The Rise of Desiccant Dehumidification

Nov. 2, 2006
Desiccant dehumidification reduces both humidity and vapor pressure, making it an effective method to dry construction materials

Moisture within a building under construction or renovation can become a real headache to owners and a danger to future occupants. All construction materials inside the structure absorb moisture, including wallboard, fireproofing, lumber, block, and concrete. Unless that moisture is reduced to acceptable tolerances, the condition can delay the construction timetable or, even worse, cause performance failure of material or lead to the formation of mold.

Surfaces affected by moisture often include freshly sprayed fireproofing, joint compound, concrete slabs, floor-leveling products, concrete block walls, and stored building materials. Water-laden fireproofing and joint compounds dry slowly inside closed areas when applied. Finished flooring products (such as vinyl composite tile and fiber-backed carpet) are applied with adhesives that are extremely sensitive to moisture. Without proper drying, large amounts of water can be trapped in concrete, preventing the installation of flooring. Hardwood flooring and trim absorb moisture easily, causing warping.

Generally, one of two methods is used to accomplish drying during the construction of commercial, industrial, and high-rise residential buildings: direct-fired heaters or desiccant-dehumidification systems. Some also attempt to dry by using the HVAC system, but that has been proven to be highly ineffective. HVAC systems are engineered for temperature control and not moisture-removal capacity. Running the blowers can spread dust and mold spores throughout the ventilation system and cause damage to HVAC equipment, coils, or filters. In addition, using HVAC systems prior to commissioning the building can lead to warranty issues and concerns. Because of these reasons, some building owners no longer allow the use of the HVAC system during the construction process.

The Seattle-based Northwest Wall and Ceiling Bureau (NWCB) says that proper temperature and humidity levels are important factors in achieving satisfactory results. It recommends desiccant-dehumidification systems for excellent results in drying applications.

Proper Drying Defined
A key factor here is “proper drying.” Principles of physics dictate that simply introducing heat into a building space is not an effective method to dry the air. Conventional HVAC systems are not designed to dry out construction-related moisture - the moisture load from wet materials is simply too large and the drying task too complex for systems that are intended for comfort.

In uncontrolled atmospheric conditions, water molecules that exist in the air exert vapor pressure on the materials of which it comes into contact. The warmer the air, the more water vapor is present.

Permeable materials (that includes almost everything inside a building) absorb water vapor to differing degrees. Unabated, this absorption process will continue to the point that equilibrium moisture content (EMC) is reached, when the material neither gives up nor takes on moisture from the surrounding air. By lowering the vapor pressure between the material and the ambient conditions around it, the material will begin to give up moisture.

Moisture will travel from areas of high vapor pressure within the material to the areas of lower vapor pressure surrounding it, which are being mechanically created. The moisture vapor will be desorbed from the material into the air and will be pushed out of the space by air movement.

Desiccant Dehumidification
An efficient, productive, reliable, and fast method of moisture abatement is aggressive drying through a desiccant-dehumidification system, which is becoming the drying method of choice during construction projects. Desiccant dehumidifiers have proven effective at creating low relative humidity and dew points when drying air at a condition far from saturation or at low temperatures.

Desiccant units used for drying are different than those used as permanent installations in commercial buildings. Portable units, delivered to the site on trailers, are designed to withstand the construction environment and to provide the drying capacity required to establish and maintain proper atmospheric conditions.

Portable, inflatable plastic ducts are used as part of the airflow system, precluding any reliance upon the HVAC-distribution system. Also, the temporary ducts can be moved easily as work progresses to other areas of the construction site. To be effective, the dry air must be contained. If a building under construction is open to the exterior, temporary enclosures may need to be erected to contain the dry air where needed.

Unlike cooling-based dehumidifiers, which cool the air to condense moisture and then draw it away, desiccants attract moisture molecules directly from the air and release them into an exhaust-air stream. Desiccants can attract and hold from 10 percent to more than 10,000 percent of their dry weight in water vapor. They are very effective in removing moisture from the air at low humidity levels and do not freeze when operated at low temperatures. The end result is an extremely dried air source capable of drying the most saturated materials.

Depending on the amount of moisture to be removed and the conditions present, the number of hourly air changes to be effective can vary greatly. Air-change rates can fluctuate depending on ceiling height, tightness of envelope, type of vapor barrier (or lack of one), outside weather conditions, and other variables.

Contractors can continue to work in the spaces being dried. However, they must be informed not to tamper with equipment, fans, or ducts. They must keep the envelope secure by closing doors and windows. If possible, the best results are achieved when work is limited in an area being dried.

In summer applications, consider combining temporary cooling systems in conjunction with the desiccant dehumidification to provide more comfortable working conditions for the workers while still creating the perfect drying conditions.

The Issue of Mold
Mold and fungus are present in almost all materials in commercial, industrial, and municipal structures. For example, just 1 square inch of surface on drywall may contain from 1 to 10 million spores.

In order to grow, mold requires air, suitable temperatures, and a moist nutrient. Of those, moisture is the major contributor as a “food medium” that sustains mold. The moisture does not need to be in liquid form. Because microscopic organisms need so little moisture, they can use what is present in solid materials, on the surfaces, or in the air as condensation or humidity. The key to mold control is moisture control.

Heat alone cannot reduce vapor pressure in an interior construction environment. As a result, materials cannot give up the moisture they retain. Because desiccant dehumidification reduces both humidity and vapor pressure, it is an effective method to dry construction materials. Materials dry in a matter of days, not weeks. The method also reduces the potential for mold growth and it establishes a more comfortable working environment.

Russ Brown is national accounts manager at Munters Moisture Control Services, Glendale Heights, IL, North America’s largest water-damage recovery/temporary humidity-control company. He can be reached at (800) 422-6379 or ([email protected]).

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