Why Two Houses on the Same Street Can Have Completely Different Radon Readings

There's a piece of radon folklore that refuses to die: if your area is flagged as a radon hotspot, every house on the street must be in more or less the same boat. Your neighbour tested low, so you'll probably test low too. Or worse, they tested high, so you're doomed as well.

It sounds logical. It's also wrong, and understanding why is far more useful than any scare story about radon itself.

What radon actually is, briefly

Radon is a naturally occurring radioactive gas produced by the decay of uranium in rocks and soil. It's colourless, odourless, and present at some level almost everywhere, seeping up from the ground and usually dispersing harmlessly into open air. Problems arise when it gets drawn into an enclosed building and concentrates. That's a straightforward physics and geology story, not a horror story, and it's worth treating it that way.

Government radon maps (in the UK, UKHSA and the BGS; in the US, the EPA) work at area level, grouping postcodes or counties into risk bands based on the underlying geology. That's genuinely useful for deciding whether testing is worthwhile. What it can't tell you is what's happening under any single house. That's where the myth falls apart.

Myth: same street, same soil, same reading

In reality, radon concentration in a home is the product of several things stacking together, and almost none of them are shared evenly between two neighbouring properties.

Soil permeability varies house to house. Radon maps describe average uranium content in the underlying rock, but how easily the gas actually travels up through the soil depends on permeability, not just source concentration. Loose, gravelly, well-drained soil lets gas move freely. Dense clay, or soil compacted during construction of one particular plot, can choke that same gas off or redirect it sideways to a neighbouring foundation instead. Two houses fifteen metres apart can sit on meaningfully different soil conditions, especially where ground has been disturbed, backfilled, or built up differently during construction.

Bedrock isn't uniform either. Even within a single "high radon" zone, geology can change over short distances. Research comparing homes on carbonate bedrock against those on siliclastic rock has found indoor radon levels averaging around 2.8 pCi/L higher on the carbonate ground, purely down to what's directly underneath.

Foundations behave differently. A basement offers a large area of contact with the soil and typically more entry points: cracks, floor-to-wall joints, service penetrations. A slab-on-grade has its own construction joints and settlement cracks. A suspended floor over a crawl space behaves differently again. Even where two neighbouring houses use the same foundation type on paper, workmanship, age, and the exact location of cracks and gaps are never identical.

The house itself pulls the gas in. This is the part people tend to miss. Radon doesn't just diffuse in politely; it's actively drawn in by what's called the stack effect. Warm air rises and leaves through the upper part of a building, and that creates gentle suction at the bottom, pulling in replacement air from wherever it can, including through the ground. The strength of that suction depends on how the house is built, sealed, heated, and ventilated. Extractor fans, open fires, tumble driers, and even how tightly doors and windows are sealed all change the pressure balance. Identical-looking semis with different owners, different renovations, and different daily habits can end up with meaningfully different pressure differentials, and therefore different radon draw.

Age and retrofit history matter too. Data comparing homes by build era shows this isn't simply "older house, leakier, worse" or "newer house, tighter, better" in one direction. Newer, more airtight homes can actually average higher readings than older ones in some studies, because a tightly sealed building that hasn't been specifically detailed for radon control keeps whatever comes in rather than letting it escape through natural draughts. Whether and how a house has been extended, insulated, or had its floors dug up and relaid all shift the picture further.

Put it together and the conclusion is simple: an area-level map tells you it's worth testing. Your neighbour's test tells you almost nothing. The only way to know your own reading is to measure your own home.

The insightful takeaway, not the scary one

None of this means radon is an unmanageable mystery. It means the opposite: it's a well-understood, physically explainable phenomenon, and once you know that entry depends on soil path, foundation detail, and building pressure rather than luck, it's obvious what actually reduces risk. You control the entry points. You don't need to control the geology.

That's the whole logic behind a radon barrier: rather than trying to guess at soil conditions or hope your particular foundation has fewer cracks than the house next door, you install a continuous layer that the gas simply can't get through, regardless of what's happening in the ground beneath you.

Where Passive Purple fits in

This is exactly the gap Passive Purple is built to close. It's a liquid-applied membrane, sprayed, rolled and brushed on to form one continuous, seam-free layer across floors, walls and the junctions between them, which is precisely where sheet membranes tend to fail, at overlaps, laps and awkward details around pipework and foundations.

It's independently tested as a radon barrier, with a measured diffusion coefficient of 3.3 × 10^-12 m²/s (ISO/TS 11665-13), alongside methane and ground-gas barrier performance (ISO 15105), and it carries BBA and Passivhaus certification. Because it's brush, roller or spray applied rather than laid as sheet, it bridges gaps and awkward junctions (up to 5mm with the brush-applied version) rather than relying on taped joints, and it can be applied in retrofit situations without lifting floors, which matters for anyone dealing with an existing house rather than a new build.

In practice, that means the same continuous protection whether the soil below you happens to be gravelly and permeable or dense clay, and whatever your foundation type and vintage of construction. The barrier does the same job regardless of what's making your specific plot different from the one next door.

The point of busting this myth isn't to alarm anyone into thinking radon is lurking uniquely in their house. It's the opposite: understanding why readings vary tells you exactly where to focus, on your own building, your own foundation, your own test result, rather than anyone else's.