Radon Mitigation in Selkirk

Selkirk sits on the Red River about 35 kilometres downstream of Winnipeg, and its spring flood risk comes from ice jams as much as from meltwater volume. When ice pans stack up in the channel, water levels can climb quickly and locally. For homeowners that means crawlspaces along the lower-lying streets take water in some years and stay dry in others, which is precisely the pattern that leads to deferred repairs — the space dries out, the smell fades, and the damage accumulates quietly in the framing.

Much of Selkirk's housing was built between 1900 and 1960, including a large stock of modest wood-frame homes on shallow foundations. We find a lot of mixed construction here: an original crawlspace under part of the house, a later basement under another part, and an addition on piers that were never taken to frost depth. Those transitions are where differential movement concentrates, and they are where we most often see cracked stem walls, sloped floors and separated sill plates. Fibreglass batts stapled between floor joists are nearly universal in the older stock, and after decades of humidity cycling they have usually collapsed, soaked and gone mouldy.

Our priority in Selkirk is getting water managed before anything is sealed. That typically means correcting exterior grading, confirming or installing interior drainage with a properly sized sump, and routing discharge far enough from the foundation that it does not recirculate. Encapsulation follows, with a reinforced vapour barrier carried up the walls and sealed at every penetration, and insulation moved from between the joists to the perimeter wall where it belongs in our climate. For homes with repeat flood exposure, we detail systems to be serviceable and recoverable rather than pretending the water will never come back.

Selkirk calls follow the river. In years with significant ice jamming or a fast melt, we hear from homeowners on the lower-lying streets whose crawlspaces have taken water, sometimes after a decade of staying dry. In between, the calls are about the slower consequences: a floor that has gone soft near an exterior wall, a smell that returns every spring, insulation that has fallen out of the joists. The intermittent nature of the flooding is exactly what makes it damaging, because the repair gets deferred while the framing accumulates wet seasons.

The mixed construction in Selkirk's older housing deserves specific attention during an inspection. It is common to find an original crawlspace under part of a house, a later basement under another part, and an addition on shallow piers — three foundation systems at three depths, moving independently as clay swells, shrinks and freezes. The transitions between them are where differential movement concentrates, and where we most often find cracked stem walls, separated sill plates and the air leakage paths that carry crawlspace air into the house.

For a Selkirk property with any history of water, our recommendation is to design the system to be recoverable rather than theoretically waterproof. That means drainage and a sump sized for real melt volume, discharge detailed so it cannot freeze or recirculate, and a barrier that can be lifted and reseated at the pit and low points. If water does arrive, the goal is extraction, drying and reinstatement — not demolishing a system that was built on the assumption it never would.

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.