Humidity is often overlooked when considering indoor air quality. Yet, moisture is a critical factor that contributes to the air quality in your home and affects energy efficiency. Humidity measures the amount of water vapor in the air; relative humidity refers to the amount of moisture in the air compared to how much water it can hold at a given temperature. Higher temperatures can hold more moisture.
Why indoor humidity levels matter?
It is essential to keep indoor humidity levels between 30-50%. When humidity levels are too high, your home could inhibit microbial growth and provide dust mites with moisture to thrive. Symptoms can worsen for people with asthma, allergies, and respiratory issues. When humidity levels are too low, it can cause dry skin, worsen symptoms of skin conditions, and aid in the airborne transmission of viruses. Changes in temperature and humidity levels can also affect focus and concentration. Low humidity levels can negatively affect furniture and decor around the home. Wood floors and wood furniture can dry out faster. Low humidity can also dry out and deteriorate paintings.
Ventilation for humidity control
Ventilation can reduce or increase humidity levels in the home. Humidity can be transferred from the indoors and outdoors when designed correctly within your HVAC system and conditions permit. Humidity can also get built up indoors via showering, cooking, and breathing, so it is essential to ensure you use your bathroom and kitchen exhaust fans when cooking or showering. A home that isn’t well ventilated can have humidity problems. Let's take a look at this psychometric chart.
Looking at the temperature on the left and relative humidity on the top + right sides, we can see that the relative humidity decreases as point A temperature rises. Looking at point B, we can see that as the temperature drops, the relative humidity increases. This can get tricky, considering that we typically want to decrease the relative humidity in a warm home in the winter and decrease the relative humidity in a cold house in the summer (depending on geographical location). However, you can essentially increase or decrease the home's relative humidity by introducing outdoor air. The downside to this is that you will be losing the energy already used to cool or heat the house. ERVs and HRVs help minimize the energy lost in the ventilation process. Learn more ventilation here.
Using your air conditioner as a dehumidifier
By design, your air conditioning system doubles as a dehumidifier. When your HVAC system is running on cooling, the indoor coil drops to temperatures of about 40 degrees. As the warm air passes through it and significantly drops in temperature, it can no longer hold the moisture, and the moisture condensates onto the coil. This process effectively removes moisture from the air in warm, humid environments. Sometimes the air conditioner alone is not enough to remove moisture from the air. You may need the help of a dedicated dehumidifier in areas where humidity is high or in well-insulated homes where the AC doesn’t run for as long.
Dehumidifiers for humidity control
A dehumidifier works similarly to an air conditioner in removing moisture from the air. However, its primary purpose is to remove water from the air instead of cooling your home. A dehumidifier's effectiveness is measured by the amount of moisture it can remove from the air per day. Portable dehumidifiers take up space in the bedroom or living room. Due to their size and fan speed limitations, they cannot remove moisture from the air and distribute it effectively throughout the home. In comparison, a whole-home dehumidifier is typically connected within the ducting system of an HVAC system. It can remove moisture more evenly and more effectively, maximizing the benefits of humidity control throughout the home.
Humidifiers for humidity control
Humidifiers release moisture in the air through one of two processes, evaporation or steam. Portable humidifiers take up space in the bedroom or living area and are less effective in humidifying the home due to their size and fan speed limitations. Whole-home evaporative humidifiers have a moisture (humidification) pad that allows moisture to evaporate as air flows through it and throughout your home. There are two mechanisms for this process: bypass humidifiers and fan-powered humidifiers. Whole-home bypass evaporative humidifiers work by connecting to your central HVAC system's return and supply ducts. When your fan is running, some of the air flowing through the ductwork bypasses the cooling or heating process, goes through the humidification pad, and then back into the supply ducts and into your home. Whole-home fan-powered evaporative humidifiers connect directly to the supply ducts and utilize a fan to force the air through the humidification pad and back into the supply duct of your system. Your HVAC system's blower fan needs to operate to distribute the humidified air into the home. A bypass humidifier doesn’t consume any additional energy as there is no motor. However, its effectiveness is reduced and your blower fan may need to run longer. Steam humidifiers generate steam via electricity and distribute the steam directly into the supply duct of your HVAC system and into your home with the help of the HVAC system blower fan.
Choosing between a dehumidifier or a humidifier?
Whether you need a humidifier or dehumidifier depends on where you live and your current systems. In the cold winter months, we want to heat our homes, and in the hot summer days, we want to cool our homes. However, heating the air will reduce the relative humidity, and cooling will increase the relative humidity (even though moisture is removed in the air conditioning process). To make matters more complicated, any unregulated air that makes its way into the home through cracks in unsealed attics, windows, and doors will affect the relative humidity due to the temperature differences. It is best to consult with a professional.
The bottom line
Humidity is a critical factor in indoor air quality and energy consumption. Humidity levels outside the recommended range of 30-50% increase the chances of spreading viruses and bacteria, inhibit microbial growth, increase allergy symptoms, asthma, and respiratory infections, and are a breeding ground for dust mites. Humidity levels that are too high or too low can cause itchy skin, focus and concentration issues, and a clammy feeling in the home. When heating or cooling your home, the humidity levels affect your energy bill. It takes more energy to heat or cool humid air. However, it also holds those temperatures for longer. Think of a desert; for example, it is very dry, and you have extreme temperature changes between the day and night because there is low humidity. There are a few ways you can regulate the humidity in your home:
Ventilation and air conditioning for humidity control are harder to regulate because they depend on external factors such as outdoor temperatures, indoor temperature, and relative humidity. It is easier to regulate humidity levels with the help of a humidifier or a dehumidifier.
Sources: The Role of Dry Winter Air in Spreading COVID-19 | University Hospitals Home Humidity Levels Can Trigger Allergy Symptoms - CRC. Moisture control and ventilation - WHO Guidelines for Indoor Air Quality - NCBI Bookshelf. Dehumidification - an overview | ScienceDirect Topics Recommended space humidity levels linked to its impact on human health . How air humidity affects how much time is needed for heating the air? - Physics Stack Exchange