Radon Mitigation in St. Boniface

St. Boniface is one of the oldest settled parts of Winnipeg, and its housing runs from pre-1900 river-lot homes to postwar infill and modern development. That range means we encounter every foundation type the city has produced: fieldstone, early unreinforced concrete, mid-century block, and modern poured walls. Crawlspaces appear throughout, most often under additions, but in the oldest homes as the original foundation under part of the structure.

Both the Red and the Seine run through the district, keeping groundwater high in many areas, and the clay soil does what it does everywhere here — holds water, swells, shrinks and heaves. In the older housing the result is masonry deterioration and rot at the wood that bears on it. In the postwar stock it is frost-heaved addition piers and crawl areas that freeze in winter and sweat in summer. The common thread is that the crawlspace is outside the conditioned envelope while being physically connected to the air inside the house.

We approach each property by era, because the right repair differs sharply. Old masonry gets assessed and stabilized first, with moisture strategies suited to the wall. Postwar additions get structural correction down to stable bearing, then sealing and insulation. In all cases the goal is the same: a continuous sealed barrier across the soil, drainage where water is active, insulation at the perimeter instead of between the joists, and controlled humidity verified by readings through a full seasonal cycle.

St. Boniface calls cover every foundation type the city has produced, because the district spans from pre-1900 river-lot homes to modern development. The recurring themes are proximity to the Red and Seine keeping groundwater high, and postwar additions on shallow piers that have been cycling with frost for decades. Both the oldest and the newest housing here generate calls, for opposite reasons.

Our approach is to sort the property by era before specifying anything. Old masonry gets assessed and stabilized first, with moisture strategies suited to the wall. Postwar additions get founding depth established and support corrected. In both cases the end state is the same — sealed floor, drainage where water arrives, insulation at the perimeter, humidity verified by readings — but the route there is quite different.

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.