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I am having trouble understanding how indoor temperature plays into what a healthy indoor relative humidity level should be. Most sources I have come across searching the topic online provide a range based on the outdoor temperature. For example, according to Therma-Stor, "The ideal relative humidity for health and comfort is about 40–50%. In the winter months, it may have to be lower than 40% RH to avoid condensation on the windows." I understand this is important for preventing condensation that can lead to mold and other problems, but I have not seen condensation forming on any windows and I think the air may be too dry in my case.

I live in a home that was built in 1938. It is not well-insulated and there have been several additions in its history. It is currently winter in my location, so the furnace is running. I do not feel any drafts coming from outside, so I believe most of the heat escapes due to the poor insulation -- the closer I am to exterior walls, the colder it feels. The thermostat in my home is set to 64 degrees Fahrenheit (17.8 $^oC$). Increasing the temperature beyond that has a minimal effect on the downstairs temperature, while the upstairs temperature increases drastically. I have looked at the air ducts, but I cannot find any place where I am able to adjust the airflow.

The outdoor temperature ranged from 19-49 degrees Fahrenheit (-7 to 9.4 $^oC$) today, with a current outdoor temperature of 21 degrees Fahrenheit (-6 $^oC$) and humidity of 79%. I just measured the temperature and humidity in my bedroom, which is downstairs, and it is 57 degrees Fahrenheit (13.9 $^oC$) with a humidity of 37%. In the office, which is upstairs, the temperature is 64 degrees Fahrenheit (17.8 $^OC$) with a humidity of 35%. I placed the thermometer/hygrometer in the center of each room.

Considering the temperature of my bedroom is quite low, I am assuming that the 37% humidity level is pretty dry -- the thermometer/hygrometer also provides a reading of "Low" next to the humidity level. It seems that most sources I have found assume a more reasonable indoor temperature in the range of 68-72 degrees Fahrenheit (20 to 22 $^oC$). I do have a portable humidifier I could run in my room, and I am wondering what would be an appropriate humidity level under these outdoor and indoor conditions? Is there some sort of an equation I could use, maybe something similar to how the thermometer/hygrometer determines when to display a reading of "Low," "OK," or "High?"

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  • $\begingroup$ Sounds normal: heat rises so the upper floors get the heat while the lower looses what heat it has due to lack of insulation. Improve the insulation and consider a vapor barrier. $\endgroup$
    – Solar Mike
    Dec 10 '19 at 7:27
  • $\begingroup$ Check out other posts on here, this is one: engineering.stackexchange.com/q/17992/10902 $\endgroup$
    – Solar Mike
    Dec 10 '19 at 7:29
  • $\begingroup$ Also see: Does a house heating system affect perceived and true indoor air humidity? $\endgroup$
    – Fred
    Dec 11 '19 at 2:46
  • $\begingroup$ Thanks, @SolarMike. Unfortunately, I cannot make any structural changes and will just have to rely on space heaters and humidifiers this winter. I will be moving out of the house after this winter; I am just trying to make it through without the low humidity causing adverse health effects (I have a case of the cold now) but also not cause any damage to the house. $\endgroup$
    – Poosh
    Dec 11 '19 at 6:07
  • $\begingroup$ @Fred, I used this to calculate the absolute humidity outside, and got 0.002 kg/m^3. I then used that value here to calculate a 14.6% RH for the indoor temperature. In the past, it has gotten down to 15% RH indoors during the winter, but so far it has not dropped below 30% RH. $\endgroup$
    – Poosh
    Dec 11 '19 at 6:29
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In the world of heating/ventilating/air conditioning engineering (a.k.a. "HVAC") there is a number called the comfort index (which you can search on) which takes both humidity and temperature into account to determine where most humans would feel comfortable. This will tell you the range of optimum humidity percentages to shoot for at a given temperature setting.

Regarding the use of the humidifier: generally speaking, at lower temperatures, the more humidity, the better your comfort. I recommend adding humidity until you just start getting condensation on your windows, then back off by 10%. Note that mold problems commonly arise in humid houses during the winter with the main culprit being chests of drawers and tall wardrobes backed up against exterior walls. The narrow gap between the furniture and the (poorly-insulated) wall and the poor circulation there will cause the temperature of the air in it to drop below the condensation point and moisturize the wallboard, leading to the rampant growth of mildew on the wall in a pattern which exactly duplicates the outline of the chest or wardrobe. It's a good idea to regularly check for mold behind the furniture, and also to pull those things a few inches away from the walls to improve circulation and prevent condensation.

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  • $\begingroup$ I am having trouble finding information on the comfort index; should that be calculated like here as the (temperature in degrees Fahrenheit + the RH%)/4? I got a comfort index value of 27.5 when the temperature is 70 degrees Fahrenheit with a 40% RH. Adjusting for a temperature of 60 degrees Fahrenheit, 50% RH also yielded a 27.5 comfort index. $\endgroup$
    – Poosh
    Dec 11 '19 at 6:59
  • $\begingroup$ Tonight I will try pulling all furniture away from the walls and put the humidifier in the center of the bedroom, setting it to 50%. Then I will check for any condensation on windows or moisture on walls. I will keep the thermometer/hygrometer several feet away because the humidifier contains an internal humidistat. I am guessing the reading it obtains will be higher since the measurement is taken so close to the source of the humidity. $\endgroup$
    – Poosh
    Dec 11 '19 at 7:09
  • $\begingroup$ It was 61 degrees Fahrenheit next to my bed and remained around 50% RH throughout the night. No condensation on the windows and the walls did not appear wet. It is actually warmer near the humidifier -- 67 degrees Fahrenheit according to the space heater that is about 1.5 feet away from it, so the humidifier was reading 37% RH in its location. Since the humidifier never stopped running, I think this is as high as I will be able to raise the humidity. $\endgroup$
    – Poosh
    Dec 11 '19 at 20:33
  • $\begingroup$ then you're good. just watch out for the mold! once you get that in your walls it's almost impossible to get rid of. $\endgroup$ Dec 11 '19 at 21:15

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