Sub-slab depressurization

If a long-term test came back above 200 Bq/m3, this is almost certainly the fix you will be quoted. Health Canada describes it in one sentence: a pipe is installed through the foundation floor to an outside wall or up to the roof line, and a small fan attached to that pipe draws the radon from below the house to the outside before it can enter (Reducing radon levels in your home). The same page says a system can be installed in less than a day and will reduce the radon level by more than 80%.

Why it works. The fan holds the space under your slab at slightly lower pressure than the basement above. Air then moves from the house down into the soil rather than the other way round, so the cracks that were letting gas in are now letting a trickle of indoor air out. Nothing is filtered or neutralized. The gas is collected before it arrives and put outside.

Step one: a diagnostic visit. A good installer does more than glance at the basement. Expect a test hole in the slab and a measurement of how far suction spreads through the material beneath it. That communication test decides the single biggest variable in your quote: whether one suction point serves the whole floor or the slab needs two or three.

Step two: agreeing the route. Where the pipe runs and where it exits is the decision you live with. Common routes are up through a closet or mechanical room and out the roof, or along the basement ceiling and out through the rim joist to a pipe on the outside wall. The exhaust has to be clear of windows, doors and air intakes. Ask to see the route marked out before anybody cuts.

Step three: the core and the pit. A hole roughly 100 to 150 mm across is cored through the slab and material dug out beneath it, making a void that lets the fan draw from a wider area. The pipe goes in and the collar where pipe meets concrete is sealed.

Step four: pipe, fan and gauge. Sealed PVC runs to the fan, mounted outside the living space in the garage, the attic or on the exterior wall, never in the basement. A U-tube manometer goes on the pipe where you can see it: when the fan pulls, the two fluid columns sit at different heights, and when they level out the fan has stopped.

Step five: sealing, then proving it. Major floor cracks, the perimeter joint and the sump lid are sealed so the fan is not wasting its pull on open holes. Then the system is proven with a measurement rather than a thumbs up.

How long it takes. Most single-family installations are a four to eight hour day for a two-person crew. Multiple suction points, or an exterior route involving work at height, can run into a second day. Add a few weeks for scheduling in autumn, which is the busy season because that is when test results land.

What it costs in Ottawa in 2026. An indicative range for a typical single-family system is about $2,800 to $3,800 before HST. That is a planning number rather than a quote. What moves it: the number of suction points, how far the pipe travels, whether it goes up through finished space or out through the wall, how easily the fan can be reached for service, and how much crack and joint sealing is included. A house with a capped builder rough-in often lands at the low end. Radon work carries no federal home grant and no City of Ottawa rebate.

What makes an Ottawa house different. Two things. First, the ground. Much of the city sits on Champlain Sea clay, often called Leda clay, and clay transmits suction poorly, which is why a slab poured directly on clay may need two or three suction points where a slab over clean granular fill needs one (NRC, Geotechnical properties of Leda clay in the Ottawa area). Second, the winter. An exhaust terminating low on an outside wall can frost over or get buried by a snowbank, and an attic-mounted fan works in temperatures reaching the minus thirties. Both belong in the design conversation rather than in a surprise call in February.

The suburbs have their own patterns. A 1970s or 1980s Orleans or Nepean basement usually has a cold-joint gap at the wall and an open floor drain, while a Stittsville or Half Moon Bay house from the last fifteen years has a sump pit and sometimes a capped rough-in.

Who should do the work. Health Canada recommends the contractor be certified as a radon mitigation professional under the Canadian National Radon Proficiency Program. Mitigation certification is a different credential from measurement certification, so ask which one the person holds and verify it on the C-NRPP certification check page rather than taking a logo on a van at face value. The Canadian standard for existing houses is CAN/CGSB-149.12, Radon mitigation options for existing low-rise residential buildings (Publications Canada). A quote should say it follows that standard.

What to ask before you hire. Will you do a suction field test before quoting. How many suction points and why. Where will the pipe run and exhaust. Which fan model, what wattage, what warranty. Is a manometer included. Does the price cover sealing the sump lid and major cracks. Is a post-installation test included, and what happens if the long-term retest still comes back above 200.

Depressurization is reliable rather than magic. A stone foundation, several disconnected slab pours or a large bare-earth crawlspace makes the job harder. The fan runs continuously at typically 50 to 100 watts, and the system works only while the fan works, which is what the gauge is for.