Radon is a colorless and odorless gas that originates from the radioactive elements in the soil. The intensity of this gas varies from location to location. When you build a house over such an area, the radon gas emitted from the soil seeps into the house through the cracks in the foundation.
The radon levels are higher in the basement because it is the lowest part of the house where radon easily seeps in. Besides, Radon is heavier when compared to air, so most of the gas stays in the basement without spreading to the other areas of the house, further increasing its concentration.
If your house has a crawl space, the radon gas’s intensity will be much higher there than in any other part of the house. If the crawl space has a dirt floor, the radon levels will be much higher. Ventilating the crawl space can mitigate the radon levels, but this can lead to mold growth.
Since radon gas is heavier than air, its intensity reduces as we go higher. The attic and the top floor of the house have no or meager amounts of radon. However, if your house uses any forced ventilation, like a whole house fan, it can create a negative pressure in the house, drawing more radon into it (source).
Why are radon levels higher in the basement
There are a few reasons why radon levels are higher in basements. They are;
Radon enters a home through the basement
The primary reason radon levels are high in the basement is that Radon enters a house primarily through its basement. The exposed earth, the cracks in the walls and the floor, the sump pump, etc., cause radon to seep into the basement from below. This is true even if the basement is sealed. If the earth of the basement is not sealed, it is much easier for the radon gas to enter the basement. This proximity and ease of access can lead to a higher concentration of radon in basement spaces.
Radon is heavier than air
Air is a mixture of gases, and its weight per cubic foot is approximately 0.0807 lbs. The exact weight varies with atmospehric conditions however.
A cubic foot of radon gas weighs 0.607 lbs.
That means Radon gas weighs approximately 7.5 times than air. This higher weight of radon causes it to occupy the lower areas of the home, pushing the air up.
So it is no wonder that the radon gas entering into the basement of the house refuses to go up. Since there is no way for the gas to move, other than upward, the radon accumulated in the basement seldom moves to other spaces. Moving the gas using forced ventilation creates negative pressure inside the basement, causing it to let in more radon gas.
Basements lack ventilation
The basement of a house is its least ventilated space. As a result, the Radon entering a basement gets stuck in it with nothing drawing it out. The use of HVAC systems and forced ventilation only further increases the radon levels of the basement as these measures create a negative pressure there.
How to lower radon levels in the basement
There are a few ways to reduce the radon levels in your basement. Here are they;
Seal all the cracks in the basement
As radon enters the basement through cracks, sealing them can reduce radon levels. Even though it is impossible to seal all cracks, it is still possible to seal most of them, and that van considerably reduces the radon levels.
Pressurize the basement
It is the negative pressure inside the basement that brings the radon gas in. By pressurizing the basement, this inflow of radon can be reduced. This can be done by installing a powered fan and properly sealing the basement from the rest of the living spaces.
Providing natural ventilation in the basement
Natural ventilation in a basement can considerably reduce its radon levels. The increased air circulation will carry away most of the radon gas stuck inside the basement.
Using a radon mitigation system
The most commonly used mitigation system is sub-slab-depressurization. This is a system that reduces the pressure under the basement with the help of a radon fan. Large pipes connected to the radon fan are installed through the basement floor to suck the air under it. This arrangement will draw out any radon from the ground and release it into the atmosphere thus preventing the gas from entering the basement. This is the most popular and widely used method for radon reduction.