Defining Wet Mechanical Insulation

July 1, 2021

Last month we discussed wet insulation in a metal building industry application. In this article, we address wet insulation in a tightly closed insulation envelope after the system has been in service for an extended period of time.

Defining “Wet”

In the mechanical insulation industry, phrases like “remove all wet insulation,” “do not install wet insulation,” etc. often appear in specifications and guide documents, raising the questions: What is the definition of “wet,” and should all insulation material system components be treated equally when it comes to those types of “wet” phrases? As expected, the answers depend on what definition you look at and whom you ask.

Merriam-Webster’s online dictionary definition of wet is “consisting of, containing, covered with, or soaked with liquid (such as water).” Though the general definition is clear, it does not answer the core question of what—from the perspective of the mechanical insulation industry—the definition of wet is. Is any insulation that contains water in any amount considered wet?

Being a liquid, as described by University of California, Santa Barbara (UCSB) ScienceLine, “water is not itself wet, but it can make other… materials wet. Wetness is the ability of a liquid to adhere to the surface of a solid.”1 So, in our industry, when we say something is wet, do we mean a “liquid is sticking to the surface of a material?”2 To borrow again from ScienceLine:

“Whether an object is wet or dry depends on a balance between cohesive and adhesive forces. Cohesive forces are attractive forces within the liquid that cause the molecules in the liquid to prefer to stick together. Cohesive forces are also responsible for surface tension. If the cohesive forces are very strong, then the liquid molecules really like to stay close together and they won’t spread out on the surface of an object very much. On the contrary, adhesive forces are the attractive forces between the liquid and the surface of the material. If the adhesive forces are strong, then the liquid will try and spread out onto the surface as much as possible. So how wet a surface is depends on the balance between these two forces. If the adhesive forces (liquid-solid) are bigger than the cohesive forces (liquid-liquid), we say the material becomes wet, and the liquid tends to spread out to maximize contact with the surface. On the other hand, if the adhesive forces (liquid-solid) are smaller than the cohesive forces (liquid-liquid), we say the material is dry, and the liquid tends to bead-up into a spherical drop and tries to minimize the contact with the surface.

Water actually has pretty high cohesive forces due to hydrogen bonding, and so is not as good at wetting surfaces as some liquids such as acetone or alcohols. However, water does wet certain surfaces like glass for example. Adding detergents can make water better at wetting by lowering the cohesive forces. [Detergents contain saturates that lower the forces in the water and improve wetting.] Water resistant materials…[are] made of material that is hydrophobic (water repellent) and so the cohesive forces within the water (liquid-liquid) are much stronger than the adhesive force (liquid-solid) and water tends to bead-up on the outside of the material and you stay dry.”

To some, “wet” usually implies saturation or soaked, but it may suggest a covering of a surface with water or something, as opposed to saturation. “Damp” implies a slight or moderate absorption or some lesser degree of moisture. “Moist” applies to what is slightly damp or not felt as dry, and “humid” applies to the presence of much water vapor in the air.

For most insulation materials, there is an ASTM test to determine the maximum allowable moisture in the material, usually expressed as a percentage by weight or volume. But that does not address the core question of defining “wet” insulation on a project site.
When you hear talk of corrosion under insulation (CUI) and wet insulation materials, you hear terms like corrosion inhibitor, hydrophobic, hygroscopic, and water resistant. Here is how NIA’s Technical Information Committee defines each of these terms.

  • Corrosion inhibitor
    A chemical or physical barrier that reduces the rate at which corrosion may occur—e.g., on pipe and equipment outer surfaces.
  • Hydrophobic
    When the material tends to repel or not to absorb water.
  • Hygroscopic
    Refers to a material’s affinity to absorb water or adsorp water vapor (humidity) from the atmosphere.
  • Water resistant
    The relative ability of a material to resist the influence of liquid water on its form or properties.


With the loss of insulation efficiency, increased energy consumption, and increased insulation weight (which leads to a whole different discussion), the discussion of wet insulation is driven by many factors, including dripping liquid, condensation, or CUI. CUI on carbon steel and alloy steel surfaces unfortunately is a common problem across all industries. CUI prevention should include routine inspection and maintenance with some form of nondestructive testing, like thermal imaging, that can identify potential moisture in the insulation and check the underlying pipe or equipment substrate. CUI often remains undetected until insulation is removed or there is a leak or other event, some of which can be extremely hazardous.

The problem occurs in carbon steels and 300-series stainless steels. On the carbon steels, CUI manifests as generalized or localized wall loss. With stainless pipes, it is often seen as pitting and corrosion-induced, stress-corrosion cracking. Though serious consequences up to and including failure can occur in a broad band of temperatures, Association for Materials Protection and Performance (, formerly known as NACE International, has concluded that CUI is more prevalent in the service temperature range of 25°F (-4°C) to 350°F (175°C) for carbon steel and 120°F (50°C) to 350°F (175°C) for austenitic or duplex stainless steel.

Insulation does not cause corrosion. In fact, to demonstrate that they do not cause corrosion, most insulation products are tested for their ability to induce or contribute to the start of corrosion. CUI is caused by the ingress of water and prolonged contact with the metal surface. The water can come from rainwater, leakage, deluge system water, wash water, or sweating from temperature cycling or low-temperature operation, such as refrigeration units. Water is the primary ingredient for CUI. In simple terms, in addition to oxygen and the unprotected metal surface, it takes three items to create CUI: temperature, prolonged exposure to moisture/water, and contaminants. Without water, corrosion will not occur. Therein lies the core issue and why the definition of “wet” becomes important.

Drying Out Insulation on the Jobsite

Some would say if insulation becomes wet in the field before installation, let it dry out before installing. That may compound the question. Not only do you have to define “wet,” but what does “dry out” mean? And who determines if it is wet or dried out?

To further complicate the question, it could be a wet pipe system and not the insulation. Sometimes a pipe or valve will develop a leak, and moisture will get trapped between the insulation and the pipe, creating the perception of wet insulation. In some cases, drains, vents, or some other method of drainage can be used on above-ambient systems, not below ambient service, to allow moisture an exit point.

There are moisture meters available to check for moisture intrusion in insulation. The challenge is where to check. It is easy to miss a pocket of moisture in insulation, whether it is a fibrous, cellular, granular, or other material. That can lead to erroneous moisture-content reading assumptions.

To compensate for this possibility, it is important to check for moisture from multiple angles and locations, whenever possible. This helps to ensure that a single space in the insulation does not give you the wrong impression about the insulation’s percentage of moisture content.

It also is a good idea to repeat measurements, moving a few inches out each time in all directions until you stop testing positive for moisture. However, that does not mean there are not additional “wet” areas.

It is also important to test the relative humidity, especially in an enclosed environment, building, etc. Moisture in the insulation system may add extra water vapor to the air, increasing the humidity. Testing for humidity in a structure may help identify potential moisture-compromised building material, including insulation.

In addition, you cannot discount the damage to some insulation materials that may be incurred in checking for moisture in the material or system, or the potential of contaminants remaining in the insulation from the moisture. Often so-called “non-destructive” testing requires the penetration or removal of insulation, which in turn requires repair or corrective action in a timely manner to restore the system and function.

The bottom line: There is no easy answer to the question of how to define “wet” when it comes to a mechanical insulation system. There are, however,
a few basic principles that may apply to all
insulation systems.

  1. Keep all insulation materials dry during storage, handling, and installation, until they are totally protected (watertight) with the specified protective covering.
  2. When in doubt, throw it out.
  3. Not all insulations are the same, and each should be addressed given its own characteristics, water resistance, hydrophobic properties, etc.
  4. Consult with the respective insulation manufacturer(s) before beginning the work for recommendations related to defining wet insulation and handling of the same. Do not assume.
  5. Take into consideration the installation
    conditions, surrounding environment, and operating temperatures.
  6. Has the surface that is being insulated been properly protected/coated?
  7. For above-ambient systems, has the insulation system been designed to allow for the escape or venting of any moisture that may have entered—or will enter—the system?
  8. Have the piping and equipment systems been designed with consideration of the insulation system, and the insulation system designed to guard against any areas where ponding or accumulation of water could occur?
  9. Installation should never take place in inclement weather or when inclement weather is anticipated to occur before the work is properly protected.
  10. On previously installed systems, all areas in which the protective covering has been compromised, and other areas that potentially allow for the entrance of moisture or other contaminants, should be examined in a timely and continual manner to determine the extent of any damage to the total insulation system; and the required repairs/replacement should be executed in a timely and proper manner by experienced craft personnel.

While water may be the “elixir of life,” as Chinese philosopher Lao Tzu said, “Nothing is softer or more flexible than water, yet nothing can resist it.” Whether it is slowly wearing down a landscape through erosion, or sweeping away a structure in a flood, water is indeed difficult to resist. This is true for insulation, as moisture can permeate many insulations and insulation systems and potentially lead to host
of concerns.

Good luck and stay dry.

2. Ibid.