Warming up to cold adhesives

• Photo courtesy of Firestone Building Products Co., Indianapolis
• Photo courtesy of Firestone Building Products Co., Indianapolis

Polymer-modified bitumen roof systems have been installed in the U.S. for more than three decades. During that time, polymer-modified sheet membranes traditionally have been mopped in hot asphalt or torch-applied; however, safety, health and environmental concerns associated with the use of hot asphalt and torch-applied membranes have led to more frequent use of alternative application methods. As a result, contractors increasingly are using cold adhesives to install polymer-modified bitumen membranes.

Defining cold adhesives

Cold adhesives first were used in Europe during the 1960s and were later introduced in the U.S. These early adhesives, typically used in re-cover and resaturant processes, have evolved into the current generation of cold adhesives that are used to install polymer-modified bitumen membranes.

Cold adhesives, as the name implies, are bitumen-based adhesives that do not require heat to be installed. Generally used to secure polymer-modified bitumen membranes to substrates, cold adhesives are composed of asphalt or coal tar that has been thinned, or cut back, with solvents or mineral spirits and blended with fillers and, sometimes, modifiers.

Solvent-based adhesives should be used for cold-applied polymer-modified system installations. There are water-based bituminous adhesives on the market; however, these bituminous adhesives typically are not recommended for use with polymer-modified bitumen membranes.

In 2003, The Construction Specifier reported more than 10 percent of all new construction and reroofing jobs involved cold-applied polymer-modified bitumen roof systems. Although cold adhesives can be used with APP- or SBS-polymer-modified bitumen membranes, they most commonly are used to install SBS products.

Application

Cold adhesives typically are differentiated by viscosity. High-viscosity cold adhesives (those highly resistant to flow) generally are spread in place with a trowel or notched squeegee, and low-viscosity cold adhesives usually are sprayed in place. And though high-viscosity cold adhesives typically are used to bond membrane flashings to vertical, or steep-slope, substrates, both cold-adhesive grades can be used to adhere polymer-modified membranes to base sheets or roof insulation surfaces or bond plies of polymer-modified membranes to form roof membrane assemblies.

It is important you consider a cold adhesive's application temperature to ensure a successful installation. A cold adhesive ideally should be at room temperature—between 70 and 80 F—at installation time. Because there are relatively wide acceptable temperature ranges for installation, consult individual cold-adhesive manufacturers about minimum and maximum application temperatures. When ambient temperatures are low enough to raise a cold adhesive's viscosity, an on-site heater generally is required to raise the cold adhesive's temperature for proper installation.

Regardless of the application method chosen, cold adhesives, when installed below their recommended temperature range, likely will be applied too thickly. On the other hand, cold adhesives, when installed above their recommended temperature range, likely will be applied too thinly. The reason for this is similar to the equiviscous temperature concept for hot asphalt used with built-up roof systems.

When a cold adhesive's temperature is within its proper application range, the installed cold adhesive should be of appropriate thickness for proper performance. Properties such as sheer resistance and resistance to slippage are affected by thickness.

Considerations

There are varying formulations of cold adhesives, and to help prevent the negative effects of incompatible materials, it is important you know a cold adhesive's specific recommended use.

For example, cold adhesives with polymer modifiers should be used with polymer-modified bitumen membranes. Cold adhesives are not interchangeable, and manufacturers' recommendations should be followed.

Additionallly, only polymer-modified bitumen sheets with sand or talc on their undersides can be used with cold adhesives. Sheets with burn-off film on their undersides should not be used with cold adhesives because the burn-off film likely will prevent proper adhesion of the polymer-modified bitumen sheets.

Specifications for cold-applied polymer-modified bitumen membranes generally require that polymer-modified bitumen sheets and base sheets be adhered in cold adhesive and insulation be either mechanically attached to a nailable or steel deck or adhered with hot asphalt or urethane adhesive to a non-nailable deck (concrete, for example). Cold-applied polymer-modified bitumen membrane roof system specifications may be appropriate for small areas or areas with tight access because some specifications do not require a kettle for hot asphalt.

However, the use of cold adhesives does not necessarily preclude the use of hot asphalt or a torch for an overall roof system installation. Installation of insulation and other substrate materials may require the use of hot asphalt, and manufacturers generally require end and side laps to be heated with an air-welder or torch at the time of application to ensure adhesion at these critical locations.

Regardless of how a membrane is secured, NRCA recommends using two layers of insulation.

Over steel decks, the first layer of insulation should be mechanically attached and the second layer should be adhered with an appropriate adhesive. The use of cold adhesives to adhere insulation boards together or to a concrete deck may not be appropriate because a cold adhesive's solvents may become trapped if too many layers are installed above the cold adhesive before it has a chance to cure, resulting in lack of adhesion and/or slipping of materials.

If you use cold adhesives, be cognizant of their locations within roof systems and provide an adequate path for the release of solvents. This may mean only using cold adhesives to adhere a membrane and base sheet.

It is important to follow polymer-modified bitumen manufacturers' recommendations for proper application procedures. Application requirements specific to sheet temperature and unrolling and relaxing of sheets are critical.

Curing issues

Cold adhesives cure as solvent is released—the solids left behind are the adhesive. Because cold adhesives are sandwiched between roof membranes, curing may take anywhere from a few days to a few weeks. Curing time also is extended when ambient temperatures are low.

Long curing times have their advantages and disadvantages. For example, unlike self-adhering membranes that essentially adhere upon contact, polymer-modified bitumen membranes installed in cold adhesives can be repositioned after being installed. I am not suggesting you intentionally manipulate a sheet's position once it has been installed, but if a sheet is incorrectly positioned, there is an opportunity for you to correct its placement.

Also, foot traffic and point loads from stored materials or equipment on a newly installed cold-applied polymer-modified bitumen membrane can mark the surface, displace the adhesive and possibly move the membrane. A polymer-modified bitumen sheet will stay soft from the solvents that pass through the membrane until it is fully cured.

Long curing times also can result in a membrane and/or roof system not achieving its desired wind resistance until the adhesives are fully cured. This can cause problems if high winds occur. Additionally, end and side laps, when adhered with cold adhesives, may not immediately be appropriately weatherproof. Obviously, this can pose a problem, especially in areas that pond water.

Solvents will migrate through or be absorbed by SBS-polymer-modified bitumen sheets. APP-polymer-modified bitumen sheets are more resistant to solvent migration and absorption and generally will take longer to fully cure than SBS-polymer-modified bitumen membranes.

Additionally, "open time" should be considered. Open time is the amount of time cold adhesives should be exposed before a polymer-modified bitumen sheet can be installed on a substrate and allows for the initial release of solvents, which prevents too much solvent from being trapped.

Cold adhesives may be installed only on a substrate, or they may be installed on a substrate and on the backs of polymer-modified bitumen sheets. If installed on both surfaces, the amount of cold adhesive on each surface should be half the total amount required. Application methods are related to ambient temperatures; check with cold adhesives manufacturers for specific instructions.

FM Global's Property Loss Prevention Data Sheet 1-29, "Roof Deck Securement and Above-deck Roof Components," was updated in August. Two new sections discuss cold-adhesive application—specifically, curing time and wind-uplift resistance. FM Global requires the installation of membranes in cold adhesives to be scheduled appropriately to allow adequate curing time before potential high-wind events. FM Global discusses curing times from 28 to 60 days, depending on the roof system. For more information about FM Global's changes, see "FM 1-29 updated again," November issue, page 26.

Many cold adhesives currently on the market are volatile organic compound-compliant; however, some cold adhesives may emit strong odors while curing resulting from solvent release. Heating of the materials will increase the rate of solvent dissipation. If the temperature of an on-site heater is too high, greater amounts of solvents will be released, resulting in more odors and, possibly, building occupant discomfort.

A few concerns

There is concern that solvents from cold adhesives may negatively affect the bond between a facer sheet and the plastic foam component of polyisocyanurate insulation. To prevent this from happening, it is recommended an appropriate cover board be installed over polyisocyanurate insulation. Cover boards may be installed in hot asphalt or urethane adhesives or mechanically attached (though mechanical attachment is not recommended because thermal shorts can result).

Cold-applied adhesives can be used to adhere all layers of a roof assembly from insulation to a roof deck. However, NRCA does not recommend attaching any materials to steel decks with cold-applied bituminous adhesives because of the long-term adhesive qualities and possible subsequent loss of wind-uplift resistance. Additionally, NRCA is concerned cold-applied bituminous adhesives could contribute to internal building fires.

Their future

Cold adhesives have proved to be an appropriate alternative to hot asphalt and torch application for polymer-modified bitumen membranes. However, there still are some concerns that need to be addressed on a job-by-job basis.

No one roof system or application method is right for all projects; you must consider a building's or project's parameters before designing and installing a roof system.

James R. Kirby, AIA, is NRCA's associate executive director of technical communications.

Cold adhesives: a quick reference

• Cold adhesives typically are differentiated by viscosity. High-viscosity cold adhesives (those highly resistant to flow) generally are spread in place with a trowel or notched squeegee, and low-viscosity cold adhesives usually are sprayed in place.
• A cold adhesive ideally should be at room temperature—between 70 and 80 F—at installation time.
• Cold adhesives can be used with APP- and SBS-polymer-modified bitumen sheets but are more commonly used with SBS sheets. Only polymer-modified bitumen sheets with sand or talc on their undersides can be used with cold adhesives.
• Cold adhesives cure as solvent is released—the solid left behind is the adhesive. Be cognizant of a cold adhesive's location within a roof system and provide an adequate path for the release of solvents.
• Cold adhesives may be installed only on a substrate, or they may be installed on a substrate and on the backs of polymer-modified bitumen sheets. If installed on both surfaces, the amount of cold adhesive on each surface should be half the total amount required.
• Open time—the amount of time cold adhesives should be exposed before a polymer-modified bitumen sheet can be installed on a substrate—allows for the initial release of solvents, which prevents too much solvent from being trapped.