Radon in Manitoba Crawlspaces: Test, Then Decide

Why this matters more here than most places

Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and rock. It is colourless, odourless and tasteless. Health Canada identifies it as the second leading cause of lung cancer after smoking, and sets a guideline of 200 becquerels per cubic metre for indoor air, recommending remedial action above that level.

Manitoba consistently records among the highest residential radon concentrations in the country. This is not an abstract national statistic — it is a local building science issue, and crawlspaces are directly implicated.

Why a crawlspace is the efficient entry route

Radon moves upward through soil and enters buildings through any opening in ground contact. A crawlspace with exposed soil, or with a thin unsealed sheet of poly over it, offers essentially no resistance.

From the crawlspace, the gas enters the living space through the same paths that carry crawlspace air generally: gaps in the subfloor, plumbing and electrical penetrations, duct chases and the rim joist.

The stack effect does the rest. During our long heating season, warm air rises and leaves the upper part of the house, drawing replacement air in from the lowest point — which is the crawlspace. That pressure difference operates for a large fraction of the year.

A house can have a well-sealed basement slab and still show elevated levels because of an open crawlspace under an addition.

Testing comes first

Do not buy mitigation before you have a number. And do not rely on your neighbour’s result — radon varies dramatically house to house, even between identical homes on the same street, because entry depends on the specific openings and pressures of each building.

Use a long-term test of at least three months. Short-term tests are useful screening tools but fluctuate too much to base a decision on. Ideally run the test through the heating season, when windows are closed and the stack effect is strongest, since that reflects your realistic exposure.

Test kits are available through Health Canada–recognized channels, provincial lung associations and certified measurement professionals. The device goes in the lowest lived-in level of the house.

What the result means

• Below 200 Bq/m³: below the Canadian guideline. Health Canada notes there is no level considered entirely risk-free, so reducing further is reasonable but not urgent.
• At or above 200 Bq/m³: remedial action is recommended. The higher the reading, the shorter the sensible timeline.

How mitigation works in a crawlspace

The standard approach is active sub-membrane depressurization, and it pairs naturally with encapsulation because it uses the same membrane.

1. A perforated collection pipe is laid on the soil, often in a gravel bed.
2. A sealed, reinforced membrane is installed over it, carried up the walls and sealed at all seams and penetrations.
3. A dedicated fan draws continuously from beneath the membrane and vents above the roofline.

The fan creates a slight negative pressure under the barrier, so soil gas is captured and exhausted before it can enter the space. The system is simple, has no moving parts inside the house, and is highly effective — but it depends entirely on how well the membrane is sealed. Installation quality matters far more than which fan you buy.

Sealing alone: helpful, not reliable

Sealing the crawlspace floor reduces radon entry, and in a moderately elevated house it sometimes brings levels below the guideline. But it is not dependable on its own, because small remaining pathways and seasonal pressure changes can keep levels up.

The combination — sealed membrane plus active depressurization — is what produces consistent results.

Plan it before you encapsulate

This is the practical point for homeowners considering crawlspace work. If you are going to encapsulate anyway, test for radon first.

The collection pipe goes in before the membrane. Installing it afterward means cutting open a barrier you just paid for. A half-day of planning avoids that entirely.

Verify afterward

Retest after the system is running and the house is back to normal operating conditions. Mitigation should be confirmed, not assumed.

Also understand what the manometer on the pipe tells you: it shows that the fan is drawing. A mitigation fan is a mechanical device with a finite life, and it usually fails silently. Check the gauge occasionally, and retest every few years — a system nobody monitors may have stopped protecting the house years ago.