Drying Methods Explored
by Ernie Storrer
President--Injectidry Systems, Inc.
Positive, negative and passive
It is now universally accepted among knowledgeable restorers that wet building material should be dried and returned to equilibrium as soon as possible after a water damage. When the unexpected water intrusion occurs, there is often confusion as to what materials are wet, how wet they are, and what if anything should be done to effect drying. For the purposes of this article, it is assumed that structural moisture content readings have been taken after a water loss and actions should be taken. Moisture has been found in locations and quantities that are not normal for this specific building. The article further, will deal with specific drying methodologies that we will call: negative, positive and passive.
First, several concepts about air and moisture as pointed out by Joe Lstiburek of Building Science Corporation: *
· Air flows from high pressure to lower. Air molecules will move to a less crowded space if you will. In the atmosphere, weather systems exhibit this tendency when air (wind) moves from zones of “high pressure” to zones of low atmospheric (barometric) pressure. In a building, the same phenomenon occurs only on a much smaller scales. A visible evidence of this air movement is the “soil filtration” lines in buildings. Contaminated air moves across the face fiber of carpeting and deposits its load.
· CFM in = CFM out. If air is moved into a space, an equal quantity has to come out in order to equalize pressure.
· The action happens at the surface. Assuming the surface material is wet, the movement of dry air over and by it, will cause the material’s moisture to transfer to the air. This air activity will act to retard mold growth. (If the structure is not dried, the relative humidity will be elevated and mold will result, just about guaranteed).
The understanding of these concepts will enable the restoration contractor to better visualize what needs to happen and how structural drying can be accomplished. Moisture is present and must be removed from affected materials that surround interstitial spaces. A vehicle, air that is dry, is used to convey the moisture from the wet surface material to the exterior of the trapped area. Air forced into or through the area of entrapment must come out with its load of moisture, resulting in drier structure.
Let’s deal with “passive” drying first. A passive attempt describes measures that do not directly affect wet areas with moving dry air. Passive attempts are essentially inactivity without the use of mechanical devices. Instead of actively opening up wet interstitial spaces and moving air through them, methods are used to let things happen more slowly, perhaps by simply allowing the wet structure to equalize on its own. An example of when this might be done would be when a wall is encountered that has no paint and is only wet up several inches from the floor. There would often be no effort that would be worth taking, as the wet material will often rapidly dry without intervention. Even if dehumidification were undertaken, “passive” would still be the definition unless the dehumidified air was concentrated and blown on the wet area.
Water vapor pressure
A more aggressive step than simply letting the structure acclimate and equalize on its own, would be to change the vapor pressure present in the structure. In almost all cases where excess water has intruded into the built environment, there is a change of and an increase in water vapor pressure. Water vapor pressure can be defined as “the exertion of force by water upon a surface.” This pressure will change if either temperature or the specific amount of moisture in the air changes. In our hypothetical water damage situation, the specific water vapor pressure will increase as the air becomes more saturated with moisture. As air moves by the wet flooring, walls or ceilings, the moisture attaches to the air molecules. This extra moisture migrates to the air if the air has the specific ability to hold more grains of moisture. Any relative humidity reading of less than 100% will mean the air is not saturated and can hold more moisture at that specific temperature. Whether or not the moisture will go to the air will depend on factors such as air movement, temperature of the air and temperature of the surface material.
Dehumidification to reduce water vapor pressure
In most cases, it is relatively easy to change the water vapor pressure by adding dehumidification to the environment. Dehumidification will remove moisture from the air, and other conditions such as temperature remaining constant, will produce reduced water vapor pressure. If the air outside the structure contains less specific grains of moisture than the inside air, the wetter inside air can be exchanged with the outside by ventilating. This is almost never practical for an extended period of time, as both the inside and the outside conditions will be changing. Within a matter of hours, relative humidity outside can change from 60 to 100%. It is practically impossible to properly monitor jobs closely for any length of time. If climatic changes are not reacted to quickly, moisture can even be added to the structure.
The reduced water vapor pressure in the air will move moisture from the wet structural material to the air. Moisture continuously seeks stability, and when there is a pressure differential, the moisture will move toward lower pressure, both vapor and barometric.
Some of the structural drying that contractors do, involves the use of moving air directly into interstitial spaces from sources such as airmovers blowing air directly into wall cavities. Usually large holes are drilled into the wall and air is blown directly at the wall. Sometimes, TurboventsÒ or Mini-Turbovents are used to better concentrate air at the surface. Simply blowing air alongside a wall that has holes drilled into it, is not positive drying. In fact, there is a negative pressure created as a result of the air velocity along the surface. (This is what keeps an airplane in the air)! Air is moved out of the holes drilled into the surface due to the lowered pressure at the surface. Another system used to deliver air to the wet areas is the Injectidry System. This system delivers air to the area of entrapment behind the surface instead of at the surface. The air is not released from the injector nozzles until it is in the area of wetness behind the surface.
Positive drying has several advantages including the delivery of air of known temperature and moisture content. The act of pressurizing a space means we capture air from a known area such as the output face of a dehumidifier where we know the humidity is controlled. This air is lower in moisture content than the air in the surrounding area. Positive drying is almost always the fastest and most efficient use of energy due to the direct concentration of processed, drier air.
The biggest problem is that air blown into the space must come out (cfm in = cfm out). The act of moving air into the cavity will cause the dispersal of dust and debris from the space. In most cases this will not be a large problem, but where there are concerns about the health of the building occupants, this drying technique can cause the spread of mold and dust. In fact, all buildings contain contamination to some degree within walls and ceilings. If those who occupy the building wish to minimize the impact of the drying process on the structure and occupants, blowing air into the walls could be unacceptable. Examples of such situations could include:
· hospitals and nursing homes
· homes with elderly people or young children
· homes with asthmatic or allergic occupants
· homes with occupants of compromised health
· food establishments
· production facilities where “clean” is the only option.
The answer to most of the above mentioned concerns is the concept of negative drying. If moving the possible contamination outside the wall into the built environment is a problem, standard positive air movement is not the answer. The effort should be to move the moisture and contamination to a known location where no damage can happen. This can be accomplished by the negative pressurization of the interstitial space by removing air from the area. The air can then be processed either through a HEPA filter system, or moved to the exterior of the built environment. This transport can be achieved by attaching a hose to the Injectidry System or other air delivery apparatus to convey the moisture laden, contaminated air to another location. This approach is the preferred one by most Industrial Hygienists, architects and building engineers. The goals of structural drying should include the goal of not spreading contamination or making the damage situation worse. The situation that exists when we arrive at the job should not degrade as a result of our actions or inactivity.
Positive and negative drying combined
In some cases, it is both practical and desired to combine the insertion of dry air into the wet spaces while simultaneously extracting air from the area. When a structure is comprised of fairly wet floor to ceiling lath and plaster, extra insertion and extraction points will facilitate drying. Additionally, the circulation of air from top to bottom and from bottom to top will accomplish drying without the problem of dust being forced from the wall from simple positive drying. This combination was recently used on a Karate studio wall which had three layers of ½” drywall and one layer of plywood (there were four 7’X10’ mirrors attached to the wall). The wall was saturated up several feet and was dried out in 8-10 days using a combination of positive and negative pressurization. This method obviously takes multiple systems.
Where does the air go?
Each stud cavity has approximately 35 linear feet of stud and drywall contact area (both sides of stud cavity). With an average of 1/100th of an inch of clearance between stud and wallboard, there is ½ sq. inch of open area per stud cavity. This is a more than adequate amount of area for the air to escape from the wet space, although there is a back-pressure exerted in the form of resistance. This resistance is due to the amount of structural material the air has to pass by before finding an exit (or entrance) point.
Under the terms and conditions of standard insurance policies, the insured is to “take such steps as are necessary to preserve and protect from further loss and damage.” This is an obligation we are fond of quoting to those who call in with damage. In the same manner, we as professionals are responsible for availing ourselves of knowledge, techniques and equipment in order to accomplish results that are readily available to the industry. The body of knowledge available to all professionals, is a measure of our responsibility and liability. What should you have known and when should you have known?
*WLI Water Loss Mitigation Specialist Workshop, October, 1996, Chicago.