Insulation Finishes—This Month’s Topic: Mastics and Coatings

March 1, 2014

Product Characteristics of Weather Barriers,
Vapor Retarders, and Finishes

Mastics are
available in numerous formulations and are designed to protect insulation from
physical, chemical, water, and weather damage. They can be broken up into
special-use classes, as described below; and the selection of the proper mastic
will depend on the insulation type, equipment, piping or duct operating
temperature, fire hazard classification required, expected service life, and
other conditions. Mastics can be applied to protect the entire insulation
system surface, facing materials over insulation, or over irregular insulation
surfaces such as sprayed polyurethane foam systems; bends and elbows; protrusions
such as flanges, valves, supports; or insulation terminations where sheet
materials cannot be effectively applied. They are most often applied by brush,
trowel, or spray in 2 coats at the manufacturers? recommended application rate,
with a reinforcing mesh embedded between the first 2 coats. Typical reinforcing
meshes are made of synthetic fibers, fiberglass scrim, or cloth and canvas
cloth. The mastic manufacturers? application guide should be consulted for
selection of the proper reinforcement to use with the mastic chosen.

Properties and
tests commonly considered in the selection of a mastic are given in ASTM C647,
Standard Guide to Properties and Tests of Mastics and Coating Finishes for
Thermal Insulation.

Mastics are
broken up into the following types and sub-types:

  • Vapor Retarder (Vapor Barrier) Mastics and

    • Solvent-based
      thermoplastic rubber/resin types.


      • Cryogenic applications (below

      • Severe chemical environments


      • Fire resistive—meet Class A flame
        and smoke

      • Highest performance of vapor

      • Lowest permeance

    • Water-based
      synthetic polymers types


      • Low-temperature piping and
        equipment (-40°F to ambient)

      • Sealing seams, punctures, and
        terminations of vapor retarder facings

      • Chilled water, air-conditioning
        duct, brines


      • Fire resistive—meet Class A flame
        and smoke

      • Low hazards during application and
        shipment—low toxicity and no fire hazard

      • Permeance:
        dependent on type—below 0.5 perms

      • Solvent-based
        asphaltic types


      • Buried pipes

      • Exterior low-service temperature


      • Chemical resistant

      • Poor fire resistivity

  • Weather Barrier (Breather) Mastics and Coatings

    • Water-based
      synthetic polymer type

      • Most common type on the market

      • Provide weather protection

      • Keep liquid water out

      • Allow water vapor to pass through
        over hot equipment

      • UV resistant

      • Protect vapor retarder facings
        (FSK, ASJ)

      • Exterior ductwork and piping

      • Weather protection

      • Physical protection against

    • Water-based
      asphalt emulsions

      • Older technology

      • Low material cost, but high labor cost

Mastic Characteristics

selecting a mastic, the following general characteristics and uses should be

Vapor retarder mastics are designed to prevent the
ingress of water vapor into cold insulation systems in addition to protecting
against mechanical abuse, liquid water intrusion, and weather. Permeance of
vapor retarder mastics will vary greatly, ranging from 0.5 perms to <0.01
perms, depending on the mastic type and performance requirements. Most
manufacturers will provide information on the mastic?s permeance on their
product data sheets. It is important to consider the test temperature, test
relative humidity, and film thickness when comparing the permeance of a mastic.
Changes in any of these properties will affect the permeance of any mastic.

insulation systems with respect to mastics can be further defined by:

  • Cryogenic
    service (operating below -40°F)

  • Low-temperature
    service (-40°F to 32°F)

  • Cool/cold
    service (33°F to ambient)

Cryogenic insulation systems
require specialized engineering beyond the scope of this column. Mastics and
coatings for these uses have very low permeability (<0.02 perms) and include
specialized vapor stop coatings with extremely low service temperature limits
(down to -320°F), and solvent-based thermoplastic rubber (Hypalon) mastics.
Contact the mastic manufacturer for assistance in selecting these materials.

service mastics should have permeance of <0.02 perms. These products include
solvent-based thermoplastic rubber and water-based synthetic rubber mastics.
The solvent-based mastics will typically have the lowest permeance, highest
chemical resistance, and longest service life; however, they may be restricted
for use in some regions, are combustible during application, and require
solvents for cleanup. Some water-based mastics have permeance values below 0.02
perms, can be used in all regions, and have the added advantages of being
non-flammable during application and easily cleaned with  water.

Cool or cold-service insulation
includes insulation of chilled water piping, air conditioning, ductwork, and
other systems operating between 33°F and ambient. The proper mastic and
permeance requirements for these systems will depend on whether the system is
interior or exterior, the facing on the insulation, the likelihood of physical
or mechanical abuse, the climate (high versus low-humidity environment), and insulation
type. There are still some solvent-based mastics used for these applications;
however, in most cases, water-based mastics will meet the required performance
and are preferable. The vapor retarder system, including any sheet facing
materials and mastics, should have permeance <0.05 perms, per ASTM C755.

In many cases insulation for duct
systems or piping in warm humid climates will be faced with a FSK, ASJ, or
other vapor-retarder jacket. In this case, water-based mastics with permeance
<0.5 perms are typically acceptable for vapor-sealing punctures (from
hangars or pins) and seams in the facing on interior applications. These
mastics can also be applied over the entire facing surface to provide
additional physical protection, if required, or physical and weather protection
of the facing on outdoor insulation. If the insulation is not faced with a
vapor retarder jacket, at insulation terminations, or over bare insulation, the
mastic should have a permeance less than 0.05 perms. Reinforcing mesh embedded
in the mastic is typically required per manufacturer?s guidelines.

Weather barrier mastics and
also commonly referred to as “breather” coatings. They are specifically
designed to provide protection of the insulation from physical abuse and/or
weathering. They are normally water-based synthetic polymer coatings. These
mastics have higher permeance, > 1.0 perm, than vapor retarders and will
allow water vapor to pass through them while repelling liquid water. This is
particularly important when used over hot equipment or piping where trapped
moisture must be allowed to pass through the mastic to avoid blistering of the
coating. Weather barrier coatings also find use on dual-temperature systems;
such as rooftop HVAC ductwork used for cooling and heating, or dual-temperature
water piping, where the insulation contains a vapor retarder facing that
requires weather protection. On exterior applications, the insulation should
always be sloped to avoiding ponding water.

interior applications on hot pipes, specialized lagging adhesives and coatings
may be used with fiberglass cloth or canvas cloth to create an insulation
cover. The lagging adhesive is used to both bond the cloth to the insulation as
well as to provide a protective finish.

Inspection, Maintenance, and Repair of Mastic Systems

Mastics are a key
component in the protection of many insulation systems and need to be
inspected, maintained, and quickly repaired to function properly. Regular
inspection of the mastic should be conducted as part of an overall insulation
system maintenance program. Inspection should include visual observation for
any cuts, tears, punctures, chemical breakdown, embrittlement from chemical
attack, or other damage to the mastic or reinforcement. Any buildup of dirt or
other chemical contaminants should be removed to ensure that underlying damage
has not occurred and to prevent deterioration of the mastic. Surface wear
should be repaired by thoroughly cleaning the surface before applying a new
finish coat of mastic. The use of reinforcing mesh may be required if there was
damage or exposure of the previous reinforcement. If damage includes a breach
of the mastic such as a puncture, tear, or through cut, the insulation system
should be closely examined to ensure that water or contaminants have not
entered the insulation system. If the insulation is wet or damaged, it must be
removed and replaced prior to re-applying any mastic. Any newly applied mastic
should be reinforced per the manufacturer?s recommendations and extend at least
3 inches over the previously sealed and cleaned surface.

generally need to be re-coated every 2?3 years. If applied to flexible
insulation products or insulation materials that will expand and contract
during service, they may “egg shell or crack,” but will not flake or peel off.
This egg-shelling effect may detract from the appearance of the application,
but it will not generally affect the UV performance of the product. It can be
re-coated for extended service life.