• cobylamarche


At its core, the effectiveness of a sub-slab depressurization system will depend on the connectivity of the sub-slab material. Imagine trying to draw water through a barrier made of marbles versus a barrier made of Play-Doh. It will be much easier to move the water through the marbles. In sub-slab depressurization, marbles are like gravel and Play-Doh is like native soil. Contemporary construction practice is to construct floor slabs on a layer of gravel. Historically floor slabs may have been constructed on native soil (or other fill).

The sub-slab may also be broken up into compartments by footings. Trying to draw soil gas from a sub-slab separated by a footing is like trying to drain your neighbours pool by pumping your own pool.

Does your home have marbles or Play-Doh under the slab? Are there multiple compartments? A diagnostic investigation can help to answer these questions. The diagnostic investigation is essentially a pilot scale sub-slab depressurization test. A high amperage vacuum is used to draw soil gas from a test hole. Pressure is monitored at other locations across the floor slab. The degree of pressure change and the flow of soil gas from the vacuum tell us about the connection beneath the slab. This data is then used to determine the type of fan that is likely to achieve the mitigation targets and if more than one suction point will be needed.

In the absence of this data, we are only guessing and hoping.

Vacuum connected to test hole.

Micro-manometer measuring sub-slab pressure.

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