Structural drying occurs when there is a discovered intrusion, or a state where the facilities have gained excessive moisture that is not typical of the environment within the building, structural materials and contents that require the need for remediation. It’s at this point the client needs to react as quickly as possible to prevent or reduce further damage by returning the environment to a dry standard by achieving agreed-upon drying goals. Ken Larsen puts it, “Restorative drying is the result of producing an imbalanced vapor pressure between the wet materials and the adjacent environment so as to encourage the release of excess moisture to the environment.”

While we learn to perform the initial inspection, we gain the ability to document and note what is no longer restorable and what can be restored. Additionally, we need to undertake specific tasks to protect the environment within the building, the structural materials and contents. That awareness makes us competent remediators. We are aware that drying standards and drying goals should be designed for each facility. If not, we are to use similar facilities within the area or rely on our understanding of the composition of the materials affected.

As we return a building to an acceptable condition after a loss, we need to manage the environment within the building, control the moisture in the structural materials and the contents. To accomplish this, we should understand how to:

  1. Manage the psychrometric properties of the environment 
  2. Control the effects of moisture movement through different materials  
  3. Promote surface evaporation from the materials
  4. Determine if it is drying
  5. Qualify whether it is dried  

What is Drying?

As mentioned earlier, the initial inspection begins with an understanding of how to evaluate and develop a drying plan that has initial drying goals conforming to drying standards. This initial evaluation is reviewed continuously as things are discovered, psychrometric changes occur, environment changes, and the designated category, classification and even condition all affect the configured drying plan. So, moisture consideration isn’t just bulk moisture removal and that’s it; we need to consider the moisture within the air and how it is being absorbed into materials, and how and what effect it has on the drying plan. We need to understand how the various materials that have absorbed this moisture are going to release it so that we can have a truly dry environment within the building, structural materials and its contents for the client.  

Simply designing a drying plan and providing direction that we deem appropriate does not mean we will meet dry standards or dry goals. The current edition of the IICRC S500 provides clarity on this. Seriously, when in doubt, always go to the standards. 

Remediation Note: Photograph and describe the environment within the building, the structural materials and contents themselves as it justifies the timeline needed for drying, the scope of work and expected cost. 

Initial Inspection and Planning Procedures

All mitigation procedures should begin as soon as it is safe and practical following the initial moisture intrusion. If building materials and structural assemblies are exposed to water and water vapor for extended periods, moisture will penetrate them based on the porosity of the materials. The more water they absorb, the more time, effort and expense will be required to dry them. 

Examining Water Source(s) 

Before remediation begins, the source or sources of moisture intrusion should be located and eliminated, repaired or contained to the extent practical. In some cases, it may be appropriate to mitigate the spread of damage by starting procedures (e.g., humidity control, extraction) that prevent further water migration, even before the source is found and contained or repaired.  

Determining the Category of Water

When we are defining the category of water 1, 2 or 3 and if regulated or hazardous materials are part of a water damage remediation project, a specialized expert may be necessary to assist in determining the category of water per the current edition of the S500 section 10.4.1 and 10.4.2. It further provides references to the range of contamination in water, considering both its originating source and its quality after it contacts materials present on the job site. We need to remember time and temperature can affect or retard the amplification of contaminants, thereby affecting its category. We should consider potential contamination, defined as the presence of undesired substances, the identity, location and quantity of which are not reflective of a given perceived normal indoor environment. Or that can produce adverse health effects, cause damage to the environment, materials and contents, or adversely affect the overall operations or the intended function of building systems.  

Remediation Note: With Category 2 or 3 water intrusions, we should perform necessary remediation prior to restorative drying. For humidity control in Category 2 or 3 contaminated structures, we will need to refer to the {S500, 12.3.5}. Category 1, we proceed without contamination controls (e.g., erecting containment barriers, establishing pressure differentials) {S500, 12.4.2}.

Determining the Class of Water Intrusion

We should estimate the amount of humidity control needed to begin the project. The term “Class of Water of Intrusion” as defined in the current edition of the S500 10.4.3 is a classification of the estimated evaporation load and is used when calculating the initial humidity control (e.g., dehumidification, ventilation) of a structure. It is based on the approximate amount of wet surface area, and the permeability and porosity of the affected materials left within the drying environment at the time humidity control or restorative drying are initiated. Initial information to determine class should be gathered during the inspection process. The classes are divided into four separate descriptions: Class 1, 2, 3 and 4. The determination of class may be dependent upon the restorability of wet materials and access to wet substrates. Depending on the project, this determination may occur at a different point of the initial remediation procedures. 

Determining the Condition of Environment

Per the current edition of the S520 10.5, we can develop a preliminary determination that is a conclusion: Identifying actual or potential mold growth, of the need for the assistance of an IEP to conduct a formal assessment of Conditions 1, 2 or 3, or whether other specialized experts are needed to perform necessary services beyond the expertise of a remediator. 

Remediation Procedures for Category 2 or 3 

We should try to remediate prior to drying. There is a need to consider potential contamination, defined as the presence of undesired substances; the identity, location and quantity of which are not reflective of a normal indoor environment; and can produce adverse health effects, cause damage to environment, materials and contents, or adversely affect the operation or function of building systems as defined in the current edition of the S500 10.   

An environment can be contaminated because of preexisting and secondary damage. We need to continue with the designated appropriate procedures that should not vary regardless of whether contaminants are the result of water intrusion or preexisting damage. We shall inspect the structure for the presence and location of contaminants as part of their site safety survey.   


Isolation or critical barriers, usually referred to as containment, should be used to control and contain the contamination within a structure, and to control the flow of dry condition to wet condition interactions.  

Work Process (Category 1, 2 and 3) 

Extract Water/Spread of Water

We need to attempt to remove all water by absorbing, draining, pumping or vacuum extraction.

Excess water removal may be required on multiple levels, in basements, crawlspaces, stairwells, interstitial spaces, HVAC systems, utility chases or elevator shafts. We should initially remove as much liquid water as is reasonably possible before any evaporative drying procedures are initiated. Repeated extraction of materials and components may be required as water seeps out of inaccessible areas, especially in multi-story water remediation projects.  

Controlling Humidity and Stabilization (Initial Humidity Control)

Humidity and temperatures within the structure should be controlled as soon as practical to minimize moisture migration, potential secondary damage and microbial amplification. While an increase in the humidity can occur at the beginning of a project, persistent humidity increases can indicate an adjustment is necessary (e.g., additional ventilation, dehumidification equipment).   

When using mechanical dehumidification equipment, we should determine an initial dehumidification capacity to establish humidity control for the anticipated evaporation load after considering project conditions and requirements. Dehumidification capacity may be modified at any point after setup based on psychrometric readings to achieve or maintain targeted conditions.  

The rate of drying also occurs in three distinct but continuous stages: a constant drying rate stage followed by two falling drying rate stages. One of the most important factors influencing surface evaporation, especially during the constant drying rate stage, is airflow velocity. Generally, continuous rapid airflow is needed to enhance the evaporation of wet surfaces. 

Controlled Demolition and Removal of Un-Restorable Components or Assemblies

When accessing the environment, we want to consider if the demolition and removal of structural materials is appropriate before setting up the drying system. Materials that are deemed non-restorable or that pose a safety hazard should be removed as soon as practical. 


Steps should be taken as quickly as practical to minimize damage to contents. This includes but is not limited to protecting contents from moisture absorption, which can result in stain release, discoloration of finish, splitting of wood components in direct contact with wet surfaces (legs, bases), staining, rusting, ringing or other forms of moisture damage. If contents restrict access to walls, ceilings or other areas, the remediator should manipulate them (e.g., move, relocate, discard).  

Open Drying System

We should exchange the indoor air at a sufficient rate to promote drying and prevent secondary damage. If the indoor humidity level increases:

  1. A greater rate of exchange may help. 
  2. Supplemental dehumidification can be installed, converting to a combination drying system.
  3. The outdoor air exchange can be stopped, converting to a closed drying system. 

Closed Drying System

Closed drying systems are commonly used as they provide the greatest amount of control over the drying environment and the best protection from varying outdoor conditions while preserving building security. We should isolate the building or affected area from the outside and install dehumidification equipment. When appropriate, the existing building’s HVAC system can help. We gain control of ambient humidity. However, it is not sufficient to achieve optimum conditions for restorative drying.   

Combination Drying System

A third approach is to use a combination of the above, especially at the beginning of a project when indoor humidity levels are at their highest. We may consider ventilating the moist air to the outside while bringing in the drier air. This is often done at the time of debris removal, extraction and the initial cleaning since security is not typically an issue during the early stage of a project while the remediator is actively working on site.  

Using Mechanical Dehumidification to Promote Drying

When a closed or combination system is planned and mechanical dehumidification is not already in use, we should determine an initial dehumidification capacity to establish humidity control for the anticipated evaporation load after considering project conditions and requirements. Dehumidification capacity may be modified at any point after setup based on psychrometric readings to achieve or maintain the targeted conditions in the drying plan. We should document factors considered to determine the initial dehumidification capacity.  

On projects involving significant water absorption into low evaporation materials and assemblies, it can be beneficial once free water has evaporated to reduce airflow velocity across the surfaces and the vapor pressure differential should be increased (e.g., increase temperature of wet materials, reduce humidity of the surrounding air, or a combination of both). 

Controlling Airflow

Air movers are used to circulate air throughout the workspace to ensure drier air is continually displacing the evaporating moisture at the surface of wet or damp materials. Several different types of air movers (e.g., centrifugal, axial, SCDS and AFDs) are available. 

Prior to implementing the restorative drying effort (e.g., rapid air movement), we should evaluate the clean materials within the work area as needed. Where necessary, we should clean visible debris, dust and soil from materials and surfaces to reduce the amount of soil or particulates that can become aerosolized. We should employ cleaning methods that minimize aerosolizing particulates. 

Where cleaning cannot sufficiently remove soil or particulates: When airflow is directed through interstitial spaces or where there are high-risk occupants, it is recommended we install one or more air filtration devices (AFDs).   

Controlling Temperature to Accelerate Evaporation

The temperature within a work area, and the temperature of wet materials themselves, also impact the rate of evaporation significantly. Increasing the temperature of wet materials can be accomplished by using the sensible (thermal) energy gained by air movers, dehumidification, portable heating equipment or existing HVAC systems. The greater the temperature of wet materials, the more energy is available for evaporation to occur. 

Using the Installed HVAC System as a Resource 

We should consider the built-in HVAC system as a resource, provided contaminants will not be spread or the drying effort will not be negatively impacted. The HVAC system can add energy to or remove it from the environment being dried.  

Verifying Drying Goals

We should use appropriate moisture meters to measure and record the moisture level or moisture content of specific structural materials and contents. The S500 recommends that the drying equipment remain in operation on a site until it has been verified and documented that the drying goals have been achieved.   

Post-Remediation Evaluation and Verification Category 2 or 3  

At the completion of the remediation process of Category 2 or 3 projects, a post-remediation evaluation should be conducted. The post-remediation evaluation (PRE) and verification (PRV) should include, at a minimum, a determination that all surfaces have met drying standards and drying goals, and all areas are visibly free of dust and debris, and that malodors have been eliminated, per the current edition of the S500 

Post-Restorative Drying Evaluation 

We should evaluate structural materials, assemblies and contents that have been cleaned and dried to ensure predetermined drying standards and drying goals have been met, per the current edition of the S500 12.6.1. 


We know that drying a structure is not a one-step process. In fact it is a multiple-step process that requires an understanding of how to manage the environment within the building, the moisture in the structural materials and contents. The intent of this article is to provide an understanding of what structural drying is, and provide an educated and informed conclusion related to the restorability of environment, materials and contents.  


  • The current edition of IICRC S500 Water Remediation Standard (2021)
  • The current edition of IICRC S520 Microbial Remediation Standard (2015)
  • Leadership in Restorative Drying