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Restoration / Disaster Restoration

The physics of structural drying — part 3

July 27, 2012
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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:

Vapor Pressure Differential: The water vapor pressure on each side of the material is controlled to ensure that there is precisely a one inch mercury (Hg) differential. This translates to a difference of approximately 150 grains of water per pound of dry air (gpp) from one side of the material to the other. Material Surface Area: Permeability is assessed on a material that is precisely one square foot in size.

Time: The measurement is taken over a one hour period.

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:

Material Vapor Pressure (Vm): Using a hygrometer, measure the water activity (aW) or surface relative humidity of the target material. This is most efficiently done by containing a hygrometer on the surface of the material with a small sheet of poly sheeting (see image). Combine this humidity measurement with the material temperature on a psychrometric chart to obtain the vapor pressure associated with the water in the material.

Air Vapor Pressure (Va): Using the same hygrometer, measure the relative humidity of the air surrounding the material. For this measurement, it is important to allow the hygrometer to remain near or on the surface of the material for best accuracy – just remove the plastic barrier you used for the Vm measurement.

Permeance (P): Identify the material (e.g. plywood, softwood framing, gypsum wallboard) and do some research online. There are a variety of online resources that can be used to look up the known permeability rating for common building materials. Just make sure you find the imperial (inch, foot) measurement and not the metric (meter), as the formula I''ve included below is in imperial units.

Area (A): Measure (or estimate) the total surface area affected in square feet. For example, if you are trying to assess the evaporation from a concrete slab that measures 20 feet by 30 feet where about half is wet, use 300 square feet (1/2 of the total 600 square feet).

After finding the information defined above, use the following to determine how permeability will affect your evaporation rate:

E = (Vm – Va) × P × A


E = the calculated drying rate you will achieve (in grains of water per hour) Vm = the vapor pressure within the material (in Hg) Va = the vapor pressure of the air around the material (in Hg) P = the defined permeability of the material being dried (in grains) A = the total surface area of the affected portion of the material (in square feet)

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:

Option A: Increase Vm of the building material: This will excite water in the material and raise the vapor pressure. How warm do you need it? You can evaluate this using your psychrometric chart. Calculate the new vapor pressure (Vm) at a higher material temperature, and then solve the formula above using that new vapor pressure. Was the new temperature enough?

Option B: Decrease Va of the air near the surface of the material. As a first step to determine how to do this most efficiently, make sure the humidity near the surface of the material is similar to the humidity in the balance of the space. If the humidity near the surface is significantly higher than the air in the balance of the room or space, increase airflow to the surface. If there is not much difference between the surface and the rest of the space, increase dehumidification.

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

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