In the first two installments of this series, we discussed the simple and complex phases of drying.
We learned that drying begins with the evaporation of surface and free water (simple phase), then begins to slow with the drying of bound water (complex phase). Finally, we identified material permeability as the one major factor that influences the rate of evaporation and drying in the complex phase.
In this installment, we will discuss the role permeability plays in the evaporation process, from a mathematical perspective.
Using the information in this article, you will be able to determine how effectively your drying environment will promote evaporation from target materials. You will also be able to use this same information to analyze situations that aren''t drying properly, and make quantitative decisions about how to adjust conditions for better results.
Permeability and restorative drying
Permeability is a rating that expresses the ability for water to travel through a material. It is established by measuring the amount of water that passes through a material under controlled conditions. Permeability takes into account three different factors:
The result of the assessment is a unit of "perms" where one perm is a single grain of water vapor (a grain is 1/7000 of a pound) per square foot per hour. In other words, if a material has a rating of 11 perms, then 11 grains of water will travel through each square foot per hour, given a one inchHg vapor pressure differential. Simple, right?
To further simplify, the information above can be used by a restoration contractor to assess precisely how well a material will dry under the existing temperature and humidity conditions throughout a property being restored. In order to do this, the restorer needs to obtain a few measurements:
After finding the information defined above, use the following to determine how permeability will affect your evaporation rate:
After running the math, evaluate the calculated drying rate (E). Remember, this number is in grains of water (1/7000th of one pound). If the calculated drying rate is not significant, changes are needed to improve the rate of drying. So, what change should you make? There are two options to improve results:
Making the choice between these two options will depend on what resources you have available and on the other limitations for the restoration project. For example, you may need to be very cautious about how warm materials become, and thus elevating the Vm may not be an option.
Brandon Burton is the technical education manager for the Restoration Sciences Academy, a part of Legend Brands. He has served the restoration community for more than 15 years as an IICRC instructor, ANSI/IICRC S500 chair, RIA restoration council member, and many other industry roles. You can contact Burton at firstname.lastname@example.org.