How Absolute & Relative Humidity Affect Your House


Humidity is a big deal when it comes to a healthy indoor environment. Seasonal changes in humidity can be the cause of high or low indoor humidity levels.

When indoor humidity is high, you can end up with health problems related to respiratory infections, asthma and allergies, and the house can have mold growth problems that can even lead to structural damage. When indoor humidity is low, you can end up with a different set of health problems related to increased exposure to bacteria and viruses. The structure of a house doesn’t suffer much from low humidity levels, but wood floors and other natural materials can shrink and crack as they dry out. Obviously, neither situation is ideal.

What’s the Relationship Between Indoor and Outdoor Humidity Levels?

Part of the relationship has to do with the transfer of air between indoors and outdoors. A house that has a leaky building envelope allows a huge amount of air transfer. This house is going to be more affected by seasonal changes in humidity than one with a tight building envelope, in all seasons and in all climates.

This is important to understand, because of the relationship that water vapor (moisture) has with temperature. There are two ways to measure humidity: absolute and relative.

Absolute Humidity (measured in grams of moisture per cubic meter of air, or g/m3) indicates how much moisture is in the air, regardless of temperature. However, at different temperatures, there is a maximum amount of water vapor that air can hold. The lower the temperature, the lower the amount of water vapor the air can hold. For example, air at 86°F (30°C) can hold about 30g/m3 of water vapor, but at 32°F (0°C) air can hold roughly 5g/m3. Relative Humidity (RH) is measured as a percentage. It indicates the actual amount of water vapor in the air compared to the total amount (the absolute humidity) of water vapor that could be held at the current temperature. When air hits its absolute humidity capacity, it also hits 100% relative humidity. The air simply cannot hold more water vapor, so you get condensation. How Does This Affect Your House?

When it’s cold out, warm, moist air in your house comes into contact with the cold surface of your windows. As the temperature of the air in contact with the glass suddenly drops, the amount of moisture the air can hold drops as well. This causes condensation on the glass. This same principle is at work on the mirror in your bathroom when you have a hot shower, and also is at work in the structure of your house.

There has to be an element of the construction assembly to stop or slow the flow of warm, moist air from the heated space through the walls to the outside in winter, or to the cooled inside in summer (heat always flows to the colder side).

Vapor barriers are required in new construction because they help stop the movement of water vapor through the structure of the house and help reduce ‘interstitial’ condensation.

Whole House Ventilation is the BEST Solution

As great as they are, vapor barriers don’t help with interior humidity loads. Washing, bathing, cooking, breathing…these activities all add moisture to the indoor environment. For health and comfort, relative humidity (RH) should be between 40 and 60 percent. In an energy efficient home, indoor humidity levels can be controlled with the whole house balanced mechanical ventilation.

Whether you choose a heat recovery ventilation (HRV) system or an energy recovery ventilation (ERV) system, installing a balanced ventilation system from Zehnder America is the best solution to controlling humidity levels throughout your house. Balanced ventilation systems recover energy from stale air extracted from the wet rooms in the house. The recovered energy is transferred into the incoming fresh air stream before being supplied to habitable rooms. The result is a balanced, comfortable living area.

Want more information? Call us today to learn more about using Zehnder systems in your home!