Energy Retrofit Mistakes That Raise Indoor Radon (And What To Do Instead)

Once a homeowner accepts that a tight house needs fresh air, the cheapest answer is usually an exhaust fan running more often. This is the one ventilation choice that can make radon worse rather than better, and the reason is pressure rather than airflow.

An exhaust-only system pushes air out of the house without deliberately bringing an equivalent volume back in. The house responds by pulling replacement air through whatever openings it has, and some of those openings are in the floor, connected to soil. The result is that the interior sits at a slightly lower pressure than the ground beneath it, which is exactly the condition that drives soil gas through a slab. Researchers at the National Research Council of Canada demonstrated this directly in an instrumented test duplex, where running mechanical exhaust ventilation continuously increased the depressurization of the basement relative to the sub-slab by between 1 and 10 pascals.

Those pressure figures come from a research facility rather than from an ordinary occupied house, so treat them as a demonstration of direction rather than as a number your basement will reproduce. The direction is the point, and it is consistent with international guidance: reviews of US weatherization research strongly recommend that non-exhaust options, meaning balanced systems or systems with dedicated air intakes, be used alongside retrofit work specifically to prevent radon from rising.

The correct practice is a balanced system that supplies and exhausts roughly equal volumes, which in most Canadian homes means a heat recovery ventilator or an energy recovery ventilator.

If you are weighing that decision or trying to understand what you already have, our guide to how HRV and ERV systems work covers the mechanics and the sizing questions in detail.

Balanced ventilation earns its reputation here, and the Canadian evidence for it is genuinely good, which is precisely why it gets over-extended into a claim it cannot support.

The National Research Council of Canada studied heat recovery ventilators in 16 occupied houses in the National Capital Region. In the 12 houses whose ventilators were connected directly to the forced-air furnace system, continuous operation at high fan speed produced a median radon reduction of 39 per cent. The benefit reached both storeys almost equally, at 43 per cent on the lower floor and 46 per cent on the upper, so this is not a basement-only effect.

How you run the unit turns out to matter as much as owning one. The same study found higher effectiveness for continuous operation than for periodic operation, and for higher fan speeds than for lower ones. An HRV switched on for twenty minutes an hour on its lowest setting, which is how a great many of them are actually run, is not delivering the result the research describes. If you have one, running it continuously through the heating season is a free adjustment with a measurable return.

Now the boundary. Those 16 houses started at radon levels between 91 and 312 Bq/m³, with a median of 175, which sits below Canada's guideline. The study demonstrates that balanced ventilation works well as a control strategy in homes with moderate concentrations, and it does not demonstrate anything about homes well above the guideline. Health Canada draws the same line from the other direction, putting a typical HRV's effect at 25 to 50 per cent and stating that the effectiveness of ventilation for radon reduction is limited and only appropriate where modest reductions are needed.

Two independent Canadian sources, one federal research council and one federal health department, land on the same range. A ventilator is a good tool for shaving a moderate number down. It is not a substitute for mitigation in a house that is meaningfully over the guideline, and treating it as one means living with a number you have decided not to fix.

Where your own reading falls decides which of those two situations you are in, and our guide to understanding radon test results walks through low, borderline and high numbers.

This is the mistake that quietly disables every other fix on this list, and it is the one with a hard deadline attached.

Testing after the retrofit tells you what you are living with now, which is worth knowing. What it cannot tell you is what changed. Without a baseline you cannot separate an elevated reading caused by the work from one that was always there, and those two point toward different conversations with different contractors.

The scheduling constraint is what turns this from advice into a decision. Health Canada does not recognize a short test as a basis for action: a valid long-term test runs at least 91 days and should be arranged so that at least 91 of those days fall within the heating season, with the detector placed in a normally occupied area of the lowest lived-in level.

The re-test rule is where people get caught. If fewer than 91 days of your test happened during the heating season and the result came back below 200 Bq/m³, that number may be underestimating your annual average, and the guidance is to test again during the next heating season. A comfortable July reading is not a clearance to seal the house. Note also that Health Canada defines the heating season by when your heating actually runs, which varies by region and climate rather than by fixed calendar months.

Work the arithmetic backward and the consequence is obvious. A test started in October produces a usable result in the new year. A homeowner who decides in February to test before spring insulation work cannot have a valid heating-season result before the crew arrives, which leaves three real options: start the test now and schedule the work for the following year, proceed with the work and test through the coming winter with no baseline, or split the project so the envelope work waits.

If you are choosing a device or a service to do this with, the comparison of DIY radon test kits and professional testing covers what each approach actually buys you.

This is the summary worth keeping. Every row below traces to a source listed at the end of this article, and where no defensible number exists for a Canadian house, the row says so rather than inventing one.

Every row in that last column depends on the calendar, and our explainer on why the testing window closes in March sets out how little of the year is actually available.

The disconnect Nicol describes is not abstract, and you can verify it yourself on any live program page.

Ontario's Home Renovation Savings Program is a useful example because it is current and specific. It pays for basement wall and slab insulation, crawl space wall and ceiling insulation, foundation header insulation, attic and exterior wall insulation, window and door replacement, and air sealing measured against an airtightness target. A registered energy advisor visits the home twice, once before the work and once after, to verify what was done.

Radon appears nowhere in that sequence. Not in the list of qualifying upgrades, not in the five steps between application and rebate, and not in either advisor visit. A professional is standing in the basement, before and after work that measurably affects how soil gas enters the house, and the question is not on the form.

At the federal level there is now nothing to point to either way, since both Canada Greener Homes instruments have closed. The grant stopped accepting documents at the end of December 2025, and the loan's funding was fully committed as of 31 March 2026, so the incentives most Canadians will encounter now are provincial and utility programs.

The asymmetry is the uncomfortable part. There is public money available for the work that raises radon and, as CBC reported in March 2026, no federal grant for the work that lowers it, leaving a patchwork of provincial and non-profit programs and most homeowners paying out of pocket. That is not a reason to skip the retrofit. It is a reason to budget for the test now and to know what mitigation would cost before you need it.

On that last point, our breakdown of what a radon mitigation system costs in Canada sets realistic expectations for the range.

A reading above 200 Bq/m³ is not an emergency, and it is not a reason to regret the retrofit. It is a defined problem with a well-established, same-day solution.

The standard remedy is sub-slab depressurization, in which a pipe is run through the foundation floor and out to an exterior wall or up through the roofline, with a fan drawing soil gas from beneath the house and releasing it outside before it can enter. Health Canada notes that a system can be installed in less than a day and in most homes will reduce the radon level by more than 80 per cent, for roughly what other common home repairs cost.

Two things matter about who does it. Health Canada recommends the contractor be certified through the Canadian National Radon Proficiency Program, and C-NRPP maintains a searchable directory of certified measurement and mitigation professionals across the country. Certification is the difference between a system sized and placed for your foundation and a fan attached to a pipe.

It is also worth knowing that a newer home is not automatically a protected one. The radon rough-in added to the National Building Code in 2010, a short capped vent pipe rising from the floor, was later judged generally insufficient, which prompted British Columbia to require a pipe vented outside. Even a passive system cannot be relied on to bring a high concentration below the guideline, and homes that have one should still be tested. Requirements vary considerably: Quebec now requires radon test results be submitted to the local building authority, and municipalities including Guelph, Kingston and Hamilton have adopted their own standards.

For the mechanics of what gets installed and why, our guide to how radon mitigation systems work covers the components and the design choices.

The warmer, quieter, cheaper house is still worth having. All the evidence asks is that you learn your number before you tighten the house, and that you keep ventilation and mitigation in separate categories. Test first, ventilate deliberately, and mitigate if the number says so.