Should You Test for Radon During a Basement Renovation?

Yes, and the test should happen before you design the mechanical layout. Radon is a radioactive gas produced by uranium decay in bedrock and soil; it rises through foundation cracks, sump pits and floor drains, then accumulates in enclosed spaces. Health Canada's guideline is 200 becquerels per cubic metre, and roughly one in ten GTA basements tests above that level. The difference between fixing it during a renovation and fixing it afterward is about $8,000 in labour and patching, because the cheapest mitigation strategy requires access under the slab before you pour it or frame over it.

If you are planning a basement renovation that includes underpinning, a new slab, or a legal second suite, you will already be opening the floor. That is the moment to install passive radon protection at almost no added cost, and to route a sleeve for an active system if the test comes back high. Waiting until the drywall is up means cutting through finished floors, chasing pipes through framed walls, and patching tile you installed six months earlier.

What radon is and why basements in the GTA have higher risk

Radon is colourless, odourless and the second leading cause of lung cancer after smoking. It comes from radium in the soil, which is present across Ontario in varying concentrations. The GTA sits on Paleozoic shale and limestone overlaid by glacial till; radon levels vary street by street depending on how fractured the bedrock is and how porous the overburden. North York, Scarborough and Markham sit on dense glacial till and clay that trap radon near the surface. The Beaches, Leslieville and the old shoreline terraces have sandier soils that vent more easily, but older homes there often have stone foundations with open joints that let the gas in anyway.

Basements concentrate radon because the gas enters through the lowest point and cold air settles. A 1950s bungalow with a poured foundation and a dirt crawlspace will show higher readings than a 2015 house with a vapour barrier under the slab, but both can test above the guideline. Finishing a basement does not create radon, but sealing it with vapour barrier and insulation can raise concentrations if the gas has no path out. That is why the test matters before you close the walls.

When to test: before you design or after you frame?

Test before you start drawings if you are underpinning or replacing the slab. A long-term radon test runs at least 91 days, which means you place the monitor in the lowest occupied level of the house during fall or winter, when windows stay closed and the stack effect is strongest. If you test in July with the windows open, you will undercount. The monitor ships back to the lab, and results arrive within two weeks of receipt.

For a finishing project where the slab stays in place, you can test during the rough-in stage, after framing but before drywall. That still gives you time to add a radon vent stack and a sump crock if needed, without ripping out tile. The mistake is testing after you move furniture into the finished space and discovering you need to cut a hole through your new floor to retrofit a system. We have been called in to do exactly that, and the homeowner spent $11,000 on mitigation work that would have cost $1,800 during construction.

How a radon test works and what the numbers mean

A long-term alpha-track detector costs about $50 to $80 from a lab accredited by the Canadian National Radon Proficiency Program. You place it in the lowest room you occupy regularly, at least 50 centimetres off the floor and away from exterior walls, and leave it untouched for three months. Short-term tests exist but are less reliable; radon levels swing with weather and season, and a 48-hour reading in May tells you almost nothing about January.

Results come back in becquerels per cubic metre. Health Canada's guideline is 200 Bq/m³; the World Health Organization recommends 100 Bq/m³. Between 200 and 600 Bq/m³ you should mitigate within two years. Above 600 Bq/m³ you should mitigate within a year. Below 200 Bq/m³ no action is required, but if you are already opening the floor we still install a passive vent stack, because it costs $400 in materials during construction and acts as insurance if levels rise after you air-seal the envelope.

Passive mitigation you can build into any basement renovation

Passive radon protection means giving the gas a path out of the soil before it enters the house. The simplest version is a layer of clear stone under the slab, covered with a polyethylene vapour barrier, and a 100-millimetre PVC vent stack that runs from the stone layer up through the roof. The stone creates an air gap; the stack vents it by convection. No fan, no power, no moving parts. This setup costs roughly $600 to $1,200 depending on how much stone you need and whether the stack can run inside a mechanical chase or needs to be boxed.

If you are underpinning or pouring a new slab anyway, the stone and poly are already in the specification for moisture control, so the only added cost is the stack itself and a sleeve through the footing. If you are finishing over an existing slab, you can still add a passive stack by coring a hole through the concrete, excavating about half a cubic metre of soil, backfilling with clear stone, and running the vent up. That approach works if the floor is exposed; it does not work if you have already laid tile. A basement conversion in Kleinburg we completed in 2024 tested at 140 Bq/m³ before renovation; we installed a passive stack during the slab pour and retested at 68 Bq/m³ twelve months later, well below the guideline.

What an active radon mitigation system costs in 2026

An active system adds a continuously running fan to the vent stack, creating negative pressure under the slab that pulls radon out before it can enter. The fan mounts in the attic or outside the building envelope, draws about 80 watts, and costs roughly $8 a month to run. Total installed cost during a renovation is about $1,800 to $2,800 if the stack rough-in is already in place; $6,000 to $12,000 if we have to cut through a finished floor, chase the pipe through framed walls, and patch drywall and tile afterward.

The fan itself is a specialized radon blower rated for continuous duty, not a bathroom exhaust fan. It needs a dedicated electrical circuit and a warning label at the disconnect. We mount it where you will not hear it, route the discharge above the roofline so it does not re-enter through a window, and install a pressure gauge in the basement so you can verify the system is pulling. A properly sized active system can drop concentrations by 80 to 95 per cent. If your test comes back at 450 Bq/m³ and you install an active system, expect post-mitigation readings between 50 and 90 Bq/m³.

When building code requires radon protection in new construction

The 2024 Ontario Building Code, which came into force on January 1, 2025, requires rough-in radon protection in all new houses and in additions with a new foundation. That means a soil gas barrier under the slab, a perimeter drainage layer, and a sealed sump or a vent stack stubbed to the roof. The requirement applies to new construction, not to renovations of existing basements, but if you are pouring a new slab as part of underpinning or a walkout conversion, the same measures make sense and most building departments will expect them.

The code does not require you to install the fan unless a post-occupancy test shows levels above 200 Bq/m³, but it does require the rough-in so the fan can be added without demolition. That is exactly the approach we take on every basement underpinning project: stone, poly, stack and a capped tee in the attic where the fan would mount. If the homeowner tests high two years later, the fan threads on in an afternoon.

How underpinning or a walkout conversion changes your radon plan

Underpinning lowers the basement floor by extending the footings downward in alternating sections, each about one metre wide, poured and cured before the next. Once the new footings are in, you excavate the interior, compact a granular base, lay clear stone and poly, and pour a new slab. That sequence gives you a clean shot at radon rough-in with no retrofit penalty. We install the stone layer at 100 to 150 millimetres, run the vapour barrier up the inside face of the foundation wall, and stub the vent stack before the slab goes down. Total added cost over a standard underpinning: about $800.

Converting a basement to a walkout on a sloped lot involves cutting through the foundation wall, excavating outside to grade, and often replacing part of the slab where the new door opening sits. That gives you access to add sub-slab stone and a vent stack on the low side of the house, where the stack can exit horizontally through the new walkout wall instead of running up through two storeys of framing. The result is the same, sub-slab depressurization, but the install is faster and the homeowner does not see a pipe on the exterior front elevation.