Groundwater moisture vapor exposure over an extended period of time in a crawl space can cause significant damage to wood frame construction. One way in which water migrates into the crawlspace is through seepage. Seepage is the flow of water through the soil. The figure below shows a soil / foundation cross-sectional view with a superimposed grid called a flow net.
The blue lines are called flow lines or stream lines. They represent the path of the water flowing through the soil. The space in between each adjacent pair of flow lines is called a flow channel. The red lines are called equipotential lines. They represent places in the soil at the same head. In other words, if a series of piezometers were installed along an equipotential line, the water would rise to the same level in all of these piezometers. Flow nets can be used to calculate the rate a flow through the soil. So, in this case, it is the rate of flow of water from outside the foundation wall into the crawlspace. The two primary factors affecting flow rate are hydraulic conductivity and the difference in the head from outside to inside.
Hydraulic conductivity is a proportionality constant relating speed or velocity of flow to the hydraulic gradient. What this essentially means is that at a given head difference between two locations a soil with a high hydraulic conductivity will have a high velocity of flow through it while a soil with a low hydraulic conductivity will cause the water to flow through it more slowly. Generally, this property of the soil is fixed as the soil present on the site is typically utilized as is in most residential construction. Therefore the flow rate of water from outside the foundation to inside the crawlspace is mostly influenced by changes in head difference.
The head difference in this situation is the pressure and elevation head outside the foundation wall minus the pressure and elevation head inside the crawl space. Therefore as the pressure difference between inside and outside increases or the elevation difference between inside and outside increases, the head difference increases, and consequently the rate of flow of water into the crawlspace increases given the same parameters.
Pore water pressure develops and increases outside the structure when the soils become saturated and in particular when water accumulates on the surface of the ground. Consequently, to reduce the pressure difference between outside and inside, the accumulation of water adjacent to the foundation wall needs to be avoided. This is one of the reasons why building codes like the International Residential Code (IRC) require grading away from the structure to drain surface water away. The 2012 IRC, for instance, requires a minimum fall of 6 inches within the first 10 feet (R401.3). Additionally, roof system rainwater runoff can cause the accumulation of water in this area if the downspout deposits the runoff in this region. This is one of the reasons it is good practice to have the water carried away by non-perforated surface or subsurface downspout extensions or diverters.
Foundation Dampproofing and Waterproofing
This accumulation of water behind the foundation wall also creates hydrostatic pressure that can force the groundwater into and through the porous concrete foundation wall materials or the footing / foundation wall joint into the crawl space. This is often visible as water staining or efflorescence on the inside face of the foundation wall and water accumulation in the inside of the footing trench. This can be particularly problematic as water that migrates through the wall can potentially accumulate on top of the vapor retarder which will then effectively hold the moisture in instead of keeping it out of the crawl space. There are a number of code compliant methods called for to dampproof or waterproof the exterior surface of the foundation to keep this from happening. However, sometimes the application is minimal or discontinuous in areas and can have a design life much less than that of the structure. One potential solution to this issue is the attachment of the vapor retarder to the inside foundation wall face at an elevation above which the water might migrate through the wall. This keeps the water below the vapor retarder.
Many times during residential construction when the site is being excavated in order to install the footings and the foundation wall, the elevation in the crawlspace ends up appreciably lower than what the finished outside grade will be once construction is complete. This elevation difference also drives the migration of groundwater into the crawlspace. In areas with a high water table or poor surface drainage characteristics, the 2012 IRC states that the grade in the under-floor space shall be as high as the outside finished grade or equipped with an approved drainage system for this reason. Although it would be good to minimize or eliminate this elevation difference during excavation another alternative that can be used during the initial construction or as a remediation technique is to add a layer of gravel or granular material over the ground surface in the crawl space and under the vapor retarder. The granular material provides not only a way of raising the surface elevation in the crawl space but also a high void ratio or high porosity material that can contain a larger volume of water and does not provide for capillary action. This keeps the water in and below this layer and in particular below the vapor retarder which rests on top of the granular material. It is important that the granular material not be too angular and consequently cause damage to the vapor retarder.
Finally, another helpful system component for eliminating or limiting the amount of water seeping into the crawl space is a perimeter foundation or footing drain. This usually includes a continuous run of perforated pipe or tile with an open end that runs out to daylight laid adjacent to the footings. The pipe is sometimes wrapped in a filter fabric sock and then lain in and encapsulated with clean, coarse rock. Also sometimes this rock is wrapped in a filter membrane. The filter membrane keeps fine soil particles from collecting and clogging the flow of water into and through the drainage system over time. These systems, when properly installed, help keep groundwater from accumulating outside the foundation wall and intercept and channel away water that might otherwise have seeped into the crawlspace.
As can be seen, a number of important details need to be considered when attempting to limit the amount of groundwater migration or seepage into the crawl space including exterior surface grading or slope, roof system rainwater runoff channelization, height of the water table, type of soil, foundation wall dampproofing or waterproofing, elevation difference between the interior and exterior of the foundation, and the perimeter foundation drain system.