Renovation & Design

Demolition likely not required to inspect for mould

Question: Over the past 15 years, we have noticed minor seepage from under our baseboard in our finished basement bathroom wall, through the spring thaw. This year, because of the wet weather, we had major flow. We are thinking of joining the lineup of homeowners requesting foundation repairs on the exterior wall, but with the overwhelming demand it may take a year. We wonder if we should tear apart our bathroom now to determine if there is any mould? Can we wait until the exterior wall is repaired?

Thank you, Diane.

Answer: Opening up a basement wall to inspect the area for mould growth may or may not be required, due to annual seepage, depending on the construction methods used to build the wall. If the wall is partially insulated with moisture resistant insulation, and a pressure treated bottom plate, then you may not have to worry. If it is insulated with the traditional fibreglass batt and poly method, especially if there are any gaps behind the insulation, mould growth is likely and repairs and mould removal will be required.

Mould may grow in our indoor living space if three essential components are present. The first is heat, which will be present, because we have to live in a warm environment to survive. The second is a cellulose-based food source. The majority of the building products used in our homes fit that criteria, including products from the environment like common dust and dirt. The final item is the key for successful growth of dormant mould spores, water. Mould spores are naturally occurring in our environment, from dozens of species, but most will remain inactive until they get wet. In relation to our homes, this moisture can come from precipitation, plumbing fixtures, human activity, or condensation.

The key to prevention of significant mould growth inside and outside our houses is ensuring that anything that does get wet, quickly dries. This applies to exterior components as well as anything inside the living space. Products on the outside that are not moisture resistant should be able to take a good soaking from a prairie thunderstorm, as long as they can easily shed that water once the sun comes back out. Inside, house components that can get wet, particularly in our kitchens and bathrooms, should not allow water to sit for more than a short time before it drains or evaporates. Also, areas where warm air can become trapped or stagnant, must have good ventilation to prevent cooling below the dew point. Otherwise, condensation on cooler surfaces and high humidity is another risk factor for mould propagation.

The area behind the wall covering in your basement bathroom may fall into the high-risk category, depending on how it is constructed, insulated, and air sealed. Because it is in a bathroom, proper ventilation with a good exhaust fan is also essential, or both surface mould and hidden growth is much more likely. Typical construction for many years has included using untreated softwood framing, fibreglass batt insulation, and polyethylene sheathing stapled to the warm side of the studs. This method is effective if the poly is fairly well sealed, over cavities completely filled with fibreglass, in a home with relatively low winter relative humidity (RH). If the poly is full of holes, or not sealed at the top between the floor joists, the fibreglass installed with gaps behind, or other poor-quality methods, warm air leakage into the wall cavity is highly likely. If that air has moderately high RH, common to bathrooms, or has no way to easily escape the cavity, condensation will often form inside the insulation or on the foundation wall. In really cold winter weather, that condensation will freeze and then melt when the weather warms. What you may have seen in previous years was this melted frost trickling out from underneath the bottom wall plate, or small amounts of seepage through the foundation walls.

If you don’t know what materials were used to construct and insulate the bathroom walls over top of the foundation, opening another area in the basement that is less obtrusive should be done before damaging the bathroom walls. If problems are noticed in those areas, or significant mould growth is visible, then more demolition is in order. If you find rigid foam insulation in the wall cavity, or better yet spray-on foam, you have less to worry about. Even if a layer of foam insulation is seen directly applied to the concrete and covered with batt insulation, that may be enough to prevent problematic condensation and frost.

If the water is indeed coming through the foundation walls, due to small cracks, rusted form ties, deteriorated damp-proofing, or damaged weeping tiles, then exterior excavation is certainly in order. That may not be fully determined without exposure of the majority of the inside of the foundation in that area. So, depending on the wall covering material and its condition, opening up the wall prior to excavation and repairs will help diagnose the cause of the leakage. That will also reveal whether you do have a mould issue to deal with, and help decide how to proceed. A small amount of mould should be within most homeowner’s skill level to clean, but a larger growth will likely require professional cleaning and remediation.

Determining how well your basement walls have been insulated and sealed should help you decide whether premature demolition is required to deal with mould growth behind. If the area is not able to properly dry after the most recent moisture intrusion, partial removal of wet or damaged wall coverings is warranted.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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