Comparing polyurethane sealants to silyl-terminated polymers
by Dave Kimball
Bob Dylan’s famous song lyric, “The times, they are a-changing,” aptly describes building science and the role of fluid-applied air barriers in construction today. Regarding fluid-applied flashing materials, we are seeing this change take place right before our very eyes. However, as times change, some things remain the same.
For example, some sealant manufacturers are currently ‘repositioning’ their polyurethane sealants as fluid-applied flashing materials for air barrier applications. The problem is while polyurethane sealants offer many good properties, they just do not have all the correct properties for this particular application—especially when compared to the silyl-terminated polymer (STP) technology used to create the first fluid-applied products 15 years ago. In addition to having virtually zero performance history in air barrier applications, polyurethane sealants can also bring a litany of inherent problems, as discussed later in this article.
The first fluid-applied products were created with the express purpose of finding a better way to weatherproof and seal a building envelope’s rough opening. Their development began with Tatley-Grund (a notable restoration contractor in Seattle), and would result in a permanent change in the industry.
Tatley-Grund found itself continually fixing young failed buildings that had been detailed with peel-and-stick membranes. At that time, the only repair method was to use the same peel-and-stick products that caused the initial failures. This set the company on a course to finding something better for these applications, and so it developed a ‘wish list’ of product capabilities to seek out.
Tatley-Grund contacted a few notable sealant manufacturers, but no one could fill their needs. However, the contractors were directed to a certain type of chemistry that could be harnessed to meet the requirements, along with a chemist with the background to help. From there, Building Envelope Innovations (BEI) was created to develop an STP product now known as FastFlash. Through a partnership with PROSOCO, it has now been used across the United States on thousands of high-performing projects.
Tatley-Grund’s ‘wish list’ of performance characteristics called for the ideal product to be:
- fluid-applied (so it could fit perfectly everywhere);
- able to bond to damp surfaces (because wet substrates are common in the real world);
- adhered without primers (to save an application step and the wait for drying);
- comprising 100% solids to avoid shrinkage (what is applied remains the same thickness upon cure);
- compliant with low volatile organic compound (VOC) requirements (minimal odor contributing to the highest indoor air quality [IAQ]);
- immediately waterproof (can withstand a rain event before cure);
- opaque when target thickness is achieved (offers easy visual inspection);
- able to be exposed for 12 months (necessary for any construction delays);
- paintable/compatible (can be used with most other construction materials);
- vapor-permeable (does not trap moisture that causes rot or mold issues);
- able to reduce steps and save time (apply-and-spread application that works on wet surfaces and requires no primer);
- competitively priced (cannot be cost-prohibitive);
- easily repaired (just apply new material, and spread over any damaged areas); and
- self-sealing around fasteners (a must-have for air barrier continuity).
To see how silyl-terminated polymer technology satisfies Tatley Grund’s wish list, watch this video series.
Fulfilling all these product capabilities was quite a tall order more than 15 years ago, but it served to provide an advancement fully supported by current building science. Fast-forward to today, there are many manufacturers realizing the benefits of fluid-applied detailing materials; there are currently at least 14 companies offering this option in their air barrier lines. Some manufacturers are trying their hand at versions of STP chemistry, but others are repurposing old sealant technology for this application. Of these, the most concerning is use of polyurethane sealants for fluid-applied flashing applications. This chemistry had been evaluated more than 15 years ago, and failed to meet Tatley-Grund’s criteria.
Polyurethane points to consider
- These products come with no mention of moisture vapor permeability—this means moisture may be trapped moisture in the underlying substrate, which is never a good thing. Industry experience suggests polyurethanes generally have poor moisture vapor permeability.
- Polyurethane sealant literature states an upper service temperature of 82 C (180 F), which is problematic as temperatures inside a wall cavity and in the rough opening can reach higher temperatures year-round in warmer climates and in the summer for cooler regions. In comparison, FastFlash has a service temperature of 150 C (300 F).
- Are polyurethane sealants immediately waterproof? Their product literature does not address this, but we do know exposure of standard polyurethane sealants to liquid water during cure can affect the cured sealant properties.
- Polyurethane sealants typically state there is “no restriction” on ultraviolet (UV) exposure, yet the industry is very aware of the limited life of polyurethane sealants with UV exposure alone, and the degradation effects are only multiplied when heat and water are added into the mix.
- Polyurethane sealants may be low-VOC, but they are not odor-free, which can be a problem on restoration projects and in new construction. In 1984, it was the reaction of products of methyl isocyanate (an ingredient of polyurethane sealants) with water that killed 8000 people and injured 558,000 in the Union Carbide disaster in Bhopal, India. It is not surprising that polyurethane sealant safety data sheets call for use only outdoors in in well-ventilated areas and state that, if inhaled, the sealants are harmful and may cause allergy or asthma symptoms or breathing difficulties.
- Polyurethane sealants typically note an installation temperature of 0 to 38 C (32 to 100 F). The PROSOCO R-Guard line of STP products can be applied at temperatures well below this range. These products will not significantly cure until 0 C (32 F) is reached, but it does not matter since they are immediately waterproof. It should also be noted getting a polyurethane out of the cartridge or sausage at 0 C (32 F) (unless using a hot box) can be very difficult because they become stiff at lower temperatures.
- Typical polyurethane sealant literature states a two-hour tack-free time. Particularly in restoration (but even in new construction), this amount of time can be restrictive to productivity.
- Polyurethane sealant literature makes no mention of National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-loadbearing Wall Assemblies Containing Combustible Components. PROSOCO R-Guard silyl-terminated polymer products, on the other hand, have extensive documentation relative to this industry standard, and a complete report is available on request.
- Opaque at target thickness for ease of inspection and quality control? This is not addressed with a polyurethane sealant as fluid-applied flashing, but it is not likely a problem since they are requiring 1 mm (40 mils). Compared to the target thickness of 0.3 to 0.4 mm (12 to 15 mils) for PROSOCO R-Guard FastFlash, this 1-mm (40-mil) requirement—which will likely be thicker because the general thought is ‘the more, the merrier’—will begin to interfere with rough opening tolerances and other detailing. Wet mil gauges can help, but they are unreliable on anything other than a perfectly flat surface. On the other hand, PROSOCO R-Guard FastFlash is applied to ‘opacity.’ This means, if you can see through it, then it is not thick enough—apply until you cannot see through it, and then you are good! Additionally, a sausage of polyurethane sealant at 1 mm (40 mils) will only go about one-third as far as a sausage of FastFlash.
- Polyurethane sealants make no claims to self-sealing capability, which is a critical component in real-world applications.
- Polyurethane sealants being used as fluid-applied flashing alternatives are touting multiple color options as an advantage. However, air barrier detailing treatments get buried in the wall, so why would color matter? In fact, having so many colors can be a disadvantage from a quality control and inspection standpoint.
- Polyurethane sealants being ‘repositioned’ as fluid-applied flashing materials for air barrier applications do not discuss performance history. PROSOCO R-Guard FastFlash was pioneered and has been used around the country for more than a decade. It comes with proven performance that goes back to the repair of a leaking condominium in Seattle, completed in 2005. This project was inspected by an independent consulting firm (with property owner approval) in 2010, and again in 2015, to observe performance. After removing a section of exterior façade for inspection, the repair assembly and FastFlash were working perfectly.
There are many manufacturers jumping on the fluid-applied flashing bandwagon that PROSOCO/BEI pioneered many years ago. Some of them are re-purposing old technology for this critically important scope of work in construction, but none has the performance history to support a quality application. It is in the interest of longevity, sustainability, and building performance that you consider your products and their pedigree. It is very important to understand the difference between products that were created and designed with intention versus others that may be thrown into the market as an afterthought.
Design professionals should be careful they read and fully understand what is being portrayed.
After 34 years at a major silicone sealant company, Dave Kimball retired and started anew with PROSOCO Inc., as a technical specialist in the Building Envelope Group, covering the North Atlantic Region from his office in Long Island City, New York. Kimball has consulted on such high-profile projects as the Pentagon (Washington, D.C.), the Transamerica Pyramid (San Francisco), and the One World Trade Center (New York City). With areas of expertise in weathersealing, structural glazing, and exterior coatings applications, he now focuses on the unique challenges presented by rough opening preparation and air and vapor barriers. Along with belonging to the Sealant Waterproofing and Restoration (SWR) Institute, Kimball is also a member of standards testing organization ASTM International.
All information listed in this section was submitted by PROSOCO.
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