How Radon Enters Homes

Radon enters a home because of one simple mechanism: pressure. The air inside your home is slightly lower in pressure than the soil gas beneath the foundation, and that pressure difference pulls radon-laden gas up through any small opening it can find. The mechanism is the same in every home; what varies is which openings the gas uses and how much accumulates inside.

This page covers the physics of radon entry (briefly), the specific pathways radon uses, why basements concentrate it, and how seasonal patterns change the entry rate.

Two forces create the pressure difference that drives radon entry:

Stack effect (the biggest driver)

Warm indoor air is less dense than cold outdoor air. In winter, warm air inside your home rises through stairwells, upper floors, and any chimney or vent stack, creating slight suction at the lowest levels. That suction pulls soil gas — including radon — up through openings in the foundation.

The stack effect is strongest in winter (largest indoor-outdoor temperature difference) and weakest in summer (temperatures roughly equalize). That's why Colorado winter readings are typically 30–50% higher than summer readings on the same home.^[1]

HVAC and exhaust systems

HVAC systems, range hoods, bathroom fans, and clothes dryers all push air out of the home. When air goes out, replacement air has to come in somewhere — typically through small foundation openings. That replacement air carries radon with it from the soil.

Combustion appliances (gas furnace, water heater, fireplace) intensify this effect by drawing combustion air from inside, again creating depressurization that pulls soil gas in.

Diffusion (small effect)

Radon also diffuses through concrete and soil naturally, even without a pressure difference. Diffusion is a much smaller effect than pressure-driven flow but contributes to baseline indoor radon in tight, well-sealed homes.

Soil gas can enter through any opening between soil and indoor air. The common pathways:

1. Hairline cracks in the basement slab. Even cracks too small to see clearly are large enough for soil gas.
2. The floor-wall joint. The seam where the basement slab meets the foundation wall is almost never airtight; it's the largest single entry pathway in most basements.
3. Sump pits. An open sump pit is a direct soil-to-air connection. Even covered sumps can leak if the cover isn't sealed.
4. Plumbing penetrations. Drain pipes, water lines, and gas lines passing through the slab leave small openings around their edges.
5. Crawlspace gaps. Crawlspaces with dirt floors have a continuous soil-air interface — every square foot is a pathway.
6. Foundation walls. Concrete block walls (less common in Colorado) have hollow cores; poured concrete walls have hairline shrinkage cracks.
7. Construction joints. Where one slab section meets another, the joint isn't fully airtight.

Radon doesn't pick one path. It uses whatever's available. Sealing a few cracks won't fix the problem — see why sealing alone isn't enough.

Three reasons basements have higher radon than upper floors:

• Largest soil-contact area. The basement floor and walls are in direct contact with the most soil. Every square foot of contact is a potential pathway.
• Lowest level = strongest stack-effect suction. The suction created by warm air rising is strongest at the bottom of the home.
• Less ventilation. Upper floors have windows that get opened, exterior doors that swing, and active air movement. Basements are often closed off and don't get the same air exchange.

A typical Colorado home's radon profile: basement reads highest, ground floor reads about half the basement level, second floor reads about a third. That's why the EPA places radon test kits in the lowest livable level — that's where exposure is highest.^[2]

An unsealed crawlspace with a dirt floor is the most direct possible path between soil and indoor air. Every square foot of crawlspace dirt is a potential entry point. The crawlspace then connects to the living space above through floor penetrations, hatches, and any unsealed seams.

This is why crawlspace mitigation is its own technique (sub-membrane depressurization) rather than the same approach used for basements. More on crawlspace systems →

Colorado has significant seasonal swings in indoor radon. Three reasons:

• Stack effect strongest in winter. Larger indoor-outdoor temperature difference = more suction at lower levels.
• Tighter house in winter. Windows closed, exterior doors closed, weatherstripping engaged. Less air exchange means radon accumulates.
• Soil conditions. Frozen ground or saturated ground (after rainfall, snowmelt) can change soil permeability and either increase or decrease radon flux into the basement.

The practical implication: a short-term test in February captures the seasonal peak. A short-term test in July captures the seasonal trough. A long-term test (90+ days) averages both — see short-term vs long-term tests.