Figure 1. A properly encapsulated a crawl space

We know that your home is the most important investment that you make, and you care about not only your house structure and possessions, but also the people inside. So let’s get into a bit of detail about why you would want to encapsulate your crawl space.

There are 4 ways that moisture can enter your crawl space: Capillary action, bulk moisture transport, air transport, and vapor diffusion. You can learn more about those ways here. It’s usually easy to see obvious signs of a water intrusion problem in a foundation, whether they are basements, crawl spaces, or slabs.

In vented crawl spaces, the dominant source of moisture is bulk water, not water vapor from indoor or outdoor air condensing in the crawlspace. Water enters the crawlspace because of improper irrigation practices, ground slope, rain runoff, high groundwater tables, rain during the construction process, and leaks in plumbing.

Figure 2. Bulk water in a crawl space

These sources can be controlled by careful site grading, installation of drainage systems, proper foundation design and waterproofing, appropriate landscaping, and other measures. Most foundation water leakage or intrusion is due to either bulk moisture leaks or capillary action. Bulk moisture is the flow of liquid water. Capillary action occurs when water wicks or is absorbed into small cracks and pores in building materials, such as masonry block, concrete, or wood. Moisture can also be carried by soil gas into the home. Moisture may cause structural decay and can contribute to human health problems.

But water in its liquid state is not the only problem that houses can face. Water vapor can also be a source of damage. Unlike moisture in its liquid form, water vapor travels wherever air flows. Where there are air leaks, there are vapor leaks. Diffusion can also force vapor through materials and into places it shouldn’t be, such as wall cavities. Differences in vapor pressure and temperature are the forces that drive diffusion. Vapor diffusion moves moisture from areas of higher vapor pressure to areas of lower vapor pressure, and from areas of higher temperature to areas of lower temperature. Air movement is by far the most important mechanism for moving water vapor.

Water vapor causes problems when it is trapped within a building assembly, such as a wall cavity. When warm air touches a cold surface, the water vapor it carries can condense, turning into its liquid form, where it can cause damage to structural components. Condensation can also form in and on ductwork, especially when air conditioning cools duct surfaces that come in contact with humid air, such as in a vented attic or crawlspace.

Figure 3. Condensation on floor joists in a crawl space

Vapor movement through a building component can be impeded by use of a vapor diffusion retarder (vapor barrier). Some moisture in crawl spaces also occurs from soil vapor. To control water vapor from soil vapor in crawl spaces, it’s important to do the following:

  • Install 6-mil polyethylene across the entire ground surface.
  • Overlap all seams by 12 inches and tape.
  • Seal the polyethylene to the walls with pressure-treated wood strapping nailed at least 6 inches up the walls or to a height equal to ground level.

Figure 4. A diagram of properly installed vapor retarder

Sealed, conditioned crawl spaces offer non-energy advantages over vented crawl spaces that are worth considering, such as minimizing bulk water intrusion and pest intrusion.

MOISTURE CONTROL STRATEGIES IN CRAWL SPACES

Before you decide on a moisture control strategy, it helps to understand that moisture or water vapor moves in and out of a home in three ways:

  • With air currents
  • By diffusion through materials
  • By heat transfer.

Of these three ways, air movement accounts for more than 98% of all water vapor movement in houses and buildings. Air naturally moves from high-pressure areas to lower pressure areas by the path of least resistance, and generally that air moves through any available hole or crack in the home or building. And moisture transfer by air movement happens quickly. Carefully and permanently sealing any unintended paths for air movement in and out of the house is a very effective moisture control strategy.

Figure 4. Air transport of moisture

The other two ways that moisture moves in and out of a home or building — diffusion through materials and heat transfer — are much slower processes. Most common building materials slow moisture diffusion to a large degree, although they never stop it completely.

CONTROLLING WATER VAPOR IN A CRAWL SPACE

The laws of physics govern how moist air reacts in various temperature conditions. The temperature and moisture concentration at which water vapor begins to condense is called the “dew point.” Relative humidity (RH) refers to the amount of moisture contained in a quantity of air compared to the maximum amount of moisture the air could hold at the same temperature. The ability of air to hold water vapor increases as it warms and decreases as it cools. Once air has reached its dew point, the moisture that the air can no longer hold condenses on the first cold surface it encounters. If this surface is within an exterior wall cavity, the result is wet insulation and framing.

FIgure 5. Condensation

In addition to air movement, you also can control temperature and moisture content. Insulation reduces heat transfer or flow, so it also moderates the effect of temperature across the building envelope. In our climate (mixed-humid Atlanta, GA), properly installed vapor diffusion retarders can be used to reduce the amount of moisture transfer. In crawl spaces, insulation and vapor diffusion retarders work together to reduce the opportunity for condensation.

When looking for opportunities to improve the energy efficiency of homes and buildings, sealing and insulating of vented crawl spaces can improve the energy efficiency, comfort, and durability of the structure, particularly in climates with cold winters and/or hot humid summers. However, before any sealing and insulating of the crawlspace walls can take place, water drainage and moisture management issues in and around the crawl space must be dealt with to ensure that the crawlspace will be dry and remain dry once enclosed.

Controlling the intrusion of water and the movement of water vapor, air, and heat through the building envelope by proper design and construction of wall assemblies are major goals in the mixed-humid climate zone.

At DriTek Solutions, we are experts at foundation waterproofing, drainage solutions, foundation repair, and crawl space encapsulation. We can help you with all your foundation needs.

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