The thermal insulation industry as a whole is now involved in a multi-year concentrated effort to educate people outside of the industry in the importance, effectiveness, and benefits of proper insulation systems. These educational efforts have brought with them a renewed focus on making sure that insulation materials and applications are correctly specified and applied.

It’s not an exaggeration to say that, in many cases, without an appropriate and well-applied jacketing or coating system on top of thermal insulation, the insulation will simply not be capable of performing as designed over its desired lifespan. This is particularly true with outdoor applications. Insulation that has been physically damaged or that has become saturated will lose its thermal efficiency rapidly. Additionally, wet insulation exposed to freeze – thaw will become damaged in a short period of time, even to the extent that it must be replaced. Moisture presence in the insulation can also lead to extremely costly corrosion of the underlying pipes and systems, and this corrosion, because it’s hidden by the insulation, can become serious by the time it’s detected.

Mastic coatings have long been proven to be an effective form of protection for insulation systems, not only as weather barriers and vapor retarders, but also as general protection against many sources of mechanical damage. Though these types of coatings have been replaced in some applications by newer products, mastics are still widely used with great success throughout the commercial and industrial insulation trades. Today’s family of mastic coatings are safer, easier to apply, and more effective than ever.

Mastic Definitions

It’s important to have an understanding of some of the terms typically used when discussing mastic coating performance characteristics. Some of these terms are still used incorrectly from time to time within the insulation industry.

Weather Barrier Coating

A weather barrier coating is a type of "breather" which on the one hand prevents water from passing through it, but at the same time also allows water vapor to pass through it. This seemingly contradictory feature makes it ideal for protecting outdoor systems operating at temperatures above ambient. This allows a certain amount of moisture to escape when the hot system is operating. This is still one of the advantages that a mastic offers over other types of jacketing, which can seal in residual moisture.

Vapor Retarder Coating (author note: This type of coating is typically referred to in the marketplace as a vapor barrier.)

This coating resists the ingress of water vapor toward a cold operating system. Vapor retarders are applied on the outside of the insulation to assist in keeping the insulation dry. The importance of using a proper vapor retarder over a cold system, even if it is just air conditioning ductwork, has become more obvious in recent years with the prevalence of mold and mildew problems occurring where moisture has been allowed to intrude.

Perm Rating

Perm is defined as "the mass of water vapor flow through one square foot of a material." The term perm rating is used to quantify the resistance of a coating to the passage of water vapor through it. The generally accepted cutoff point between a weather barrier and a vapor retarder is considered to be about 1 Perm, as tested per ASTM F 1249 or ASTM E 96. Anything achieving a rating of less than 1.0 Perm begins to fall into the vapor retarder category. The lower the perm rating, the more resistant the coating is to the passage of water vapor. Most below-ambient applications require a perm rating of .08 or lower to seal the system with any effectiveness. This depends, of course, on the system temperature and ambient conditions.

The perm rating is shown on the data sheet provided by the manufacturer of any coating intended to be used as a vapor retarder. In comparing perm ratings between different products, you should be careful to note temperature and relative humidity at which the test was performed, as well as the thickness of the coating.

Flame Spread

This is the term used to quantify the flammability or "surface burning characteristics" of a coating once it’s dry. It normally refers to the index achieved per the ASTM E 84 test method (which involves a 25-foot long tunnel in which the product is burned). A flame spread rating of 25 or less and a "smoke developed" rating of 50 or less are considered to be optimal. Manufacturer’s data sheets will provide you with this data for each of their coatings. When comparing flame spread information of products, you should note the film thickness and width at which the product was tested.

Flash Point

This is the term used to quantify the flammability danger of the coating while still wet. This is tested per ASTM D 93 or ASTM D 3278 and indicates the temperature at which the vapors coming off the product will ignite when exposed to an ignition source. The lower the flash point, the more dangerous it is from a fire/explosion standpoint.

Some History

The original protective coatings used widely up until the late 1950s were bituminous asphaltic products, which had some utilitarian features but also some drawbacks. They were cheap and had good weatherability, but they were also somewhat brittle and prone to damage and mud cracking, had poor application characteristics and had undesirable flammability issues associated with them. And, of course, they came in your choice of color: black or black.

Versions of these products are still used today in selected applications where their features are appropriate, particularly in industrial environments requiring resistance to a high degree of atmospheric corrosives and liquid chemicals, to name a few examples.

Generally, there are two types of asphaltic coatings: asphalt emulsions and asphalt cutbacks. The emulsions are classified as weather barriers and have better fire resistance, while the cutbacks are classified as vapor retarders and have poorer fire resistance. By the way, they’re still black.

Modern Mastics

Beginning in the 1950s, with the development and improvements of modern synthetic latex resins, it became possible to formulate increasingly better mastic coatings.

Today’s weather barrier mastics are typically based on polyvinyl acetate, vinyl acrylic, or acrylic latex resins, and are designed to offer good outdoor protection against rain, sleet, temperature changes and damage caused by ultraviolet exposure. They’re also very effective when used indoors as a general protective coating and often find other uses not related to thermal insulation.

These coatings are elastomeric, meaning that they offer a high degree of flexibility, and are typically fibrated to give them added strength to resist mechanical stresses. Because they’re mostly water based latex emulsions, they’re safe for personnel in the application process, both from a flammability and a hazardous ingredient standpoint. They should always be used on sloped surfaces, however, because latex emulsions don’t offer good longevity when submerged under ponded water on a flat surface.

From a performance standpoint, they can be remarkably effective when properly applied so that an adequate film thickness is achieved universally across the covered surface. Though long runs of pipe insulation are more typically protected with ASJ, aluminum, stainless steel or PVC jacketing, mastics are still optimal for protecting irregularly shaped surfaces around fittings, flanges, valves and elbows. The lifespan of today’s mastics can be many years, provided that they’re inspected occasionally and maintained where needed. The durability of the mastic protective system can be enhanced by embedding reinforcing mesh within the coating. Typically this is done between the first and second coats of an application. A typical mastic application will have a dried film thickness of between 35 to 60 mils, depending upon the nature of the application and whether it’s indoors or outdoors. When applied with reinforcing mesh, it’s applied so that the outline of the mesh will be slightly visible in the surface of the topcoat.

Though most mastic applications are done in either white or gray, today’s coatings can also be supplied in a wide variety of colors, although this policy varies among manufacturers. Special colors can be useful in color-coding pipes, tanks, or systems, or they may appeal simply from an aesthetic standpoint. Application of today’s weather barrier mastics can be done by brush, trowel, spray, or even by palm (as some applicators still prefer). The most common application method on medium to small jobs is by brush, so the ease of brushability offered by vinyl acrylic coatings makes them the most popular.

Most mastics sold today are formulated to have Class A flame spreads, though some manufacturers offer extra fire resistant versions of their products for applications if the end user so desires.

Variety of Choices

There are a number of excellent vapor retarder mastic coatings also available today. There are several vapor retarder coatings still in use which contain solvents. These products are less desirable from a health and safety standpoint, but, being solvent based, they can offer higher performance characteristics as a dried coating and they feature a durability not readily available from a water-based coating. They’re expensive when compared to standard mastics.

There are also several very good water-based vapor retarder coatings on the market, which offer surprisingly low perm ratings and the ability to be used on job sites where solvents aren’t acceptable. They have a milder, less noxious odor than solvent-based materials and don’t have the dangerously low flash points associated with solvents. The better performing water-based vapor barrier mastics are typically emulsions formulated with a styrene butadiene latex, which is what gives the coating its ability to achieve a low perm rating even though it’s water based. They’re normally brushed on and have good brushability, but they can also be sprayed or toweled. These coatings can be suitable for industrial cold system applications requiring a perm rating of .05 or less. Also, they’re often used on commercial applications such as HVAC ducts or chilled water lines where a perm rating of .08 is considered acceptable.

It’s important when applying a vapor retarder coating that it be done correctly, per the manufacturer’s recommendations, and to the correct thickness since even small and seemingly invisible voids in film integrity can allow additional water vapor into the system. Water-based vapor retarder products are generally preferred today on many applications because they combine safety and effectiveness. Typically, they’re slightly more expensive than weather barrier mastics, but not as costly as the solvent-containing products.

Today’s mastic coatings continue to offer a tried-and-true method of protecting a wide variety of insulation types, and they provide a durable and cost-effective method of finishing an insulation system.