Radon Mitigation in The Maples

The Maples developed from the 1960s through the 1980s in the northwest part of the city, on the heavy clay that covers the whole Red River valley. Original construction is mid-century and later, generally with full basements, and the crawlspaces that bring us out are additions and secondary spaces — sunrooms, extensions, enclosed porches and the occasional mechanical crawl.

The pattern is consistent with every Winnipeg neighbourhood of this vintage. Additions were founded shallow, frequently on piers that do not approach the design frost depth, and they move every year with the freezing and thawing clay. Interior finishes crack, the junction with the main house opens, and the crawl area underneath — vented, uninsulated or batt-insulated at the ceiling — swings between frost in January and condensation in July. Fibreglass batts in that environment absorb moisture, lose their R-value and support mould growth on the sheathing above them.

Our scope addresses cause rather than symptom. Support goes down to stable bearing so the seasonal cycling stops. Exterior water management removes the easy part of the load. Interior drainage and a properly discharged sump handle active groundwater where it exists. Then the space is encapsulated with a reinforced barrier sealed to the walls and penetrations, insulated at the perimeter, air sealed at the rim joist, and checked with humidity and framing moisture readings through a full year.

The Maples calls are about additions and secondary spaces: sunrooms, extensions and enclosed porches on shallow support, with vented crawl areas below. Owners describe cold floors, a musty smell in summer, and batts that have fallen out of the joists onto the soil.

Our first recommendation is founding depth assessment followed by support correction where piers are cycling, then exterior water management, then sealing and perimeter insulation. On flat clay the exterior work does real work for little money, so we always look at downspouts and grade before specifying anything interior.

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.