If a basement’s concrete cracks, will radon get into the air inside the house?

Yes. Cracks in a basement floor or wall give radon gas a path indoors, and a crack widens entry that is already happening through the concrete. Concrete is not airtight to begin with. Radon comes up from the soil and rock under your house, finds the gaps, and rides indoor air pressure into the room. A crack just makes the path bigger. Patching every crack is not the fix. A radon mitigation system is, and ours starts at $900 to $6,000, and most homes land around $1,950 to $2,250.

Here is what is going on. Radon is a gas that forms naturally as uranium in soil and rock breaks down. It seeps up through the ground under and around your foundation. Your basement sits a little lower in air pressure than the soil below it, so the soil gas gets pulled up and in. That pull does not need a big opening. It uses the floor wall joint, gaps around pipes, hairline cracks, and the natural pores in the slab. When a crack opens up, you have handed the gas a wider door.

Caulking the crack feels like the answer. It is not, at least not on its own. You can seal a crack you can see and still have radon coming through the slab and the joint you cannot see. EPA says sealing alone is not a reliable way to lower radon, because you can rarely find and close every entry point. Sealing is a helper. The system does the work.

The fix moves the gas before it ever reaches the room. An active sub-slab system puts a pipe through the floor into the gravel or soil under the slab, then a quiet inline fan runs all the time and pulls the soil gas out from under the house and vents it up and out above the roofline. The crack stops mattering, because the air under the slab is now being pulled away from your living space instead of pushed into it. If your basement is a dirt or fieldstone crawl with no real slab to work with, we use sub-membrane depressurization, which lays a sealed membrane over the floor and pulls from underneath that. We install both.

You do not need to test before you worry, and you do not need to panic over one crack. Here is the part most homeowners get wrong. The crack is not the cause. The soil under your house is. A house with a perfectly smooth slab can still test high. So the smart first step is a radon test, which runs $50, credited toward the job if you move ahead. EPA’s action level is 4.0 pCi/L, and they say to consider fixing between 2 and 4. If you test below that, you may not need a system yet, and we will tell you so.

Why do cracks show up around here in the first place? In New Hampshire a lot of it is the ground. The soil packed back against your foundation when the house was built is disturbed backfill, looser than the ground around it, so it drains slow and holds water. Then our winters freeze that wet soil and frost heave pushes on the walls. That movement is what cracks slabs and walls. New England also sits on granite, and granite-heavy ground tends to give off more radon, which is why this state takes it seriously.

A note on cost, because it depends. A standard active sub-slab system in a finished, accessible basement is the lower end. A fieldstone crawl, a long pipe run, or a tricky vent route costs more. We come out, look at the slab, find where the gas is getting in, and price the right system. The inspection and the estimate are free, and you get the quote within 24 hours. Financing is available through Hearth, subject to approval, and we make nothing on it.

Branden runs our radon work. The crew calls him B-Radon, which tells you how often he does this. We are licensed and insured, OSHA-certified, and hold state radon certification RMS-113966. Our radon mitigation carries a 10-year workmanship warranty. We do not put a number guarantee on your radon level unless it is written into your contract, because the gas comes from your ground and we will not promise a reading we cannot control.

If you have a crack and you are worried about the air, the move is simple. Test first, then mitigate if the number says so.

Call us at 603-610-1770 or book your free inspection. We serve New Hampshire, Maine, and Massachusetts.

See our radon mitigation page, our crawl space services if yours is a dirt or fieldstone floor, our basement waterproofing page if you also have water at that crack, our structural and foundation repair page if the crack is moving, and the 603 guarantee.

External reading worth your time: EPA radon, NH DHHS radon program ([verify]), and DOE Building America on foundation air movement.

Related questions

Will sealing the crack lower my radon by itself? Not reliably. EPA says sealing alone rarely works, because you cannot find and close every entry point. Sealing helps a mitigation system run better, but the system is the fix.

Does a crack mean my radon is definitely high? No. A crack widens an entry path, but radon comes from the soil under the house, and a smooth slab can still test high. Test first. A radon test runs $50, credited toward the job if you move ahead.

Do you retest after installing a system? We install and warranty the system for 10 years. The post-install radon retest is the homeowner’s to run, and EPA tells you how. We can point you to it.

What if my basement is dirt or fieldstone with no slab? Then we use sub-membrane depressurization. We seal a membrane over the floor and pull the soil gas out from under it. We install these too.