relative humidity

Humidity in my home: what is relative humidity and how do I manage it?

Humidity (or relative humidity) in the home is a common topic, especially in these northern climates where we get significant temperature swings from season to season. Let’s get into it!

What is relative humidity?

Without getting too technical, relative humidity is how we express the amount of water vapor that air at a certain temperature is holding compared to how much it is capable of holding. Hot air holds more humidity than cold air and the warmer the air is, the more humidity it can potentially hold. If you are using a humidifier with a humidistat to control the humidity in your home you may need to periodically adjust it to maintain the desired level of humidity, as you can see on the chart below.

Outside Air Temp (°C)Maximum Indoor Relative Humidity at 20 °C (68 °F)
-30 °C or below15%
-30 °C to -24 °C20%
-24 °C to -18 °C25%
-18 °C to -12 °C35%
-12 °C to 040%

How can I tell my home’s relative humidity?

While some more state-of-the-art thermostats might be able to give you an exact percentage of the relative humidity, it’s not a luxury we all currently have. But there are a couple of other ways to get an indication without having to buy a brand new stat. Assuming you have a humidifier in your home, checking your humidistat which controls the appliance can be an easy way to determine where the relative humidity level in your home is sitting. Commonly located on the return air ductwork near your furnace, by turning the dial and waiting to hear a ‘click’ you will know roughly where your home humidity level is. The point at which the dial clicks, is slightly higher than the current level. You may not get the most accurate reading, but it is a good indicator. Another option is using a hygrometer, or an indoor humidity monitor. These are fairly inexpensive and can give you a reading to the percentage.

What is the proper humidity level for my home?

When it comes to achieving a comfortable home environment, relative humidity is one of the most important factors to control. In the summer, we use dehumidifiers and air conditioners to remove heat and humidity from our air to keep our homes from getting swampy, and when conditions are cold or dry it might become necessary to add humidity to the air. Again, the chart above can give you a good indication as to what the level of humidity should be, but consider it a guideline as personal comfort can vary and how much humidity you need to add may even depend on the kind of home you live in. Modern homes are constructed to be air-tight and can require up to three gallons of humidity to be added per day, with older homes needing as much as five to maintain the right relative humidity. But even with that in mind setting the right humidity level can sometimes be a balancing act and finding and maintaining that balance can depend on a number of things. Cooking, showering, outdoor temperature and even the number of people in your home at any given time can all impact relative humidity.

What happens when my home’s relative humidity is too high?

However, as our indoor relative humidity climbs above 55%, then viruses and bacteria begin to grow, asthma symptoms may be triggered and the growth of what everyone fears the most: mold. Dust mites, mold, and fungi all exist as a result of high humidity. These contaminants are organisms and like all life, they require 4 components in order to exist: oxygen, heat, food, and water. If we remove any one of these components, then the organism dies. The easiest by far to remove from your home is water by reducing humidity to below 55%. When we do this, mold, fungi and dust mites will dehydrate and die.

What happens when my home’s relative humidity is too low?

When humidity drops below 30%, viruses and bacteria begin to grow, asthma symptoms worsen, and a general discomfort emerges. Dry eyes and skin, nosebleeds, static electricity, and dry throat and cough are all triggers from low indoor humidity. In these cases, we should all have good fan powered central humidifier installed that can actively produce enough water vapour to increase our humidity to above 30%. When we get our humidity above 30% but still below 55%, most of us consider our homes to be comfortable and healthy. In other words we are not experiencing any adverse effects as a result of low humidity.

What can I do to control humidity in my home?

Aside from installing a humidifier or an air conditioner to your HVAC system here are a few more steps you can take to dial in your desired humidity level:

  1. If you have a Heat Recovery Ventilator (HRV) in your home, understanding how to properly use it will prevent trading indoor humidity with the humidity of the outdoor air, making sure that your system is working efficiently.
  2. When humidity is already high in your home, using exhaust fans in your bathroom during a shower or your kitchen exhaust while cooking can help keep your levels where you want them. Try, however, to avoid doing this excessively or for long periods of time as it can dry out your home.
  3. Setting your thermostat to around 22 degrees Celsius (72 Fahrenheit) can help the humidifier absorb more water and allow that water to evaporate into the warm air ductwork more easily. Remember, the warmer the air is, the more humidity it will be able to hold.