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You might wonder why we talk about relative humidity in the wintertime. The story has to go back to a USGBC seminar I attended during the first year of the pandemic. One of the speakers, Dr. Taylor, explained why indoor air with 40% to 60% relative humidity benefits for our immune system.
What is Relative Humidity?
First, understand the difference between relative humidity and humidity ratio. To simplify the differences, Relative Humidity is pounds of steam per pound of wet air; the Humidity Ratio is the measurement we typically describe as pounds of moisture per pound of dry air.
To understand this further, we need to look at the Psychrometric charts. This graphic shows important moist air properties such as wet-bulb/ dry bulb temperature(1), enthalpy, dew point, and relative humidity. The cooler air will hold less moisture than warmer air, as a rule of thumb. Since the atmosphere of higher temperature can have more water, it also makes people "feel" dryer.
Assume your dry-bulb temperature shows 72°F, and your wet-bulb temperature shows 60°F; then the RH in your room is 50%. But if you heat the same amount of air to 79°F, you will see the RH in your space drop to 40%.
(See graphic below.)
Why does Relative Humidity matter (in the winter)?
It is inevitable to use a heater during the winter in northern America. Turning up the heat and increasing the indoor air exchange rate (2) might result in low humidity in the space and further impact our respiratory and immune systems. There is growing scientific literature showing that the human respiratory-immune system is more robust when the relative humidity is between 40% and 60%(3). Besides purchasing a regular air humidifier, there are a few other ways to increase the overall humidity level without mechanical devices.
Simple tips to help increase the Relative Humidity (RH):
You could level up your indoor relative humidity by simply getting a humidifier. However, some simple tips can humidify your place without buying a machine.
Increase RH + Beautify your space at the same time
Plant above is Golden Kudzu. It is placed by a window and under a ceiling register.
Set up containers with water close to the room registers: Fill flower vases or potteries with water (and glass beads if you like). Then, place them close to the air register (or sunny window). That will beautify your space and help increase the humidity level.
Place indoor plants: studies show evapotranspiration from spider and jade plants can improve relative humidity in an interior environment. Spider plants transpire more during lighted periods. In comparison, the jade plants have more excellent evapotranspiration rates during dark periods. Therefore, you could place more jade plants in your bedrooms or bathrooms and spider plants in your brighter areas, such as the living room or sunroom. That will benefit the overall indoor humidity level.
Making seasonal ornaments: I found out that Bernice L. Rocque, the author of Until the Robin Walks on Snow, teaches people how to make seasonal pinecone wreaths without gluing or wiring. Her method was to soak the pinecones in advance to bind the pinecones together. Pinecones release air moisture during the pinecone's hygromorph process (from cold/ wet to warm/ dry), thus increasing the overall indoor humidity level.
A typical spider plant. Image powered by Unsplash
A typical jade plant. Image powered by Unsplash
Increase RH + Save energy at the same time
Open the door after using the shower: the warm steam will carry the moisture throughout the house (due to convection). Therefore, it increases the indoors' overall humidity.
Using a drying rack inside a room: I place the rack directly below the ceiling registers and lay the clothes for a day before putting them in the dryer. That not only helps me with drying clothes faster, but it also increases the overall humidity level and cuts utility bills.
Using the stovetop rather than the microwave oven to warm up food: the steam will help carry moisture around the space while cooking food on a stovetop.
Last update on: 12/20/2021, 10/30/2023
Notes and References:
The dry bulb temperature is typically the ambient temperature. If you are wrapping a wet wick around the bulb of a thermometer, then you will get a wet-bulb temperature.
If you use an air purifier or have HVAC devices, you will see CFM on the product specification. CFM means Cubic Feet per Minute. Let’s say if you have a product with 250 CFM and put it in a 125 square feet room with standard ceiling height of 8 feet. You will get 15 air changes per hour (ACH). Here is how it calculated:
250 CFM x 60 (minute)=15,000 Cubic Feet per Hour.
Your room volume is 125 (square feet) x8 (feet)=1,000 cubic feet
15,000 (Cubic Feet per Hour)/1000 (Cubic Feet) =15 times per Hour
(Typically, 6 minimum ACH is recommended.)
The earliest literature I found was in 1986 “Indirect health effects of relative humidity in indoor environments.” by Environmental Health Perspectives; 65: 351-361.
Recent years, Dr. Stephanie Taylor has published several findings on how indoor air moisture impact our health. One of her recent published paper is “Low indoor-air humidity in an assisted living facility is correlated with increased patient illness and cognitive decline.” By Taylor, S., and M. Tasi. 2018. Proceedings, Indoor Air 2018 744:1–8. You can visit 40To60RH.COM for more in depth information.
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