A Comparison: Dehumidification Versus Heat in Construction Drying Applications

Garrett Freeman

September 2009

The effects of moisture inside a building under construction can become a serious problem for contractors as well as a danger to future occupants. Many of the materials used today to construct buildings absorb moisture. As a result, building materials such as wallboard, fireproofing, millwork and concrete can experience a number of problems.

Unless this moisture is reduced to acceptable tolerances, a contractor may be confronted with a variety of problems. They may be as small as construction schedule delays resulting from slow dry times of building materials or as large as flooring failures resulting from elevated moisture content in a concrete slab. In addition to the fact that most flooring materials respond poorly to elevated moisture levels, water-based adhesives used to attach flooring product to the slab are sensitive and will not cure unless moisture content in the concrete is very low.

As a result, specifications for a number of flooring products demand that moisture content in a slab be reduced to exceptionally low levels in order to apply adhesives and finished flooring products such as vinyl composite tile or fiber-backed carpet. Similarly, hardwood flooring and cabinetry can absorb moisture. As these wood products take on moisture they will expand, which can dramatically affect the structural composition and integrity of the material. This results in difficulty with installation as well as finish appearance. While these problems are certainly costly as well as time consuming, another problem that can be considerably more severe is mold growth, which can adversely affect the building and its occupants during and post-construction.

To make matters worse, the building materials that absorb moisture and cause this wide range of construction issues are the same materials that require water as a part of their installation, application or finishing. Materials such as freshly sprayed fireproofing, joint compounds and tapping mud as well as concrete slabs and floor-leveling products all utilize a considerable amount of water to improve their application. Often, the ambient humidity resulting from the application of these products can negatively affect material dry times, and in some cases encourage mold contamination.

Without properly removing the necessary amount of moisture, water can remain trapped in building materials causing very serious bottle necks in the construction process.

Drying Building Materials While many people would assume that relative humidity is the key to drying a building material, there is actually a more important factor. If you plot the relative humidity and temperature of your construction space on a Psychrometric Chart (Psychrometrics is defined as the study of the dynamic properties of water-air mixture), you can determine the humidity ratio or actual amount of moisture that is present in your airspace. With this information, you are able to determine (through direct correlation) that the vapor pressure is the key to moisture management.

In uncontrolled atmospheric conditions, such as an interior construction space, water molecules that are present in the air exert vapor pressure on the materials with which they contact. Permeable materials, which include the majority of the materials inside a building, absorb water vapor at differing degrees.

By nature, moisture will travel from areas of high vapor pressure to areas of lower vapor pressure. If you are able to decrease the vapor pressure in the construction space to a level that is less than the vapor pressure of the building materials, the moisture vapor will be desorbed or released from these materials and forced out of the space through positive air movement. Under the right environmental conditions, you can naturally force excess moisture content out of your construction materials until they have achieved equilibrium moisture content or the point where a material is not releasing or taking on ambient moisture from the surrounding air.

A Comparison of Technologies
In an attempt to manage moisture, contractors typically use one of two methods to dry building materials during the construction process: heat or desiccant dehumidifiers. While there has been much debate as to the most effective technology for construction drying applications, scientific fact cannot be argued.

If one were to evaluate a dehumidifier and heater with respect to their moisture removal capabilities, a heater does not compare. To simply introduce heat into a building space will not remove the actual moisture that is present in the air. By design, a heat-only system will increase the temperature in an airstream by forcing air over some type of heating apparatus. For example, a direct-fired heater passes air directly over a flame that is fueled by propane or natural gas; in comparison, an indirect-fired heater will pass air through a heat exchanger so that the supply-air does not come into contact with the combustible materials being emitted from the fuel source. In either case there is one constant: a heat-only system does not have an internal component that removes, or does anything, to address moisture and subsequently, vapor pressure.

Having established this, a logical question is, why have we used heaters to help dry construction projects in years past? Although simple, the answer can be quite confusing. As the heater raises the temperature in a construction space, the amount of water in the air becomes lower relative to the increased air temperature. There is no actual moisture removal that occurs as a result of increasing the air temperature; however, the humidity is lower relative to the increased air temperature. As a result, (relatively) improved environmental conditions are created in the construction space.

In direct comparison to a heating system, as air passes through the desiccant material inside of a dehumidifier, the moisture is literally assumed by the desiccant material and consequently removed from the airstream. Because there is a direct correlation between moisture content and vapor pressure, by removing moisture from the airstream, one simultaneously lowers the vapor pressure of that airstream.

Finally, when comparing heat and dehumidification it is imperative to evaluate the operating costs associated with each technology. As highlighted earlier, a heating system requires a fuel source that is either passed directly or indirectly over a flame to generate heat. The boilers or heat exchangers required for these systems consume large quantities of diesel, propane or natural gas to generate high temperatures.

While upfront costs for heat-only systems are relatively low, the additional expense from large operating costs resulting from fuel consumption dramatically increase the overall costs associated with using this type of technology. In comparison, the initial costs associated with desiccant dehumidification are justifiably higher as the technology is much more complex.

Despite the initial expense, operating costs associated with dehumidifiers are comparatively lower than a heat-only system as they require much less fuel to perform their function. Additionally, recent innovations like remote monitoring/control of equipment and energy efficient dehumidification equipment have created additional savings with respect to operating costs.

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

An inflatable plastic ducting system is attached to the dehumidifier and used to evenly distribute dehumidified air throughout the construction space. This creates a uniform drying condition in all areas of the project and eliminates the need to utilize the permanent HVAC distribution system. For flexibility, this temporary ducting system can be moved or redirected easily as work progresses to other areas of the construction site.

To maximize the effectiveness of a desiccant system, the dry air being supplied to the construction space should be contained and imperfections in the building envelope should be minimized to reduce penetration from outside ambient conditions. While this may sound like an unrealistic requirement, building envelope enclosure and/or temporary containment are becoming much more important as the concerns of the construction industry shift toward things like LEED® design, indoor air quality and mold prevention.

Unlike cooling-based dehumidifiers, which cool the air to condense moisture out of the airstream, desiccants attract moisture molecules directly from the air and release them into an exhaust airstream. Desiccants can attract and hold as much as 10,000 percent of their dry weight in water vapor making them very effective at removing moisture regardless of low humidity levels or freezing temperatures. The end result is an extremely dry air capable of moisture removal in the most saturated of materials.

Because the moisture removal capacity of a desiccant dehumidifier is so abundant, on-site environmental conditions (temperature and relative humidity) and the specific material you are attempting to dry play a part in determining the amount of dehumidification you require. Additionally, time of year (weather), project dimensions, vapor barrier issues and project schedules are all factors that affect sizing of a dehumidification system. Whether it is to satisfy a LEED credit, accelerate a project schedule or solve a moisture problem, every construction drying application requires a specific moisture level. Depending on the time of year, one might consider combining temporary cooling or a heating system in conjunction with a desiccant dehumidifier to provide a more comfortable working environment while still creating ideal drying conditions.

Heat-Only Systems
Heat can temporarily reduce relative humidity but it does not remove moisture from the air and, most importantly, it does not reduce vapor pressure. In fact, heating the space with particular types of heat-only systems, such as direct-fired heaters, will actually add moisture to a space as combustibles such as propane or natural gas are burned. In addition, it is typical that some type of fuel storage tank be located next to the direct-fired heater either close to the building to minimize heat loss or inside of the construction space. As a result, safety has become a huge concern and increasingly, fire codes prevent the use of direct-fired heating systems on construction projects.

As the general construction industry continues to increase its focus on indoor air quality, the use of this type of heater will likely decrease or become much more restrictive. Likely the largest concern with respect to heating systems is that there is no moisture removal taking place when utilizing this technology. For this reason, heaters are very ineffective at maintaining conditions and eliminating mold growth.

Trade Organizations Weigh In
For most professional trade organizations there is no longer debate as to which technology is most effective for drying building materials. For example, the Northwest Wall and Ceiling Bureau (www.nwcb.org), which serves general contractors, material manufacturers, dealers as well as labor groups, in a statement about wallboard in weighed in technical bulletin #303, said, "Cold damp weather contributes to joint bond failure, delayed shrinkage, ridging, nail pops, joint flashing and board sagging. Proper temperature and humidity levels (environmental conditions) are important factors in achieving satisfactory results. Some sources of temporary heat will cause high humidity. The NWCB recommends a desiccant dehumidification system for best results in joint finishing and final decoration of gypsum wallboard.”

A Common Misconception
While dehumidification and heat are very common in construction drying applications, there are contractors who will attempt to utilize the building’s permanent HVAC system to address moisture problems during the construction process.

An HVAC system is engineered (primarily) for temperature control (thermal loads) with much less attention paid to moisture removal. As such, an HVAC system does not have the moisture removal capacity to address the elevated ambient moisture that is generated during the construction process.

In addition, early commissioning of the HVAC system can spread construction dust and mold spores throughout the permanent ducting system, compromising the permanent system, and affect the warranty of the HVAC system. For these and other reasons, many building owners and HVAC manufacturers do not allow the use of a building’s permanent HVAC system during construction.

Moisture Control = Mold Control
The facts regarding mold growth are well established. We know that in order to grow, mold requires spores, moisture, temperature and a food source. Unfortunately, spores are everywhere and rarely can be seen by the human eye, and are capable of withstanding freezing conditions as well as temperatures as high as 160 degrees Fahrenheit. In addition, mold spores will eat anything organic, which includes most everything on a construction site.

One requirement that can be controlled is moisture. By lowering the amount of moisture in the construction space to particular levels, it is possible to create an environment within the space where mold growth, reproduction and sustainability are impossible.

In summary, moisture can wreak havoc on a construction site affecting all types of building materials. By factually comparing the two most common technologies used for construction drying applications, it is evident that heat alone cannot reduce the moisture content or vapor pressure that is present in a construction space. The materials throughout a site that naturally absorb moisture will not dry in a timely fashion, and in some instances, this unabated moisture can cause serious construction bottlenecks or larger, long-term problems.

As the construction industry has evolved, and through practical application as well as scientific fact, it is evident that a desiccant dehumidifier is much more effective at drying construction materials than a heater. Because a desiccant dehumidifier reduces both humidity and vapor pressure, it is the most efficient method for providing a construction environment where materials dry at an accelerated pace and the potential for mold growth is considerably decreased.

Garrett Freeman is industry sector manager, North America with Munters Moisture Control Services. MCS is North America’s largest temporary humidity control and water/property damage restoration company, providing full restoration and emergency services through 30 offices in the United States and Canada. Freeman can be reached at 800.MUNTERS (686.8377) or by e-mail: garrett_freeman@munters.com.