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How do you seal cracks in a basement floor for radon?

By Radon411 Editorial TeamUpdated How we research

Short answer

Seal cracks in the floor and walls, floor-wall joints and gaps around pipes with plaster, caulk or other materials made for the purpose, then test again. Sealing is a basic part of most radon fixes, but on its own it has not been shown to lower radon significantly or consistently.

Next step: a certified professional designs and installs the fix. Find a mitigation professional

Radon enters through cracks in floors, floor-wall junctions, gaps around pipes or cables, hollow-block walls, and sumps or drains, and those are the openings sealing targets. Seal them with plaster, caulk or other materials designed for this purpose.

Sealing limits the flow of radon into the home, which makes other reduction techniques more effective and cost-efficient. On its own it falls short: it is difficult to identify and permanently seal every place radon enters, and normal settling opens new entry routes and reopens old ones.

Always test radon levels again after making changes like these. If the level is still high, the fix is a soil suction system, with subslab suction the most common and usually the most reliable method, and sealing as part of it.

What to do next: if your home tests high, get quotes for a full system from certified professionals, and if you seal cracks in the meantime, run a new test once the work is done.

What sealing can and cannot do

Sealing cracks and other openings in the foundation is a basic part of most approaches to radon reduction. It limits the flow of radon into the home, which makes other radon reduction techniques more effective and cost-efficient. Sealing alone is not recommended, though, because by itself it has not been shown to lower radon levels significantly or consistently.

Sealing, on its own and as part of a system
ApproachWhat to expect
Sealing aloneNot recommended: not shown to lower radon significantly or consistently
Sealing with a soil suction systemMakes the system more effective and cost-efficient
Sealing as listed among reduction methodsNormally only used with other techniques; proper materials and installation required

Think of sealing as a supporting step. A soil suction system does the main work, and sealing makes it more effective and cost-efficient.

Why sealing alone falls short

It is difficult to identify and permanently seal the places where radon is entering, and normal settling of a home opens new entry routes and reopens old ones.

Where to seal

Sealing targets the openings radon uses to get in: cracks in the floors or at floor-wall junctions, gaps around pipes or cables, small pores in hollow-block walls, cavity walls, and sumps or drains.

Cross-section drawing of a house with a basement and a crawl space, numbered at the seven ways EPA lists for soil gas to get in: cracks in solid floors, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, the water supply.
Read this figure as text
House in cross-section showing where soil gas entersGround levelSoil and rockBasementSolid floorCrawl spaceSuspended floor1234567
  1. Cracks in solid floors
  2. Construction joints
  3. Cracks in walls
  4. Gaps in suspended floors
  5. Gaps around service pipes
  6. Cavities inside walls
  7. The water supply
The openings in a foundation that soil gas uses, and that sealing targets.
Common openings and how they are handled
OpeningHow it is usually handled
Cracks in the floor and wallsSealed with plaster, caulk or other materials designed for this purpose
Floor-wall junctionsSealed along the joint
Gaps around pipes and cablesSealed where they pass through the floor or wall
SumpCapped so it can still drain water and serve as a suction point
Hollow-block wallsBlock-wall suction can be used, often in combination with subslab suction

How to seal floor cracks

  1. Test first

    Know your radon level before you start, so you can tell whether the work helped.

  2. Find the openings

    Look for cracks in the floor and walls, the joint where they meet, and gaps around pipes, cables and the sump.

  3. Use the right material

    Seal with plaster, caulk or other materials designed for this purpose, following the product's instructions.

  4. Test again

    Retest after the work is done to see whether the level came down.

When sealing is not enough

If a test shows a high level, the fix is a system. The method primarily used is a vent pipe and fan that pulls radon from beneath the house and vents it outside; subslab suction is the most common and usually the most reliable method. Sealing foundation cracks and other openings makes this kind of system more effective.

Cross-section of a house with a sub-slab depressurization system: soil gas drawn from beneath the slab into a suction point, up a vent pipe, through a fan outside the living space and out above the roofline.
Read this figure as text
Sub-slab depressurization system in a house cross-sectionGround levelLiving spaceBasement12345
  1. Soil gas drawn from beneath the slab
  2. Suction point through the slab
  3. Vent pipe
  4. Fan, outside the living space
  5. Discharge above the roofline
A sub-slab system with its suction point sealed into the slab.

Use a contractor trained to fix radon problems: without the proper equipment or technical knowledge, you could actually increase your radon level or create other hazards and costs. If you are considering doing it yourself, first contact your state radon office. See do-it-yourself radon mitigation and can radon come through concrete.

Questions about sealing cracks for radon

Does sealing basement cracks reduce radon?

It helps as part of a system, but sealing alone has not been shown to lower radon levels significantly or consistently.

What do you use to seal cracks for radon?

Plaster, caulk or other materials designed for this purpose, following the product's instructions.

Should I seal the sump pit?

A sump can be capped so it can continue to drain water and serve as the location for a radon suction pipe.

Will sealing cracks last?

Normal settling of a home opens new entry routes and reopens old ones, which is one reason sealing alone falls short.

Do I need to test after sealing?

Yes. Always test radon levels again after making changes like these.

Can I seal radon cracks myself?

You can seal cracks yourself, but for a radon fix use a contractor trained to fix radon problems. If you plan to do the work yourself, first contact your state radon office for guidance.

Is a radon system better than sealing?

Yes. A soil suction system is the method primarily used, and sealing is a basic part of it rather than a substitute.

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