Sub-slab depressurization

If a representative long-term result is above the 200 Bq/m³ guideline, ask a C-NRPP certified mitigator to assess the home. Active soil depressurization is a common option, but the appropriate method and expected result depend on the property.

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.

Ask the mitigator what diagnostic work is needed to assess suction coverage, possible entry paths and the number of suction points. The proposed design should be explained in the written quote.

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.

The installer should explain the proposed suction point, pipe route and sealing method. Hole dimensions and details depend on the system design and applicable standard.

Ask where the pipe, fan and system indicator will go, and how to check that the system is operating. The certified mitigator should confirm compliant placement for this home.

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. Installation time and crew size depend on the home, system design and provider schedule. Ask for a project-specific timeline, particularly in autumn when testing demand may rise.

Costs vary with the property, testing method, system design and provider. Ask for a current itemized quote and confirm what testing and follow-up are included.

Ground conditions, fill beneath the slab, and the number of connected slab areas can affect system design. Ask a certified mitigator to assess suction coverage, exhaust location and winter exposure at the specific home.

Do not assume a home has a particular drain, sump, slab joint or radon rough-in because of its age or neighborhood. Ask the mitigator to inspect and document the actual entry paths.

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. An active system uses electricity continuously while operating. Ask the provider for the proposed fan model and its rated power use; check the system indicator as instructed.