# Radon and Energy-Efficient Homes: The Toronto Irony

Here is a fact that surprises most Toronto homeowners: a 2020-built energy-efficient home or a deeply retrofitted older home often tests higher for radon than the same home would have before the upgrades. The same envelope tightness that drives down heating bills also reduces passive air exchange, and radon that previously leaked out at low concentration now accumulates to action-level concentrations.

This is the central irony of energy-efficiency retrofits in Toronto, and it is the reason the Ontario Building Code now mandates radon rough-ins in new construction. This post lays out the physics, the OBC response, and what homeowners doing envelope retrofits should know.

The Physics of the Irony

Three factors interact.

1. Radon Production Is Constant Per House

Radon comes out of the soil under a home at a rate determined by soil uranium content and slab permeability. Insulating the attic or replacing the windows does not change the radon production rate one bit. The same becquerels per second are entering the home before and after the retrofit.

2. Air Exchange Determines Concentration

The radon concentration inside the home is the production rate divided by the air exchange rate. If the home doubles its air exchange (from 0.5 to 1.0 air changes per hour, for example), the concentration roughly halves. If the home halves its air exchange (from 0.5 to 0.25 ACH), the concentration roughly doubles.

3. Energy-Efficiency Retrofits Reduce Air Exchange

A 1950s Toronto bungalow with single-pane windows, leaky rim joists, no weather-stripping, and a drafty attic might have run at 0.6-1.0 ACH passively. The same home after attic insulation upgrade, window replacement, rim-joist spray foam, and weather-stripping might run at 0.15-0.30 ACH passively.

Combine the three: radon production is unchanged, air exchange has dropped 3-4x, and the indoor concentration rises 3-4x. A home that read 80 Bq/m3 before the retrofit now reads 240-320 Bq/m3 after.

This is not a flaw in energy-efficiency retrofits. It is a known, predictable interaction. The fix is to add controlled mechanical ventilation (HRV/ERV) and, if testing warrants, active radon mitigation.

What This Means for Toronto Retrofit Projects

Two consequences for homeowners doing envelope retrofits in 2026:

Consequence 1: Test Before and After

Run a long-term alpha-track baseline test before the retrofit, and another long-term test 6-12 months after. The before-after comparison tells you whether the retrofit pushed the home from low-radon to high-radon territory.

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If the before-test was, say, 120 Bq/m3 and the after-test is 280 Bq/m3, the retrofit moved you above the Health Canada guideline. Mitigate.

Consequence 2: Add Mechanical Ventilation

Any meaningful energy-efficiency retrofit in 2026 should include an HRV or ERV. The reason is broader than radon — indoor CO2, moisture, VOCs, and combustion byproducts all need a controlled ventilation pathway. But radon is the most consequential of these for long-term health, and HRV at design ventilation rates contributes a 20-40% radon reduction in tightened envelopes.

HRV is a complement to active mitigation, not a substitute. See [Radon Resealing Cracks vs Active Mitigation](/blog/radon-resealing-cracks-vs-active-mitigation).

The OBC Response: Mandatory Rough-Ins in New Builds

The Ontario Building Code now mandates radon rough-ins in new residential construction (effective Jan 1, 2025 with regional variation). The rough-in is a stub of pipe through the slab, sealed at the slab penetration, terminated below the slab and capped.

If post-occupancy testing warrants, the rough-in is activated — fan added, pipe extended through the framing to the attic, manometer installed. Activation cost is dramatically lower than retrofit:

OBC rough-in activation: $1,200-$2,000.
Full retrofit on a comparable home: $2,500-$4,000.

The savings is $1,000-$2,000, which approximately covers the post-occupancy long-term test plus a substantial fraction of the activation cost.

The OBC rough-in is a recognition by the regulator that energy-efficient new construction will produce some homes above 200 Bq/m3 and that pre-installing the rough-in is dramatically cheaper than retrofitting.

What New-Build Homeowners Should Do

If you are buying or building a new home in Toronto in 2026 with the OBC rough-in in place:

1. Test 91+ days after move-in during the heating season. A long-term alpha-track kit, $60.
2. If the result is below 200 Bq/m3: leave the rough-in capped. Re-test in 5 years.
3. If the result is 200-599 Bq/m3: activate the rough-in. Engage a C-NRPP-certified Mitigation Specialist. Budget $1,200-$2,000.
4. If the result is 600+ Bq/m3: activate the rough-in promptly. Same process.
5. Run a post-activation long-term re-test to verify.

What Retrofit Homeowners Should Do

If you are doing energy-efficiency upgrades on an existing Toronto home:

1. Long-term baseline test before the retrofit. During the heating season.
2. Include HRV or ERV in the retrofit scope. This is non-negotiable in 2026.
3. Long-term post-retrofit test 6-12 months after upgrades complete, during the next heating season.
4. If the post-retrofit result is above 200 Bq/m3: install active mitigation. The HRV alone will not reach guideline.
5. Run a post-mitigation long-term re-test to verify.

For sequencing radon work with broader renovation, see [Radon During Basement Finishing in Toronto](/blog/radon-during-basement-finishing-toronto).

The HRV-Only Mitigation Trap

A pattern we see repeatedly: homeowner does deep retrofit including HRV. Tests after retrofit. Reads 320 Bq/m3. HVAC contractor proposes "running the HRV harder" as the mitigation. Homeowner runs HRV at boost mode 24/7. Re-tests. Reads 250 Bq/m3.

The math:

HRV at design ventilation rate: 20-40% radon reduction.
HRV at maximum continuous boost: 30-50% radon reduction (with significant energy penalty).
Active SSD: 80-99% reduction.

A home reading 320 Bq/m3 might come down to 200-250 with maximum HRV operation. That is just barely at guideline, with no margin for seasonal variability, and at significant ongoing energy cost.

Active SSD on the same home brings it to 30-80 Bq/m3 with a 60-90 W fan that draws less power than the increased HRV operation it replaces. SSD is the right answer.

How RenoHouse Coordinates Energy-Efficient Retrofits With Radon

Our pattern on a deep-retrofit project:

Long-term baseline test before envelope work begins.
Envelope work proceeds (insulation, air-sealing, windows, HRV install).
Post-envelope long-term test 6-12 months after completion.
If post-test exceeds 200 Bq/m3, C-NRPP-certified mitigation specialist designs SSD into the home as a follow-on phase.
Post-mitigation long-term re-test to verify.

The total project cost increase versus a retrofit-without-radon scope is typically $2,500-$4,000 if mitigation is needed, and $0 if testing comes back below guideline. Either way, the homeowner has documented testing and a clear path forward.