How Much Airflow is Really needed?
I have felt for a long time that we are taking “shots in the dark” when we make decisions about how many airmovers to deploy on a drying job. We have some very basic industry standards about airflow, but I believe more specific information is needed. Too many water restorers follow the belief that bigger is always better when it comes to the amount of airflow on a drying project. There are times when a lot of airflow is needed, but there are other times when it would be best to dial back the numbers of airmovers and their speed. The reality is that many restorers are not placing enough airflow at the beginning of a project, and on the same loss have too much airflow at the end of the project.
Airmovers are Necessary
No matter how you look at the drying process, it is easy to understand that airmovers are needed on virtually every drying project. For a basic understanding, consider the “drying pie.” The drying pie includes three factors for successful drying: low humidity, adequate temperature (in the air and in the affected materials) and, of course, airflow. All three aspects must be controlled to promote rapid evaporation. A more advanced understanding of the drying process promotes vapor pressure differentials between the wet materials and the air. If you are in doubt about the importance of air movement, consider the fact that the mathematical equation for evaporation of a liquid uses airflow as a multiplier! Airflow multiplies the effectiveness of the other tools you use on the project.
Airmovers are most important at the beginning of the water project, when evaporation is most rapid. Materials that absorb water quickly (i.e., carpet, pad, drywall) also release that water quickly. During this phase of drying, airspeed is critical – and more is better. Here are the IICRC’s specific recommendations: “Airmoving devices used to create a general drying environment, when practical, should be set up so that continuous rapid airflow is provided across wet surfaces. In many structural drying situations, one of the most difficult areas to dry is the lower part of the wall where it meets the floor. Restorers should install one airmover for each 10 to 16 linear feet of wall, with the outlet of each airmover pointing in the same direction.” (IICRC S-500 2006 p. 51)
It is significant that the word “should” is used in the previous quote. This means that placing an airmover every 10–16 linear feet is the accepted standard of care to be followed by professional restorers. Airmovers should be placed in this manner when there are wet walls and floors. The standard goes on to state that small rooms need an airmover as well.
Bottom line: When dealing with saturated materials and high evaporation rates, bigger airflow is better. Later I will give specifics on how big this airflow needs to be for good evaporation. Unfortunately, many restorers have too few airmovers at the beginning of the process.
But, As The Project Progresses…
…the need for rapid airflow decreases. Earlier, I referenced the evaporation equation which states that airflow is a multiplier. But it is important to understand that the evaporation equation assumes that materials are saturated. It assumes that that liquid water is continuously available at the surface of the material.
As any self-respecting restorer knows, most of the surface water is gone within the first 24 hours of a dry down. (If you are not getting these results on your projects, please read up on the extraction process!) So, if surface water is gone, evaporation rates fall, and less airflow is needed.
The rapid initial phase of drying is called the “constant rate period.” Airflow is a multiplier to the effect of evaporation during this phase. The basic formula is this:
Evaporation = Airflow × Vapor Pressure Differential
After surface water has evaporated, we enter the “falling rate period” which is a much slower rate of evaporation. Airflow is no longer multiplying the evaporation. Now, the basic formula is:
Evaporation = Vapor Pressure Differential
Practically Speaking: How Much Airflow is Really Needed?
In the restorative drying industry, there is no real peer-reviewed science that says how much air movement is really needed. Thankfully, much research has been completed on industrial wood products as well as food products. The science from these industries is very clear, and it directly relates to what we do because wood and food products are hygroscopic, just like our wet structures. The research says that more airflow is needed at the beginning of the process, and less is needed at the end. You can see the references for my research at the end of this article. Basically, the studies show the following:
- During the constant rate period most studies specified airflow rates between 600-700 feet per minute (6.8 – 8.0 mph).
- During the falling rate period all studies specified airflow rates between 100 – 250 feet per minute (1.1 – 2.8 mph).
Based on this research, the best approach is to start drying jobs with airmovers placed every 10-16 lineal feet. This easily achieves over 600 feet per minute along wet surfaces. Of course, flow rates near the outlet of the airmover will be moving faster – in some cases as high as 2,000 feet per minute. This airflow can be verified using an anemometer.
As the job progresses and materials no longer have surface moisture, the airflow can be reduced to one airmover per room or area. This small amount of airflow is needed for circulation in the drying environment. How can a restorer know when materials no longer have surface moisture? Here are a number of ways, listed in order from most difficult (and most accurate) to easiest (and least accurate):
- Perform an equilibrium relative humidity (ERH) test on the wet material. If the ERH is below 85%, the material no longer has surface moisture. This test is done with a hygrometer placed on the surface and sealed over with plastic and taped down for several hours.
- Test the surface of material with a penetrating moisture meter. If the meter shows a reading of less than 30% moisture content, it no longer has surface moisture.
- Hold a hygrometer probe near the affected surfaces while airflow is applied. If the temperature and RH near the surface correspond to the ambient conditions, you have enough airflow. If the temperature is cooler and RH is higher, than more airflow is needed.
- Touch the surface to be tested with moisture detection paper strips. These are papers infused with cobalt chloride and are very inexpensive. They change color when they are wet and leave no residue.
- Sensory test. If it looks or feels wet, there is still surface moisture.
In my opinion, any of these methods of searching for surface water would be appropriate. Common areas to test for surface moisture include:
- Wall and baseboard surfaces.
- On the surface of the carpet.
- In between the carpet and pad.
- In between the pad and subfloor
Of all the areas in this list, the space between the pad and subfloor will be the last to feel dry to the touch. Let me be absolutely clear! I am not saying these areas are “dry,” only that they are in the falling rate period when they no longer have surface moisture.
Boost Your Confidence
As I was reading the research from other drying industries, I found that our restorative drying best practices match up well to what the PhDs are saying. The most efficient restorers are going to use proper airflow to start a project and note when surface water is no longer present and reduce airflow. Efficiencies like these are often rewarded with more business and higher profits.
1. Lamb, Dr. Fred M. The Importance of Air Velocity in Drying. Modern Woodworking. July 2002.
2. Steiner, Ylva. Optimizing the Air Velocity in an Industrial Wood Drying Process. Norwegian University of Life Sciences – Dept of Ecology and Natural Resource Management. 2008.
3. Salamon, M and McIntyre, S. 1969 Manipulation of Air Velocity Permits Drying Time Savings. Canadian Forest Industries. Volume 89.
4. Simpson, William T. Effect of Air Velocity on the Drying Rate of Single Eastern White Pine Boards. Res. Note FPL RN 266. Madison, WI: US Department of Agriculture, Forest Service, Forest Products Laboratory 1997.
5. Xuai-Kang Yi, et al. Thin Layer Drying characteristics and Modeling of Chinese Jujubes. Mathematical Problems in Engineering. 2012.