Radon Mitigation in Point Douglas

Point Douglas is the oldest residential area in Winnipeg, sitting inside a tight bend of the Red River just north of downtown. Much of the housing predates 1920, and some predates 1900. Foundations from that period were rubble stone or early unreinforced concrete, built shallow, without footings as we would recognize them, and with no damp-proofing. Crawl areas are common under additions and under sections of houses that were never fully excavated.

River proximity on three sides keeps groundwater high, and the clay soil holds it against these old walls for weeks every spring. The resulting conditions are the most severe we encounter in the city: displaced and bowing stone, mortar washed out entirely in places, standing water on soil floors, rot well advanced at sill plates and post bases, and mould throughout the joist bays. In houses this old the crawlspace is frequently also the main radon entry point, with bare soil directly beneath the living space.

Work here requires judgment about what can be sealed and what must first be rebuilt. We assess masonry bearing and condition before specifying anything, and we choose moisture strategies appropriate to historic walls rather than wrapping them tight. Typically the sequence is masonry and structural stabilization, then drainage with a sump sized for sustained spring operation, then a sealed floor barrier to cut off vapour and soil gas, with perimeter insulation where the wall construction permits. Radon is tested after completion.

Point Douglas calls involve the oldest and most severe conditions we see in the city: displaced rubble stone, mortar washed out, standing water on soil floors, advanced rot at sill plates and post bases. Owners frequently call about a smell or a soft floor and are not aware of the foundation's condition until someone goes in with a light.

Our first recommendation is always masonry and structural assessment before anything is sealed, and sometimes engineering involvement, which we will say plainly. Stabilization comes first, then drainage with a sump sized for sustained spring operation, then a sealed floor to cut off vapour and radon. Perimeter insulation follows where the wall construction permits it.

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.