Sump and crawlspace sealing

An open sump pit is a hole straight into the ground under your house. So is an unsealed perimeter joint where the slab meets the foundation wall, and so is a bare-earth crawlspace floor. Sealing work closes those openings. On its own it rarely brings a high reading under the guideline, and it should not be sold to you as a standalone cure, but it makes a depressurization system dramatically more effective and it sometimes makes the difference between one suction point and three.

What gets sealed, and in what order. The sump pit comes first, because it is usually the largest single opening. A sealed lid is a rigid cover gasketed to the slab, with a clear port so you can see the pump, grommeted pass-throughs for the discharge pipe and power cord, and a way to service the pump without breaking the seal. A lid held down with silicone and good intentions does not qualify.

The perimeter joint. Many Ottawa basements have a deliberate gap where the floor slab meets the wall, left so the slab can move. In some houses it is covered by baseboard or flooring and you have never seen it. That joint is often the single biggest entry path by area. It gets cleaned out and filled with a flexible polyurethane sealant that can stretch as the slab moves rather than a rigid product that cracks the first winter.

Floor cracks and penetrations. Shrinkage cracks in the slab, the collars around plumbing stacks, the gap around a floor drain, and any abandoned hole from an old post or a removed fixture all get routed out where necessary and sealed. Floor drains need care: the drain still has to work, so the answer is usually a trap primer or a one-way drain insert rather than filling it.

Crawlspaces. A crawlspace with a dirt or gravel floor is an unlimited source of soil gas. The standard treatment is a heavy polyethylene membrane laid over the floor, lapped at the seams, sealed to itself and run up and sealed to the walls and piers. Where the crawlspace forms part of a mitigation system, perforated pipe or a drainage mat goes under the membrane and connects to the fan. Doing this properly means emptying the crawlspace first, which is the part most homeowners underestimate.

How long it takes. A sump lid alone is typically one to two hours. Perimeter joint and crack sealing across an average basement is usually half a day, and most sealant needs several hours to cure before a fan is commissioned against it. A crawlspace membrane is the variable one: a clean, empty, head-height crawlspace might be a day, while a low, cluttered, damp one with piers and ducts in the way can run to two or three days.

What it costs in Ottawa in 2026. These are indicative ranges. A gasketed sump lid supplied and fitted is generally around $200 to $600. Crack and joint sealing across a basement is often $300 to $900 depending on the metres involved. A sealed crawlspace membrane sits in a much wider band, roughly $1,000 to $5,000 or more, driven by area, headroom, access and how much has to be cleared out first. Bought as add-ons alongside a depressurization system they cost less than as separate visits. Ask for them itemized on the quote.

What makes an Ottawa house different. Sump pits are close to standard in the newer suburbs, because Champlain Sea clay drains badly and builders design for it. Barrhaven, Riverside South, Stittsville and Half Moon Bay houses very often have one, so sealing the sump is usually the first conversation rather than an afterthought. Older Nepean, Alta Vista and Orleans houses more often have a floor drain and a wide perimeter gap instead. The older core, the Glebe and Old Ottawa South among it, is where you find rubble or stone foundations and partial crawlspaces, the hardest combination to seal well.

Freeze-thaw matters here too. Ottawa slabs move seasonally, so a rigid sealant in a moving joint will fail. That is the practical reason flexible polyurethane is specified over cheaper rigid products, and it is fair to ask a contractor to confirm it.

Who should do the work. Sealing is not licensed work and a capable homeowner can seal a floor crack. The credential matters when sealing forms part of a radon repair. Health Canada recommends a mitigation professional certified under the Canadian National Radon Proficiency Program, and the details are covered by CAN/CGSB-149.12 (Publications Canada). Check certification on the C-NRPP certification check page. If a contractor offers sealing alone as the answer to a result over 200 Bq/m3, get a second opinion.

What to ask before you hire. Which sealant product, and is it flexible. Will the sump lid let me service the pump without breaking the seal, and does it have a view port. For a crawlspace, what membrane thickness, how are the seams joined, how is it sealed at the walls, and do I have to empty the space first. And the important one: will sealing alone get me below the guideline, or is this part of a system.

A last word on limits. Sealing reduces entry paths. It does not reverse the pressure difference pulling soil gas in, which is what a fan does. Health Canada treats sealing as a supporting measure alongside depressurization. If your result was only slightly above 200, sealing may be enough, and a retest will tell you. If it was well above, plan on a system.