Test for radon before you finish the basement

Radon is a radioactive gas from uranium in the ground. You cannot see or smell it, and Health Canada says every home has some. Basements usually have the most, because they sit closest to the soil. Finishing one turns that room into the place you spend your evenings.

This is planning information, not medical or engineering advice. Health Canada and a certified radon professional are the sources that count.

The Canadian guideline

Health Canada's guideline is 200 becquerels per cubic metre (Bq/m³), averaged over a year in the part of the house where someone spends more than four hours a day. It also says no level is risk free.

That definition is why the basement plan matters. Health Canada counts a finished basement with a family room, office, guest room, or a basement apartment as a normal occupancy area. An unfinished basement and a furnace room do not count. Finish the space and the basement becomes the level the guideline is about.

How to test

Radon moves around through the year, so Health Canada asks for a long-term test of at least three months, ideally in fall or winter. Its measurement guide says tests shorter than 91 days in the heating season can underestimate the annual level. Place the detector in the lowest lived-in level. If you plan to finish the basement, test there.

You can buy a do-it-yourself long-term test kit, or hire a measurement professional certified under the Canadian National Radon Proficiency Program (C-NRPP).

1. Start the test in early fall if you want to build over the winter.
2. Leave it in place for at least three months. A two-day reading is not this test.
3. Send it to the lab and read the result in Bq/m³.
4. Decide on mitigation before you order framing lumber.

If the result is over 200

Health Canada recommends lowering the level within one year, and sooner the higher it is. It recommends a C-NRPP certified mitigation professional.

The most common fix is sub-slab depressurization, also called active soil depressurization. A pipe goes through the basement floor to an outside wall or up past the roof line, and a small fan draws radon from under the slab and vents it outside. Health Canada says a system can usually be installed in less than a day, typically cuts radon by more than 80 percent, and costs about the same as other common home repairs such as replacing a furnace. It does not publish a dollar range, so get a written quote rather than trusting a number online.

Ventilation alone, with an HRV or ERV, helps less. Health Canada puts the typical reduction at 20 to 50 percent.

Why the order matters

The pipe goes through the slab and has to reach the outside. Before the basement is finished, that is a hole in bare concrete and a run along an open wall. After, it means cutting through new flooring, opening a finished wall or ceiling, and boxing the pipe in. Sealing cracks and the sump lid is also easier before the floor assembly covers them.

So test first, mitigate if you need to, then frame. If the result is under the guideline but close, ask your mitigation contractor whether a sealed pipe stub through the slab is worth roughing in now, while it is a small job. That is a planning judgement, not a Health Canada rule.

Test again after

• Right after a system is installed, a certified professional should run a short-term test to confirm it works.
• In the first heating season after installation, run a long-term test in the same spot as the original one.
• Keep the fan running all the time, and retest every five years.

Do not let the radon test become the step that gets skipped because the drywaller is booked. Three months of waiting is cheaper than cutting open a finished basement.