IICRC WRT - Water Damage Restoration Technician (WRT)

The IICRC WRT body of knowledge defineshumidity ratioas theamount (or weight) of water vapor contained in a given weight of dry air. It is typically expressed as grains per pound (GPP) or grams per kilogram and represents an absolute measurement of moisture in the air.

Unlike relative humidity, humidity ratio does not change with temperature unless moisture is added or removed. This makes it one of the most reliable psychrometric measurements for evaluating drying potential and comparing indoor and outdoor air conditions.

The WRT manual emphasizes that humidity ratio is critical for determining vapor pressure, dew point, and the suitability of ventilation drying. Restorers frequently rely on humidity ratio to decide whether introducing outdoor air will improve or hinder drying.

Moisture content applies to materials, not air, and relative humidity is a percentage comparison rather than a mass measurement. Therefore, humidity ratio is the correct and precise term under WRT psychrometric science.

The IICRC WRT body of knowledge explains thatrelative humidity decreaseswhen air temperature increases and no additional moisture is added. This occurs because warmer air can hold more water vapor; therefore, the same amount of moisture represents a smaller percentage of the air’s total capacity.

This principle is foundational in psychrometry and directly applied in restoration drying. By increasing temperature while controlling moisture content, restorers lower relative humidity and vapor pressure, increasing evaporation potential.

Relative humidity does not remain constant with temperature changes, nor does it increase unless moisture is added. Dew point remains unchanged unless moisture content changes.

Understanding this relationship allows restorers to use controlled heat strategically to improve drying efficiency without introducing excess moisture.

The IICRC WRT body of knowledge identifiesrelative humidity at or above approximately 70%as presenting the greatest risk for structural and microbial damage to hygroscopic materials. At this level, many materials readily absorb moisture from the air, increasing moisture content even without direct liquid water contact.

The WRT manual explains that hygroscopic materials such as wood, paper, drywall, and textiles reach higher equilibrium moisture contents as RH increases. When RH exceeds safe thresholds, these materials may swell, deform, lose structural integrity, or support microbial growth.

Microbial amplification risk also increases significantly at higher RH levels. While mold growth depends on multiple factors, sustained RH above approximately 60–70% greatly increases the likelihood of microbial activity on organic materials.

This is why restorers are trained to aggressively control humidity during drying and to monitor RH as part of daily documentation. Maintaining RH well below damaging thresholds protects unaffected materials and limits secondary damage during the restoration process.

The IICRC WRT body of knowledge clearly identifiestimeas one of the most critical variables influencing the extent of damage in a water loss. The longer materials remain wet, the greater the likelihood of primary damage, secondary damage, and microbial amplification. For this reason, the WRT standard emphasizes that mitigation activities should beginas soon as it is safe to do so, following an initial hazard assessment.

Beginning mitigation promptly limits moisture migration, reduces absorption into hygroscopic materials, and decreases the duration materials remain above safe moisture thresholds. Early actions such as stopping the water source, removing bulk water, and initiating controlled drying significantly reduce structural deterioration and restoration costs. The WRT manual repeatedly reinforces thatdelays increase damage, regardless of water category or class.

Waiting for adjuster authorization or focusing on antimicrobial use before drying does not align with the standard of care. Antimicrobials are supplemental and do not replace drying. Likewise, baseboard removal may be necessary but is not the primary factor in minimizing drying time.

The ANSI/IICRC S500 standard supports emergency mitigation to prevent further damage and explicitly recognizes that restorers may need to act before third-party approvals when necessary to protect the structure and occupants. Prompt mitigation is therefore both a technical and professional responsibility.

The IICRC WRT body of knowledge states that verification of drying completion must be supported byobjective, measurable documentation. This includesmoisture content or moisture level records,moisture maps, anddocumented drying conditionssuch as temperature, relative humidity, humidity ratio, and dew point.

These records demonstrate that affected materials were dried to established drying goals, typically based on comparison with unaffected reference materials. The WRT manual emphasizes that documentation must show trends over time, not just final readings, to confirm effective drying.

Electrical usage records, contracts, or remediation protocols alone do not verify drying success. While they may be relevant administratively, they do not demonstrate moisture removal.

Comprehensive drying documentation is essential for transparency, defensibility, and compliance with the ANSI/IICRC S500 Standard and is a cornerstone of professional restoration practice.

Once bulk water has been physically removed, the IICRC WRT body of knowledge identifies therate of evaporationas the most significant factor influencing drying time. Evaporation is the process by which remaining moisture within materials changes from liquid to vapor and enters the surrounding air.

The WRT curriculum explains that evaporation is controlled by several interrelated variables, including vapor pressure differential, airflow across wet surfaces, surface temperature, and ambient humidity conditions. If evaporation is slow, drying time increases regardless of how effective extraction was initially.

While extraction method plays a critical role in reducing the initial moisture load, it does not control the drying phase once free water has been removed. Similarly, fiber saturation point describes moisture conditions within materials but does not dictate drying speed. Condensation, conversely, inhibits drying and adds moisture.

The WRT body of knowledge reinforces that successful drying requires creating conditions that maximize evaporation while simultaneously removing evaporated moisture through dehumidification or ventilation. Monitoring evaporation effectiveness is therefore a core responsibility of the restorer during daily inspections.

The IICRC WRT body of knowledge teaches that initial dehumidification capacity for LGR dehumidifiers is based oncubic footage and class of water intrusion. Class 2 intrusions involve a larger amount of moisture absorption than Class 1 but do not reach the full saturation of Class 3.

First, calculate the affected volume:

1,300 sq ft × 8 ft =10,400 cubic feet.

ForClass 2 losses, a commonly accepted WRT guideline is approximatelyone LGR dehumidifier (≈200–210 PPD)per10,000–12,000 cubic feet. This capacity balances evaporation demand without over-drying or inefficiency.

A recommendation of 208 PPD aligns directly with this guidance and reflects standard WRT training tables used for initial equipment placement. Lower values (26 or 99 PPD) are insufficient for the moisture load, while 303 PPD exceeds the initial requirement for a Class 2 loss and would require justification through monitoring data.

The WRT manual emphasizes that this is aninitial recommendationand must be validated by daily psychrometric and material moisture monitoring. Equipment may be adjusted as drying progresses.

Athermo-hygrometeris the instrument identified in the IICRC WRT body of knowledge for measuring bothair temperature and relative humidity. These two measurements are fundamental inputs for psychrometric evaluation and drying documentation.

The WRT curriculum explains that accurate air readings allow restorers to calculate additional psychrometric values such as humidity ratio, dew point, and vapor pressure—either manually or using built-in instrument calculations. These values are critical for assessing drying conditions, equipment performance, and the effectiveness of the drying strategy.

Moisture meters and moisture sensors are used to measure moisture in materials, not air. A thermometer measures temperature only and cannot determine moisture content or humidity conditions. The thermo-hygrometer integrates both functions into a single instrument, making it a required tool for daily monitoring under the WRT standard of care.

The WRT manual further stresses consistency in air measurements, recommending similar measurement locations and procedures during each monitoring visit to ensure defensible documentation.

Vapor pressureis defined in the IICRC WRT body of knowledge as the force exerted by water vapor molecules in the air against surrounding surfaces. It represents the energy level of moisture in the air and is a key driver of moisture movement.

The WRT manual explains that water vapor moves from areas of higher vapor pressure to areas of lower vapor pressure, whether between materials and air or between different air masses. This principle governs evaporation, condensation, and moisture redistribution within a drying chamber.

Relative humidity describes a percentage relationship, humidity ratio measures moisture mass, and dew point identifies saturation temperature—but vapor pressure quantifies the actualdriving force. Because vapor pressure is directly influenced by both temperature and humidity ratio, it is considered one of the most precise indicators of drying potential.

Effective drying systems focus on lowering air vapor pressure relative to wet materials, ensuring continuous moisture migration out of structural components.