Walking through a mechanical room, you are surprised to find a small puddle on the floor  in front of you. “How did that get there?” you wonder, as you start looking around for the source. Something catches your eye, and you see a drop of water falling from above you. Does the roof have a leak? Did someone spill something above you?

Wait, is that pipe sweating?

Water vapor is part of the air around you, as water moves through the water cycle, and it is an important part of transferring heat and energy around the world.¹ All air has some water vapor in it, meaning it is always present in the air around your mechanical systems. Given the right conditions, this water vapor will condense into a liquid and will greatly affect the performance of your system.

To determine whether the conditions will lead to condensation, knowledge of the relative humidity and dew point are key. The amount of moisture in the air can be measured by the relative humidity (sometimes abbreviated “RH”), defined as the percentage of water vapor in the air compared to the maximum amount of water vapor that air at that temperature could hold. For instance, in Las Vegas, Nevada, the most arid of the major cities in the United States, the average relative humidity is only 30%, meaning that, on average, only 30% of the maximum amount of water vapor at that temperature is held in the air. Most major U.S. cities average about 70% relative humidity. See Figure 1 for a sample.²

The dew point is the temperature at which water vapor in the air condenses into a liquid. The higher the relative humidity, the closer the dew point will be to the temperature of the air. Conversely, the lower the relative humidity, the cooler the dew point temperature is. For instance, at 68°F and 70% relative humidity, the dew point is 58°F, while at that same 68°F temperature and only 30% relative humidity, the dew point is a crisp 35°F.³

If a surface is below the dew point temperature, the air around it will cool, and the water vapor will condense into a liquid on the surface. Thus, maintaining the surfaces of below-ambient mechanical systems above the dew point temperature is paramount to controlling the formation of condensation.

Condensation: Raining Indoors

Below-ambient systems—such as chilled water, refrigeration, and cool air duct systems—are highly susceptible to the formation of condensation on their surfaces. With surface temperatures far below the average indoor dew point, these systems can quickly perspire and create enough condensation to form indoor rain.

Take, for example, the conditions in Figure 2: a 40°F chilled water pipe in a warm, humid space with relative humidity of 75%. The water vapor molecules in the 80°F air will condense into a liquid, as the surface temperature (Ts) of 40°F is far below the dew point of 72°F of the space.

This is obviously not an acceptable condition for a system, but what can be done to prevent it from happening?

Preventing Condensation: Use Insulation!

Keeping the surface temperature above the dew point, 72°F in the example, is of the utmost importance in preventing condensation. Adding insulation in the proper thickness to the system not only saves energy by preventing heat gains throughout the system, but it also raises the surface temperature above the dew point (Figure 3). However, if the insulation is porous, the water vapor can still find its way through the insulation and will condense on the cold surface of the pipe, regardless of the insulation thickness. If using a porous insulation material, a vapor retarder is necessary to prevent the water vapor from passing through the insulation and condensing.

These principles also hold true for duct systems. As air-conditioning systems not only cool the space but also remove humidity, condensation control is important with duct systems. The correct insulation thickness, with vapor retarder, if necessary, will prevent condensation formation on the duct surfaces, just like the below-ambient piping systems.

Why Is Condensation Forming, Even with Insulation?

Even after a system is insulated, condensation may still form as a result of a miscalculation or poor installation. If the mechanical system designer does not take the extreme humidity conditions of the space into account, or the system functions outside the normal design parameters, the insulation thickness will not be enough to make up for the increase in water vapor in the air, and condensation will form as the surface temperature falls below the dew point. The insulation also must be installed correctly: Any gap in insulation, or any small opening in the vapor retarder, will lead to condensation and must be sealed immediately.

In order to control condensation within a below-ambient system, an insulation material with low water vapor permeability must be chosen to prevent water vapor from passing through the material and condensing on the system. The proper thickness must be determined from the worst-case conditions in the space and can be confirmed with industry or manufacturers’ calculation tools. With the right thickness, low water vapor permeability, and proper installation, a system will be protected from the effects of condensation.

The Problem with Condensation

Condensation in a mechanical system does not just bring the nuisance of dripping water; it can also lead to devastating effects to the insulation or system itself. Moisture ingress is the absorption of water in a porous material that leads to an increase in thermal conductivity and the deterioration of the insulation system. Corrosion under insulation (CUI) can form when water gets trapped between the system and the insulation, heavily corroding the metal underneath. With the presence of water and a food source, mold is apt to follow any condensation that forms in the system.

Moisture Ingress: Soaking up Water Like a Sponge

Porous insulation materials rely on a vapor retarder to protect from water vapor accumulation. Unfortunately, these vapor retarders are not completely impenetrable and often get nicked or torn open during the regular maintenance process, or they are not completely sealed during installation due to difficult configurations or space constraints. With any gap in the vapor retarder, water vapor will start accumulating between the voids like a sponge soaking up water. For every 1% increase in moisture content in the material, a 7.5% loss in thermal value can be expected. After all the voids are filled, condensate will start to accumulate on the exterior surface of the insulation and the system itself, forming a thermal bridge with a thermal conductivity of water (4.1 Btu/(hr. °F. ft2/in) at 75°F mean
temperature). This thermal bridge causes large heat gains within the below-ambient system as the wet insulation allows heat to conduct to the system. When this occurs, the insulation that was used to prevent heat gain is now accelerating it, dropping the system efficiency precipitously. The water being held in close proximity of the system also can lead
to other issues, affecting the material you were trying to protect in the first place (see Figure 4 through Figure 6).

Corrosion Under Insulation

As alluded to earlier, one issue that can result from moisture ingress is CUI, or the formation of corrosion on the system surface when water is trapped between the surface of the system and the insulation. While CUI can form because of a system failure (leaks) or improper weather protection, it can also occur when condensation finds its way to the surface of a pipe through a break in the vapor barrier. Moisture ingress in porous materials can lead to CUI, as the insulation holds onto the water right next to the system itself, wrapping the metal with a wet covering and providing the means to form corrosion. CUI also can form if the water vapor finds a gap in the vapor barrier and proceeds to condense under the insulation. Any system that is subjected to corrosion will not operate as expected, as the metal starts degrading, and the maintenance cost of replacing the damaged system increases. Left alone long enough, this corrosion may lead to a complete failure of the system and the potential for catastrophic events.

Mold

Mold is various types of fungi that can grow on almost any surface that stays between 32°F and 120°F (optimally between 70°F and 90°F) with no air flow and that is damp with moisture.⁴ If condensation forms within the insulation and the insulation stays damp, it creates the perfect environment for mold growth to begin, often without any trace on the outside of the insulation. The mold can then propagate throughout the insulation and start to form on the surface, where it can travel throughout the air space and cause overall poor air quality within the space, potentially leading to allergies, rashes, and asthma attacks.

Conclusion: Do it Right the First Time

With below-ambient systems, condensation formation is always a risk. If the system is not insulated correctly, moisture ingress, CUI, and mold will soon follow the first drop of condensation. Not only will the insulation have to be replaced if the condensation formation is not identified in time, but the system piping, ductwork, or other components—along with any surrounding equipment that the condensation was dripping on—will have to be replaced as well. It is important to make sure the system is insulated with the right thickness of insulation so that the surface temperature is always above the dew point, and to use a complete vapor barrier to avoid condensation risks.

References
1. https://earthobservatory.nasa.gov/global-maps/MYDAL2_M_SKY_WV
2. Relative Humidity Averages in US Cities – Current Results
3. http://www.dpcalc.org/
4. Michael Pugliese, The Homeowner’s Guide to Mold, Reed Construction Data, Inc ©2006