How Fast Does Radon Drop After Mitigation?

What is Radon Mitigation and how does it work?

Radon levels typically drop within a few hours of the system turning on, but you need to wait at least 24 hours before a follow-up test to get a reliable reading. That's the short version.

We see this question on almost every job. A homeowner gets their test results back, sees a number above 4 pCi/L, and wants it fixed yesterday. They want to know: when can I breathe easy? The fan starts working the moment we flip the switch. The pressure under your slab changes immediately, and radon stops being pulled into the house. But the air inside your home takes time to cycle through.

Here in Huntsville, we deal with a mix of soil types. Some neighborhoods sit on limestone, which can have pockets of higher radon. Others are on clay. The soil under your slab matters, but the system itself works the same way. We drill a hole through the slab, install a pipe that runs up through the house or outside, and put a fan on that pipe. The fan creates negative pressure under the slab, so soil gases get sucked into the pipe and vented above the roofline instead of seeping into your living space.

The fan itself is a workhorse. It runs 24/7 and pulls a constant vacuum. You'll hear a low hum if you're near it, but most people forget it's there after a day or two. The system has no moving parts besides the fan motor, so there's not much to go wrong.

How Radon Mitigation works, start to finish

A standard sub-slab depressurization system uses a fan to pull radon from under your foundation and vent it safely above the roof. The whole installation takes a day, but the hours after are when the system starts working.

We start by finding the best spot to core through the slab. That's usually in a utility room, garage, or closet where the pipe can run up without being an eyesore. We cut a 4 to 6 inch hole, dig out some dirt underneath, and set a pipe that connects to the fan. The fan mounts either in the attic or on the outside wall. We seal the hole around the pipe with a special caulk that stays flexible.

Once the fan is wired and running, the pressure under the slab drops. That change is immediate. Radon molecules that were being pushed into your house by soil pressure now get pulled into the pipe instead. But the air already inside your home needs to be replaced. That's where the 24 to 48 hour wait comes from.

Your home's air changes per hour (ACH) rate determines how fast the old air gets swapped out. A drafty old house might turn over its air every hour. A tight, modern home with good insulation might take three or four hours. Radon is heavier than air, so it tends to settle in basements and crawl spaces. Those areas take longer to clear because the air doesn't move as much.

We always tell homeowners not to run their HVAC system on recirculate during the first 24 hours. Let it pull in outside air. Open a couple of windows if the weather cooperates. That speeds up the air exchange and helps the system clear out the old radon faster.

1. Core the slab.We drill a hole in the lowest floor, usually in a utility area or garage. This gives us access to the soil underneath.
2. Install the pipe.PVC pipe runs from the hole up through the house or outside. Every joint is glued and sealed tight.
3. Mount the fan.The fan goes in the attic or on an exterior wall. It creates negative pressure under the slab.
4. Test and wait.We turn the system on and leave it running. A post-mitigation test happens no sooner than 24 hours later.

Why Radon Mitigation matters for your home

Radon is the second leading cause of lung cancer after smoking, and it's a gas you can't see, smell, or taste. Mitigation is the only way to bring levels down to a safe range.

We've pulled into driveways all over Huntsville, from the older homes near Five Points to new construction in Hampton Cove. Radon doesn't care about the neighborhood. It comes from the natural breakdown of uranium in the soil. Some areas have more than others, but any home can have a problem.

The EPA action level is 4 pCi/L. That doesn't mean 3.9 is fine and 4.1 is dangerous. It's a threshold where mitigation is recommended because the risk starts to add up over years of exposure. We've tested homes reading 20 pCi/L and higher. Those homeowners didn't know until they tested.

Mitigation is not a luxury. It's a straightforward fix that costs a fraction of what you'd spend on a new roof or HVAC system. And once it's in, it works for the life of the home as long as the fan keeps running. Fans typically last 5 to 10 years. Replacing one is a simple service call.

Radon doesn't care about the neighborhood. It comes from the ground under your house, and mitigation is the only way to stop it.

Common problems and what causes them

Most issues after mitigation come from three things: a system that wasn't designed right, a home that's too tight, or weather that messes with the pressure. Here's what we see and how to fix it.

The biggest mistake we see is someone installing a fan that's too small for the house. A 3-inch pipe and a weak fan might work on a 1,000 square foot slab, but a 3,000 square foot home with multiple foundation sections needs a bigger setup. We always calculate the square footage and the number of separate slabs before we pick the fan.

Weather can throw things off too. A cold front moving through Huntsville in January drops barometric pressure. That can make radon levels spike temporarily, even with a working system. It's not a failure. It's physics. The system still works, but the soil gases are under more pressure than usual. We tell homeowners to test during stable weather if they want a baseline reading.

Another issue is a home that's been tightened up with new windows, spray foam, or a radon barrier. That's good for energy efficiency, but it can trap radon inside if the system isn't sized for it. We've had calls where a homeowner sealed their crawl space and their radon levels went up because the gas had nowhere to go except into the living space. The fix is usually a larger fan or an additional suction point.

If levels don't drop after 48 hours, the first thing we check is the manometer. That's the U-shaped tube on the pipe that shows pressure. If it reads zero, the fan isn't running. If it reads low, there might be a leak in the pipe or the seal around the slab. We've also seen cases where the soil under the slab is so porous that the fan can't hold a vacuum. That calls for a different approach, like a membrane or a sump-pump conversion.

Common questions about Radon Mitigation

Here are the questions we hear most often from Huntsville homeowners, answered straight.

We get calls every week from people who just bought a house and the inspection turned up radon. Or from someone who has lived in their home for twenty years and finally tested. The questions are always the same: how long, how much, and does it work at all?

The answers are pretty consistent. Mitigation works. It takes a day to install and a day to confirm. The cost varies by house, but it's a one-time expense that protects your family for years. We stand behind every system we put in.

Frequently Asked Questions