Radon Mitigation in Grande Pointe

Grande Pointe is a low-lying acreage community in the RM of Ritchot, just south of Winnipeg, and flood management is part of daily life here. Much of the area relies on raised building pads and local drainage works, and the 1997 and 2009 Red River events are still reference points for what the ground can do. For crawlspaces, the relevant fact is that the water table can sit high for weeks during melt while the clay subsoil stays effectively impermeable, so any below-grade space has to be designed with active water management rather than passive hope.

The housing stock skews newer and larger than in the older river villages, with substantial homes built from the 1990s onward on big lots. That changes the failure pattern. Rather than century-old rubble foundations, we are usually working with sound modern concrete where the problems are detailing problems: a crawlspace mechanical room that was never air sealed at the rim joist, a floor barrier that stops short of the wall, insulation installed between joists instead of at the perimeter, a dehumidifier with no permanent drain, or a sump discharge that freezes solid in January and backs up.

Because the soil is heavy clay and the water table is high, we treat drainage and discharge as the core of the system here, not an accessory. Sumps are specified for sustained running, with backup power considered on properties that have a history of taking water. Discharge is routed well away from the foundation and clear of septic infrastructure. Encapsulation then seals the space against vapour and radon, perimeter insulation keeps it conditioned through winter, and monitoring confirms the space holds a stable humidity through both the melt and the summer humidity peak.

Grande Pointe calls come mostly from owners of larger, newer homes, and the problems are detailing problems rather than deterioration. A crawlspace mechanical room that was never air sealed at the rim joist and runs cold all winter. A floor barrier that stops short of the wall, so soil moisture and radon move around the edges. A dehumidifier draining into a bucket. A sump discharge that froze solid in January and backed water into the space. Modern construction, incomplete specification.

Flood context is unavoidable here and it should inform the design. Much of the area relies on raised pads and local drainage works, and the 1997 and 2009 events remain reference points. For a crawlspace that means drainage and discharge are the core of the system, sumps should be specified for sustained running, and backup power deserves serious consideration rather than a brochure mention. We also keep discharge routing clear of septic infrastructure, which is a real constraint on acreage lots.

Our first recommendation for most Grande Pointe properties is a commissioning review rather than a rebuild: confirm the barrier is sealed and terminated at the wall, confirm the insulation is in the correct plane, confirm the rim joist is air sealed, confirm the sump is sized and its discharge will survive a cold snap, and test radon. Each item is modest on its own. Together they are the difference between a crawlspace that performs and one that merely looks finished.

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.