Renovation & Design

Battle mould by eliminating moisture


I am wondering if you could help me out with directions on how to deal with mould under my cottage. I use the cottage year round. My cottage is built on four foot piers around the exterior and a few interior supports. The cottage is on a small hill and on some rock and soil, so the dirt in the crawl space is always damp. Over the last few years, I have poured a small footing around the exterior of the cottage. This helps to keep out the critters and winterizes the space. I framed up a knee-wall to the underside of the floor joists, which was then insulated with spray foam. I also put in a few vents for the summer and a baseboard heater.

The mould in the sealed space is so bad this year I need to remove it or spray it immediately. The uneven, dirt floor is always wet and I need help with controlling the mould. The moist dirt will always be there, due to the hill on which the cottage is built and the grade, which is sloping behind the cottage to the cottage wall.

Is poly on the dirt a good idea, or an HRV to control the air in the crawl space, should I turn on the heat all year or add more vents?


Help! Jim Bathe



You have sent me a perfect inquiry for the subject of my annual cottage crawl-space article. This is one of the most common questions I receive, but yours has the twist of the mould buildup due to this issue. You appear to have taken the proper steps with the enclosure of your crawl space, but there a couple of omissions that will surely lead to excess moisture and mould growth.

By now, almost every homeowner knows excessive mould growth in confined spaces is due to an excess of moisture. Controlling or minimizing water in your enclosed crawl space is the key to preventing the mould, as you are obviously aware. While it appears you have done a proper job of enclosing the area beneath your cottage, and using excellent insulation on the inside of the knee-wall, you forgot about the dirt floor. Whether your place is on a hill, or a flat grade, excess moisture can be drawn out of the soil beneath the building if you heat the crawl space in the winter. The soil contains moisture and heating the air in the totally enclosed space will draw the moisture into that area. The summer vents are also an excellent idea, but the real problem likely occurs during the heating season, when those are closed.

You are absolutely correct that installation of polyethylene sheathing over the entire dirt floor will help prevent this moisture being drawn into the crawl space. To be effective, this sheathing should be at least six-millimetre poly, cover the entire floor and footing and be caulked around all protrusions and sealed to the insulated knee-walls. This is more easily accomplished before foaming, but should still be possible with a little extra effort. Once installed and sealed, putting a few centimeters of sand over the poly will keep it on the soil and prevent damage when it is walked on. While this step may help with a substantial amount of the moisture in this area, it may not be enough.

Because your crawl space is situated on a hill, as you have noted, water will often seep into the soil underneath, no matter what you do. Because of this, you will have to take extra steps in proper water management outside the building. Do you have complete eavestroughs and downspouts on the building? Are they draining to downspouts on the low side of the structure, only? Are the downspout extensions channelling the water far enough away from the cottage to prevent seepage into the crawl space? Addressing all of these issues successfully will certainly help with the amount of moisture entering the soil under your cottage. Finally, building the soil up near the middle of the high side and gradually grading it away to the corners may also help divert excess water around the crawl space, rather than inside.

If you already have these items in place and in good working order, and still get so much water under the building that mould growth is troublesome, mechanical systems may also be required to dry out the space. The simplest thing to install may be a sump pit and pump. Before installing the poly over the dirt floor, digging a hole and installing a perforated plastic sump, with properly plumbed pump, may help get rid of excessive ground water. To aid in the collection of this water, installing perforated weeping tile both outside the high side of the crawl space, and radiating out from the pit underneath the cottage would be a good idea. While the weeping tile may be difficult to install at this stage, it may be the ultimate answer if the sump pit and pump, combined with the poly, don't do the trick.

Maintaining a dry crawl space is difficult enough on a well-drained, flat prairie lot, but can be very tricky on sloped property like yours. Installation of a proper layer of poly over the floor and proper water management outside the building may be enough to accomplish this task, but installation of a sump pit, pump, and weeping tiles may be a final solution if the other measures fall short.


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 emailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at


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