The Key to Accelerated Drying


By Brandon Burton

There are more four- and five-syllable terms in the technical papers on drying rates than I’d care to count — from psychrometry to thermodynamics, and hygroscopicity to sorption isotherms. The truth is, although these terms and the principles they describe are critical to understanding the drying process on a specific and molecular level, you don’t need to memorize the Academic Press Dictionary of Science and Technology to master accelerated drying.

The principles you need are actually pretty simple, and you likely already have the gear you need. The most important thing to understand is there are three things you control that affect drying rates and three stages in the drying process. Learn to identify the drying stage correctly and adjust your conditions to match, and presto — you’ll maximize the drying rate for the given condition.

Three stages of the drying process

During the drying process, there are three general stages. Each has a significant difference in both the methods of drying that are most efficient and the manner in which water moves through and exits the material. There are several ways these stages are described depending on the document or expert you refer to. In simple terms, they are best understood by using the type of water present.

Stage 1: Surface water

The first stage in the drying process is the “surface water” phase. Water is present at the surface of the material where your drying resources have direct access and uninhibited impact on the drying process. It can be identified easily because surfaces are extremely cool and possibly even sensibly damp to the touch, especially in the presence of airflow. During surface water stage, there are several things that are critical to consider:

Airmovers are most important in the first stage of the drying process.

Water on the surface is not only being evaporated, it is also continuing to absorb into the material. Any delay in removing this water will result in a greater degree of absorption and will increase the time and energy required in the latter phases of drying.

Evaporation rates during the surface water stage will be extremely high. It is important to closely monitor the resulting humidity in the space to ensure conditions are not supporting secondary damage to otherwise unaffected materials.

During this initial stage, the most impactful influence on evaporation is airflow along the wet surface. At no other point in the restoration process will airmovers be more important or influential. To ensure air movement at the surface is sufficient, use a hygrometer to measure the relative humidity of the air in direct contact with the material being dried, and verify that it is no wetter than the air two to three feet away from the surface. If you observe that the air touching the material has a higher humidity, increase the airflow along that surface.

Stage 2: Free water

The drying process transitions to the second stage when surface water has been removed and your target becomes moisture within
the pores and spaced within the material. Here, liquid water is contained like the water in a sponge. It can still move and evaporate without tremendous effort, but the rate of overall evaporation will begin to decrease. This is because your drying efforts are no longer in direct contact with much of the water you are targeting. This challenge increases the further into the stage you progress.

There is typically a noticeable difference in this stage as the humidity in the general area begins to decrease. During this stage, your efforts should continue to focus on airflow; however, the amount of air movement needed will gradually reduce.

Continue to evaluate the humidity immediately in contact with the surface to ensure it is equal to that of the surrounding air. If you observe, at any location, that the humidity is elevated at the surface, increase airflow.

Stage 3: Bound water

A dramatic difference is observed as free water nears completion. Humidity in the general space will likely fall sharply, indicating that the overall evaporation rate also is falling sharply. Air movement in this stage becomes much less impactful, and the importance of low humidity and higher energy (temperature) begins to take over as the priority.

To maximize drying rates, it’s critical to make significant changes to your drying approach as you enter this phase. In simple terms, you need to leverage any opportunities to lower the overall humidity and ensure all target materials are as warm as practical.

Reduce air movement in the space to approximately one for each small room or area and one for every 100 to 150 feet in larger areas. This will provide ample circulation, which is all that is needed.

Next, evaluate the complexity and density of the remaining wet materials and assemblies, and focus your drying equipment to add energy to those materials that are the densest and/or comprised of the most layers. Consider all sides of the material, and add the highest energy air to the areas that are the smallest in volume. For example, direct the warm air from dehumidification to ceiling or wall cavities as opposed to the large open space in the room. This will result in much better heat transfer to the target wall board. Or, focus energy in a crawlspace as opposed to the living space above — there’s less volume in the crawlspace, and the same energy will result in a much higher temperature gain to the subfloor.


The nature of a water damage restoration project changes as the job progresses. The best drying results will come from a system that adapts in order to respond to those changes. Initially, high velocity along wet surfaces will generate the best return for effort. Use a hygrometer to verify that surfaces are constantly supplied with dry air to keep up with evaporation.

Once liquid water is removed (surface and free), the effort should shift to basic circulation (fewer airmovers) and a more focused use of humidity and temperature control devices (e.g., dehumidifiers). Get creative, and focus your systems to control smaller air spaces around target materials. They’ll give you much more impact if the air is not diluted into a larger, general space.

Throughout the process, use your hygrometer to understand how well you are placing warm, dry air where it’s needed. Your hygrometer should just be used to measure the air in the middle of the room. Use it on surfaces — that’s where the action is.

Brandon Burton is the technical director for Legend Brands and an instructor with the Restoration Sciences Academy. He teaches IICRC-approved classes in the categories of Applied Structural Drying and Water Damage Restoration. He has served as an ANSI/IICRC S500 chapter chair, RIA restoration council member, and in many other industry roles. Contact him at [email protected].

Cleanfax Staff

Cleanfax provides cleaning and restoration professionals with information designed to help them manage and grow their businesses.

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