Why is Sealing Important for Radon Mitigation? - American Radon Mitigation

Why is Sealing Important for Radon Mitigation?

Did you know that sealing gaps and cracks open to the soil is an important aspect of radon mitigation? Not only does sealing increase the effectiveness of a system, but it can also save you money on operating costs over time. Let’s take a look at the impact that sealing can have on radon mitigation. 


To fully appreciate the importance of sealing, we must first understand how radon mitigation works. Radon is a radioactive gas released by the decay of uranium in rocks and soil. This gas can rise from the ground and collect in your home. Radon exposure is the second leading cause of lung cancer in the United States. Because radon is colorless and odorless, testing is the only way to determine if it is in your home. You can learn more about radon testing here.

A radon mitigation system is the best way to reduce radon levels. Mitigation systems work by sucking the radon-laden air out from under your house and exhausting it out the roof, so it doesn’t enter your home. Therefore, for a system to be as effective as possible, it must create a vacuum under every area of the house.


Sealing is important because it increases the amount of suction that we are able to achieve under the house. It also allows us to use a smaller fan with a lower operating cost and prevents conditioned air from leaving the home. Let’s look at the impact that sealing gaps and cracks had on one customer’s unfinished basement.


To determine how much suction our system would create under the home, we performed what’s known as pressure field extension testing. To start, we drilled small test holes through the floor in a few areas of the basement and hooked up tubes that we ran back to micromanometers—devices that allow us to measure the amount of suction created in each test hole.

Micromanometer Tubes

Tubes connected to Micromanometer

We also drilled a hole for our suction point. Then we attached a pitot tube to the suction point to measure and control the amount of air that we were moving. Next, we hooked a shop vac up to the pitot tube to simulate the suction created by a radon fan. Finally, we referred to our micromanometer readings to see how much suction we were creating.

Suction Point

Despite running 90 cfm, our furthest test hole was only reading about -0.3 pascals. So, to create the most effective system, we wanted to get at least -5 Pa in our furthest, weakest test hole. We needed to improve suction.



The basement had lots of gaps and cracks, which were weakening our pressure field extension. In addition to the visible cracks in the floor, we also had a gap behind this Thermax foam. While the foam was sealed to the floor, there was still a gap behind the wall.

Gap behind Thermax foam

 We were also losing air through a gap at the top of the Thermax.

Gap at the top of Thermax

To better access the floor-to-wall joint, we cut off the bottom of the Thermax.

Access floor-to-wall joint

Smoke helps to visualize the way that cracks pull air from the house. In the photo below, we have our shop vac running to create suction under the home. You can see the crack pulling in air from the house. That’s air that the customer is paying to heat and cool leaving their home.

Smoke Test Gaps

To create the most efficient and effective system possible, we had to seal around the entire basement, including along the walkout wall. In hard-to-reach places like the floor-to-wall joint, we used Great Stuff Pro-gun and foam. You can see it below in orange.

Great Stuff Pro Foam

In the areas that were more accessible, like the control joints, we used a wire brush followed by a shop vac to clean out the cracks. Next, we sealed with a low-VOC radon sealant.

Radon Sealant

We also sealed along:

Load-bearing walls.

Sealing Load-Bearing Walls


Plumbing penetrations.

Sealing Plumbing Penetrations

Under the steps. There’s often an opening to the soil where the step stringers go down into the concrete, so we were sure to foam that.

Sealing under stairs



Once we sealed everything that we could reach, it was time to rerun diagnostics. When we started, we ran about 90 cfm and achieved about -0.3 Pa in our furthest test hole. After sealing, we only needed about 11 cfm to achieve our -5 Pa in that same test hole. That means about 80 cfm was coming from the house initially. Without sealing, we would have needed a much bigger fan, which would have been much more expensive to run. We would also have the added cost of pulling conditioned air from the home. 

Micromanometer Results

Sealing alone greatly improved the efficiency and effectiveness of this customer’s system. While it takes a bit more work upfront, it’s an important step in creating a quality system that lowers radon levels without costing you later.

Contact us at 612-474-1004 to learn more about testing or to set up a free radon mitigation estimate.

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