Why do I need a Methane Vapor Mitigation System?

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Methane Mitigation Systems include a series of redundant design approaches to prevent the migration of gas vapors into the interior space of a structure. One of the main components to Methane Vapor Systems is the Methane Gas Vapor Barrier.  This Methane Membrane is installed below the foundation of a structure, and acts as a seal for the concrete slab.

Why can’t my concrete slab prevent methane vapor migration? 

Any vapor depending on the size of the molecule can diffuse through a surface. Diffusion is the movement of a substance through a surface, typically driven by pressure and concentration gradients. A relatable example would be a helium balloon deflating over a period of time.  The helium pressure and concentration is significantly higher then the atmosphere, and the size of helium molecules are extremely small.  So, naturally the helium wants to migrate through the balloon surface to reach the atmosphere.  Over a period of time, helium finds its way through the small crevasses of the balloon material, until ultimately a majority of helium is diffused resulting in a deflated balloon.  This phenomenon is called Mass Diffusion.  The same concept of mass diffusion of helium through a balloon is applied for Methane Gas through a concrete slab.  Besides the fact that concrete slabs can have cracks which are open pathway for gas vapors, over a period, the methane gas can diffuse through the concrete slab introducing explosion risks inside a structure.

Methane Mitigation Designers implement a material that is installed beneath the foundation of a structure.  This material is designed specially to reduce the amount of diffusion of methane gas vapor.  The reduction of diffusion is driven by several variables including Methane Gas Vapor Pressure (Industry standard Measures in Inches of Water), Methane Gas Construction, typically measure is parts per million (PPMV), Methane Barrier Material and Vapor Barrier Thickness.

Referring back to the helium balloon example, if the balloon membrane was thicker, it could reduce the diffusion rate accordingly, also if a different material was used, it could reduced the helium Gas Migration.  Similarly, Methane Barriers are designed disitnvtly to minimize the diffusion rates of methane gas.  The thickness of the barrier can change accordingly for projects and anticipated risks.  This proves that that not all methane barriers have the same diffusion prevention capabilities.  

Vapor Gas Migration Engineering Comparison

The study Methane Mitigation is in line with the study of heat transfer and the thermal sciences. The term used to measure to diffusion capability of a material is termed the coefficient of Mass diffusion. Mass Diffusion values can be calculated using Fick’s Law which has a close analysis methodology to Fourier’s Law of Conduction for Heat Transfer. 

One may ask what values of mass diffusion or appropriate for use? The Los Angeles building and safety and the DTSC has establish safety values for Mass Diffusion through Methane Barrier’s.  All approved Methane Barriers need to meet the standards to ensure that they can sufficiently prevent the diffusion of methane at a reasonable rate. This ideology and theory is only appropriate if you have a perfectly sealed foundation to the sub surface. For this reason, it is important to emphasize that a licensed Methane Mitigation Contractor must install vapor mitigation systems to ensure that the seal of the foundation is perfect. Any loose ends holes or obstructions can create a path for methane to intrude into a building without the protection of the Methane Barrier.

Sub Slab Depressurization System

The sub slab vent system is another redundant methane mitigation design approach that is implemented in Methane Mitigation Construction.  As previously noted, Mass Diffusion is not only a function of concentration differential, but it is also a function of pressure differential. The larger the pressure and concentration gradients, the higher the mass diffusion. This is where the subslab vent system comes to help. Implementing a sub slab vent system allows the depressurization of the subfloor to ensure that the pressure differentials are minimal, this reduces the mass diffusion and also introduces relief in the case of an obstruction on the membrane.  The Sub slab vent system allows the methane gas to route through vents rather than through the structure. Methane vent pipes consist of perforated pipe along with a gravel blanket below the surface of a building. These systems need to be designed by a licensed professional engineer who has experience in the design of methane depressurization vent systems.

Vapor Mitigation Design

The details utilized prepared on the Vapor Mitigation Design  is of upmost importance, often time consultants use standard details that are prepared by manufacturers or city officials.  This is poor practice that leads to issues during construction for having lack of details and direction during the Vapor Mitigation Construction phase. 

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