Understanding precured silicone sealants: The secret to leak-free claddings

by Katie Daniel | March 8, 2016 4:44 pm

by Chris Bovee
When it comes to a leak-free building, the sealants are the backbone of the whole cladding system. Even the best exterior aluminum, masonry, glass, steel, or exterior insulation and finish systems (EIFS) are only as good as the sealants used to weatherproof the joints, penetrations, and transitions of the cladding system being specified.

Silicone has been used to seal building interiors and exteriors for close to a century because silicone sealants are ultraviolet (UV)-resistant and have an operational temperature range of −50 to 150 C (−60 to 300 F). However, construction specifiers have been turning to precured silicone seals for the past few decades because a precured sealant system can handle at least 100 percent more expansion and at least 25 percent more compression movement than traditional ‘liquid-applied’ elastomeric silicone sealants. (This comes based on a comparison of product data sheets from three liquid-applied sealant manufacturers with a validation certificate of a precured sealant on the Sealant, Waterproofing, and Restoration Institute (SWRI) website).

To oversimplify, a cladding system is the building’s outer rain gear, and sealants are like the zippers, cuffs, and drawstrings pulling the whole thing together. Precured seals either replace or work in conjunction with more traditional liquid-applied, elastomeric sealant or mechanically fastened weatherproofing systems to keep water out and heating and cooling in.

Not only are preformed sealants used to waterproof the exterior of a cladding system, but they are also available in a translucent form (extruded strips and molded corner pieces) that provide continuous transition and bridging to air and vapor materials behind the cladding—similar to another layer of waterproof clothing underneath the rain gear.

Possibilities with precured sealant
To fully appreciate the different applications of precured sealants with various claddings, one requires a basic understanding of the material’s features and benefits. Precured sealants start out as 208-L (55-gal) drums of silicone similar to the caulk found in tubes, without the curing agents. However, this sealant gets color and fast-curing ingredients mixed in; it is then extruded through a die, that was cut to exact specifications, allowing a continuous, consistent strip to feed through a long water trough. The result is a uniform, stretchable strip free of bubbles, voids, or any of the weak spots occurring when liquid or tape sealant is applied by hand or tooled into place onsite.

By the time the sealant reaches the end of the trough in the factory, it is fully cured and typically spooled onto rolls 15 to 45 m (50 to 150 ft) long. The precured seal is then boxed up and delivered along with a case of its manufacturer’s specially formulated high-performance, neutral-cure silicone adhesive.

At the jobsite, the seal is rolled out, separated from a packing backer, and cut to length with a utility knife or scissors. There is no need for special splice kits, as ends are just lapped and sealed. (Silicone sticks to silicone; manufacturers should have a list of tested silicone sealants where the adhesion has been tested with precured extrusions.)

Only a thin bead of silicone bonding adhesive underneath the outer edges of the precured seal is needed to set the sealant system into place, and keep water and weather out for the long haul. To finish the process, one simply presses the seal into the adhesive bonding bead (i.e. silicone sealant for which adhesion has been tested) with a block of wood or a finger. Since there are no mechanical fasteners, holes do not need to be made in the roof, wall, or building transition being sealed up against the elements.

Instead of waiting days (or even weeks) for thick liquid-applied sealants to cure, the thin bead of adhesive underneath the precured seal sets within a few hours under most weather conditions. The fast curing time of the silicone bonding adhesive means the seal installed in the morning will be set up to handle the expansion occurring with the heat of the afternoon and the contraction of the cool night.

This is vital to a good seal because many liquid sealant failures occur—even if they do not appear as leaks until years later—during the multiple cycles of expansion and contraction that happen during the curing period. With liquid sealants, a new or repaired joint can take days or weeks to fully cure; this can lead to loss of adhesion (i.e. tears or voids between sealant and substrate) and cohesive failure (i.e. tears or voids within the sealant bead).

By contrast, the precured sealant is able to withstand joint movement of +200 percent expansion and −75 percent contraction. Liquid-applied sealants range from +50 to 100 percent expansion and −50 percent contraction, depending on brand and formula. When choosing between liquid-applied and pre-cured sealant, the main consideration, just in terms of material costs, is the width of the expansion joint. A wider joint requiring lots of liquid-applied sealant may cost as much or more than a precured seal—the reason why the latter is mostly employed for niche applications in new construction.

How precured and liquid-applied sealants compare with respect to service life depends on the application and the chemistry. If both materials are silicone, then the only difference is the application. Due to the way which precured sealants are installed, there is a neutral area that is not bonded. This is a similar effect as utilizing bond-breaker tape and tooling sealant over an undersized joint.

When comparing typical expansion joints, precured sealants impose little to no stress on the substrate during a compression cycle. Certain substrates can be damaged from these forces, allowing water penetration during the expansion cycle. If a joint is properly designed and installed, the service life is probably the same. When the joints are improperly designed or installed, precured sealants could outlast their liquid-applied counterparts.

Exterior insulation and finish systems
Using precured seals with EIFS panels that have stucco or other-textured finish coats can have important dividends. Roy Conrad, a project manager with Gorman Moisture Protection Inc. of El Paso, Texas, has been using precured seals for about 30 years. (Gorman Moisture Protection is a regular presenter at the events held by SWRI, which offers results of long-term studies on how various sealants age in the field—real world versus manufacturer claims. Visit www.swrionline.org for more information). It is currently in the process of installing up to 518 m (1700 lineal ft) of precured seals on the $648-million Fort Bliss Replacement Hospital. Gorman is the subcontractor to general contractor Clark McCarthy, and the client is the United States Army Corps of Engineers (USACE)–Fort Worth District.

“We are technicians and our primary focus is on doing it right the first time,” Conrad says. “The physical consistency of precured seals is such that there is low bond-line stress on the substrate.”

Other liquid-applied, elastomeric sealant systems can generate higher bond-line stress on EIFS walls because of the thickness of the hourglass bead needed to bridge the joint. For a typical 19-mm (3⁄4-in.) wide expansion or perimeter joint, the sealant bead thickness ranges from about 5 to 13 mm (3⁄16 to 1⁄2 in.), whereas the precured seal is only 2 mm (1⁄16 in.) thick across the entire joint. The thicker the sealant, the less it can expand and contract with temperature and load changes. (This is akin to stretching a rubber band versus trying to stretch a rubber tire.)

If too much bond-line stress is exerted on the thin stucco finish coat of an EIFS panel, then the substrate is likely to crack and the seal is likely to fail. As mentioned, the precured seals, by contrast, can withstand movement of +200 percent expansion to −75 percent compression because they are thinner. They also offer customization potential, with notched profiles for bending and wrapping around edges and other difficult-to-seal details.

Masonry and stucco
Precured seals work well with masonry and stucco applications because of the aforementioned low bond-line stress. The thin bead of silicone adhesive used underneath the outer edges of the precured seals forms a chemical bond with the substrate.

Like their liquid-applied counterparts, precured sealants have the ability to match color and texture. A matte surface is the standard profile for most precured seals, but special surface texturing and custom color can be added to match almost any masonry or stucco substrate.

The molecular structure and high-bond energy of silicone is resistant to UV light and weathering, which means the colors selected for the project will resist fading for decades, just as with any quality liquid-applied silicone sealant. Precured or liquid-applied sealants that are polyurethane-, polysulfide-, or butyl-based tend to not have as effective UV and weathering resistance properties, however.

Metal building
Precured silicone sealants are primarily used on metal buildings to help keep weather out of hard-to-seal areas of both roof and cladding systems:

• roof-to-wall transitions;
• roof height change details;
• roof curb seals;
• ridge applications;
• vertical and horizontal expansion joints;
• joints between new buildings and existing buildings;
• pipe and other penetrations; and
• repairs.

Span Construction (Madera, California) is building about 90 Costco warehouses around the world. Its crews use precured seals for a vital detail on its projects—rain gutter joints. Warehouse manager Jerry Christian says these gutters are basically big troughs capable of holding a lot of water pouring off the roofs. His crews use 3.1-m (10-ft) long lengths of 76-mm (3-in.) wide, precured silicone sealant to seal up the gutter joints.

Span has been employing precured seals for about four years because of the material’s ability to handle a wide range of multi-directional expansion and contraction. The precured seals also eliminate the need to put additional holes into the gutters because no mechanical fasteners are needed to hold the seals in place.

Precured seals are a suitable solution for many specialty applications, but they are often just one component of the sealant system specified on a building project. Brushed-on or rolled-on elastomeric liquid-applied sealant may be more a cost-effective option for whole-roof coatings, for example, due to basic per-area material prices. Further, some highly contoured, detail applications may be better-suited to sealant systems that use mechanical fasteners to hold membranes in place while the bonding adhesive cures. A precured seal should lay flat on the bonding adhesive for at least an hour for the chemical bond to set up sufficiently. More detailed applications with contours and edge wrapping may require weights or clamps to hold the pre-cured seal in place during initial curing.

Glass, aluminum, and steel
When it comes to new office buildings, high rises, and modern structures with glass and aluminum or steel cladding, the most common application for precured silicone sealants are: expansion joints and window weatherproofing.

Expansion joints
Precured seals provide some additional advantages over traditional expansion-joint sealants. Greater movement capability, faster cure times, higher tear resistance, wider operational temperature range, and ease of installation are just a few of the factors spurring engineers and architects to consider specifying precured seals instead of the traditional closed-cell-foam backer and tooled-on, liquid-applied silicone sealant systems.

Precured silicone sealant virtually eliminates the need for the traditional hour-glass-shaped bead of caulk with foam backer system that has been in use for more than a half-century. Precured seals come in standard widths of 25 to 300 mm (1 to 12 in.), so they can be used across a wide array of expansion joint configurations, such as:

• traditional bridge joints;
• recessed bridge joints;
• concave bridge joints;
• recessed concave bridge joints; and
• U-joints.

Windows
Precured silicone seals are predominantly used to repair failed liquid-applied structural glazing systems. In terms of cost and aesthetics, precured seals save time and money in situations where the old, failed structural glazing would have to be removed by hand in order to re-glaze the mullion-to-glass or glass-to-perimeter transition. This kind of labor-intensive repair also poses a risk of damaging the glass or mullion.

With precured sealant, however, the contractor can install the new seal right over the top of the failed structural glazing without having to scrape and dig out the old material. The profile of each strip of precured sealant can be customized with notches to form easy-to-install, weathertight covers for mullions and window transitions.

Generally speaking, time and labor costs are very job-specific. Some applications require much more work than others in order to prepare an expansion joint/seal prior to re-applying a wet-applied sealant. The labor saved from not removing the old/failed sealant and surface preparation is the reason there is a market for this technology.

Building remediation and retrofitting
By far, the most common use of precured silicone sealant is for remediation. Conrad uses precured seals to bridge over joints where there is a risk of damage to the substrate.

“The ability to leave existing failed seals in place” is a key advantage, he says.

While basic preparation work is still required to create a clean, dry surface for the precured seal’s bonding adhesive, the crew does not have to remove the old sealant—and risk damage to the joint and substrate—before installing the precured seal. This is especially true with EIFS, where the exterior stucco-like coating is thin and easily damaged when contractors try to remove old liquid-applied sealants.

The precured sealant is installed with a silicone sealant or adhesive, bonded to the substrate on either side of the failed expansion joint or seal. The preparation required depends on the surface on which it is being installed—ultimately, it needs to be clean, dry, and frost-free. Precured joints are wider than the original joints, with the extent depending on the width of the material being installed and the installer. For smaller extrusions, it might be 9.5 mm (3⁄8 in.), or as much as 25 mm (1 in.).

For new construction, its appearance can be viewed as a negative by architects desiring recessed or flush seals, but most applications are very large multistory buildings—in other words, seals on the third floor and beyond tend to be only visible to the window cleaners. Further, there are engineering/architectural ways of using precured extrusions with a flush seal.

During building remediation, owners try to get the most out of their building with as little disruption to their tenants and as low a cost as possible. This is where precured joints fit in.

Continuous air and vapor transitions
To return to this article’s original metaphor, these aforementioned applications are the outer rain gear of the cladding system, but the use of translucent extruded silicone strips along with molded silicone pieces for curtain-wall transitions is a bit more like a good pair of long underwear underneath. Not only does a continuous air and vapor transition system provide another layer of protection against water infiltration, it can also be considered a part of a ‘green’ energy-efficient design that creates a leak-free air barrier.

The translucent feature of preformed transition strips and pieces allows inspectors to easily see and verify the bonding adhesive forms a continuous seal. Preformed transition strips and pieces are used to seal transitions in curtain wall, façade, punched windows, and storefront cladding systems. Molded transition pieces are available for inside and outside corners and can be installed in-shop or in the field.

Preformed transition strips and corner pieces can help contribute to successful Leadership in Energy & Environmental Design (LEED) certification, ranging from low-volatile-organic-compound (VOC) material use to thermal comfort. According to U.S. Department of Energy (DOE) EnergyStar estimates, up to 30 percent of the costs of heating and cooling a building are wasted due to building-envelope air leaks. Preformed silicone transition strips and molded pieces can be used to considerably reduce that waste.(Visit www.energystar.gov/buildings/facility-owners-and-managers/existing-buildings/save-energy[1]).

For building remediation projects, it is important to remember how far the technology has come. One of the first large projects where precured extrusions were installed, the workers started on the top floor and progressed toward the bottom. By the time they got within three floors from the bottom, there was so much air coming out of the original failed joints that the extrusion was blowing off the building before the adhesive sealant could cure. They had to drastically reduce the HVAC make-up air in order to complete the installation. The old solution was to pressurize the building to prevent water infiltration.

Conclusion
No matter what type of cladding specified for a project, precured silicon sealant can play a vital role in keeping outside weather out and indoor climate in. The material’s use continues to grow throughout the construction industry. Its large potential for marine, transportation, and other construction-related applications has barely been tapped, from certain residential uses to window manufacturing applications.

Chris Bovee is vice president of Sealex Inc., a manufacturer and developer of precured sealant technology. He holds degrees from Ferris State University in both plastics and rubber engineering. Bovee can be reached at chrisbovee@sealexinc.com[2].

Source URL: https://www.constructionspecifier.com/understanding-precured-silicone-sealants-the-secret-to-leak-free-claddings/

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