Condensation Control not a Matter of Chance

Winfield T. Irwin

July 1, 2001

Along with energy conservation, personnel protection, and process control, thermal insulation plays an important role in condensation control. The most common applications where condensation control is important are on pipes and ducts carrying water, air and other fluids at temperatures that are lower than that of the ambient air.

Unless surface temperatures on the outer surfaces of these pipes and ducts are higher than the dew point temperature of the ambient air, water will condense from the air onto these surfaces. Without a continuous vapor-retarder membrane on the outer surfaces of the insulation sealing the insulation, joints, seams, and penetrations, condensation will occur between insulation sections and condense and collect between the insulation and the cold metal surfaces of pipes and ducts.

Water poses potential problems for the owners whether it collects on the surfaces or behind the vapor retarder or both. These problems include deterioration of the thermal performance of the insulated system, metal corrosion, wood deterioration, mildew and mold growth, objectionable odors, complaints on the indoor air quality, and callbacks for insulation contractors.

The Blame Game

Many of these callback sessions can slide into blame assessment meetings involving the owner (who has an obvious problem that needs corrective action at someone else’s expense), the engineer, the insulation contractor, and the manufacturers of the insulation and accessory materials. Most of us have been there and done that.

During these callback sessions, the engineer blames the contractor for not following the specifications and for using inferior workmanship, and the engineer also blames the manufacturer for supplying inferior materials. The contractor blames the engineer for ambiguous specifications and blames the manufacturers for inferior products. The manufacturers blame the engineer for inadequate design and specifications, the contractor for inferior workmanship, and the owner for the way the systems are operated.

It is too bad that meetings of the interested parties too often occur after a problem is thrust on the owner. None of the parties involved anticipate that condensation problems will happen on the job when the systems are placed in service. They all feel confident that their individual contributions to the final installation are excellent. It follows, then, that any problem must be attributed to others. The proper time for a meeting is while the project is in the planning stages before the designs are completed and specifications are written.

Effective condensation control is not a matter for chance, compromise, or cost minimization. The laws of physics are not subject to the wishes of engineers, contractors, manufacturers, or owners.

Communication is Key

Open and frank discussions among all parties about the operating conditions at the building site can help the engineer select the proper design criteria for the ambient air, define the pipe and duct dimensions, and select the insulation materials, facings or jackets. In addition, the discourse will aid the engineer in calculating the thicknesses required for the intended service, and in selecting the method by which a continuous and effective vapor-retarder membrane is to be provided on the exterior surfaces of insulated pipes and ducts intended for cold service.

As a result of the interaction among the parties, insulation contractors may become more sensitive to the importance (during installation) of providing a continuous vapor-retarder membrane at seams, joints, penetrations, hangers, and supports.

Insulation manufacturers have the opportunity and obligation to provide their recommendations on how their insulations must be installed and maintained for long life in cold service. Some insulation materials may require a factory-applied effective vapor-retarder facing or jacket. Installation recommendations must be provided for all field-applied, vapor-retarder treatments including the closure for all joints, seams, and penetrations.

Owners should become more sensitive to how their operational procedures may adversely affect the ambient air conditions in service. Decreasing the operating temperatures of the fluids in pipes or ducts, increasing the relative humidity of the ambient air, and decreasing the dry-bulb temperature of the ambient air are examples of changes that can lead to condensation problems.

Series Heat Flow Path

There is no reason for any of the interested parties to remain uninformed on the basic principles of condensation control. The laws of physics involved are not so complicated that we, as interested parties, cannot learn and use them.

Pipes and ducts that carry cold fluids have heat transferred from the ambient air to the cold fluids inside the pipes and ducts. This heat must pass through an outside surface film which offers some resistance to that flow of heat. Then, that heat must pass through the insulation, if present, through the metal walls of pipes and ducts, through an inside surface film, and into the cold fluid. This path is called a series heat flow path because the heat moves through each of the components in sequence from the warm side to the colder side.

For our purposes, the thermal resistances of the pipe and duct walls and the inside surface film are so low that they have insignificant impact on the amount of heat transferred. Uninsulated pipes and ducts have only the resistance of the outside surface air film to impede the heat flow.

Open joints and seams in insulation provide a heat flow path parallel to the path through the insulation. Most of us pay little heed to this low-resistance parallel path because we think our specifications and good intentions will result in an installed insulation system where these gaps do not occur.

The extent of these parallel heat flow paths is in the skilled hands of the insulation contractor. Engineers must address this in their specifications. The owners must consider this and take corrective action when installed systems are mechanically abused and the vapor-retarder system is compromised and the insulation is damaged.

Determining Insulation Thickness

Engineers are expected to select the type and thickness of the insulation and the vapor-retarder system to be used for condensation control. They have many insulations from which to choose. The manufacturer of each type of insulation extols the virtues of its insulation and vapor-retarder treatment. Sometimes the zeal that stresses the inherent vapor-retarder properties of an insulation overshadows the requirement that these insulation systems must be provided with a vapor-retarder membrane sealing all joints, seams, and penetrations of the insulation.

Once the insulation material is selected, and the insulation outer surface is identified, the engineer must calculate the thickness of insulation needed to keep the surface temperature of the outer insulation surfaces higher than the design dew point temperature of the ambient air.

Determination of the design dew point temperature of the ambient air is a critical step in the process. The use of average values from weather data for dry-bulb and wet-bulb temperatures, or percent relative humidity for design purposes is almost certain to result in surface condensation problems in service whenever the ambient air has a wet-bulb temperature or percent relative humidity higher than the design value.

Instead, the designer should select for design purposes those values that would reflect the worst case conditions. The dew point occurs at that temperature where the wet-bulb and dry-bulb temperatures are the same. At this condition, the air can hold no more water vapor. Any decrease in a surface temperature below this point will cause water to be condensed.

Next, the engineer must determine the thermal resistance of the air film. This resistance is a function of the surface emittance of the exposed surface, and the velocity of the air around the pipe or duct. Those things which increase this resistance are low emittance (such as shiny aluminum at 0.05 for FSK versus 0.90 for ASJ) and zero wind speed (still air).

Engineers and contractors should not overlook this important technical point: the higher this surface resistance is, the greater will be the insulation thickness required to keep the surface temperature above the dew point temperature. This is a potential problem for engineers concerned over designs for condensation control and personnel protection. An evaluation may be needed to determine the cost impact for increased insulation thickness versus selecting finishes of high emittance and providing air circulation around pipes and ducts.

Once the design parameters are defined, the engineer has access to several computer programs that use ASTM C 680 to calculate the insulation thickness required. Keep in mind, that in the case of pipe insulations, the thickness required to prevent condensation is also affected by the pipe size. The technical explanation for this is that the heat transfer area per unit length on the outside of the insulation is greater than the heat transfer area of the pipe.

Finally, the engineer must write a clear, unambiguous specification. Insulation contractors must read, understand, and build a quotation that provides the materials selected and accepts stipulations on installation requirements. Insulation contractors must then order the specified materials from the manufacturers who must provide quality assurance for the products shipped to a job site. The engineer must inspect the quality of materials delivered, and the quality of the workmanship during installation.

Irwin Rule of I’s

After going into service, the owner must not alter operating conditions without first confirming that condensation control has not been compromised. So what should happen if a condensation problem occurs despite cooperative meetings before construction? There is no single simple solution. I like to implement the Irwin Rule of I’s.

Interrogate the owner about operating condition Xs in effect when the problem appeared, and highlight any discrepancies compared to the design assumptions.

Investigate to confirm that the insulated systems are installed as specified, and highlight any discrepancies. Insist that the insulated system be examined to determine if the condensation is confined to the exterior surfaces, or if present also behind the vapor-retarder membrane. Should the condensation be limited to the outer surfaces, the contributing factor may well be that the ambient air conditions are more severe than design parameters. Should the insulation be wet, or the space between the insulation and the pipe or duct is wet, the contributing factor is most likely a degradation of the vapor-retarder system especially at joints, seams, or penetrations.

Insist that wet insulation be removed, discarded, and replaced with dry insulation, and insist that the vapor-retarder system be repaired or restored to design levels. Wet insulation systems will not dry out in service. Condensation on or in insulation systems provides sites for mold and mildew growth and propagation. Presence of water on the cold side of the vapor-retarder membrane degrades the thermal performance of the insulation. This, in turn, contributes to a decrease in the surface temperature and permits condensation at an even lower wet-bulb temperature and relative humidity.

Indicate, after interrogation and investigation, the probable cause for the problem. Once the problem origin is identified, a fair and reasonable assignment for the cost of remedial actions should be possible. Our past experience should tell us that there is usually enough blame to go around.

Not a Matter of Chance

The owner, engineer, insulation contractor and manufacturers must be in thorough communication from the very beginning of the project in order to prevent condensation problems. Each party should learn from one another the factors affecting condensation control. By gaining familiarity with the entire process, each party can ensure that none of the work is compromised. For example, contractors will know there’s a possible moisture problem before the insulation is installed, if the vapor-retarder membrane is specified to be provided on the cool side of the assembly.

It is critical that the design parameters are accurate. Determining the correct insulation thickness and selecting the appropriate vapor-retarder system are paramount to successfully controlling condensation.

Effective condensation control is not a matter of chance, it takes planning, communication and cooperation among all parties.