ICS Magazine

Drying In High Humidity

February 1, 2007


Through many years’ experience we’ve learned what a remarkable difference location makes in determining the correct approach to drying a water-damaged structure. From restorers in Florida who have learned from experience that high humidity requires a closed drying system to restorers from Colorado who can often use an open drying system, customer experiences clearly point to the importance of understanding the critical role of humidity in the drying process.

Outside humidity produces dramatic differences on a drying job. With dry outside conditions, a restorer may be able to open the windows and doors and dry quickly. A drying job with extreme outdoor humidity can drag on for days battling outside humidity as well as the water intrusion inside the structure.

Dealing with High Humidity

When humidity is higher outdoors, a closed drying system is most effective. Create a drying chamber with the lowest possible humidity. This will cause the most-rapid drying. Nature dictates that wet goes to dry, and the drier the air the faster materials will reach pre-loss condition. However, at the same time the moisture is leaving the materials, humidity is literally pouring though the walls of the structure. The difference between the dry air inside and the wet air outside causes infiltration of moisture.

Closed drying systems can be sabotaged by:
  • Customers opening doors and windows
  • Customers turning off dehumidifiers
  • Ventilation fans (bath and kitchen vents) turned on
  • Doors and windows left open by technicians

When dealing with high humidity outside, make the air inside as thirsty as possible by raising the temperature within the limits of indoor comfort and equipment operation. Thirsty air is air that can hold the most moisture. More thirst leads to more evaporation.

Excessively high heat in the structure may dry some materials faster, but has two major problems. If occupants are staying in the structure, they will complain. Excess heat may not be an option. The other problem is dehumidifiers. Many refrigerant dehumidifiers operate best at temperatures between 70 and 90 degrees. If the air inside the structure is too hot, dehumidifiers will lose effectiveness. Grain depression will be low. To enhance the effectiveness of dehumidifiers in hot structures, consider feeding dehumidifiers cold air from the air conditioning system.

When It's Too Hot

In cases where the temperatures are too high on the inside, there are several solutions. Portable air conditioners can be used to control inside heat. These have an added benefit in that they are also essentially a refrigerant dehumidifier. It’s possible to measure water removal (e.g. grain depression) on these units just like a dehumidifier.

If the temperature inside is still elevated, consider pulling some drying equipment and using a more disruptive drying approach. Because of the significant electricity used, drying equipment adds quite a bit of heat to the structure.

Preventing Heat Problems

The amount of heat drying equipment will add to the structure can be estimated by using this formula:

Amps x Volts x 3.4 = BTUs per hour

Prevent heat problems by providing enough cooling capacity (HVAC, etc.) to offset the heat produced by the equipment. For example:

14 Carpet dryers (5 amps each) = 70 amps

2 LGR dehumidifiers (10 amps each) = 20 amps

1 Air scrubber (3 amps) = 3 amps

Total amps = 93 amps

93 amps x 115 volts x 3.4 = 36,363 BTUs per hour.

One ton of air conditioning capacity will offset 12,000 BTUs of heat every hour. If the structure has a 2-ton HVAC unit, will that be enough? No, because the equipment produces more heat than the HVAC can remove. Also, some heat will come from other sources, such as the outside air temperature and occupant activities. In this case, a portable air conditioner would be needed.

Other Effects of High Humidity

With high humidity outside (high GPP), moist air infiltrates into the structure. Vapor pressure differences cause this infiltration into the structure. Moisture will pass through materials with no effect on the moisture content of the material. This phenomenon is known as vapor diffusion. This means that along with the moisture from the water loss, nature continues to add more moisture into the structure. This is the reason that dehumidifiers continue pumping water late in the drying job after the boundary layer is gone and the material readings are close to dry standard.

Remember that the indication that the job is done is when the materials reach dry standard, not how much water the dehumidifier is removing.

Drying Faster

When starting the job and the humidity is high, you need to be close to 90 degrees for your dehumidifiers to work best. Warmer air is much thirstier and will be able to hold more of the moisture that is evaporating from wet materials. Refrigerant dehumidifiers perform very effectively in warm, wet conditions like this. They will produce even thirstier air that is ready to collect even more moisture.

As the materials dry, and evaporation slows down, the air is collecting less moisture. Humidity (e.g., GPP) will go down. Dew point will go down. This makes it harder for dehumidifiers to remove moisture. Toward the end of the job, cooling the air will help dehumidifiers work better.

Desiccants in High Humidity

Another problem when drying in high humidity is using desiccant dehumidifiers for specialty drying situations. When using desiccants it is necessary to feed the desiccant dehumidifier with an air temperature of 65 degrees or less for best operating results. In the summer months it is difficult or impossible to get the temperature down that low inside a structure. If a desiccant is run in high temperatures and high humidity, it will remove little water when compared to refrigerant dehumidifiers.

In hot conditions, setting up containment is an advantage in keeping out some heat problems. For example, tenting a hardwood floor or containing the affected room are methods of helping desiccants while providing thirsty air to help dry affected materials. This keeps the heat at the surface of the material while preventing the hot air from entering the dehumidifier’s intake air stream.

Another way to help your desiccant dehumidifier in hot, wet conditions is to place the desiccant intake near an air conditioner vent. This allows the cool air from the AC to enter the desiccant, bringing the intake air temperature closer to 65 degrees. If the structure has AC vents in the ceiling, use lay-flat ducting to direct the air into the desiccant.

Also, be sure to duct the reactivation air (hot, moist air) from the desiccant out of the structure. This is the most common mistake made by restorers when using desiccant dehumidifiers.

In any drying job, no matter where the structure is, you must use your knowledge of psychrometrics (the science and practice associated with humidity, its evaluation, and control and affect on materials). Using this knowledge allows you to scope the job properly and move forward using the best drying techniques to overcome any drying challenge. If you liked this article, circle 141 on the Reader Inquiry Card.