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Specifying Silicone Sealants: Providing Weather Sealing and Flexibility Between Building Components

Silicone sealants offer long-term durability under extreme weather and building conditions for both new construction and renovation applications.

November 2008
Sponsored by Dow Corning Corporation

By Peter J. Arsenault, AIA, NCARB, LEED-AP

Continuing Education

Use the following learning objectives to focus your study while reading this month’s Continuing Education article.

Learning Objectives - After reading this article, you will be able to:

  1. Differentiate between silicone and organic sealant chemistries.
  2. Explain the functions of non-structural and structural glazing sealants and select appropriate sealing systems for new construction and renovation applications.
  3. Specify appropriate requirements, products, and installation for weather-sealing vs. structural sealant joints.

Credits: 1.00 HSW

This test is no longer available for credit

In very basic terms, what is the primary role of a sealant? Fundamentally it is to keep unwanted water and air from infiltrating into a building, particularly along joints and seams of building materials. This function is compounded by the many different types of materials used on a building's exterior, all of which have different coefficients of thermal expansion. This means that as the temperature changes, the different materials move at different rates. For example, aluminum moves at twice the rate of glass and concrete. When these materials abut each other at window perimeters and other locations, a sealant that can accommodate the range of movement (expansion and contraction) of each of these materials is required to keep the joints, and subsequently the building, watertight. It is worth noting that this role of stopping water and air intrusion is the fundamental characteristic of a "sealant" as opposed to the decorative or interior finishing roles of "caulking."

Introduction to Sealants

Properties of Sealants
In specifying sealants, there are some important properties that need to be considered.

• Adhesion: First, and perhaps most important, is the ability of the sealant to adhere or permanently "stick" to different types of building materials. Every sealant, depending on its formulation, has a different adhesion profile. Certain sealants adhere better to porous materials or substrates, while other sealants adhere better to non-porous materials. In many cases, the sealant manufacturer will require the use of a primer to enhance a sealant's adhesion to a specific material by acting as an intermediary between the adhesion properties of the sealant and the characteristics of the substrate material.

Solano County Government Center, Fairfield CA (See Solano County case study on p.5 of this course.)

Photo © 2008 Michael O'Callahan


• Modulus is used in terms of sealants to identify the degree of elasticity that different sealants exhibit. This is important because it helps determine the stress-strain relationship between the joint and substrates. A high-modulus sealant is a stiffer sealant. This type of sealant usually generates more stress within the joint and bond line during expansion and contraction or conversely resists high applied forces to reduce the deflection of the joint.

High-modulus sealants are often used where strength is important, such as in structural glazing where a sealant is used to adhere glass to the outside of a building. A low- or ultra-low modulus sealant has high elasticity that allows the sealant to expand with minimal force. Typically these sealants are used with substrates that may be sensitive to high stress (EIFS, composite cladding systems, etc…) to reduce the potential for damage to the substrate during joint movement. Medium modulus sealants bridge the realm of available sealants and are commonly used in weatherseal applications; but, can be used in structural applications depending on the needs of the application and performance characteristics of the sealant.

• Movement capability is usually a function of modulus and indicates the ability of the sealant to perform through the life of the joint dependent on the expected movement of the joint. Typically, high-modulus sealants have lower joint-movement capability and low-modulus sealants have higher movement capability. Low-modulus sealants used for weather sealing may have movement capability of up to +100 percent expansion (stretching) and -50 percent contraction (compression) of the original joint width. Movement capability is determined through the ASTM C-719 test method.

• Durability. A sealant may have excellent physical properties, initially, but what will happen to those properties after 5, 10 or 20 years on the building? Also, what effect will temperature have on the sealant's movement capability and modulus? The ability of a sealant to be durable enough to still provide air and water infiltration protection over time is clearly an important property to consider.


Originally published in the November 2008 issue of Architectural Record
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