Radon in Your Basement: Measure, Seal, Protect

There is a paradox in how we protect our homes: we inspect the roof, watch for moisture, insulate the attic — while the basement breathes the very ground the house sits on. Radon, a radioactive gas produced by the natural breakdown of uranium in soil, takes advantage of every crack and every unsealed joint to accumulate there. Virtually every building in Canada contains a trace of it; about one in ten exceeds the recommended guideline. Here is what you need to know — and the role the basement envelope plays.

A Documented Risk, Not a Trend

Radon is neither a recent discovery nor an invented sales pitch: Health Canada identifies it as the second leading cause of lung cancer in the country after smoking, and the first among people who have never smoked. It is linked to roughly 3,000 deaths per year in Canada — more than road accidents.

What makes radon particular is that it leaves no clues: no smell, no colour, no immediate symptom. An impeccable, recent, well-maintained home can show a high level while the seemingly identical house next door stays under the guideline. Everything depends on the geology beneath the lot, the foundation type and the airtightness of the envelope. The only serious answer is measurement.

Where Radon Gets In — and Why the Basement Is the Key

In winter, warm air rises through the house and creates negative pressure at the lowest level: the house draws in soil gas like a straw. Every discontinuity in the basement envelope becomes an entry point:

• the joint between the slab and the foundation walls — the home's entire perimeter, almost never sealed at construction;
• shrinkage cracks in the slab, normal in any concrete, but open to the soil;
• plumbing and drain penetrations through the slab;
• the sump pit without an airtight lid;
• the dirt-floor crawl space — no barrier at all between the soil and the home's air.

Newer homes, tighter up top, exchange less air: radon entering from below stays more concentrated. And a finished basement — a teenager's bedroom, an office, a family room — turns the most exposed level of the house into a room where people spend hours every day.

Measure Before Acting: The Long-Term Test

The protocol Health Canada recognizes is simple and inexpensive: a certified dosimeter (about $50, lab analysis included), placed at the lowest lived-in level, for at least three months during the heating season. Readings taken over a few days fluctuate too much to support any decision.

Result	Meaning	Recommended action
Below 200 Bq/m³	Under the Canadian guideline	No remediation required; retest after major renovations
200 to 600 Bq/m³	Above the guideline	Remediate within 2 years
Above 600 Bq/m³	High concentration	Remediate within 1 year

Worth noting: the World Health Organization recommends aiming for 100 Bq/m³. Radon risk is proportional to exposure — there is no threshold below which it becomes zero, only levels where it becomes negligible.

Sealing the Envelope: The Role of Spray Foam

Health Canada recommends sealing radon entry points as a first-line measure. That is building-envelope work — our trade. Spray polyurethane foam is the most effective tool for the job, because it does more than cover a crack: it forms a continuous, seamless membrane bonded to the concrete, treating the slab-to-foundation joint, cracks, rim joist and penetrations in a single pass.

Three typical intervention zones:

1. The rim joist and top of the foundation — the most active air path in the basement, sealed and insulated in one application.
2. The slab perimeter and cracks — spray foam penetrates and seals openings that conventional caulking does not hold over the long term.
3. The crawl space — over a dirt floor, a membrane sealed with spray foam cuts the direct contact between the soil and the home's air, and eliminates rising moisture in the process.

And because spray foam is first and foremost a high-performance insulator (R-6 per inch, air barrier and vapour barrier in a single step), the same work cuts a basement's heat loss — often the most energy-hungry zone of the house after the attic. Radon sealing is not an extra on the invoice: it is a property of work done right.

One precision that separates a serious contractor from a salesman: sealing reduces radon entry — it does not by itself guarantee a return below 200 Bq/m³ when the starting level is very high. In those cases, the recognized corrective measure is a sub-slab depressurization system installed by a certified radon mitigation specialist (C-NRPP) — generally $2,000 to $3,500. The two measures reinforce each other: a tight envelope makes depressurization more effective, and vice versa. The honest sequence stays the same: test, seal, retest.

Our Approach: Document, Seal, Verify

As with all our insulation work, the process starts with a documented inspection: condition of the slab, the perimeter joint, the rim joist and the crawl space, photos included. You receive a detailed, firm-price quote describing precisely which zones will be sealed and insulated, line by line — not a verbal estimate that inflates on site.

If your project already involves insulating the basement or crawl space, sealing radon entry points is part of the job. And if a test reveals a level beyond what sealing can correct, we will tell you — and point you to the right certified resource rather than sell you a result we cannot guarantee. That is how we work on attics, roofing and basements alike: the right diagnosis first, the right tool second.