What material beneath a slab is likely to give you the best success with a sub-slab depressurization system?

The best material beneath a slab for the effectiveness of a sub-slab depressurization system is clean aggregate. This is primarily because clean aggregate provides a highly permeable medium that allows for efficient airflow, facilitating the movement of air from beneath the slab to the ventilation system. A well-designed sub-slab depressurization system relies on the ability to effectively create a negative pressure beneath the slab, which is much more achievable with a material like clean aggregate compared to denser or less permeable materials.

Clean aggregate typically consists of crushed stone or gravel, which not only allows air to flow freely but also can help transport radon gas away from the slab and into the mitigation system before it can enter the living space. When air can move easily under the slab, the depressurization system is more effective in reducing radon levels indoors.

In contrast, materials like clay and granite tend to be denser and less permeable, making it harder for air to flow and for the system to achieve the necessary pressure gradient. Sand may also provide some permeability but is generally not as effective as clean aggregate because it can compact over time and lose its ability to facilitate airflow. Therefore, the choice of clean aggregate significantly enhances the performance of a sub-slab depressur