April 12, 2026

Radon and Real Estate in Ottawa: What Buyers and Sellers Need to Know

Buying or selling a home in Ottawa? Learn what radon means for real estate transactions — CARST screening thresholds, seller disclosure, how to negotiate, and what to do after you move in.

Radon has become an increasingly important factor in Ottawa real estate transactions. Buyers want to know what they are moving into; sellers want to disclose accurately and avoid post-sale disputes; and realtors need to understand what testing options are practical within a transaction timeline. This article explains what radon means at each stage of an Ottawa home purchase or sale — from pre-listing testing through post-purchase follow-up — and clarifies the difference between the screening standards used in real estate and the long-term measurement Health Canada recommends for all homes.

Why Radon Matters in Ottawa Real Estate

Ottawa’s geology places a meaningful proportion of homes — approximately 1 in 6 in the Ottawa-Gatineau area — above Health Canada’s 200 Bq/m³ guideline (Cross-Canada Survey of Radon Concentrations in Homes, 2024). This is not a marginal risk. For buyers, an unmitigated radon problem represents both a health concern and a future cost. For sellers, undisclosed elevated radon can become a source of post-sale liability.

Radon is also invisible, tasteless, and odourless — there is no physical indication of its presence. A home can look and feel perfectly normal while radon accumulates in the basement. Only a test reveals the actual level.

The Two Types of Radon Tests in a Real Estate Context

Understanding the difference between a real estate screening test and a long-term radon measurement is essential for both buyers and sellers.

Long-Term Test (the gold standard)

Health Canada recommends a minimum 91-day test conducted during the heating season (October through April) as the most accurate way to measure a home’s average annual radon concentration. This is the test that produces the number most directly comparable to Health Canada’s 200 Bq/m³ guideline.

Ideally, every home listed for sale would already have a current long-term radon test on file. The Canadian Association of Radon Scientists and Technologists (CARST) states that a radon measurement report should be provided as part of the seller’s declaration/disclosure, indicating that a long-term measurement of at least three months was performed.

If a long-term test has been conducted by a qualified professional during the heating season in the last two years, it may be used in lieu of a new screening assessment for a real estate transaction.

Real Estate Screening Assessment (the practical alternative)

When a long-term test is not available — which is common, as most homes have never been tested — a radon screening assessment can be conducted during the transaction period. The CARST guideline for real estate radon screening sets a minimum duration of 4 days. Because this is shorter than a long-term test, the result is interpreted conservatively, using lower threshold values to account for the uncertainty.

The minimum 4-day screening test is specifically designed to provide useful information within the time constraints of a real estate transaction. It is not a replacement for the long-term test that the new owner should still conduct after taking possession.

The CARST Green / Yellow / Red System

CARST’s guideline for real estate radon screening uses a three-colour classification system with threshold values that are more conservative than Health Canada’s 200 Bq/m³ action level. This conservatism is intentional — a short screening test is less accurate than a long-term measurement, so the thresholds are set lower to compensate.

Green — ≤75 Bq/m³ (heating season) / ≤50 Bq/m³ (outside heating season) A Green result indicates a low likelihood that the annual average radon concentration exceeds 200 Bq/m³. No further radon action is required before purchase. However, a Green result does not guarantee the annual average is below 200 Bq/m³ — a long-term test during the first heating season after possession is still required.

Yellow — 75–400 Bq/m³ (heating season) / 50–400 Bq/m³ (outside heating season) A Yellow result indicates a higher likelihood that the annual average radon concentration is above 200 Bq/m³. CARST recommends that purchase negotiations take the potential cost of a mitigation system into account — for example, by setting aside funds in escrow.

Red — above 400 Bq/m³ A Red result indicates a strong likelihood that the annual average radon concentration is above 200 Bq/m³. CARST recommends that the potential cost of mitigation be reflected in purchase negotiations, and that a long-term test be conducted after the new owner takes possession.

Important note: These thresholds apply specifically to short-term screening tests. They are not the same as Health Canada’s long-term guideline of 200 Bq/m³. A home that screens Yellow or Red during a real estate assessment has not necessarily been confirmed to exceed the Health Canada guideline — but the probability is elevated and the buyer should plan accordingly.

For Sellers: Disclosure and Pre-Listing Testing

What Ontario law requires: Ontario does not have a specific legislative requirement mandating radon disclosure in all residential real estate transactions. However, under Ontario’s general real estate disclosure framework, sellers are expected to disclose known material latent defects — which could include a known elevated radon level.

The most defensible position for an Ottawa seller is to test before listing. If your home tests below 200 Bq/m³ on a long-term test, you have documentation to provide to buyers. If it tests above 200 Bq/m³, you can mitigate before listing — turning a potential liability into a selling point — or disclose the result and price accordingly.

Mitigation before listing: A mitigated home with documented post-mitigation test results is a stronger listing than an untested one. Buyers increasingly understand radon; a seller who can show that the issue was identified and professionally resolved, with verification testing to confirm, demonstrates transparency and reduces negotiating leverage on the buyer’s side.

Providing a recent long-term test: CARST’s guideline recommends that sellers provide a radon measurement report as part of disclosure documentation. If a qualifying long-term test was conducted during the heating season within the last two years, it can serve as the radon record for the transaction.

For Buyers: Due Diligence Before and After Purchase

Request existing test results: Before making an offer, ask whether the seller has a recent radon test on file. If a qualifying long-term test exists (at least 3 months, conducted during the heating season, within the last 2 years), review the result.

Request a screening assessment as a condition of sale: If no long-term test exists, make a radon screening assessment a condition of your offer — or arrange one during your inspection period. A minimum 4-day CARST-protocol screening provides useful information within a standard real estate timeline.

Interpreting the result: If the screening comes back Green, the likelihood of the annual average exceeding 200 Bq/m³ is low — but you still need to conduct a long-term test during your first heating season in the home. If it comes back Yellow or Red, factor mitigation costs into your negotiations or request that the seller mitigate before closing.

Conduct a long-term test after possession: Regardless of the screening result, CARST’s guideline requires that the new owner conduct a long-term radon test (91 days or longer) during the first heating season following possession. This is the definitive measurement of the home’s actual annual average radon level. Retesting should be repeated every two years, and after any major renovations.

New Homes: Ontario Building Code and Tarion Warranty

New Ontario homes are built with radon-resistant features under the Ontario Building Code, including soil gas barriers and heat recovery ventilators. These measures reduce, but do not eliminate, radon risk — testing is still recommended for new homes.

The Tarion New Home Warranty Program treats elevated radon as a major structural defect. Ottawa Public Health notes that Tarion considers elevated radon to be “an environmentally harmful substance or hazard” covered under its major structural defect provisions. Buyers of new Ottawa homes who discover elevated radon should consult Tarion’s warranty process and timelines for submitting a claim.

When a Home Has an Existing Mitigation System

Some Ottawa homes come with an Active Soil Depressurization (ASD) system already installed. For buyers, this is generally a positive sign — it means a previous owner identified a radon problem and took action. However, it also means you should:

  • Confirm the system is operational (the fan is running and the system indicator shows proper function)
  • Request any documentation of post-mitigation testing to confirm the system achieved its intended reduction
  • Conduct a long-term test after possession to verify current performance
  • Plan to retest periodically — ASD fans typically operate for 10–15 years before potential replacementA mitigation system in good working order is not a reason to skip post-purchase radon testing. It is a reason to test with confidence that the result is likely to be good.

FAQ

Q: Do I have to disclose radon when selling a house in Ontario? A: Ontario does not have a specific statute requiring radon disclosure in all cases. However, sellers are generally required to disclose known material latent defects. If you are aware of elevated radon in your home, non-disclosure carries potential legal risk. The practical recommendation is to test before listing and provide documentation of results — and to mitigate before sale if levels are above 200 Bq/m³.

Q: How long does a radon test take for a real estate transaction? A: A CARST-protocol real estate screening assessment requires a minimum of 4 days. The longer the test runs within the available window, the more reliable the result. This is distinct from a standard long-term test (91 days minimum), which is required after the new owner takes possession.

Q: What is the difference between 75 Bq/m³ (CARST) and 200 Bq/m³ (Health Canada)? A: These are two different thresholds for two different purposes. Health Canada’s 200 Bq/m³ is the action level for long-term measurements. CARST’s 75 Bq/m³ is a conservative trigger used in short-term real estate screening — it is set lower specifically because short-term tests are less accurate than long-term measurements. A screening result of 76 Bq/m³ does not confirm the annual average exceeds 200 Bq/m³; it indicates the probability is elevated enough to warrant follow-up.

Q: Who pays for radon mitigation in a real estate transaction? A: This is a matter of negotiation between buyer and seller. Common approaches include: the seller mitigates before closing; the seller provides a price reduction or credit to cover mitigation costs; funds are set aside in escrow pending post-possession long-term testing results. There is no fixed legal requirement in Ontario governing who bears this cost — it is determined by the terms of the Agreement of Purchase and Sale.

Q: Can I use a DIY test kit for a real estate transaction? A: CARST’s guideline specifies that real estate screening assessments be conducted by a C-NRPP Measurement Professional using approved devices and protocols, including anti-interference measures and a non-interference agreement signed by the seller. A DIY kit does not meet these requirements and would not produce a CARST-compliant screening report acceptable to most buyers and sellers in a formal transaction.

Q: The home I’m buying has a radon mitigation system — do I still need to test? A: Yes. Confirm the system is operating correctly, request any available post-mitigation test documentation, and conduct a long-term test during your first heating season. Systems can degrade over time, and an independent test gives you current, verified data.

Q: If a home screens Green, is radon no longer a concern? A: A Green result is a positive indicator, but it does not guarantee the annual average is below 200 Bq/m³ — CARST’s guideline is explicit on this point. A long-term test during the first heating season after possession is still required, regardless of the screening colour. The Green result simply means elevated radon is unlikely, not impossible.

Q: My home’s radon test came back high after I moved in — what are my options? A: If you recently purchased a home and a post-possession long-term test shows levels above 200 Bq/m³, your options depend on what was disclosed during the transaction and what your Agreement of Purchase and Sale includes. Consulting a real estate lawyer is advisable. For new homes, contact Tarion about warranty coverage. For all homes, arranging professional radon mitigation is the immediate practical step to protect your family’s health.

Radon Shouldn’t Be a Surprise After You Move In

The Ottawa real estate market moves quickly, but radon doesn’t have to be left as a post-possession surprise. Sellers who test and disclose — or better, test and mitigate — are better positioned in negotiations. Buyers who make radon assessment a condition of offer have the information they need to proceed with confidence.

Ottawa Radon Expert works with Ottawa buyers, sellers, and real estate professionals to provide accurate, professionally conducted radon testing and mitigation. Whether you need a real estate screening assessment during a transaction or a long-term test after taking possession, our experienced local team provides documentation that meets the standards buyers and sellers expect.

Contact Ottawa Radon Expert to discuss your radon testing or mitigation needs — before, during, or after your Ottawa real estate transaction.

Internal links: Learn about how to test for radon or radon mitigation options.

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