Although relatively new to the insulation industry, the use of rubberized asphalt membranes to provide moisture protection isn’t new. In one form or another, membranes have been in the pipeline and construction industry since the 1970s. The original membranes were developed for the underground gas and oil pipeline industry in the form of tar-like paints, wax tapes, and finally rubberized bitumen "tapes" (called tapes because of their width) which were, and are, used to shield and protect pipes from the moisture which causes corrosion on uninsulated pipelines.
Insulated underground piping requires a moisture barrier (waterproofing), to prevent moisture from entering the insulation. Wet insulation is a damaged insulator, and the water held in the insulation creates an environment promoting rapid corrosion of the piping surface. There’s a defined difference between "damp-proofing" and "waterproofing" and it’s stipulated by the American Society for Testing and Materials (ASTM) that waterproofing must resist a "standing head" of water, while damp-proofing has no such requirement. The pipeline industry began using polyethylene with laminated to rubberized asphalt compounds began their history in the 1970s as a waterproofing film.
In the recent past, the preferred underground waterproofing system over insulation consisted of roofing felt (asphalt saturated felt) sealed with a gloved-on asphalt based, fibrated emulsion. Another method of waterproofing was to apply an asphaltic fibrated emulsion coating with fiberglass fabric mesh (glassfab) imbedded in the first coat, and a top coat of asphalt emulsion applied over the glassfab. This was quite effective if installed properly, but tended to be a messy job which could be applied in inconsistent coating thickness’. The required drying time between and after coats also made it necessary to keep the trench open for days while the drying process took place.
The early forms of rubberized asphalt membranes contained a higher percentage of asphalt than today. The result was a more brittle compound that required the application of heat to soften the compound and establish a sealing bond to the insulation and at the laps. Today, peel and stick membranes are the most widely specified used products for below grade waterproofing on insulated systems.
In the commercial and residential construction industry, "peel and stick" membranes are the fastest growing type of flashing materials. Rubberized asphalt membranes are being used as waterproofing for foundations and roof decks; as a seal for window installations against moisture and wind intrusion; as a moisture barrier underlayment for highways and runways; and most recently as a termite barrier that has proven in testing to completely deter termite infestation. The insulation industry is just now benefiting from the introduction of construction grade "peel and stick" membranes.
Today’s rubberized asphalt compounds are carefully engineered blends of very specialized rubber polymers and asphalt. Other components are added to alter tack, cohesiveness, and "softness" of a compound. Not all membranes use the same rubberized asphalt compound. Membranes are sold as roofing products, insulation weather barriers, insulation vapor barriers, foundation waterproofing membranes, window flashing, pipeline tapes, and corrosion control tapes. The compound used in each product is formulated for each that particular application.
For example, many roofing membrane compounds are blended with oils. Since they are adhered to porous substrates such as wooden roof decking or roofing felt, the oils penetrate into the porous substrate, improving the adherence of the membrane. These same compounds, when applied to a metallic or foil surface, could delaminate because the oils, which assist in adhesion to a porous substrate, do the opposite when applied to a non-porous surface. Applying a bituminous membrane to a substrate other than that for which it’s designed can be risky. You don’t waterproof a foundation using window seal tape, and you shouldn’t install roofing membranes on oil and gas pipelines. Membranes are specifically formulated for each particular application.
One of the greatest benefits of rubberized asphalt compounds is that they have a greater cohesive strength than adhesive strength. This is the basis for their "cold flow" self healing properties. If a rubberized asphalt membrane is punctured or cut, the compound "cold flows" back together, re-sealing itself and retaining its waterproofing properties. The same property allows the compound to cold flow around screws or mechanical fasteners, which puncture the membrane under some installation criteria. The compounds will cold flow around penetrations sealing to the flashing screw, or mechanical fastener. A patch is then applied over the washer head of the mechanical fastener to prevent it from rusting and assisting in minimal cold transfer to the pin head.
While the rubberized bitumen compounds themselves are a moisture barrier retarder (approximately 0.03.4 US PERMS), a "skin" (or backing, as the industry refers to it) is required to improve not only the waterproofing and vapor barrier retarder qualities, but also the membrane’s puncture resistance. Polyethylene films are the most commonly used films, cross laminated layers of polyethylene or reinforced polyester scrim for additional strength. The compound is laminated to the backing at approximately 300 degrees fahrenheit (F), and a silicone coated release liner is applied to the compound for ease of field application. These silicone release liners are as specific in nature as the backings. Using the correct silicone is necessary to achieve good release in cold weather, yet not stick to the compound in hot weather, and in the process not transferring silicone to the compound surface.
The Skins Game
Membranes have enjoyed their greatest advances by employing the use of very specialized films (also known as backings). Examples of high-tech backings include low density polyethylene, high density polyethylene, polyester, Mylar®, polyester laminated foil, and low density polyethylene loaded with titanium dioxide for maximum ultraviolet (UV) resistance.
The majority of membranes used for direct burial are laminated to high strength, low-density polyethylene films which have good perm ratings and high puncture resistance. All membranes, if being buried in rocky backfill, should have some sort of mechanical protection applied. There are plastic mat products on the market for this purpose. They are constructed from recycled plastics (such as soda bottles) and provide approximately 400 PSI puncture resistance. They are installed by taping them over the rubberized asphalt membrane with filament tape, and the pressure of the backfill material will holds them in place.
More specialized films are available such as polyesters and Mylar® which improve the membrane’s vapor barrier retarding properties. Recent introductions of "zero" perm peel and stick, self healing membranes for the cold piping industry are examples. These films are UV stable indoors and can tolerate wash down conditions, making them excellent membranes for the food industry.
New membranes for exterior insulation protection have been introduced which are UV stable outdoors. These are marketed as substitutes for metal or polyvinyl chloride (PVC) lagging over ductwork and piping systems. The most popular combination system is an aluminum foil laminated to high strength polyethylene for vapor barrier retarding and UV resistance.
The introduction of these membranes improves a contractor’s ability to seal a cold insulation system without having to rely on mastics, caulks, or adhesives to seal joints. Unlike mastics and caulks, membranes need no further maintenance over time to retain their waterproofing and UV stability. Again, the greatest advantage of rubberized asphalt membranes is their "cold flow" capability.
Cold piping systems jacketed with metal or PVC require "slip joints" (a joint in the metal or PVC that’s designed to move with the expansion and contraction of a cold system). These joints are usually silicone caulk sealed to keep out moisture. The requirement for "slip joints" in piping systems is eliminated when using membranes, as membranes expand and contract with the piping system (up to 400 percent elongation before break). Employing membranes, a cold system can be completely sealed. The most detrimental effect of expansion and contraction to a membrane is wrinkling at the expansion and contraction site.
The cold industry favors white jackets because of their emissivity, and because painting aluminum jacketing is difficult. There’s a rubberized asphalt membrane which solves this problem by heat laminating a low density polyethylene heavily loaded with titanium dioxide onto the foil membrane, giving the membrane a better perm rating, excellent UV resistance, and a high gloss white appearance similar to PVC jacketing.
These exterior rubberized asphalt membranes can be used on hot systems over calcium silicate and perlite insulation, when used with a liquid activator, or over cellular glass without the use of an activator. The critical factor on waterproofing hot systems with a rubberized asphalt membrane is that the surface temperature of the insulation must remain under 150 degrees (F) to prevent the softening of the rubberized asphalt compound. If temperatures exceed 180 degrees (F), softening of the compound and sagging can occur.
The ease of application of rubberized asphalt membranes is as great a benefit as is their performance. Specially formulated rubberized asphalt membranes stick to all types of insulations and facings, with the more rigid the insulation the better (for appearance sake). The term "peel and stick" may sound a little too easy, but essentially, that’s the application method installers will need to utilize their metal layout skills to form fittings, transitions, gores, and appurtenances. Like insulation tapes, they require pressure to ensure complete adhesion. This is done by rolling the surface with a laminate roller to press the membrane onto the substrate.
The underside of large expanses of ductwork (more than 24 inches) must be pinned with mechanical fasteners (insulated weld pins) through the membrane and insulation, to the duct itself, to prevent sagging or delamination from the insulation. When used as tank top lagging, membranes must be pinned through to the tank to prevent "lift" when a large tank top expanse is subject to high winds. The wind across the large expanse of a tank top creates lift, similar to the effect of wind flowing over an airplane wing. Additionally, membranes seal so well that if air leaks exist in the ductwork, the air pressure can balloon the membrane and separate the insulation facing from the insulation.
One shortcoming of rubberized asphalt membranes is its loss of tack in cool weather (50 degrees [F] and below). Membrane compounds are made up of rubber and asphalt and, similar to roads and automobile tires, when it gets cold, they get harder and subsequently less tacky the colder they become. (On the other hand, testing has shown that membranes adhered in warm temperatures will not loose adhesion in cold weather.)
There are two ways to overcome this problem. One is to apply heat to warm the substrate and membrane to enhance tack (this is a time consuming and sometimes messy process). The second way was developed by a company that specializes in rubberized asphalt membranes. They have developed a paint roller applied "cold weather activator." When applied to the substrate, it remains tacky all day, it doesn’t hamper installation time and it works by chemically softening the rubberized asphalt compound and making it tacky. Application of the activator enhances adhesion in the 20 degree (F) to 50 degree (F) temperature range, extending the application season in cold climates.
Another shortcoming of rubberized asphalt membranes is their flame and smoke properties (ASTM E-84). Being comprised of rubber and asphalt, when they burn, they develop large amounts of black smoke (similar to a burning automobile tire). Membranes are first tested supported by wire mesh and metal rods, and most rubberized asphalt membranes pass this test in the 5-15 smoke developed range. However, there’s a published ASTM stipulation to this test. It reads as follows:
"X1.8.1 Single-layer membranes or thin laminates consisting of a limited number of similar or dissimilar layers not intended for adherence to another surface may be supported on poultry netting placed on steel rods in accordance with X22.214.171.124 and X126.96.36.199. Netting shall be 20-gage, 2-in. (51mm) hexagonal galvanized steel poultry netting conforming to specification A 390. If so tested, the specimen shall be additionally tested, bonded to a substrate representative of a field installation."
The ASTM stipulation virtually eliminates rubberized asphalt membranes from 25-50 compliance. When a membrane is adhered to a substrate (for example FSK faced Fiberglass board), the smoke developed during the test jumps from 5-15 (as tested supported) to over 400 when applied to a substrate. (The flame spread normally remains constant between tests.)
Most manufacturers test their product both ways, and technically a portion of ASTM E-84 is satisfied when you test a membrane supported as stated previously. However, membranes aren’t installed supported by wire mesh, they’re installed to an insulation substrate, and ASTM requires additional testing when applied to a substrate. Rubberized membranes can’t pass this second test. Membranes could be advertised as ASTM E-84 compliant, but the statement would only be accurate if the membrane was installed supported by wire mesh.
Benefits to Industry
Thanks to the pipeline and construction industry, the insulation industry is now benefiting from the use of rubberized asphalt "peel and stick" self-healing membranes. These membranes are now available in many configurations, to meet the many specific needs of the HVAC, refrigeration, insulated pipeline and food industries.
Specifiers and contractors should investigate these rubberized asphalt membranes, as they often will give you a better result on the job, save money, and last considerably longer than most systems on the market today.