January 04, 2023

What Are Pre-Formed Silicone Transition Membranes & How Do They Work?

Hello and welcome to another round of Pecora’s Constructive Conversations blog! This is the beginning of year four and it feels like we are just getting started. Over the past few years, we have highlighted topics pertaining to our three product lines: Air Barriers, Deck Coatings, and Sealants. We have also introduced new products, explored various project spotlights, and took a dive into how Covid has affected the industry. For 2023, we are changing up our look, but will continue to provide the educational content you have grown accustomed to reading. Planning is currently underway, and we want to thank you for your continued support in our blog and hope you enjoy!

For the first Construction Conversations blog post of 2023, we want to discuss pre-formed silicone transition membranes and how they work. A continuous air, moisture vapor, and water-resistant barrier is key when considering building envelope performance and durability. Exterior glazing and window systems, curtain wall systems, storefront systems, louvers, and door frames must connect and seal to adjourning opaque building façade components. This is to prevent air, moisture vapor, and water infiltration or exfiltration. As building codes and regulations become more stringent, most buildings leverage some type of air barrier system to ensure energy efficient building envelopes. The creation of a continuous monolithic membrane remains a key factor to the success of any air barrier system which typically includes challenges in connecting dissimilar components and materials. For example, if you have a masonry wall butting up to a steel column and there is a 3-, 4-, 5-inch gap, you can address this using a pre-formed silicone transition membrane. Luckily for our readers, Pecora offers a pre-formed silicone transition system, Pecora XL-Span.

Pecora XL-Span is a pre-formed silicone transition membrane that utilizes a translucent high-molecular weight, 100% silicone composition. XL-Span Pre-Formed Silicone Transition Membrane is extruded to create a flat, ribbed profile which is designed to create an airtight transition seal between curtainwalls and the building envelope. These flat profiles are provided in a ribbed configuration in order to control adhesive thickness and maintain a consistent sealant configuration. XL-Span may also be used as a flashing material as well as a transition membrane from below grade or roofing membranes to façade air barrier system. XL-Span comes in 50-foot rolls and is even provided in 4-, 6-, and 9-inch-wide sections. The Pecora AVB Silicone Sealant/Adhesive is utilized as the pre-formed XL-Span membrane adhesive. The AVB Silicone Sealant is a single component, 100% silicone sealant specifically designed to adhere without primer to a wide array of low surface energy air barrier components as well as common building substrates. When incorporated into the building envelope, XL-Span allows for a continual air and watertight transition seal between dissimilar materials, curtain wall, and window systems.

We recommend Pecora XL-Span for use in the following applications:

  • As a weather seal spanning wide joints (>2”)
  • As a weather seal spanning undersized joints (<1/4”) where dynamic joint movement may occur
  • As a weather seal spanning dissimilar building substrates such as roof- and foundation-to-wall transitions
  • As a field-applied flashing material in rough window openings
  • In-shop or field-applied as a weather seal in aluminum mullion assemblies within storefronts and curtainwalls
  • Skylight leak repair
  • Other applications as approved by Pecora Technical Services

To learn more about our air barrier products or for information on our air barrier application guide, please visit our website at You can also reach out to one of our Technical Service Members by calling (800) 523-6688 or emailing at


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