How Does Humidity Move in a House?


Although there are many ways of controlling humidity, it always seems to move around a house unchecked.

So, how exactly does humidity move in a house? Despite popular belief, humid air is less dense than regular air. Therefore humid air will always rise and not sink. However, if you were to look at its movement in-depth, there are two specific ways humidity travels in a home. One is through open space while the other is by diffusing through materials.

To better understand how humid air moves in a house, you need to know the factors tied to humidity. This would also help you control humidity better and avoid moisture-related issues.


Understanding Humidity

Humidity is the amount of water vapor in the air. The amount of water vapor the air holds will affect the way it travels in a home. Naturally, dense substances will sink while those that are not will rise. If so, shouldn’t humid air be heavier than dry air because it contains water vapor?

Although it may seem counter-intuitive, the answer to this is “No”. This is because water vapor is lighter than Oxygen and Nitrogen (Oxygen and Nitrogen are the main constituents of regular air). So, when the amount of water vapor increases, the quantity of Oxygen and Nitrogen will decrease for a fixed volume of air. Thus making humid air less dense than dry air.

Another factor that affects the density of air is its temperature. The density of a substance is related to the volume and mass of it and is expressed by the following equation:

Density = Mass/Volume

For this reason, the density of air will vary with its volume when the mass is constant.

However, the volume of air depends on its temperature. Therefore, an increase in temperature will cause the air to expand, which then leads to an increase in volume and ultimately decreases the density of air. Conversely, a decrease in air temperature will reduce its volume and increase its density.

So, in summary, the movement of air depends on its density and the density depends on its temperature and moisture content.


Humidity Moves in Two Ways

It is clear that the movement of humid air depends on its density.

But it’s difficult for humid air to rise because of all the obstructions a regular home offers. The structure of the house such as the walls and the insulation will impede the movement of humidity.

Thus, there are two ways in which moisture-filled air bypass these obstacles.


1) Movement through Open space

This method results in over 98% of the total water vapor movement within a home.

Air naturally flows from high-pressure areas to low-pressure areas. The moving air created by this process is called an air current. So whenever there is a build-up of humid air, it will try to escape through the easiest way possible. Cracks, openings, and other gaps in the structure of your home make this possible. This is a reason many professionals suggest you insulate your crawl space.

The crawl space of a house is typically very moist. Therefore, the humid air created here will naturally rise and seep into the rooms above through the cracks and openings of the floor. Making sure that these gaps are sealed tight will prevent moisture-filled air from infiltrating the living spaces of your home.

Recommended Reading:


2) Diffusion through Materials

The next way water vapor can move in a house is by diffusion. Although this process is not as fast as air currents, it can affect the material of a building in the long run. Diffusion causes water vapor to travel through a material.

Just like air currents, pressure difference on either side of a material can cause diffusion. The difference in pressure will diffuse the water from the high-pressure side to the low-pressure side.

The temperature difference also aids this process. Water vapor tends to move from high-temperature zones to low-temperature zones. So if the temperature between two sides of a material is different, the water vapor will move through it. This is called “Thermal diffusion”.


Problems Caused By Moving Humidity

High humidity can cause problems in a home. If the movement of humid air is not controlled, these issues will spread throughout the house given enough time. The following are a few common problems a household with high levels of humidity faces.


Health Issues

The humid air will start accumulating contaminants of the floors below as rises within a house. A good percentage of humid air rises from the crawl space or basement. So a crawl space or basement that is infested with an issue like mold will add pollutants like mold spores into the air you breathe. This is dangerous to those suffering from conditions like asthma.

High humidity levels cause a significant increase in temperature as well. This would make your body do extra work to cool and maintain its temperature, creating that commonly known “sticky” feeling on our skin.

The primary mechanism for regulating body temperature is sweat. So when the atmospheric temperature increases, your body is going to sweat profusely to cool itself. This can lead to dehydration, which can cause dizziness, headaches, diarrhea, nausea, fatigue, and many other things.


Structural Damage

High humidity levels cause problems like wood rot and rust. If you see any instances of these issues at home, you need to treat them and better control the humidity levels. Because as humid air flows throughout the entire home, you can expect to see these problems in other rooms as well sooner or later.

It might seem like a minor issue at first, but with time it can spiral out of control and cost you a lot to fix.

Routinely examining your wooden/metal fixtures and products for rot or rust will help you catch an issue before it spreads.


Insect Infestation

Insects like dust mites and cockroaches thrive in humid environments. Cavities and other spaces will retain moisture as humidity moves in a house. These moisture-filled spaces then become perfect breeding grounds for insects.

Most insects also feed on mold and mildew, which are direct results of high humidity. So if you only have a mold infestation, for now, it’s only a matter of time before the pests come along.


Controlling Humidity

The most common ways of controlling humidity are condensation, insulation, and ventilation.

Condensation occurs when the water vapor in the atmosphere turns into water droplets. Appliances like air conditioners and dehumidifiers use this process to maintain the humidity level at home.

To better understand condensation, let’s have a quick look at what Relative Humidity is.

Relative Humidity is the amount of water vapor in the air compared to how much it can hold for a certain temperature. It is usually expressed as a percentage. Furthermore, 100% Relative Humidity means that the air cannot hold any more water vapor. When the air is at 100% relative humidity, it is said to be saturated.

Notice that Relative Humidity is defined related to temperature. This is because the Relative Humidity of air changes with temperature.
Warm air holds more water vapor than cold air. This is the working principle of most dehumidifiers and air conditioners when removing water vapor from the atmosphere. They cool the surrounding so that the amount of water vapor that the air can hold drops. The excess water vapor then becomes water droplets and is removed from the atmosphere.

Recommended Reading:

Insulation, on the other hand, does not remove water vapor from the air but controls the flow of humid air in a house. Proper insulation will prevent moisture-filled air from moving around your home unchecked. Air sealing areas like crawl spaces, bathrooms, and laundry rooms will control humidity flow to a certain extent.

But air sealing should be done along with proper ventilation. One major reason for humid air lingering indoors is the lack of proper ventilation. Use ventilation fans to exhaust the humid air outdoors. You can also open your doors and windows for ventilation. But make sure it’s dry outside.


Related Questions

Does opening windows increase humidity?
Opening a window can increase humidity indoors. But this depends on the condition outside. If it’s dry and less humid outside, you will decrease the indoor humidity levels. However, if it’s more humid outside opening your windows will increase humidity.

Should I place my dehumidifier upstairs?
Placing a dehumidifier downstairs is more effective than placing it upstairs. This because humid air will naturally rise. So, dehumidifying it at the bottom will prevent it from spreading throughout the house. More about it in this post.

Does the humidity in a house remain constant?
Theoretically, the humidity in a tightly sealed house will remain constant. It will cause all the rooms to maintain the same level of humidity as well. However, in reality, this usually isn’t the case unless your home is so saturated and humid that it’s virtually impossible to get it below a high relative humidity without any drastic measures.

The total humidity within a house usually can vary slightly because of certain factors such as the outside humidity levels changing and leaks in insulation.