Submembrane soil depressurization systems are essentially the same as subslab depressurization systems, but without the cover of a concrete slab. The membrane shall be protected from wind uplift in accordance with locally adopted codes. Systems may be of suction pit or continuous ventilation mat design.
A membrane soil-gas retarder shall consist of a 8 mil or thicker single ply polyethylene sheet or other sheeting material of equal or lower permeability and equal or greater strength. Place sheeting to minimize seams and to cover all of the soil below the building floor. Retarders must provide excellent environmental stress crack resistance, impact strength and high tensile strength including additives to retard polymer oxidation and UV degradation. Where pipes, columns or other objects penetrate the soil-gas retarder, it shall be cut and sealed to the pipe, column or penetration. All seams of the membrane shall be lapped at least 12 inches (305 mm). Punctures or tears in the membrane shall be repaired with the same or compatible material.
Submembrane soil depressurization systems covering sand or other granular soils shall meet the requirements of Section C602.1, with the suction pits filled with 1-inch (25 mm) size gravel which shall be covered by 1/8-inch (3.2 mm) thick steel plate, 16 gage corrugated sheet metal, or equivalent sheets of other termite resistant structural materials, in compliance with existing and applicable codes.
Depressurization systems in sands or other granular soils and utilizing a continuous ventilation mat shall have at least 216 square inches (0.14 m2) of suction area per lineal foot and shall meet the following requirements.
Suction points shall be equally distributed as follows:
- The suction point should be centrally located along the length of each unconnected strip of mat; and
- Mat strips should be oriented along the central axis of the longest dimension of the crawlspace; and
- A minimum of one strip shall be used for crawlspaces having widths up to 50 feet (15 240 mm) [additional strips should be added for each additional crawlspace width of up to 50 feet (15 240 mm) width]; and
- The mat strip shall extend to not closer than 6 feet (1828 mm) of the inner stemwall at both ends of the building; and
- A separate suction point and fan shall be installed for each 100 feet (30 480 mm) linear length of ventilation mat.
Suction pipe shall be a minimum 3-inch (76 mm) diameter and shall be carried full size to the final termination point.
Suction fans must be capable of developing minimum flows of at least 100 cfm (.047 m3/s), at 1-inch water column (.2488 kPa) pressure.