Where Is This Water Coming From?

By Thomas MacKinnon - EIT

All air on earth contains at least a little bit of moisture in the form of water vapor due to the earth’s atmosphere and climate¹. This means that water vapor is always present in the air around you and can condense into a liquid — with the right conditions. 

The amount of moisture in the air can be measured by the relative humidity, or the percentage of water vapor in the air compared to the maximum amount of water vapor that air of that particular temperature could hold. For instance in Las Vegas, NV, the most arid of the major cities in the USA, the average relative humidity is 30% meaning at that temperature only 30% of the maximum amount of water vapor is being held in the air. Phoenix, AZ is the next on the list of arid major cities with a relative humidity of less than 40% with most major cities averaging around 70%².

CityHumidity
Las Vegas, Nevada30.3
Phoenix, Arizona36.6
Denver, Colorado52.0
Salt Lake City, Utah55.3
New York, New York63.0
Washington, DC64.3
Oklahoma City, Oklohoma64.9
Dallas, TX65.4
Baltimore, Maryland65.8
Hartford, Conneticut66.0

Figure 1: Average Relative Humidity (%) for the Least Humid US Cities

The Dew Point is the temperature at which this water vapor being held in the air condenses into a liquid. The higher the relative humidity, the closer the dew point is to the temperature of the air; conversely, the lower the relative humidity the cooler the dew point is. For instance, at 68°F and 70% 

relative humidity, the dew point is 58°F while at that same temperature but 30% relative humidity, the dew point is 35F³.

If a surface is below this dew point temperature, the air around it will cool and the water vapor will condense into a liquid. 

Condensation: It’s 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 a temperature far below the average indoor dew point, these systems can quickly sweat and create an apparent indoor rain. 

Figure 2: Chilled Water Pipe in Hot, Humid Environment

Take for example the following 40°F chilled water pipe in a hot, humid space. The water vapor molecules in the 80°F air will condense into a liquid as the surface temperature (Ts) is far below the Dew Point of 72°F:

This is never an acceptable condition within any occupied space or even a mechanical room, so what can be done to prevent this from happening?

Preventing Condensation: Insulation

Keeping the surface temperature above the Dew Point, 72°F in this case, is of the utmost importance in preventing condensation from occurring. By adding insulation of the proper thickness to the system, not only are you saving energy by preventing heat gains throughout but also raising the surface temperature above the Dew Point (Figure 2). However, if the insulation is porous (such as a fibrous insulation), the water vapor will find its way through the insulation and condense on the cold surface of the pipe regardless of the insulation thickness! Therefore, when using a porous insulation material, a vapor retarder jacket is absolutely necessary to prevent the water vapor from passing through the insulation and condensing. 

Figure 3: Proper Insulation Thickness Prevents Condensation. If the insulation is porous, a vapor retarder jacket is also needed.

As Air Conditioning systems not only cool the space but also remove humidity, among other things, condensation control is also important with duct systems. The correct insulation thickness, with vapor retarder jacket if necessary, will prevent the humidity from condensing within or outside the duct system and prevent the same sweating as 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 engineer does not take into account the extreme humidity conditions of the space or if the system functions in an abnormal way, the insulation thickness will not be enough to make up for the increase in water vapor and condensation will form as the surface temperature falls below the Dew Point. The insulation must also be installed correctly; any gap in insulation or any small opening in the vapor retarder jacket will lead to 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. 

Part 2 of our series “Condensation and the Risks” will dive further into the causes and effects of these issues.  

Figure 4: Moisture Ingress

Figure 5: Corrosion Under Insulation (CUI)

Figure 6: Mold

Tom MacKinnon is the Eastern Market Development Manager for Armacell and works closely with Engineers, Owners, and Specifiers, answering their insulation questions and helping to create insulation specifications that meet their project needs. He is a Mechanical Engineer from Stevens and currently lives in Northern New Jersey. 

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¹http://articles.chicagotribune.com/2011-12-16/news/ct-wea-1216-asktom-20111216_1_relative-humidity-zero-dew-point

²https://www.currentresults.com/Weather-Extremes/US/low-humidity-cities.php

³http://www.dpcalc.org/