature is lower than 20-below...".

    In my over-ventilated office it's currently 15%RH @ 70F, and the outdoor temperature is 23F ABOVE 0F.

  2. Expert Member
    BILL WICHERS | | #2

    At 15%, you’re likely to feel the dryness. I have family in the Denver area, so I’m out there several times a year. That area is high elevation plains, and it can get really dry, below 15% in the winter. You start seeing health issues with levels that low, I notice it most myself in my eyes and nose (easy nose bleeds, and a burning dry feeling in my sinuses). Even now, they are having higher than usual flu levels out there and I can tell you that the dryness does make people more prone to infection due to the effects of very low humidity levels on mucus membranes in the respiratory system.

    In extreme conditions, you have to balance concerns for your structure with concerns for the inhabitants. Very low humidity may be good for keeping condensation under control, but it’s bad for the occupants of the structure. There are good arguments to be made on both sides so you need to decide which priority is most important to you.

    In my own house, I’m gradually doing an insulation/air seal renovation which is ultimately to include new windows and siding. I’m going for 3” of exterior foam, probably all polyiso with a well deserved nod to Dana and his recommendations, and that’s in zone 5. The primary reason for using so much in my case is so that I can maintain higher indoor humidity levels (35-40%) at lower outdoor temperatures without any structural concerns. My wife and I both have chronic sinus issues so the humidity is a concern for us. Sometimes energy efficiency isn’t the only reason for insulating a particual way, although in this case it’s a win/win since we’ll be saving on heating and air conditioning as well as being able to safely maintain our desired humidity level.


  3. GBA Editor
    Martin Holladay | | #3

    Q. "How does one even get indoor humidity level to the 15% range?"

    A. You take very cold outdoor air during the winter, bring the air inside, and heat it up.

  4. Expert Member
    Dana Dorsett | | #4

    >" have family in the Denver area, so I’m out there several times a year. That area is high elevation plains, and it can get really dry, below 15% in the winter. "

    In Denver the outdoor dew points in winter are still higher than in northern MN in winter- it's all about ventilation rates. In a tight, occupied house of average size the RH won't drop to 15% unless ventilated at a substantial rate.

    Right now this minute the dew point in my area (eastern MA) is running -7F, whereas in Denver it's +12F. Overnight outdoor temperature trumps the "high plains" factor.

    I'm definitely feeling the low humidity in my office every winter. For me it's the chapped lips, brittle fingernails, dry skin, and dry nose/sinuses. At home it's been hanging in the mid-30s RH during the same weather, even with the Lunos running 24/7 at 22 cfm, and diligent use of bath/kitchen exhaust fans.

    1. Expert Member
      BILL WICHERS | | #6

      Dana, you’re correct regarding dew points, but dew points deal with condensation. That’s the structural concern — and I mean “related to the entire structure”, so windows and the like included, not “structural” in regards to only strength elements like framing and sheathing.

      Humidity regarding the occupants is all about %RH, since rates of evaporation depend on relative saturation levels of two “things”, for lack of a better term. If the air has a very low concentration of moisture relative to its carrying capacity at a given temperature, which is basically what %RH represents, then moisture will tend to migrate from body tissues, which tend to have a very high relative moisture content, towards the air. That’s where the health concerns come in, when body tissues dry too fast for the body to react.

      I would argue that Denver, northern MN, and eastern MA all expierience conditions that can bring the humidity levels down to uncomfortable levels. How well someone can personally handle those low humidity levels will depend on the particular person. Improved insulation and air sealing can help to bring indoor humidity levels up to more comfortable levels as well as having benefits related to lower overall energy costs.

      This is an interesting discussion, and why I like participating in these forums.


  5. Jon_R | | #5

    Robust building design will allow higher humidity than those recommendations - avoiding the negative health effects of too low humidity.

    Some building partitions have been tested to work well at 50% RH for the entire season.

  6. walta100 | | #7

    Mostly it happens naturally and tends to follow your chart.

    Tight houses should have power ventilation and the rest leak. When the -20° 100% air is heated to 70° it will be have less than 4.5% humidity. It does not take but a day or two to get down there.

    The window will also condense water out of the air as soon as the glass surface falls below the dew point. Only the most costly windows will be dry when it gets under -15°.

    That is not to say I would find 15% comfortable.


  7. jackofalltrades777 | | #8

    That brings insight as to why flu viruses, colds and respiratory diseases explode during winter weather. Anything below 40% indoor humidity is when viruses begin to flourish. When levels drop down to < 25%, they explode and thrive. Enter flu and cold epidemics during winter.

    One homeowner I know likes indoor humidity to be in the 25% range during winter, even when outdoor temps are in the 30-50F degree range. That's just too dry. Also shows that the house is very leaky since it's not being ventilated. So the windows and doors are leaking air.

    My house is very tight. If I don't run the ERV during winter, with nobody home, the indoor levels get to 40-45%, even if its in the teens or single digits outside. Once I turn on the ERV, levels drop.

    1. GBA Editor
      Martin Holladay | | #9

      Q. "That brings insight as to why flu viruses, colds and respiratory diseases explode during winter weather."

      A. The medical community doesn't have a single answer to this question. It's certainly not as simple as you imply -- that the cause is due to low indoor relative humidity.

      According to Medical News Today, "The peak month for flu activity in the seasons spanning 1982–1983 to 2015–2016 was February, followed by December, March, and January. Other temperate locations across the globe see similar patterns, with cold temperatures and low humidity cited as the prime factors, a 2013 analysis showed. The same can't be said for tropical areas, however. There, you might see outbreaks during rainy, humid months, or relatively consistent levels of flu cases all year round."

      Another source notes,

      "Some people have argued that it is not cold temperatures that make the flu more common in the winter. Rather, they attest that the lack of sunlight or the different lifestyles people lead in winter months are the primary contributing factors. Here are the most popular theories about why the flu strikes in winter:

      1) During the winter, people spend more time indoors with the windows sealed, so they are more likely to breathe the same air as someone who has the flu and thus contract the virus.

      2) Days are shorter during the winter, and lack of sunlight leads to low levels of vitamin D and melatonin, both of which require sunlight for their generation. This compromises our immune systems, which in turn decreases ability to fight the virus.

      3) The influenza virus may survive better in colder, drier climates, and therefore be able to infect more people."

    2. Expert Member
      BILL WICHERS | | #10

      To clarify my earlier post, I used the flu as an example. It is known that very low humidity levels are not good for the mucus membranes in people, which are in the respiratory system among other places. It’s also known that people tend to be more prone to infection when these mucus membranes are damaged or impaired. That’s not to imply that low humidity is the only cause of flu cycles.

      In the winter, people due tend to remain indoors more, so they stay closer to other people which will make it more likely for illness to spread. That’s only one example though. Immunology is an entire science of its own, it’s very complex. I had only intended to point out that very low humidity levels, while in many ways are good for a structure, they are not good for the occupants.


    3. Expert Member
      MALCOLM TAYLOR | | #11

      " If I don't run the ERV during winter, with nobody home, the indoor levels get to 40-45%"

      Peter, that's interesting. What's the source of the humidity? I have a heated, well-sealed and insulated outbuilding we use quite infrequently. I use it as a proxy for what the humidity levels in our house would be if we didn't contribute to them. My assumption is that I couldn't get the levels down below those in the outbuilding. Maybe I'm wrong?

      1. jackofalltrades777 | | #12

        Not sure. No plumbing water leaks. Only have 2 household plants. House is pretty airtight. ICF walls (12" thick - 6" concrete, 5" EPS, 1/2"drywall, 1/2" stucco). Slab on grade design with a 4" concrete slab, 3" EPS foam, taped vapor barrier, tile and some carpet. I can't see the 2 bathrooms which each have a toilet releasing that much moisture into the air. House is about 1.5 years old.

        For example, right now it's 68F inside with a 42% humidity level. Outside temp is 40F with 75% humidity and a 32F dew point.

  8. Expert Member
    Peter Yost | | #13

    HI Peter -

    How are you measuring your temp and RH. There is a wide range of equipment out there; take a look at this GBA resource:

    1. jackofalltrades777 | | #14

      I am using the wireless Davis Vantage Pro 2 console which reads outside data but the display unit inside reads the indoor humidity levels.

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