Renovation & Design

ASK THE INSPECTOR: Solid flooring materials best for basement

QUESTION: Can I ask for your input on an issue in our 1950's Winnipeg bungalow?

We are renovating our basement and will be installing carpet in some of the rooms. The basement floor is a concrete slab. The construction's a bit unusual -- before the floor was poured, the substrate was covered with a tar paper. The interior partition walls were then built by suspending them about four inches above the ground, baseboard added and then the concrete floor was poured.

During renovation, we tested for Radon and found the levels a bit high. To reduce the levels we undercut all of the interior walls, laid a 15-mil poly into the spaces and used adhesive to attach it to the sides of the concrete slab. We accessed all spaces along the exterior of the building and used a polyurethane foam to seal.

The results were a reduction in the radon levels below Health Canada guidelines. We added an HRV for good measure in this hot water radiant-heat home and further reduced levels to around 100 bq/m3.

When we took the old carpet up, we noticed that all of the concrete floors were covered with a sheet of plastic, presumably to reduce moisture from getting into the underlay and carpet. There must have been tiles on the floor prior to the carpet as the floor has black tar-like residue on it. Before we had estimators in to quote the work, we removed the old carpet along with the plastic.

Two companies quoted on installation of a new wool carpet. Company A included replacing the plastic, "to avoid any black material coming through the underlay". Company B made no mention of laying plastic before installing the carpet.

Should we be replacing the plastic before installing the rubber padding and wool carpet?

Thanks, Joel Kay

ANSWER: Replacement of flooring in basements can be a tricky subject, due to the possibility of moisture or dampness in this area. Some of the discrepancy between flooring installers may be due to a lack of familiarity with your in-floor heating system, which may help prevent condensation common with carpets on basement concrete floor slabs.

I don't think that putting polyethylene sheathing under a carpet is a good idea and will offer some alternatives for you to explore.

I'm not a proponent of soft floor coverings in any basements installed directly over concrete slabs. This is because I've seen far to many cases where the carpet has become wet or damp from water sources or condensation. Even in a relatively dry basement, the concrete floor slab may be cool enough to cause condensation from slow-moving air.

If this condensation is trapped in the carpet fibres, mould growth is a strong possibility. Many times, a damp, musty smell associated with basements can be due to old floor coverings. With your in-floor heating system, this is less of a possibility, but it could still occur if there's a water leak.

Putting plastic sheathing underneath a carpet may help prevent some condensation, but may also trap moisture that develops underneath. It may have helped prevent staining from black adhesive used with the previous flooring, but it would have been a problem if water had seeped underneath the poly from minor basement seepage, a leaking water heater or washer, sweating water-supply pipes or sewer back-up. All of these things could have happened at one time or another over the many decades since your home was built.

A better alternative to carpet, especially one made from natural fibres like wool, is to use flooring that is moisture-resistant. Solid vinyl or ceramic tiles, which both come in multiple styles and applications, may be a better choice. However, since there's very little flexibility to ceramic flooring, it could crack or become damaged if your basement floor slab shifts or moves seasonally. For that reason, vinyl flooring may be a better alternative, due to the excellent flexibility if floor movement is an issue.

If you're dead-set on carpeting your renovated basement, installing a subfloor would be a much better alternative to a typical underpad and poly. This would not only prevent bleeding through of the old floor adhesive, but would also remove the carpeting from contact with the concrete, eliminating condensation as an issue. Even with a heated floor, condensation could still occur in areas where the heating pipes are poorly distributed or air circulation is poor.

The subfloor should be made from moisture-resistant material, such as pressure-treated plywood or other specialized subfloor sheathing. This could be installed directly over the concrete to ensure maximum headroom, or with treated "sleepers" to help level an uneven floor.

So, while solid flooring materials are preferable to fibre-based coverings in a basement, a proper subfloor with your heated floor slab means that carpeting could be relatively problem-free in your home.

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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