Short answer
Radon is drawn into homes by a small pressure difference: warm air rising and escaping through the upper floors lowers the pressure at the basement, so soil gas is pulled in through cracks, floor–wall joints, sumps, drains, and gaps around pipes. This “stack effect” is strongest in winter, which is why radon levels are usually highest in the heating season and on the lowest floor.
Key facts
- The stack effect is the main driver: heated homes pull soil gas in at the bottom.
- Common entry points: slab cracks, floor–wall joints, sumps, floor drains, pipe penetrations, crawlspaces.
- Radon is usually highest in winter and on the lowest lived-in level.
- Soil permeability, not just uranium content, controls how much radon reaches a home.
The stack effect
Warm air rises. In a heated house it escapes through the upper floors and attic, and to replace it, air is drawn in low down: through windows and doors, but also through the foundation. The pressure difference is tiny, just a few pascals, but it acts continuously for months. Soil gas, which can carry high concentrations of radon, is pulled straight into the basement.
Exhaust fans, clothes dryers, fireplaces and some furnaces add to the effect by pushing air out of the house. Wind and changes in atmospheric pressure make it fluctuate hour by hour, which is why radon levels can swing widely within a single day.
Where radon gets in
- Cracks in the basement slab, which almost every slab develops as concrete cures and settles.
- The floor–wall joint, where the slab meets the foundation wall, often a gap all the way around.
- Sump pits, open to the gravel and weeping tile around the foundation.
- Floor drains, especially if the trap dries out.
- Gaps around pipes and wires that pass through the slab or walls.
- Hollow block walls, which can carry soil gas up inside the cores.
- Crawlspaces and earth floors, common in older and lakeshore homes.
Why winter is worse
The bigger the temperature difference between inside and outside, the stronger the stack effect. In a Southern Ontario January, it’s strong around the clock. Windows stay closed, so less fresh air dilutes what comes in. Frozen or saturated ground can also cap the soil around a house, leaving the foundation as the easiest path out for soil gas. That’s why Health Canada recommends long-term tests that include the heating season, ideally between October and April.
Why the lowest floor is highest
Radon enters from below and is most concentrated where it enters. Basements and slab-on-grade floors typically have the highest levels; upper floors are often lower but not radon-free. Health Canada’s guideline applies to the lowest level where someone spends more than four hours a day, which is where a test detector belongs.
Why neighbours get different results
Two similar houses side by side can test very differently. Radon levels depend on:
- The ground: how much uranium is in the soil and rock directly below, and how permeable it is. Sand and gravel let soil gas move freely; clay slows it but cracks when dry.
- The foundation: the number and size of openings, the condition of the slab, whether there’s a sump or crawlspace.
- The house: height, airtightness, heating system and ventilation. Taller, tighter homes have a stronger stack effect.
- How it’s lived in: exhaust fans, open or closed windows, and which rooms are used.
This is why maps and neighbourhood averages can’t predict your home’s level. Only a test can.
Does a new or energy-efficient home help?
Not necessarily. Making a house more airtight saves energy, but it can also reduce the dilution of radon that enters, and taller modern homes generate a strong stack effect. Canadian research has found newer homes have tended to test higher than older ones. Homes built under the 2024 Ontario Building Code have a soil gas barrier and a capped rough-in pipe, which makes mitigation easier, but they still need testing.
Common questions
Do HRVs reduce radon?
A balanced heat recovery ventilator can lower radon somewhat by diluting indoor air and reducing negative pressure, but results vary and it’s rarely enough when levels are well above the guideline. Sub-slab depressurization is the most reliable fix.
Will sealing cracks stop radon?
Sealing helps and is part of every good mitigation, but on its own it rarely brings levels down reliably. There are too many small openings, and new cracks form over time.
Are radon levels higher at night?
They often fluctuate through the day with temperature, wind and how the house is used. That’s exactly why long-term tests average over months.