The Science of Drying a Structure

 

Mary was so embarrassed. She was sure that everything was dry from her "little flood," but looking at the red markers on the wall, she realized she was very wrong. The contractor had used some kind of meter and showed her where the walls were still wet. But how could the drywall be wet a foot up when the wall didn't feel wet or cold? And how would they be able to remove the water without tearing the wall apart?

For decades, the restoration industry used historical evidence in drying a structure. Basically, if it worked once, we would use it again. Everyone knew that moving air dried clothes and when the air was heated, it dried them even faster. No one really knew exactly why, but that didn't matter as long as it worked. So contractors put fans in houses for three days and said,"It is dry."

Today, using sensitive measuring devices, the industry has applied the laws of physics and chemistry to drying. Now, we not only know how the drying process works, we can predict with a great degree of accuracy, how long it will take and which technique will work best for a given situation.

There are three basic principles needed to dry a structure. Leave one out and the drying is delayed; leave two out, and drying grinds to a halt.

The first principle is heat - The more heat there is, the more energy is transferred to the water molecules and the more primed they are to move from one area to another. In most situations, the optimum temperature for drying a structure is between 90-105 degrees Fahrenheit. If it is much hotter, damage to the home can result. If it is too much cooler, the molecules will not be excited enough to move. Often, supplemental heat will need to be added to raise the temperature to the desired level.

The second principle is pressure - We control the direction of the water movement with air or atmospheric pressure and vapor pressure. The Bernoulli principle of physics explains that the faster a fluid is moving, the lower the pressures are inside the fluid. Air is a fluid, therefore, the faster it moves, the lower the pressure in the air. When the pressure of the ambient air in the room is less than the vapor pressure of the wet carpet, pad or drywall, the water molecules move from higher pressure to lower pressure or from the wet materials into the air. This migration is known as evaporation. The greater the pressure differential, the faster evaporation will occur.

The third principle is evacuation - Once the first two principles are successfully implemented, the air will quickly become saturated with water and evaporation will cease. Water molecules need to be evacuated from the room before evaporation can continue. In many situations, opening windows and doors to the outside will allow for the water to move out of the structure, lowering the vapor pressure in the room so evaporation can resume. This is especially effective on warm summer days. But when the temperature drops 30 degrees at night, high levels of humidity exist out of doors or security issues are present, other means of evacuation need to be employed. The most common option is a refrigerant dehumidifier.

The refrigerant dehumidifier cools the moist air to the dew point, forcing the water molecules to condense and fall out of the air. This water is collected in a holding tank and purged into a sink or floor drain when the tank is full. As the water is removed from the air, the pressure in the air is reduced allowing for more water to move from the wet materials into the air. Most refrigerant dehumidifiers work best in the 90-105 degree range.

There are several variations on these principles that can increase success in drying. Sometimes, hot, dry air is blown in to the building to create high pressure inside the room. This forces the air to leave through cracks and openings in the exterior, taking moisture with it. Other times hot, dry air will be pumped in to pressurize the walls while blowing fans lower the pressure of the room.

The best protocol to use will be determined by considering the following:

• Amount of water present
• Security concerns
• Weather
• Season of the year
• Indoor and outdoor temperatures
• Nature of the wet materials
• Construction of the building

Using proper tools, including; moisture meters, thermal hygrometers and thermal imaging cameras, restoration contractors can dry most structures where the extent of damage does not require replacement.