Radon Mitigation in Southdale

Southdale was built out mainly through the 1970s and 1980s on the southeast side of the city, on the same heavy clay found throughout the Red River valley. The original construction is generally sound, with full basements and reasonable drainage for its era. The crawlspaces we work on are almost entirely later additions: sunrooms, family room extensions, enclosed decks converted to year-round use, and mechanical spaces.

These additions share a pattern. They were built to add living space at modest cost, which usually meant shallow footings or sonotube piers, minimal insulation, and no vapour barrier over the soil. In a climate with a design frost depth near two metres, piers that stop short of that depth will move every year. We document the results constantly here: sloping floors in sunrooms, cracked tile and grout, doors that bind seasonally, and crawl areas with frost on the subfloor in winter and condensation and mould in summer.

Clay movement compounds the frost issue. In a dry late summer the clay shrinks away from foundation walls, opening a path for the next heavy rain to run straight down to the footing. Our work combines structural correction — support brought down to stable bearing below frost depth — with exterior water management and then a sealed, insulated crawlspace that is part of the conditioned envelope. We verify with humidity and moisture readings rather than assuming the sealing worked.

Southdale calls are dominated by sunrooms and converted decks. The original houses are sound, so what we hear about is the addition: cracked grout, a floor that slopes toward the house, a patio door that will not latch in February, and a crawlspace underneath with frost on the sheathing in winter and condensation in July.

Our first recommendation is to establish how deep the addition's piers go, because sonotube piers from this era are usually well above our frost depth. Correcting support stops the movement; sealing and perimeter insulation then fix the cold floor and the condensation. Exterior grading is worth checking at the same time, since clay backfill from the 1970s has had plenty of time to settle back toward the house.

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.