I always read your articles in the newspaper and find them very informative and relative to our environment in Winnipeg. I was hoping you could help me with a problem.
Our 2-year-old bungalow has a walk out lower level with a structural wood floor. The crawl space under the wood floor is easily accessible through a hatch door. The last 2 Springs, the sand in the crawl space gets very wet around the sump pump, which is in the middle of the crawl space, and extending in a circle roughly 25 feet in diameter. The sump pump seems to be working properly. Last winter, this area dried up completely, but once again this year it is wet. It smells musty, mouldy and I am concerned about the long-term implications of this situation.
Construction of homes in Winnipeg with crawlspaces and structural wood floors is becoming a more common way of building, in the last decade. This provides a much more comfortable living space in the lower level, or basement, of the home, which has major benefits, but you have identified one of the possible major problems with this method. Moisture intrusion into homes is the number one complaint of homeowners, according to various studies. Unfortunately, the likelihood of this occurring is increased with the presence of a crawlspace below the basement floor structure, as in your home. I will attempt to explain some of the reasons that this may be occurring and provide some items to check to help minimize or eliminate this problem.
For those who are not familiar with structural wood floors in basements they are simply a wood, or manufactured wood composite, floor structure that replaces the concrete floor slab in a conventional home. The floor is similar in construction to the main floor with supporting beams/piers and columns. The benefit of this design is to eliminate the cracking and heaving commonly seen in concrete floor slabs. This also allows for heating of the area below the floor and providing a warm lower level floor as opposed to a cool and often damp concrete basement floor. To accommodate this style of floor structure, the area below must be excavated with an accessible crawlspace installed. The purpose of this crawlspace is to prevent moisture damage to the wooden floor structure as well as allowing room for installation of heating ducts, plumbing drains, and a sump pit and pump.
In your home, the soil in the bottom of the crawlspace should be covered by a thick sheet of polyethylene that is caulked at the seams, the air-vapour barrier on the foundation walls, and to any protrusions. This is very difficult to accomplish perfectly and often is poorly done. This poly is normally covered with a thin layer of sand for protection, which is what you are experiencing as the damp area. There may be several reasons for the damp sand above the poly, but the simplest explanation may be that it is poorly sealed or has several holes or damaged areas from traffic during construction. This can be visually checked by going in to the crawlspace and brushing away several areas of the sand to inspect for damage or missing caulking. If this is the case, the moisture from the damp soil in the crawlspace is evaporating into the area and making the sand wet.
The exterior of the home should be inspected to ensure that the downspouts from the eavestroughs are also extended well away from the foundation walls and the grading sheds rainwater away from the home. This is normally the largest source of moisture entering the crawlspace and ensuring proper grading and runoff is essential. With a walk-out style of basement, like yours, this may be complicated by the natural slope of the land and the depth of the crawlspace below grade. You must try to eliminate areas that channel water toward the crawlspace.
Regardless of grading, the sump pit and pump in the crawlspace should be designed, along with the weeping tile system, to collect enough moisture from around the home to ensure a dry crawlspace. This may not be the case if the weeping tile is inadequate or improperly installed, or the there is a problem with the sump pump. The lid to the sump pit should be opened and the inside inspected with a flashlight. The sump pump should be installed to come on when the water in the pit is below the bottom of the weeping tile terminations. If the pump is too high, or the float switch improperly mounted, the weeping tile may not be draining properly and may allow excess moisture to remain in the soil around the home and in the crawlspace. I saw this situation recently in a home under construction that was nearing completion. Once the float switch was lowered, the weeping tile drained properly and the moisture around the pit quickly disappeared.
The other possibility is a sump pit that is too small or a pump that cannot handle the volume of water for the size of house. In this situation, a larger pump or second sump pit and pump may be the answer, but this should be the last course of action to attempt.