Radon Mitigation in St. James

St. James is one of Winnipeg's established west-end districts, with housing that runs from pre-war homes through extensive wartime and postwar construction. The smaller houses from the 1940s and early 1950s are the most relevant to crawlspace work: built quickly and economically, many have shallow foundations, and a significant number have crawlspaces as the original foundation under part or all of the structure rather than as a later addition.

Those original crawlspaces are usually unimproved. We find bare soil or a cracked thin slab, open or haphazardly blocked vents, minimal and deteriorated insulation, and framing that has been cycling through damp summers and frozen winters for seventy years. Sill plates and post bases are the first things to go, followed by the sheathing above the wettest part of the floor. The clay beneath heaves with frost at these shallow depths, so the structure has usually been moving the entire time as well.

For houses like these, the work is genuine restoration rather than finishing touches. We assess bearing and sill condition first and repair what has failed, bring support down to stable founding depth, manage water at the exterior and with interior drainage where required, then encapsulate with reinforced material sealed to the walls, move insulation to the perimeter, and bring the space into the conditioned envelope. Documented moisture readings before and after give the homeowner a clear picture of what changed.

St. James calls frequently involve wartime and early postwar houses where the crawlspace is the original foundation rather than a later addition. Those spaces have usually never been improved: bare soil or a cracked thin slab, open vents, deteriorated insulation, and framing that has been cycling through damp summers and frozen winters for seventy years.

Our first recommendation for these homes is a structural and bearing assessment before any sealing, because sill plates and post bases are where we most often find section loss. Once support and decayed wood are addressed, water is managed, then the space is sealed, insulated at the perimeter and conditioned. We document moisture readings before and after so the change is measurable rather than cosmetic.

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.