Radon Mitigation in La Salle

La Salle sits southwest of Winnipeg along the La Salle River, and the river sets the groundwater conditions for the community. Properties near the channel have a seasonally high water table that rises with spring flows, and the surrounding soil is expansive Red River valley clay that holds water rather than shedding it. The combination produces sustained pressure against crawlspace walls through melt season, followed by a shrinking, cracking soil mass through a dry summer that opens new paths for the next wet spell.

Most residential growth here has come since the 1970s, with heavy development in the past two decades. Newer homes are well built, but crawlspaces under additions, sunrooms and garages are routinely left unheated and incompletely sealed, which is where we find frost on the underside of subfloors in winter and condensation in summer. Older acreage properties have the familiar rural profile: shallow crawlspaces, soil floors, open vents, sagging batts, and well and septic lines penetrating the envelope without proper sealing or insulation.

We size systems in La Salle for melt volume rather than nuisance seepage, because the week the river peaks is the week that matters. That usually means interior drainage with a properly specified sump and a discharge route that stays clear of the foundation, the septic field and freezing. Encapsulation follows with reinforced material sealed to the walls and penetrations, perimeter insulation rated for our winter design temperature, and humidity control where the space cannot hold its target passively. Everything gets documented with before-and-after moisture readings so the improvement is measurable, not just visible.

La Salle calls peak when the river does. Properties near the channel see the water table rise with spring flows, and homeowners describe water arriving gradually over days and persisting for weeks — a rising table rather than a surface event. Away from the river, the calls are the familiar ones for newer housing: an unheated crawlspace under a sunroom or bonus room, frost on the subfloor in winter, condensation in summer, and a floor above that never feels warm.

We size systems here for melt and peak river conditions rather than for nuisance seepage, because the week the river crests is the week that matters. That means a sump with genuine capacity at the actual lift height, a pit large enough to avoid short-cycling, and a discharge line detailed to shed back out of the freeze zone and terminate well clear of the foundation and any septic infrastructure. Backup power is worth discussing on properties with a water history.

Our first recommendation for a La Salle property is to establish the relationship between the crawlspace floor and the seasonal high water table, and then to look at founding depth under any addition. Those two facts determine whether the project is drainage-led or structure-led, and they prevent the common error of sealing a space that will be undermined in its first spring.

Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and rock. It is colourless and odourless, it moves upward through soil, and it enters buildings through any opening in contact with the ground. 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 some of the highest residential readings in Canada, which makes this a local issue rather than an abstract one.

A crawlspace is the most efficient radon entry point a house can have. Where there is exposed soil, or a thin unsealed membrane, soil gas moves into the space essentially unimpeded — and from there into the living area, because the stack effect pulls air upward through a house continuously during the heating season. Our long heating season means that pressure difference operates for a large part of the year. A house with a sealed basement slab but an open crawlspace under an addition can still have substantially elevated levels.

Testing has to come before mitigation, and it has to be done correctly. A long-term test of at least three months, ideally during the heating season when windows are closed and the stack effect is strongest, gives a representative result. Short-term tests are useful as a screening indicator but fluctuate too much to base a decision on. We will arrange or guide testing, and we would rather you test first than buy a mitigation system you may not need — though in a crawlspace with exposed soil, sealing the floor is worthwhile on moisture grounds regardless of the radon number.

Where mitigation is needed, the standard approach in a crawlspace is active sub-membrane depressurization. A sealed membrane is installed across the floor and up the walls — the same membrane used for encapsulation, which is why these two projects belong together — and a dedicated fan draws continuously from beneath it through a perforated collection pipe, venting above the roofline. That negative pressure under the membrane captures soil gas before it can enter the space. The system is simple and highly effective, but it depends entirely on how well the membrane is sealed, which is why installation quality matters more than equipment selection.

Post-mitigation verification is not optional in our view. After the system is running and the building has returned to normal operating conditions, the space and the living area should be retested to confirm the reduction. We also label the system, explain what the manometer reading means, and tell you what to check — because a mitigation fan is a mechanical device that will eventually fail, usually silently. A system nobody monitors is a system that may have stopped protecting you years ago.

Pressure and building dynamics explain why two apparently identical houses can test very differently, and why your neighbour's result tells you nothing about yours. Radon entry depends on the pressure difference between the soil and the building interior, and that depends on stack effect strength, exhaust appliances, mechanical ventilation balance, wind, and how tight the building is. A house with powerful kitchen and bathroom exhaust and no make-up air runs at a greater negative pressure, and draws correspondingly more soil gas. This is also why sealing work and mechanical changes can shift a reading in either direction, and why testing after any significant envelope work is sensible.

System design details separate a mitigation that works from one that merely hums. The collection point beneath the membrane needs a permeable layer — clean gravel or a drainage mat — so the fan can draw from the whole footprint rather than a single spot. The fan has to be sized to the soil's permeability and the area served: too little suction leaves parts of the footprint uncaptured, too much wastes energy and can pull conditioned air if the membrane has gaps. The exhaust must terminate above the roofline and away from windows and air intakes. And the pipe needs a manometer, so the system reports its own status.

Timing relative to encapsulation is the practical advice we give most often. If you are sealing a crawlspace for moisture reasons and you have not tested for radon, test first. The collection pipe and gravel layer belong under the membrane, installed before it goes down; retrofitting them means cutting open a barrier you just paid for and compromising its continuity. A long-term test takes three months, which is usually compatible with planning a project anyway. If the number comes back low, you have lost nothing but gained a documented baseline. If it comes back high, you have saved yourself from doing the job twice.