Environmental Engineering and Contracting

Arroyo Geoscience Inc., Geotechnical Engineering, Glendale, CA
Arroyo Geoscience Inc., Geotechnical Engineering, Glendale, CA

Office: 818-967-5977

A-HAZ Engineering Contractor Lic. No. 1062092

Guide To Vapor Mitigation Systems

We often think of the foundation of a building as being absolutely solid. While this is the ultimate goal, it is not often true. Buildings can sometimes be faced with a phenomenon known as vapor intrusion. Contaminated groundwater or soil can emit dangerous gasses that find their way up and into structures.
Illustration of home with methane vapor from gound

What Is Vapor Intrusion?

We often think of the foundation of a building as being absolutely solid. While this is the ultimate goal, it is not often true. Buildings can sometimes be faced with a phenomenon known as vapor intrusion. Contaminated groundwater or soil can emit dangerous gasses that find their way up and into structures. Various factors can allow those harmful gasses and vapors to pass through a foundation and accumulate in a household or business. Vapor’s natural inclination is to rise, and follow the path of least resistance. Even perfectly poured concrete is naturally porous once dried. Combined with crevices that form around pipes, or sumps, and a structure is not naturally protected from rising vapor intrusion.

Factors that affect the amount of vapor entering a building:

  • Condition of the basement floor and foundation walls
  • Design and operation of heating and cooling systems
  • Outdoor temperature (a layer of frost traps soil vapor in the ground during the winter)
  • Barometric pressure changes and wind
  • Building construction (the type of foundation and tightness/leakiness of the building)

Depending on the types of vapor rising up through a structure, the situation could range from harmless to explosive. Some vapors are harmful to humans when concentrated in high enough densities, and some can even create a combustible environment.

Examples of VOCs (volatile organic compounds) found to have seeped into structures via vapor intrusion:

  • Methane
  • Gasoline
  • Diesel
  • Dry cleaning chemicals
  • Paint thinners
  • Solvents
  • Radon

What is Vapor Intrusion Mitigation?

Vapor Intrusion Mitigation is the process, either during construction or when retrofitting a building, of constructing a system to dispel these harmful vapors. The surest way to mitigate vapor is to clean up the source, but this can be a long and expensive process. Vapor Intrusion Mitigation systems buy time for the occupants of a building by dispersing the vapor away from the building, or actively negating the vapor. This mitigation can be passive, through structural considerations, or active, as in mechanisms or machinery that expeditiously remove harmful vapors. The methodology used is determined by a variety of factors, including expense, the breadth of the problem, and environmental factors of the building site itself. Some building zones require vapor testing to take place before construction can begin.

Passive Vapor Intrusion Mitigation Methods

Passive mitigation methods are materials or structures installed below a building to block or redirect the entry of vapors. Passive vapor intrusion mitigation devices are usually installed during construction, but they can be installed in existing buildings with a crawl space, if needed. Usually a passive system will be used in a space where the vapor intrusion isn’t a grave concern, as it is generally less effective than an active mitigation method.

Examples of passive vapor mitigation methods include:

  • Vapor barrier – Geomembranes or polyethylene plastic sheets that physically block the vapor from rising past. Alternatively, a spray-on rubberized asphalt emulsion can replace a geomembrane sheet.
  • Caulking or otherwise sealing openings created by pipe installation, sump usage, or even cracks in the foundation.
  • In the case of unfinished dirt floors at the foundation point, poured concrete can help baffle vapor rise.
  • Passive venting – As previously stated, vapors follow the path of least resistance. Venting, combined with a vapor barrier, gives the vapor a trajectory that leads to the outside air. These can be perforated collection pipes that let the vapor move laterally. Generally, this cannot be done retroactively, but rather must be done before construction.

Active Vapor Intrusion Mitigation Methods

Active approaches to mitigating vapor intrusion remove the driving force behind vapor migration, which is the higher pressure that exists underneath construction relative to indoors. By lowering the pressure beneath the sub-slab or passive barrier or inducing a higher pressure in the building, vapor flow is neutralized or reversed. This can be done by either lowering the pressure of the sub-slab, or raising the pressure in the building.

Examples of active vapor mitigation methods include:

  • Sub-slab depressurization (SSD) – The idea here is similar to the passive vent system, except a fan is used to draw the soil gas through the sub-slab venting layer rather than allowing it to rise naturally. This is more effective as outside pressure and wind conditions don’t challenge its effectiveness.
  • Submembrane depressurization (SMD) – This combines a passive vapor barrier with a depressurization directly in the soil, dispersing the vapor before it gets a chance to leave the soil.
  • Building Pressurization/HVAC Optimization – Typically used in larger buildings, this uses a building’s HVAC operations to actively overpressure the air inside the building, effectively creating an air seal that vapor can’t permeate. This requires the HVAC units to run even when the building is unoccupied but is very effective for mitigating vapor intrusion.

Do I need Vapor Intrusion Mitigation?

If there’s a chance that chemical vapor or soil gas is seeping into your home or business, testing your soil can determine whether it really is, and to what extent. A geotechnical engineer can test your soil for a variety of contaminants that can release vapor into your building.

Some building zones even require soil testing, as they lie on top of lands that were previously gas fields, or are known to have contaminants. California especially has a high number of zones where soil gas testing is required before construction can take place.
It is also not uncommon to add vapor intrusion mitigation mechanisms into new construction to protect against possible future contaminations, as being proactive is easier than retroactive when it comes to vapor mitigation. Once the structure is built, the mitigation options are reduced (but not eliminated!)
A qualified geotechnical engineer can also help design either a passive or active vapor intrusion mitigation system, and in some cases even install it themselves.
Vapor intrusion is a real threat and must be treated as such. Soil vapor is frequently odorless, and can cause harm before you’re even aware there is a problem. Investigate your vapor intrusion mitigation options, and protect the people who occupy your buildings.