# How does indoor temperature factor in when determining ideal relative humidity in a home?

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?"

• 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. Dec 10, 2019 at 7:27
• Check out other posts on here, this is one: engineering.stackexchange.com/q/17992/10902 Dec 10, 2019 at 7:29
• – Fred
Dec 11, 2019 at 2:46
• 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. Dec 11, 2019 at 6:07
• @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. Dec 11, 2019 at 6:29