# Radon in Toronto Condos: Feasibility, Limits, and What Works

Toronto's condo stock — over 180,000 units built between 2015 and 2025 alone — is generally considered low-radon-risk because most units sit several floors above grade. The cliche is roughly right but not universally true. Ground-floor and basement-level units in some buildings test above the 200 Bq/m3 guideline, and the mitigation options in a multi-unit building are very different from a detached home. This post lays out the realistic picture.

The Condo-Specific Risk Profile

Three categories of Toronto condo unit, ranked by typical radon risk.

Higher Risk: Ground-Floor and Below-Grade Units

Townhouse-style units with at-grade slabs.
Live-work units on the building ground floor.
Below-grade units (rare, but they exist in some converted-warehouse and stacked-townhouse projects).
Walk-out basement units in townhouse complexes.

These units have direct slab contact with soil and behave more like detached homes for radon purposes. Higher share of testing above 200 Bq/m3.

Moderate Risk: Low-Rise Stacked Townhouses, Boutique 4-8 Storey Buildings

Lower units in stacked-townhouse complexes.
Lower floors of small-format boutique condos.

Some slab-contact pathway, often elevated podium construction with parking garages buffering. Mixed test results — worth testing if the unit is on the lowest occupied level of the stack.

Lower Risk: Mid-Rise and High-Rise Above the Podium

Floors 3+ in high-rise towers, with parking garage(s) below the residential podium.

The parking garage acts as a thermal and pressure buffer; soil-gas radon dissipates into the garage ventilation rather than entering residential units above. Most high-rise units test well below 100 Bq/m3.

Why Some High-Rise Units Still Test High

Three exception cases worth knowing:

1. Mechanical-shaft pathways. Plumbing chases, elevator shafts, and combined-mechanical risers can carry soil gas from the parking garage upward through the stack. Higher units adjacent to such risers occasionally test elevated even though they are 10+ floors up.
2. Make-up air drawn from contaminated locations. Buildings with fresh-air intakes near grade or near garage exhaust can pull radon-bearing air into corridor pressurization systems. This is a building-design issue, not a unit issue, but it shows up at the unit level.
3. Ground-source heat pumps and geothermal loops. Buildings with extensive ground-source mechanical can occasionally show elevated radon in mechanical rooms, with secondary leakage to adjacent units. Rare but documented.

For any condo unit testing above 200 Bq/m3, the diagnosis usually requires building-level investigation, not just unit-level intervention.

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What Mitigation Looks Like in a Condo

This is where condos diverge sharply from detached homes.

Unit-Level Sub-Slab Depressurization: Rarely Feasible

In a detached home, SSD pulls air from beneath the basement slab. In a condo, "beneath the slab" of any individual unit is either another unit's ceiling, a parking garage, or a structural deck. You cannot core-drill the slab and depressurize beneath it without affecting the building's structural and mechanical systems and almost no condo board will permit it.

The exception: at-grade townhouse-style units with their own slab-on-grade pad. These can sometimes accept SSD with condo-board approval and proper engineering.

HRV/ERV-Based Dilution: The Realistic Option for Most Units

For most affected condo units, the realistic mitigation pathway is:

Increase ventilation rates via the unit's HRV/ERV.
Add or upgrade a unit-level continuous mechanical ventilation system if none exists.
Address any building-side make-up air contamination (a condo-board issue).

This is a 30-50% reduction tool, not an 80-99% reduction tool. For a unit reading 350 Bq/m3, HRV-only dilution may bring it to 200-250 — close to the guideline but not comfortably below.

Building-Level Mitigation: A Condo-Board Project

Where the issue is building-design (parking garage exhaust geometry, mechanical riser shaft pathways, fresh-air-intake placement), the fix is a building-level project requiring:

Engineering assessment by a C-NRPP-certified specialist familiar with multi-unit construction.
Condo-board approval.
Reserve-fund allocation or special assessment.
Building-permit and ESA work.

These projects exist but are uncommon. They are appropriate when multiple units in a building test high and the source is clearly building-level rather than unit-level.

The Condo Owner's Realistic Path

If you own a Toronto condo and want to act on radon:

1. Run a long-term alpha-track test in the unit's lowest occupied area, during the heating season. Cost: $60.
2. If below 200 Bq/m3: no action required.
3. If 200-350 Bq/m3: optimize HRV/ERV operation. Address any obvious sealing issues at slab penetrations (rare but possible in townhouse-style units). Re-test.
4. If above 350 Bq/m3 and the unit has slab-on-grade contact: bring in a C-NRPP-certified specialist for diagnostic assessment. Engage the condo board if SSD is technically feasible.
5. If above 350 Bq/m3 and the unit is high-rise: investigate building-level pathways. Raise the issue with the condo board. Consider relocating sleeping areas to higher-floor rooms if the unit allows.

What Condo Boards Should Be Doing

A condo board acting responsibly on radon would:

Run baseline testing in a representative sample of ground-floor and lowest-occupied-level units every 5 years.
Test parking-garage radon levels and verify garage-exhaust ventilation is operating to design.
Respond to individual-unit elevated test results with diagnostic engineering rather than dismissal.
Document any building-level mitigation in the status certificate.

Most boards do not do this proactively. Owners who care about the issue can request testing through the board.

How RenoHouse Approaches Condo Radon Work

RenoHouse does condo renovation work and we have coordinated C-NRPP-certified specialists into condo unit projects where the unit configuration allows it — typically townhouse-style ground-floor units. The fundamental constraint is that condo radon work is board-permission gated, not just engineering gated. We help homeowners frame the conversation with the board, but we do not pretend the engineering is the bottleneck. The board approval is.