“The fundamental principle of water management is to shed water by layering materials in such a way that water is directed downwards and outwards out of the building or away from the building. The key to this fundamental principle is drainage. The most elegant expression of this concept is flashing. Flashings are the most under-rated building enclosure component and arguably the most important.” -Joseph Lstiburek, Research Report – 0103: Water Management

Flashing membranes play a critical role in the construction industry. Available in many sizes and forms, all flashing membranes serve the same purpose: preserving the integrity of a structure by preventing water penetration and providing an extra layer of protection against mold, mildew, and rot. Commonly, you may see flashings in the form of flexible peel-and-stick tapes, liquid flashings, or rigid flashings. Two of the most notable types of peel-and-stick membranes today are SBS (styrene butadiene styrene) modified bitumen and butyl-based. Let's examine the differences between these two types.

Anatomy of Flashing Tape

First, let’s break down the fundamentals of peel-and-stick flashing tapes. The anatomy of a flashing tape is composed of 3 parts: the facer, the release liner, and the membrane/adhesive. Developing a deeper understanding of these components will better prepare you to make informed decisions about your installation and how current products on the market may impact it.

 

Facers

The facer of a flashing tape is the protective topside of the membrane/adhesive layer, often coated with plastic, aluminum, copper, and other durable materials. While many variations of facers exist, aluminum and plastic-based facers are some of the most common. Aluminum is a popular option for its thin and moldable form, holding its shape well around edges and corners, making it an excellent air and water barrier. However, because it lacks elasticity, aluminum is prone to cracking during installation, increasing the chance of water penetration. Polypropylene and polyethylene-based facers are more elastic and seal around edges and fasteners with less probability of tearing, but have shorter exposure times compared to aluminum facers.

 

Release Liner

The release liner is attached to the membrane/adhesive side and removed before installation. These might be composed of a coated paper material or poly-based material. Generally, release liners come off without issue, but when cheaper materials are used in production to lower costs, micro-tearing can occur, resulting in an excessively time-consuming installation. To avoid this issue, spring for a poly-based liner, as they feature super-resistance to micro-tearing and are sure to peel off in one piece.

 

Membrane/Adhesive

SBS-modified bitumen is an emerging star player in the waterproofing industry. It comes from a mixture of raw asphalt and synthetic rubber derived from styrene and butadiene. SBS is a tough thermoplastic elastomer that, through a precise blending process, gives modified bitumen its rubber-like properties, resulting in a product with exceptional elasticity, abrasion resistance, aging stability, and weatherability. (Bear in mind, since no two manufacturers produce the same SBS-modified bitumen blend, consider your options carefully. Properties may vary based on the asphalt sourcing, the fillers used, and the time and temperature used in the heating process). Additionally, the wide availability of asphalt-based construction materials makes SBS-modified bitumen an easy choice for its compatibility and accessibility.

 

We recommend Protecto Wrap BT 25 XL and Protecto Wrap 100/40.

 

Butyl is a synthetic rubber made from a copolymer of isobutylene and isoprene. Unlike SBS-modified bitumen, butyl flashing is solvent-based and was engineered in 1937 to enhance the properties of natural rubber adhesives. Often chosen for its superior performance characteristics, Butyl-based flashing will apply to most substrates. Impermeable to air, waterproof, and cold-resistant, butyl products maintain tack and flexibility in extreme conditions and have excellent self-healing properties.

 

We recommend Protecto Wrap BT20 Butyl Tape and Henry Fortiflash Butyl 20Mil.

 

So how do they compare?

Price

One key difference between SBS-modified bitumen membranes and butyl membranes is the price. Butyl-based membranes often come with a higher price tag, sometimes double the cost of their counterpart - this is attributed to the higher performance and versatility of butyl membranes but can be cost-prohibitive. Despite the affordability of SBS-bituminous membranes (making them a popular option for those working within a budget), the savings gap is steadily closing with better technology becoming more conventional.

 

Weather and Climate

SBS-modified bitumen membranes offer a much broader temperature range than other modified bitumen products. When modified bitumen combines with SBS, it gains cold-resistant properties, retaining its elasticity and allowing asphalt to flex without cracking or breaking. The maximum application temperature of SBS-modified bitumen (around 250°F) is plenty adequate for most applications, and the flexibility in cold weather drops below -20°F without compromising its composition.

 

Butyl flashings can also withstand extreme temperatures but do have a threshold. Unless labeled for high-temperature use, butyl flashing tapes may begin to soften anywhere between 120°F and 180°F. Butyl flashing formulated for commercial use, such as Henry Fortiflash Butyl, will withstand temperatures up to 300°F, although others may have less adhesion and can be harder to find. Because of its ultra-cold-resistant properties, butyl flashings are most suitable for cold-weather installations, withstanding temperatures as low as 32°F during application and -45°F while in service.

 

Neither SBS-modified bitumen nor butyl flashings should be applied in the rain or left exposed to UV radiation for more than 30 days. While both exhibit UV resistance, it is highly recommended the exterior covering be applied the same day to avoid compromising the integrity of the flashing. If prolonged UV exposure is a legitimate concern for your project, consider a flashing membrane with at least a 120-day exposure rating.

 

Always refer to manufacturer instructions for information on temperature limits and application in different climate conditions.

 

Compatibility

Another difference between a butyl flashing and a bituminous flashing lies in its compatibility. SBS-modified bitumen flashings are incompatible with materials containing plasticizers. Plasticizers are chemicals that help maintain material flexibility, commonly found in flexible polyurethane sealants, roofing membranes, and some window flanges. A chemical reaction occurs when modified bitumen meets a plasticizer, deteriorating the adhesive and compromising the performance of the flashing. Important to note is that not all PVC materials contain plasticizers, and SBS-modified bitumen will perform excellently in their absence.

 

Butyl flashings are ideal for use on more difficult substrates and stand subject to different limitations than bitumen flashings. They are compatible with all acrylics, polyurethane, and neutral-cure silicone sealants. However, since butyl is a solvent-based material, it will dissolve if it encounters strong solvents like gasoline, turpentine, or mineral oil. Additionally, butyl flashing should not pair with modified bitumen or other asphalt-based products. Doing so risks deterioration of the butyl and interference with adhesion.

 

Always refer to manufacturer instructions for information on suitable substrates.

 

The Breakdown

The Bottom Line

Choosing a flashing membrane isn't difficult if you consider your application carefully. Ask yourself:

What substrates will this be applied to?

What is the application temperature range? What about the service temperature range?

What other materials are being used in this application? Will there be any compatibility issues?

What are the application requirements?

How does the product hold up over time?

Does cost matter?

Assessing these questions and paying close attention to the manufacturer’s technical information will ensure the longevity of your project and secure its efficacy for years to come.