Ways to Minimize Water Vapor Migration

Ed Sore

November 1, 2006

The insulation industry has undergone profound changes over the past 50 years. Increasing energy conservation needs and evolving technologies have required major advances in insulation materials, application, and cost-efficient systems for end users. Industry advancements have included new jacketing materials that have self-sealing laps, improvements in vapor-retarder materials, removable and reusable insulation covers, and increased material thickness to provide greater energy savings and return on investment. Specifications calling for lower-perm products, as well as safety and closure procedures in installation, are tangible evidence of how the industry has responded to challenges like corrosion under insulation (CUI). Here is a brief review of CUI, with a discussion of some ways to address it.

Maintenance engineers and coordinators, process and project engineers, and operation managers continue to grapple with the problem of CUI, one of the most expensive facility-related problems facing industry today. Recent studies suggest that CUI costs industry millions of dollars annually, and CUI problems account for more unexpected facility downtime than all other causes combined. The increase in corrosion failures under thermal insulation makes CUI a particular concern for petroleum, chemical, food-processing, and other types of industrial operations.

CUI is caused by the ingress of water into insulation, which then traps the water like a sponge, keeping it in direct contact with the metal surface. The source can be rainwater, leakage, deluge-systems water, wash water, or sweating from temperature cycling or low-temperature operation, as in refrigeration units. It also occurs because of the availability of oxygen trapped in the insulation. As temperatures increase, moisture cannot escape. Chlorides and ions in insulation also greatly promote corrosion.

Without a 100-percent vapor seal, moisture seeps in, causing cracking and subsequent corrosion. With metal jacketing, caulking is required to seal the joints. When temperatures change, materials expand and contract, causing cracks in the caulking and allowing moisture to permeate. Similarly, materials without zero permeability, such as mastic, can crack. This also allows moisture into the insulation system.

The American Petroleum Institute (API) code 570, “Inspection, Repair, Alteration, and Rerating of In-service Piping Systems” (the piping code first published in 1993), identifies CUI as a special concern. This code is one of the industry standards, along with NACE’s RP098-98, and the OSHA 1910 rule demands that organizations maintain a program to meet the standard.

Hidden Problems

CUI is often difficult to detect because the insulation cover can mask the problem until it is too late. It is expensive to remove the insulation, especially if asbestos is involved. Various methods are used today to inspect for CUI, including radiography, ultrasonic spot reading, and X-ray. New diagnostic technologies—online, real-time corrosion monitoring systems, and others—are rapidly reaching the market. Clearly, however, prevention is still the most cost-effective approach to controlling CUI.

The problem is pervasive because most installations involve a host of environmental factors. Once water ingress occurs, there’s no stopping corrosion. Since many facility or maintenance managers cannot continuously monitor the roof, piping, or ductwork, the problem eventually makes itself known. Then costly, disruptive repair is required.

Methods of Sealing Insulation

There are many methods of sealing insulation to prevent outside water from seeping through the insulation material. Vapor-barrier mastics can be used for this purpose. Applied by spray, brush, or roller, these come in a variety of colors and are applied in several layers. Using metal jacketing—both stainless steel and aluminum—in various thicknesses is the most common method of protecting insulation materials. This is now provided with inner linings that offer tremendous chemical resistance. Once the metal jacketing is applied, the seams are often caulked with a silicone or other type of sealant to prevent the ingress of water through the lap. Other new jacketing products that provide extremely low perm ratings include polyvinylidene chloride resins, which lock out
oxygen and moisture, frequently used on urethane, Styrofoam, and foam glass because of their excellent vapor-barrier properties.

Materials used to circumvent CUI must be capable of withstanding thermal cycling and continuous exposure to elevated temperatures. They must also be resistant to ultraviolet (UV) rays. If a coating is not UV resistant, it will begin to degrade, and its effectiveness will be reduced, eventually causing failure.

New Facing Materials

Installing insulation is difficult, time-consuming, and labor intensive. Typically, insulation will have to be replaced two to three times during the lifetime of a plant, at high cost and with significant disruption. While no product can guarantee total CUI prevention, new zero-permeability facing materials are gaining increased acceptance by engineers and installers. And some contractors are now adding these insulation jacketing systems to project specifications because they believe that they can deliver a quality, cost-effective solution.

The all-purpose insulation jacketing systems designed for sealing insulation over ductwork in harsher environments offer 100-percent self-adhesive overlap and bonding capabilities, and provide complete coverage over insulation. The material is weather resistant and tough, but it is easier to install. Pressure-sensitive jacketing requires no additional rivets, strapping, sealants, or off-site fabrication. And when it is installed properly, the chance of moisture getting in is low.

Reducing cost by eliminating or mitigating corrosion damage and failures while also increasing run time and productivity are goals on which everyone can agree. The perfect water-tight system remains the ultimate goal of engineers, specifiers, and owners. And with innovative products like zero-permeability, pressure-sensitive jacketing systems, they are one step closer to solving the CUI dilemma.