Radon mitigation for crawl spaces

Why does a crawl space need a different approach from a basement?

Because nothing solid sits between the soil and the air of the house. A slab system works by lowering the pressure under concrete. Bare ground has no lid, so the installer has to build one first.

Health Canada names sub-slab depressurization as the most common and effective way to lower radon, and says that added ventilation or sealing do only a limited job when used alonesource. Sub-membrane depressurization carries that idea over to open soil. The sheet plays the part of the slab, and everything else follows the layout described on our main page about radon mitigation systems for Calgary houses.

Air in a crawl space does not stay put. The floor above it is full of openings for plumbing, wiring and ducts, and a furnace may pull return air from close by. Health Canada explains that soil gas is drawn indoors because the pressure inside a house is usually lower than the pressure in the ground, and that the stack effect strengthens the pull in cold monthssource. A low, unheated space that nobody visits can therefore influence the reading in the rooms over it.

What are the parts of a sub-membrane system?

Four things: a sealed sheet over the soil, a path for air to travel beneath it, a pipe and a fan. Each one depends on the others, which is why the sheet is treated as part of a machine and not as a ground cover.

The drawing shows the idea in section. Soil gas collects under the sheet, moves toward the suction fitting and leaves through the pipe. The fan keeps the underside of the sheet at a slightly lower pressure than the crawl space, so any small leak lets crawl space air move down into the soil side and not the reverse.

How is the membrane sealed so that it holds suction?

By treating every edge as a joint that has to be airtight. The material matters less than the detailing, and most of the labour in a crawl space job goes into the seals.

Preparation comes first. Stored items come out. Debris, offcuts of lumber and sharp stones are cleared so nothing can puncture the sheet from below, and high spots in the soil are raked down. If the design calls for a perforated pipe or a mat under the sheet, it is laid at this stage and run toward the place where the suction fitting will sit.

The sheets are then rolled out with generous overlaps and the seams are taped and sealed along their full length. At the perimeter the sheet turns up the foundation wall and is fixed there with sealant and a mechanical strip, so it cannot peel away as the building moves through the seasons. Posts and piers get a fitted collar each. Plumbing lines, gas lines and electrical conduits that rise from the soil are sealed the same way. The sheet is left with a little slack: once the fan starts, it is drawn down against the ground, and a sheet stretched too tight can pull at its own seams.

The last check is done with the fan running. The installer listens and feels along the seams and the wall line for air being drawn in, and reseals any spot that hisses.

What does the installer check before designing the system?

The space itself, from the inside. Photos taken through the hatch rarely show the far corners, the posts or what the floor is made of, and those three things decide most of the design.

The installer also notes where a fan could sit, where power is available and which side of the house offers a route outdoors. Bring your test report to the visit. A reading taken on the main floor tells the designer how much reduction the system has to deliver.

How are a basement and a crawl space combined into one design?

Each foundation gets its own way of reaching the soil, and the two are usually piped to a single fan. Split levels, additions built on a shallow footing and older houses with a partial basement all fall into this group.

The reason for treating them separately is the footing between them. Suction applied under a basement slab seldom travels past that concrete barrier into the soil under the neighbouring crawl space. A diagnostic test with a vacuum and a small test hole shows how far the suction reaches. If it stops at the footing, the crawl space needs its own membrane and fitting.

The pipe from the slab and the pipe from the membrane then meet at a tee below the fan. The two branches rarely need the same amount of air. Gravel under a slab gives up air freely, while a sheet lying on clay does not, so the installer may fit a damper on one branch to share the suction properly. In a large house, or where the two areas sit far apart, separate fans can be the cleaner answer. Homes with a pit in the basement floor can use it as the slab-side suction point; see mitigation through a sealed sump lid.

Where do the fan and the exhaust end up?

In the layout Plinio Neto uses most, the fan sits on the pipe inside the basement and the exhaust leaves through the rim joist at the side of the house. A crawl space branch simply joins that pipe below the fan, with the U-tube manometer mounted where it can be read without crawling anywhere.

The Canadian mitigation standard permits discharge either above the roof or through a side wall close to grade, and states that the side-wall route shall be favoured in cold weather areassource. A house that has only a crawl space, with no basement at all, still follows the same logic. The fan has to be somewhere a person can reach it for service, close to power, and the outlet position is settled on site.

What happens on installation day?

The membrane goes in first and the fan is switched on last. Health Canada says a radon reduction system can usually be fitted in less than a daysource. A crawl space adds hand work to that, so the assessment is where you learn whether your house fits the usual pattern.

1. Clearing. Stored items are moved out and the soil is cleaned of anything sharp.
2. Air path. Perforated pipe or matting is placed where the design needs suction to travel.
3. Membrane. Sheets are laid, overlapped, sealed at the seams and fixed to the foundation wall.
4. Collars. Every post, pier and service line is sealed individually.
5. Suction fitting. The pipe is connected through the sheet with an airtight flange.
6. Pipe run. The pipe is supported and carried to the fan, joining any slab branch along the way.
7. Fan, gauge and outlet. The fan is wired, the manometer is fitted and the exhaust is taken outdoors.
8. Leak check. With the fan running, seams and edges are inspected and touched up.

What decides the price of a crawl space system?

Mostly labour, and labour follows the space. Floor area sets how much material is needed. Headroom and the size of the hatch set how quickly a person can work. The number of posts and pipes sets how many collars have to be made by hand.

A mixed foundation costs more than either system alone, because it is two pieces of work that share a fan. Preparation you can do yourself helps: emptying the space before the visit removes time from the job. The written quote follows the on-site look. Our Calgary cost guide sets the crawl space figure beside the other system types, and the mitigation cost calculator gives a first estimate from a few questions.

What should Calgary owners with a crawl space know?

That the foundation type does not predict the result, and that the test goes in a different place than many people expect. In the Cross-Canada Survey of Radon Exposure, about 1 in 6 Calgary-area houses measured 200 Bq/m³ or moresource. Health Canada notes that levels differ from house to house, even between similar homes side by sidesource. A crawl space neither clears a house nor condemns it.

The detector does not belong in the crawl space. Health Canada asks for it to sit on the lowest level that people actually live on, in a room used for four or more hours a daysource. In a bungalow over a crawl space that is the main floor. In a split level it is the lowest finished room. The measurement should include at least 91 days of the heating season, when windows stay shut and indoor radon tends to peaksource. Our page on radon testing for Calgary homes covers the choice of test.

Some properties outside the city limits sit on a crawl space too: farmhouses, cabins and manufactured homes among them. A 2024 study of 42,051 Canadian homes found average residential radon as much as 31.2% greater in rural, lower-density communities than in urban onessource. That is a national comparison and says nothing about a single property, but it is a reason not to skip the test on an acreage. See radon in rural and acreage homes.

How do I know the membrane is still doing its job?

Look at the gauge often and at the sheet now and then. A U-tube manometer on the pipe shows the system is running: the liquid stands higher on one side because of the pressure difference the fan createssource. If the two columns move closer together than they were on installation day, air may be leaking in under the sheet. If they sit level, there is no suction at the gauge: the fan may have stopped, or the small gauge tube may have come loose. Our guide to reading a manometer and system alarm explains each case.

The sheet is most at risk when another trade works under the house. After a plumber, a furnace technician or a cable installer has been in the crawl space, check the route they took. A tear can be patched with the same material and sealant. Left open, it weakens suction across a much wider area than the hole itself.

The gauge measures suction, not radon. Only a test shows the level in the rooms you use. Health Canada recommends a new test once a system has been installed, again every five years, and after any renovation that alters the structure or ventilation of the homesource. Timing and method are set out under testing after a mitigation system is installed, and the radon test result checker reads the new figure against the guideline.

What result is realistic, and who does the work?

No honest installer names the final figure in advance. Health Canada reports that radon reduction systems lower the level by more than 80% in most homessource. That statement covers systems in general. A crawl space result depends on how completely the soil can be covered and sealed, and an area that cannot be reached cannot be treated. Where part of the space is inaccessible, that limit should be written into the quote.

The reference point is the Canadian guideline of 200 Bq/m³, and Health Canada recommends corrective action when the annual average is above itsource. No level of radon is considered risk-free, and Health Canada's stated aim is a level as low as reasonably achievablesource. How to read a result against the Canadian guideline explains the ranges.

Health Canada advises homeowners to use a professional certified under the Canadian National Radon Proficiency Program, known as C-NRPPsource. Crawl space systems arranged through Calgary Radon Pro are designed and installed by Plinio Neto, C-NRPP certified mitigation professional (CRMT-202966). The credential belongs to him as an individual and can be looked up in the C-NRPP directory of professionals. The Canadian standard for this kind of work is CAN/CGSB-149.12-2024, which sets out radon mitigation options for existing buildingssource.