Principles of Drying: An Excerpt from the IICRC S500

There are four general principles used in drying water-damaged structures and materials. They may be used singularly or, more effectively, in combination. Drying is a necessity in order to restore water-damaged materials to a pre-loss condition. The four principles of drying include Excess Water Removal, Evaporation, Dehumidification and Temperature Control.

Excess Water Removal

It is highly recommended that excess water be removed at the beginning of restoration procedures. Removal of excess water may be achieved by physical means such as mopping or soaking up excess moisture from hard surfaces or furnishings. However, water removal usually involves the use of more sophisticated techniques and equipment such as pumps, or specially designed, commercial wet vacuuming equipment.


Once excess water is removed, remaining water must be changed from a liquid to a vapor by promoting evaporation. Normally, this is accomplished efficiently with specialized air moving equipment.


Once moisture is evaporated from structural material and contents into the air, the moisture must be removed from the air through dehumidification, or it must be externally exhausted. Failure to dehumidify may result in substantial secondary damage and present a significant health hazard.

Temperature Control

Both evaporation and dehumidification are greatly enhanced by controlling the temperature in a confined environment. Additionally, micro-organism growth is temperature related. Thus, temperature modification and control is an important basic principle for safe, effective drying.

Structural and Contents Restoration

Following the application of drying principles to remove excess water, additional restoration procedures may be necessary for structural materials and contents. Items that should be examined for damage and necessary restoration measures include, but are not necessarily limited to, the following:

  • Heating, ventilation and air conditioning (HVAC) systems
  • Basements
  • Crawl spaces
  • Attics
  • Structural comports and materials


The damaged structure must be monitored starting with the initial assessment and evaluation, and continuing throughout the restoration process. It is highly recommended that technicians establish moisture content or drying goal for affected building materials and contents items.

Monitoring Procedures may include, but are not necessarily limited to, temperature and humidity readings, updating progress reports and checking the moisture content of structural wood and other materials with a moisture meter. When applicable, monitoring also must include checking equipment operation, work progress and indoor environmental quality.

Final Inspection and Completion

At the conclusion of a restoration project, a final inspection, including the implementation of completion procedures, must be performed. During final inspection and completion procedures, it is important to confirm the structure and contents have been returned to a pre-loss condition or that materials have met the predetermined drying goals.

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