You have noticed some of these signs of stress:

• Cracks in walls
• Cracks in floors
• Cracks in the ceiling
• Doors out of square
• Windows out of square
• Slopes in floors
• Cupping of trusses
• Gaps under baseboard

Wet Climate Scenario

Arid Climate: Four Mechanisms of Moisture Accumulation

The above scenario is what most people envision when foundation problems are brought up. In a wet climate, the soil is already saturated and unlikely to absorb additional moisture unless there are plumbing leaks. If a drier cycle is encountered, the edges dry out… and if they are expansive clays, they will shrink and settle…. Requiring underpinning as shown above.

However, in drier climates, the paradigm flips. Extremely dry soils attract moisture from repeated cycles of poor drainage events. Below, I outline 4 mechanisms of moisture accumulation under a foundation.

1. Electro-osmosis

Clay soil carries a negative ionic charge, water a positive one. They are electrochemically attracted. Water migrates toward the drier soil beneath the slab, with each wetting event cumulatively increasing the effect that does not fully reverse when edge drying occurs.

2. Temperature Differential

Air-conditioned interiors create soil temperature differentials under slabs of up to 100°F compared to exterior soil. Research by Dr. Claudia Zapata at Arizona State University has documented water migrating toward cooler soils, directly pulling moisture inward from the perimeter.

3. Stack Effect 

As hot air rises and exits attic vents, it is replaced by air drawn upward from under the foundation. Water vapor follows this path, condenses on cooler interior soils, and accumulates as liquid because it can no longer move farther as a gas, becoming trapped beneath the slab and building up over time.

4. Thornthwaite Effect 

Upward movement of water vapor from the water table and underground sources through evapotranspiration, demonstrable by noting condensation accumulation under buried plastic sheeting.

Profile of Dome Heave: Moisture Migration

These mechanisms drive moisture toward the same destination: the cool, shielded center of the slab. The perimeter soil cycles between wet and dry because it is exposed to evaporation and surface drying, limiting net accumulation, while the center has no escape pathway and continuously receives moisture drawn inward from all directions. 

Over time, the center clay accumulates moisture, swells, and pushes the slab upward from below, while the perimeter remains relatively stable, producing the characteristic dome pattern of interior floor heave that is the signature foundation problem in arid, expansive-clay environments like the Phoenix Valley.

Options to Remediate

Cutoff walls

Vertical or horizontal barriers can be effective at slowing additional moisture from entering beneath the foundation. However, the slab is a dome heave, meaning the center is already wetter than the perimeter. Drying out the perimeter may dry it more than the center, exacerbating the differential.

Grading and drainage

Improving grading and drainage are almost always recommended. However, like barriers, if there is a dome heave, you will be drying out the perimeter where it is already drier.

Removal and replacement of floor slabs are so invasive and expensive that they are almost never done.

Excavation lowering

Excavation lowering is a successful technique developed by the author to address extreme edge lift in post-tension foundations. It’s not really appropriate for dome heave problems.

This is a typical example of the basic thinking of non-PE analysts: raising the perimeter creates problems with exterior-adjacent concrete transitions. Additionally, it endangers existing underground plumbing. (The opposite performance of raising sunken perimeters.)

Understanding Active Soil Depressurization

Stage 1: 

The laminar airflow driven by pressure differential moves through the porous gravel or aggregate layer beneath the slab, contacting the moist clay surface below. 

The moving air carries a vapor pressure deficit relative to the wet clay, so moisture evaporates from the clay surface into the airstream and is evacuated outward. 

This is the rapid surface-drying phase, relatively fast because the clay surface is hydraulically connected to moisture below, and the airstream continuously removes vapor as it forms.

Stage 2: 

As the surface clay dries, a suction gradient develops between the now-drier surface zone and the still-moist clay deeper in the profile. Capillary forces drive liquid water upward through the connected pore network from the wetter deeper clay toward the dried surface zone, wicking against gravity because capillary suction exceeds gravitational potential. 

This replenishes the surface with moisture drawn from depth, temporarily maintaining the drying rate but progressively depleting the deeper layers.

Stage 3: 

The freshly wicked moisture arriving at the surface is now exposed to the same laminar airflow that dried the original surface, and the evaporation cycle repeats. 

Each cycle depletes moisture from progressively deeper clay layers, advancing the drying front downward over time. The critical consequence is that this is a self-propagating rather than self-limiting process. 

Each drying and wicking cycle reaches deeper into the clay profile, continuously steepening the suction gradient that drives the next cycle, until hydraulic connectivity to deeper moisture sources or the water table sustains the process indefinitely.

Stage 4: 

As the progressive drying front advances deeper and cumulative moisture loss increases, the clay begins to shrink volumetrically beyond its plastic limit, and desiccation cracks form. 

These cracks open preferentially along planes of weakness in the clay fabric and propagate downward from the dried surface zone into the still-moist clay below. The cracks are transformative because they fundamentally change the drying geometry. What was previously a slow vapor diffusion process through the intact clay matrix becomes direct airflow penetration into the crack network, exposing fresh moist clay surfaces at depth to the same laminar drying mechanism that operated at the surface in Stage 1. 

The drying front no longer advances slowly by diffusion but jumps discontinuously to the crack-tip depth, and the entire three-stage wicking and evaporation cycle restarts simultaneously from these new, deeper surfaces. 

In severe cases, crack networks interconnect, creating a three-dimensional pathway system that enables drying and desiccation to reach depths far beyond what diffusion alone could achieve. In the Phoenix Valley, expansive clays, this crack-facilitated drying can ultimately extend several meters below the surface, creating a deep, persistent moisture deficit that drives the long-term center-heave dome pattern described earlier.

As well as Desiccation – Illustration of Desiccated Soil and How It Facilitates More Drying

Illustration of a Moisture Management System

Differences with the Radon System

• Intakes deliberately bring in drier air at strategic locations.
• Location of the suction pit at the apex of the dome.
• More air flow: optimized to move 4 to 5 times more air than a radon system. 
• Feedback sensor and auto-switch control for drying.

After an exhaustive search, I have found supporting literature that directly demonstrates moisture removal in radon systems… essentially the predecessor to the MoistureLevel System. 

Bearing in mind that the MoistureLevel system is designed and optimized to specifically remove moisture in the clay soils that are aggravating expansive soil dome heave.

The Key Finding: Peer-Reviewed Evidence of Moisture Removal

Boardman, C.R. and Glass, S.V. “Basement Radon Entry and Stack-Driven Moisture Infiltration Reduced by Active Sub-Slab Depressurization,” Building and Environment, Vol. 85, 2015, pp. 220–232, USDA Forest Products Laboratory

This peer-reviewed study directly applied active soil depressurization and showed it reduces both radon and soil moisture infiltration simultaneously. The study focused specifically on the neglected topic of soil moisture infiltration, which affects not only occupant health but also building durability, demonstrating that stack effect-driven air exchange correlates with soil moisture infiltration and that active sub-slab depressurization interrupts both mechanisms simultaneously. USDA

This is the single most important citation for MoistureLevel validation because it is independent, peer-reviewed, and directly measures moisture removal as a primary outcome of the same technology, not just as an incidental observation.

EPA Exploratory Study: Incidental Moisture Removal Documented

United States Environmental Protection Agency  Exploratory Study of Basement Moisture During Operation of ASD Radon Control, EPA Research Document

This EPA study specifically investigated moisture response during active sub-slab depressurization operations, monitoring over 115 parameters at each house and documenting that the systems depressurize and draw air from the soil and materials surrounding the basement, directly reducing moisture levels as a documented, measured effect of system operation. EPA

Systematic Review and WHO Evidence

A systematic review published in PMC documented that the active sub-slab depressurization system is the most effective radon mitigation technique reviewed across studies from Europe, Australia, and North America, with WHO evidence showing active SSDS reduces exposure by up to 98%, and that any mitigation system used should reduce radon levels, control moisture, and eliminate mold in basements. Active SSDS, together with a radon-proof membrane, has been shown to serve all these purposes simultaneously. PubMed Central

Sub-Slab Pressure Field Mechanics: Directly Supporting Moisture Transport

Bonnefous, Y.C., Gadgil, A.J., Fisk, W.J., Prill, R.J., and Nematollahi, A.R. “Relative Effectiveness of Sub-Slab Pressurization and Depressurization Systems for Indoor Radon Mitigation,” Indoor Environment, Lawrence Berkeley National Laboratory, 1994

This numerically verified study documented that sub-slab ventilation systems create pressure fields that distinguish between the sub-slab gravel and underlying soil, with SSD mechanisms including inversion of the pressure gradient across the basement slab and reduction of soil gas concentration. This is the same pressure differential that drives moisture-laden air through the gravel layer and out of the sub-slab soil environment. Lbl

EPA: Moisture Reduction Formally Listed as System Benefit

The EPA’s own technical documentation on sub-slab depressurization systems formally states that SSD systems can protect buildings from radon gas and reduce moisture levels in damp basements, establishing moisture removal as an officially recognized and documented co-benefit of the technology. Itrcweb

Multiple independent sources confirm that sub-slab depressurization systems not only mitigate radon but also help reduce moisture levels in basements and mitigate VOC vapors, with protection against multiple contaminants listed as a formal system benefit. Lung Radon

Long-Term System Performance

SSD systems have proven track records based on extensive radon mitigation experience with performance data indicating typical ability to reduce concentrations by 90 to 95%, with long-term monitoring data indicating that performance remains stable within a relatively narrow range over time, establishing the technology’s durability as a continuous active moisture and gas management system. CLU-IN

Summary of the Evidentiary Chain

The literature now provides a complete chain of support for a moisture management system’s effectiveness:

The Boardman and Glass 2015 paper is the cornerstone citation. It is precisely the independent, peer-reviewed evidence that directly validates the MoistureLevel mechanism by demonstrating moisture removal as a primary measured outcome of the same technology used for radon mitigation.

Summary of Alternative Costs

The system, including the controller and sensor, costs about $400. Everything else can be purchased off the shelf. If anyone wishes to design this system, the author is happy to help at no cost.