Renovation & Design

ASK THE INSPECTOR: Cap that sump pit to combat humidity, radon

QUESTION: I have an interesting question for you about our house, which is in the country and about eight years old.

Should a sump pit in a basement, next to a forced-air electric furnace, be capped to vent outside?

I ask this because I have just capped ours with a vent pipe leading outside the house. I have done this because it seemed to have a great capacity to push cool/damp air upwards and out onto the basement floor. I believe this is what is happening because the plastic vapour barrier that I covered it with seems to "dome up" with internal pressure.

I'm afraid that this may lead to excess moisture in the house during the cold winter months. Is it a good idea or not? We have two pumps, one being a back-up, battery-powered unit. Thank you, James Temple

ANSWER: Having a properly sealed sump pit in your newer home is important for a couple of reasons. While it should have been done when your home was built, it may be fairly easy to accomplish even now.

Initially, I was not clear what you meant by having your sump pit "capped". I originally thought you meant putting a cap on the discharge pipe outside, which would have been a major mistake. Upon further reading, I realize that you simply put a plastic over the pit in an attempt to properly seal it. Again, I'm not sure how you have installed a vent with the pit sealed in this way, but it's something that should not be necessary once you properly fit and seal the lid.

In fact, if you have a properly sealed sump, there should be no need to vent any gasses to the exterior of the home, as they should remain in the pit, and may even partially vent to the outside once the pump comes on.

There are two fundamental reasons why all sumps installed in new homes are required to be sealed. The first is a safety-related issue: to prevent soil gas, or radon, from entering the living space of the home.

Radon is a colourless, odourless gas that is naturally occurring due to the decay of uranium in the soil. Uranium is radioactive and radon gas is one of the bi-products that emanate from these mineral deposits. In some types of soil and areas it can be a serious issue and others it may hardly be a concern. The problem with radon gas in the living space is that it has been strongly linked to lung cancer.

For that reason, all homes built in Canada in the last few decades have required polyethylene sheathing to be installed under the basement floor slab, to prevent soil gas intrusion. While this can be quite effective if properly installed and sealed, there is still a problem with one other component, the sump.

The sump pit in most newer homes is installed simply as a location to collect the water from the terminal ends of the weeping tiles. To accomplish this, we must have the weeping tiles running through the soil beneath and around our home.

Research conducted over the last couple of decades has found that the majority or radon entering our homes is from beneath the concrete basement floor or sub-slab soil. Because modern weeping tiles are now plastic tubes running through this soil, they can be a main conduit not only for water drainage, but for radon gas. Since these terminate in the sump, it may be the number-one location where this dangerous gas seeps into our basements.

The simplest way to ensure this does not happen is to tightly seal the lid of the sump and caulk around the pit to ensure there is no communication between the air inside the pit and the home.

The second reason to seal the sump is the one you have cited. Because there may be a constant flow of fresh groundwater into the sump pit, and typically some standing water, it's a large potential source of moisture intrusion into our homes. When water is constantly present inside a living space, without adequate ventilation, relative humidity will increase inside the home.

Sealing the sump pit lid not only will prevent dangerous soil gasses from leaking into the home, but also water vapour. This is not only a concern in the heating season, as you suggest, but year-round. The air movement inside your plastic "dome" may be more noticeable in the winter, due to all the doors and windows being shut, but can occur even in warm weather.

Properly air-sealing a modern sump pit can be as simple as replacing or modifying the existing lid. Most newly installed pits have an integral weatherstripping, either on the underside of the lid or the top lip of the plastic sump. This, combined with metal bolts to secure the lid, is often enough to provide a proper seal except if there is something protruding through the lid.

Some homes have the ABS plastic discharge pipe sticking out through the top of the lid, while others wisely have this buried in the basement floor slab. If your pipes, and the wires for the sump pump power sources, enter the sump through this area they have to properly sealed, as well. Rubber gaskets or stoppers can be purchased that fit snugly into the holes of the lid while maintaining a tight fit around these pipes or wires.

If your current sump pit has a loose-fitting top or large openings around the discharge pipes or electrical cords, replacing the lid with a one that has the appropriate gaskets may be the simplest way to seal the area. And that's very important to prevent excessive humidity in the basement and protect against dangerous radon gas from the soil.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors - Manitoba ( Questions can be e-mailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at


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