Should I test for radon under a new Ottawa addition?

Radon testing is absolutely something you should do before, during, and after adding to your Ottawa home — and the local context makes this more important than most homeowners realize.

Radon and Ottawa Home Additions

Ottawa and the surrounding region sit in one of Canada's higher-risk zones for radon gas. Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and rock. It seeps upward through the ground and accumulates in enclosed spaces, particularly basements and lower-level rooms. Health Canada's action level is 200 becquerels per cubic metre (Bq/m³) — concentrations above this level warrant mitigation. Ottawa's geology, including the clay-rich Leda soils deposited by the ancient Champlain Sea, creates conditions where radon can migrate unpredictably, and studies have found elevated readings across many Ottawa neighbourhoods, including areas in Gloucester, Nepean, Kanata, and Barrhaven.

When you build a home addition, you are creating new pathways for radon to enter your home. Any new foundation — whether a full poured concrete foundation, a crawl space, or even a slab-on-grade — introduces new contact between your living space and the soil below. Ottawa's 1.2 to 1.5 metre frost line means your footings go deep, and that depth puts the foundation in direct contact with soil layers where radon concentrations tend to be higher. Even a well-poured concrete foundation has micro-cracks, cold joints, and penetrations for drains and utilities that radon can exploit.

The timing of your testing matters significantly. You should ideally test your existing home before construction begins to establish a baseline reading. This tells you whether you already have a radon problem and informs the design of your addition. If your existing basement already reads above 100 Bq/m³, you should plan radon-resistant construction into the addition from the start — it is far less expensive to install a sub-slab depressurization rough-in during construction (typically $500 to $1,500) than to retrofit one after the fact (typically $2,500 to $5,000 or more). A rough-in involves laying perforated pipe beneath the slab and running a sealed pipe to the exterior, so a fan can be added later if needed.

After your addition is complete and the space has been enclosed and conditioned for at least three months, you should run a long-term radon test using a certified alpha-track detector. Long-term tests (90 days or more) give you a far more accurate picture than short-term tests, which can be skewed by weather, seasonal pressure changes, and construction-related soil disturbance. Ottawa's cold winters actually tend to increase radon accumulation because homes are sealed tight against the cold, reducing natural air exchange — so testing through the winter months gives you a realistic worst-case reading.

From a practical standpoint, radon-resistant construction features add very little cost when built in from the start. A gravel sub-base beneath the slab, a polyethylene vapour barrier, sealed penetrations, and a passive depressurization pipe rough-in are standard good-practice details that many experienced Ottawa contractors already include. Ask your addition contractor directly whether they incorporate radon-resistant construction practices — it is a reasonable and increasingly common expectation.

Radon testing kits are available through Health Canada's online portal and at many Ottawa hardware stores for $30 to $60 for a long-term test. Certified radon measurement professionals and mitigation contractors can be found through the Canadian National Radon Proficiency Program (C-NRPP) directory. For your addition contractor and any HVAC or mechanical trades involved in your project, you can also browse the Ottawa Construction Network directory at justynrookcontracting.com — a well-rounded addition team will understand radon's role in Ottawa construction and can help you plan accordingly.