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Reviewed by Marcus Aldridge — Senior Editorial Reviewer — Plumbing, HVAC & Wet Trades

Who to call · Foundation Specialist

High radon levels in basement: call a foundation specialist — or fix it yourself

Radon is an odorless, colorless radioactive gas that seeps into homes through cracks and gaps in the foundation where the building contacts the soil. It is the second leading cause of lung cancer after smoking, responsible for roughly 21,000 deaths per year in the United States alone. The EPA recommends taking action when indoor radon levels reach or exceed 4 pCi/L (picocuries per liter). A short-term test kit (2–7 days) provides a screening result, but a long-term test (90+ days) gives a more accurate annual average. The standard mitigation method is sub-slab depressurization: a pipe drilled through the basement slab connects to a fan that draws radon-laden air from beneath the foundation and vents it above the roofline. A foundation specialist or radon mitigation contractor installs the system, seals visible cracks in the slab and walls, and re-tests to confirm levels have dropped below 4 pCi/L. The installation typically costs $800–$2,500 depending on home size and foundation type.

Is this your problem?

Symptoms to look for

  1. 01

    Short-term or long-term radon test result at or above 4 pCi/L

  2. 02

    Visible cracks in the basement slab or foundation walls that could admit soil gas

  3. 03

    Unfinished or unsealed sump pit without an airtight cover

  4. 04

    Home built in a known high-radon zone (check EPA radon map for your county)

  5. 05

    Persistent musty or damp smell in the basement unrelated to water intrusion

Who to call

Foundation Specialist

A certified radon mitigator installs a sub-slab depressurization system with a fan and roof vent, sizes it to the slab, and tests to prove the level is below the action limit.

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Fix it yourself

How to fix high radon levels in basement

High radon levels in basement

easy 1–2 h

Tools: radon test kit (short-term, then long-term), caulk gun, flashlight · Materials: polyurethane sealant for slab and wall cracks, an airtight sump cover with a gasket, weatherstripping for the basement door

Steps

  1. Confirm with a second test — a long-term kit over 90 days on the lowest lived-in level, or a continuous monitor — before spending money; single short tests swing with the weather.
  2. Seal what you can see: caulk the floor-to-wall joint, slab cracks and pipe penetrations, and fit a gasketed airtight cover on the sump pit; this reduces radon modestly and is required for a mitigation system anyway.
  3. Do not rely on opening windows — it lowers the reading for a day and changes nothing.
  4. Keep the basement door closed and weatherstripped so basement air is not drawn upstairs by the furnace.
  5. Get quotes for sub-slab depressurization — a pipe through the slab to a fan venting above the roof — which is the fix that brings levels down reliably.
  6. Retest 30 days after any work and every two years afterwards, or after finishing the basement.

Stop and call a pro if

  • always for the mitigation system itself: the fan and vent are installed by a certified radon mitigator
  • the reading is over 8 pCi/L (300 Bq/m³) — act now, sealing alone will not get you there
  • the house has a crawl space, which needs a membrane and a different system design

Rules in the US: Many states license radon mitigators; the EPA action level is 4 pCi/L, and a mitigation system usually costs $800–$2,500.

Rules in the Netherlands: Nederlandse niveaus zijn meestal laag; het RIVM-referentieniveau is 100 Bq/m³ — meet met een gecertificeerde detector voordat je maatregelen neemt.

What a pro does that you can't

A certified radon mitigator installs a sub-slab depressurization system with a fan and roof vent, sizes it to the slab, and tests to prove the level is below the action limit.

DIY cost: $15–40 for test kits; $20–60 in sealant and a sump cover

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FAQ

Frequently asked questions

Who should I call for: High radon levels in basement?

Call a licensed foundation specialist. Foundation Specialists have the specialized training, tools, and insurance to diagnose high radon levels in basement safely and stand behind the fix. Most offer free quotes and can visit the same day or the next.

Can I fix this myself?

Often, yes: the guide above has 6 steps (easy, 1–2 h, $15–40 for test kits; $20–60 in sealant and a sump cover). Stop and call a foundation specialist if always for the mitigation system itself: the fan and vent are installed by a certified radon mitigator.

How fast can a pro come out?

Most foundation specialists offer same-day or next-day visits for non-emergency issues. True emergencies (burst pipes, gas smell, no heat in winter) are typically handled within 1–4 hours.

How much does it cost to fix?

Costs vary by severity and your location. Submit a free quote request to get real pricing from local pros in your area.

What are the signs of: High radon levels in basement?

Short-term or long-term radon test result at or above 4 pCi/L. Visible cracks in the basement slab or foundation walls that could admit soil gas. Unfinished or unsealed sump pit without an airtight cover. Home built in a known high-radon zone (check EPA radon map for your county). Persistent musty or damp smell in the basement unrelated to water intrusion.

Who to call

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