Radon Mitigation in Headingley

Headingley runs along the Assiniboine River west of Winnipeg, and the river is the dominant factor in nearly every crawlspace we inspect here. Properties on the river flats sit on alluvial sand and silt layered over clay, which drains faster than the clay found in the city but also lets groundwater move laterally with the river level. When the Assiniboine runs high in spring, or when the Portage Diversion is operating, the water table beneath riverside homes rises quickly. Crawlspaces that stay dry for years can take water in a single week.

The housing stock is a mix that complicates matters. Older farmhouses and cottages converted to year-round use often have shallow crawlspaces with minimal footings, sometimes only partially excavated, and almost never a continuous vapour barrier. Newer estate homes on larger lots are better built but frequently have crawlspace mechanical rooms or unheated crawl areas under additions and garages, where freezing pipes and frost-heaved piers are the recurring complaints. Because most of Headingley is on well and septic, we also pay attention to how septic fields and well lines are sealed where they penetrate the crawlspace envelope.

Our approach here starts with establishing where water actually comes from before anything gets sealed. We check grading away from the foundation, confirm whether weeping tile exists and functions, and look at how far the crawlspace floor sits relative to the seasonal high water table. In many Headingley homes the correct sequence is drainage and a properly discharged sump first, encapsulation second. Sealing a crawlspace that still takes groundwater simply traps the water inside the barrier, and we would rather solve the cause than hide it.

Most Headingley calls come in two forms. River-side properties call in spring, when the Assiniboine is high or the Portage Diversion is operating, and water has appeared in a crawlspace that was dry for years. Properties further from the river call in winter, usually about a frozen line in an unheated crawlspace under an addition or a garage-adjacent mechanical space. The first group needs drainage and discharge designed for a rising water table; the second needs the space brought inside the heated envelope. Both are solvable, and they are not the same job.

Because Headingley is largely on private services, there are practical details a city property does not have. Well lines, pressure tanks and septic penetrations all pass through or near the crawlspace envelope and need sealing and insulating properly, both to maintain the envelope and to prevent freezing. Sump discharge has to be routed clear of the septic field as well as the foundation. And the RM of Headingley handles its own permitting, so structural work needs to be confirmed against their process rather than the City of Winnipeg's.

Our usual first recommendation here is to establish the relationship between your crawlspace floor and the seasonal high water table, because everything else follows from it. If the floor sits below the spring water table, no amount of sealing will keep the space dry without drainage and a pump — and sealing first would simply trap the water. If it sits above, the problem is likely surface water and the fix may be as simple as grading and downspout extensions. That single measurement determines whether you are looking at a modest project or a substantial one.

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.