9 Features of a Canadian Home That Predict Higher Radon (And What Each One Means)

Taller rooms pull harder on what is below

Ceiling height seems like the least likely thing on this list to matter, which is exactly why it is interesting. Warm air rises and escapes through the upper part of a house, and the air that replaces it has to come from somewhere. In a taller space, that upward movement is stronger, and the easiest replacement air in the building is very often the air sitting under the basement slab. The taller the column, the harder the house pulls.

Evict Radon's research series reports that taller ceilings show the highest average radon of the heights studied. Its own summary of the pattern names a specific house: the Canadian bungalow, with its large footprint, high ceilings, and fewest points of dilution, showed the highest average radon of any residential form in the dataset. As with footprint, the direction is published and the effect size is not, so treat it as a tiebreaker rather than a headline.

If you have a vaulted great room, a two-storey entry, or nine- and ten-foot basement ceilings — all standard in Canadian construction since the 2000s — this feature and the build-era feature are describing the same house from two angles.

What to do: Count a point for vaulted ceilings, cathedral ceilings, or a basement finished with nine-foot walls or higher. Standard eight-foot ceilings throughout do not earn one.

The upgrade nobody expects to show up here

This is the feature that makes people argue with me, and I understand why. Replacing drafty windows is one of the most sensible things a Canadian homeowner can do. It is also, on the radon side of the ledger, a change that takes away one of the routes stale indoor air used to leave by.

Health Canada names this directly. Its guidance on energy retrofits lists increased air tightness, sealing cracks and openings, replacing windows and doors, and adding insulation as measures that reduce air exchange and can inadvertently raise radon levels indoors, and it cites American research showing that reducing energy costs raised radon by 22% on average, in its publication on radon and energy retrofits. That same page also reports single-country findings from Switzerland, the United Kingdom, and Germany, each describing a different national retrofit program in a different housing stock — so read them as evidence that the effect is real and widespread, not as a forecast for your own house.

The clearest measurement anywhere comes from a UK study linking a national energy-efficiency database to radon results. Homes with double glazing installed showed a geometric mean radon level 67% higher than homes with no recorded fabric retrofit, as reported in Indoor Air. That is UK data on UK housing, and I would not want you to read it as a Canadian number. Evict Radon's Canadian analysis found the same direction — properties with newer windows carried higher average radon than those with older ones — without publishing a magnitude.

None of this is an argument against new windows. It is an argument for testing after you install them.

Our guide to energy retrofit mistakes that raise indoor radon walks through how to sequence the work so you get the efficiency without the surprise.

What to do: Count a point if the windows were replaced, or the house was air-sealed, within the last twenty years and you have not tested since.

Sealing the top changes the pressure at the bottom

Attic insulation belongs in the same family as windows, and for the same reason. Evict Radon found that roof insulation with high R-values was associated with the highest average radon levels in its dataset. The UK study that measured windows also measured this: loft insulation was associated with radon readings 47% higher, and wall insulation 32% higher, than homes without a recorded retrofit. Again, that is British housing stock, and again, the Canadian evidence gives the direction without the size.

The framing I find most useful here belongs to Robert Stainforth, a radiation coordination specialist at Health Canada. As he puts it, it has become far more accessible to update the top part of a home — the windows and doors — and less obvious to improve the bottom, generally the foundation, in a Government of Canada science feature. His colleague Pawel Mekarski, who heads Health Canada's radon technical operations, puts the consequence plainly: homes are becoming more airtight, and radon levels are rising.

That asymmetry is the whole story of this feature. We have spent twenty years perfecting the lid and almost no time on the floor.

What to do: Count a point if your attic has been topped up to a modern R-value, or if the house was built to a recent energy standard. If you are planning attic work this fall, start a radon test before the crew arrives so you have a genuine before-and-after.

Basement, crawl space, or slab

Here is where the biggest single structural difference shows up. Canadian houses with a basement average roughly twice the radon of houses without one, and measured concentrations correlate significantly with the type of basement, according to the Health Physics review of Canadian survey data. Roughly twice is a large effect by the standards of anything else on this list.

A basement is not just contact area, it is contact area plus depth. The deeper the floor sits, the more soil surrounds it and the greater the pressure difference between the ground and the room. Crawl spaces behave similarly and are often worse per square metre, because many of them have exposed earth floors or a thin, unsealed membrane rather than a poured slab.

It helps to know exactly what radon uses to get in. Health Canada's list is specific: cracks in foundation walls and floor slabs, construction joints, gaps around service pipes, support posts, window casements, floor drains, sumps, and cavities inside walls — anywhere the house meets the ground, as it explains in its homeowner guidance. Walk down and look at yours. A sump pit with no lid, or a floor drain with no trap, is a wide-open door.

Where you put a detector matters as much as whether you own one, and our guide to placing a radon detector in a Canadian home covers the sump pit and HVAC traps that produce misleading results.

What to do: Count a point for a full basement, and count one for a crawl space with an exposed earth floor. A slab-on-grade home with no basement starts from the most favourable position on this feature.

The feature that changes exposure rather than concentration

This one is different from the eight around it, and the difference is worth naming. Finishing a basement does not necessarily raise the radon concentration down there. What it changes is how many hours a week a person breathes it — and since radon risk is a function of concentration multiplied by time, that is the same thing from your lungs' point of view.

An unfinished basement is a place you visit to move laundry. A finished basement is a home office, a teenager's bedroom, or the room where a family watches television every evening. The concentration is highest at the lowest level of the house, and a renovation quietly moves people into it. Public-health guidance for Canadian audiences makes the point directly: basement living and sleeping areas are particularly vulnerable to elevated radon, and precautions taken during a retrofit only prevent levels from rising — they do not bring down a level that was already high, as an evidence brief for the National Collaborating Centre for Environmental Health sets out.

There is also a sequencing rule most people never hear. Health Canada advises that if you convert unfinished basement space into living space, you should retest afterward in the least lived-in area to confirm the work has not undercut whatever protection the house had, in its radon reduction guide. Framing, insulating, and sealing a basement changes its air movement, and sometimes not in the direction you would hope.

If a test does come back high, the fix is well understood and quick, and our explainer on how radon mitigation systems work covers what actually gets installed.

What to do: Count a point if anyone sleeps, works, or spends more than four hours a day below grade. Count it twice in your own mind if that person is a child.

A scorecard you can total at the kitchen table

Here is the whole list in one place. Read down the middle column, count your points, and take the total to the next section.

A prior is not a measurement

I want to be careful here, because a scorecard is a persuasive object and it would be easy to let it carry more weight than the evidence does.

Start with what the score leaves out entirely: the ground. Regional variation is larger than anything the building contributes. One in three properties in Atlantic Canada and one in three in the Pacific Interior and Yukon sit at or above 200 Bq/m³, against one in five in the Prairies and Northwest Territories, one in six in Central Canada, and one in seventy-five on the Pacific Coast, in Evict Radon's regional breakdown. A low-scoring house in Atlantic Canada can easily beat a high-scoring house in coastal British Columbia.

Our breakdown of radon levels by province is the other half of this question, and it is worth reading beside your own total.

Then there is the evidence itself, which is uneven across the nine. Three features carry real Canadian magnitudes: build era, storey count, and ground contact. Two carry magnitudes measured only in the United Kingdom. Two — footprint and ceiling height — have a published direction and no published effect size at all. And the same Halifax and Winnipeg study that gave us the build-era number tested storeys, ground contact area, heating type, and ventilation type in its own sample and found none of them statistically significant there. One null result in one two-city study does not overturn national findings, but it does tell you how much noise sits around each of these associations.

So treat your total the way a doctor treats family history. It changes how urgently you act and what you check first. It does not tell you what is in the room. Health Canada's position leaves no wiggle room on this: no matter the age, type of construction, or location of a home, the only way to be sure of its radon level is to test, in its homeowner guidance.

The decision rule

Here is the rule I would give a friend. Three or more points, or a single point for a home built after 2010, means start a long-term test this heating season. Zero to two points in a low-prevalence region means test anyway, but you can queue it behind other jobs. Nobody scores their way out of testing.

The reason the timing matters is a detail almost nobody mentions. A valid Canadian radon test must span at least 91 days, with a minimum of 91 of those days falling during the heating season, and it must sit in a normal occupancy area on the lowest lived-in level of the home. If fewer than 91 heating-season days are captured and the result comes back under 200 Bq/m³, that result may be underestimating the annual average and should be repeated the following heating season, according to Health Canada's measurement guide.

Read that again with a calendar open. Heating season is defined by when your home heating actually runs, so it varies by region, but in most of the country a test that starts in late September or October comfortably clears the requirement. A test started after the new year usually will not, which means waiting costs you an entire year rather than a few weeks.

If it helps to know what you are buying into, our comparison of DIY radon test kits and professional radon testing lays out what each approach gets you.

And if the number comes back high, the path from there is short and well travelled. A sub-slab depressurization system — a pipe through the foundation floor with a small fan drawing soil gas outside before it reaches you — can usually be installed in less than a day and cuts levels in most homes by more than 80%, using a contractor certified through the Canadian National Radon Proficiency Program.

Once your result arrives, our guide to understanding radon test results explains what to do with low, borderline, and high readings.