"You Mean There's a Difference Between a Moisture Barrier and a Vapor Barrier??!"

The terms Weather Barrier and Vapor Barrier are often used interchangeably. This is because their primary function is to keep out the ingress of water. Water, in any of it's forms: moisture, frost, ice, etc., is the primary enemy of an insulation system. However, Weather and Vapor Barriers represent two very different functions.

Weather barriers protect the insulation and any jacket which may be applied to the insulation from the elements; such as rain, snow, etc. A weather barrier appears in either jacket (cladding) or mastic form. It may cover (protect) a vapor barrier on a cold system or, be used alone (jacket/cladding) on an ambient or above ambient system. Typically ambient and above ambient operating systems are not totally sealed. They need to "breathe"; or allow any moisture to escape.

A cold system is defined as any system operating below ambient or below the dew point temperature. Vapor barriers are installed on cold systems. As moisture (humidity) always moves from warm to cold; the vapor barrier retards the flow of moisture (humidity) from passing through the insulation to the cold pipe. To qualify as a vapor barrier, the perm rating must be below 0.1.

To clarify the effect of moisture on insulation in an ambient or above ambient operating system: Between +32 °F and +212° F, water will stay in the liquid phase if allowed to migrate within the insulating system. Water can adversely impact the K Value of those insulations susceptible of retaining water; as the K Value of water (4.15) is much greater than the K Value of insulation (usually less than .35).

Insulation products that are not impervious to water that are installed on systems operating above +212 °F (above the boiling point of water) typically retain moisture through a portion of the insulation thickness. This is because enough insulation must be installed to provide a surface temperature which is below +140° F to protect personnel from the possibility of a burn. At a temperature of +140° F, the pipeline is no longer hot enough to "steam" away the moisture. The result is reduced efficiency of the insulation when compared to a totally dry insulation material.

To clarify the effect of moisture on insulation in below ambient operating systems: For pipe temperature ranges of less than +32° F, it is important to install a vapor barrier over insulation prevent moisture ingress, as water will freeze if allowed to migrate to a point in the insulating system at or below +32 °F. The K Value of ice is 15.4. The formation of ice provides an internal destructive force on the insulating system. Ice can quickly destroy the insulating envelope through freeze thaw cycling caused by ambient air temperature changes.

Vapor and moisture barriers are available as a mastic coating or as a jacket (cladding) system. It is important to identify the characteristics of each product when selecting the components of your insulation system and install the products per the manufacturers direction.

Typically mastics are applied on vessel side walls or tank top heads, irregular shapes such as fittings, valves and flanges and sometimes on pipe covering; especially in a cryogenic type service. Mastic coatings, when installed, are installed in two layers. This requires brushing, troweling or spraying a tack coat; usually 3-4 gallons per 100 square feet. A mesh fabric is then embedded into the mastic to provide strength to the product. Then a finish coat of 3-4 gallons per 100 square feet is then applied in the opposite direction of the first coat.

Vapor barrier coatings will keep moisture (humidity) from entering the system. Weather barrier coatings will allow any moisture that has been trapped in to exit. The moisture will exit in the form of water vapor.

As the effects of moisture (water) are much more dramatic on a cold system versus a hot system, it is imperative to install and maintain the vapor barrier. It is critical that no pinholes be present when completed.

Vapor barrier jackets are found mostly on pipeline insulation. Laminated membrane products are the products of choice, although ASJ is often used. Usually they are protected with a cladding or weather barrier jacket; such as PVC for indoor and Aluminum for outdoor locations. Again, proper sealing of each jacket is of the utmost importance. Note: the kraft paper liner on aluminum is a moisture barrier, not a vapor barrier. The kraft paper is a liner to provide protection from the effect of dissimilar metals.

The best products may be specified and purchased. However, if the installer is not careful and conscientious, the system will not perform as intended. Additionally, as the line operates, it must be continuously inspected for any breaches in the vapor and/or weather barrier to protect the insulation from moisture infiltration. If any damage is sighted, it is imperative to take action immediately and repair it.

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