What is Negative pressure
Definition of Negative pressure in Construction
Negative pressure is used in the construction industry to relate to the differential of pressure gradients between two different areas of a project or assembly. Negative pressure is an area that exhibits a pressure within the space, of less than the adjacent space. For example if a stairwell is positively pressurized, this means that a fan is forcing air into a confined, enclosed area such as a stairwell, and causing a positive pressure to build up in the stairwell. This positive pressure will cause a negative pressure in the adjacent hallways or foyers immediately outside the stairwell. Negative pressure and positive pressure are very important characteristics of the environment on construction projects, due to smoke transmission and control. If there is a fire in an office or residential unit in a large complex, the smoke that is produced from the fire must be contained within the area of the fire. For this to occur, the hallways must have a greater air pressure than the adjacent office, or residential unit, therefore keeping the smoke from entering the hallway. Positive pressure, as well as negative pressure is carefully engineered in large commercial and residential structures to properly engineer the distribution of smoke and fire throughout the project. Negative pressure is also important with under-slab assemblies, which are engineered to keep any environmental gases or fumes from entering the structure. A negative pressure must be maintained with a series of perforated pipes and porous gravel under a slab on grade, which are connected to a fan assembly that is drawing the air out of the system, and establishing a negative air pressure under the slab. This negative air pressure keeps any gases from elevating into the structure, due to the negative verses positive pressures encountered.