Renovation & Design

ASK THE INSPECTOR: Moisture under subfloor causes mould smell

QUESTION: I'm hoping you can give us some help with our basement floor. Our home is in North Kildonan and was built in the mid-'80s. It is a bungalow in excellent condition with an unfinished basement.

We have decided to partially finish the basement and my husband started laying down Dri-Core floor tiles as a base, prior to installing flooring. We had heard that this is the solution to providing good air flow underneath, as well as protection from moisture and cold on the surface.

Since laying down a small area last summer, at the bottom of the basement stairs, we started noticing a musty mildew smell. When my husband lifted a section of the tiles, along with sow bugs and spiders there was a distinct mould smell and the presence of white powder, effervescence, around the base of the walls. Before we continue with installing any flooring we would like to know if we are doing something wrong here.

We have a new furnace, a top-of-the-line Lennox, that was installed 18 months ago. We keep a dehumidifier running all summer long, together with the air conditioner, but no other air modification. Any help you can give us would be greatly appreciated.

Thank you in advance. Judi Parry

ANSWER: The answer to your question may be difficult to pinpoint, but the goal is to stop water seepage into your basement. While it may not be readily apparent, the cause of the smell is from mould that occurs due to moisture under the subfloor. To eliminate this, you will have to find out where it is leaking and stop the water intrusion to prevent further occurrences.

The subfloor material you have installed in your basement is a good product to keep your flooring dry and make the floor warmer, but it has limited abilities to manage water. The slightly elevated plastic surface on the underside of this material will provide a small air space to allow some air movement, but may not be adequate if a moderate to large amount of moisture leakage occurs.

If small amounts of moisture get underneath, due to condensation or minor seepage, the airspace may provide enough airflow to help this moisture evaporate. If more frequent or larger wetting occurs, this water may not be able to fully escape this small cavity. When that occurs, dirt and dust or other debris under the subfloor could become a substrate for mould growth.

The efflorescence you are seeing when you lift the individual subfloor tiles is caused by minerals leaching from the concrete. This occurs from water that either leaks under the subfloor, or from moisture within the concrete slab, itself. These salty crystals themselves are harmless, but they are a sign of potentially larger issues.

The musty smell you notice is likely caused by mould growth, which may be forming on debris under this area or in the OSB sheathing of the subfloor itself. While the plastic sheathing on the underside may help prevent this, it will not eliminate the possibility if water sits under the subfloor panels for an extended time. Adding more ventilation may be the first step to fixing this issue.

Most manufacturers of this style of subflooring material requires additional vents to be cut in the surface of the subfloor at various intervals. This may be required mainly around the perimeter of the completed subfloor, or along partition walls. These openings can be covered with typical heating registers or other vented covers, but they are essential to allow proper airflow below the subflooring.

I would recommend cutting several of these vent holes, as per the manufacturer's specifications, to improve airflow under this surface. While this may help prevent problems in the future, looking outside the foundation is the next step.

Many seepage incidents into basements are cause by a combination of poor exterior grading and drainage. If the soil has eroded and appears depressed outside the foundation in the area where you're seeing your problem, some landscaping is required. Building up the soil in this area to provide a small slope away from the foundation is the answer. Planting grass seed, sod or low vegetation after the soil installation may also help prevent further erosion.

Since this is happening near the basement stairs, there is a chance that there may be a large hidden void underneath a landing or stairway outside this area. If you have a back or side door at the top of these stairs, that may be the case. Again, the solution to the issue should be to dig under this area and fill the void to prevent further seepage.

Checking to see that eavestroughs are in good condition and clean is the next task. Also, if downspouts are dumping near the foundation, extensions should immediately be installed to divert the water one to three metres away from your house. The majority of moisture intrusion I see in basements occurs near corners or other areas where the downspouts are terminating.

The plastic-coated, tongue-and-groove subflooring material you have chosen is a good product for keeping flooring off concrete basement floor slabs, which has several benefits, but may not be effective if the floor is consistently wet. It will create a thermal break between the flooring and the cool concrete, preventing wicking of moisture into the flooring, and will also make the surface warmer. Installing additional vents, particularly around the perimeter of the floor, and exterior water management is still necessary to help prevent significant seepage that can allow mould and damage to occur to the subfloor.

If the subflooring continues to smell, or become mouldy, after addressing these issues, a more costly foundation waterproofing repair may be required.

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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