Choosing A Construction Sealant Technology

Sealants come in many different formulations with varying performance capabilities. They are a critical component in safeguarding the building envelope’s integrity, and it’s important to understand their basic characteristics, functions and applications.  This article will provide an overview of sealants.

What exactly is a sealant?

A sealant is an elastomeric material with adhesive properties that is applied between building materials to provide a barrier to the outside environment and enhance the aesthetics of the transition. Sealants contain an inert filler material and an elastomer to deliver both flexibility and elongation. When applied, they have a paste-like consistency, and as it cures, the sealant will solidify to a pre-determined hardness.  Sealant products either shrink or maintain their body during the curing process.  This variance will depend on the exact formulation and the type of technology used.

Sealants are extremely versatile.  And while they contain many similar properties of adhesives, caulks, and putties, they remain very different. Typically, sealants have lower adhesive strength but higher flexibility than adhesives. They also serve more uses than caulks and putties, which are only intended to fill small voids. Regardless of application, sealants have three basic functions:

  1. To improve the appearance of joints by filling gaps between substrates
  2. To provide a barrier against water, moisture, air, dust, dirt, chemicals, noise and vibration.  And in some cases, to provide electrical or thermal insulation.
  3. To maintain a seal for the expected lifetime, service conditions and environments.

Installers must prioritize a number of application and performance needs when choosing a sealant.  The decision takes into consideration 1) the type of material the sealant will be applied to, 2) the joint dimension, 3) joint construction, and the 4) sealant’s performance characteristics. Let’s look at each of these in more detail.

1. Material Type

Most sealant technologies don’t adhere equally to all material types under all conditions, so it’s important to understand the substrate characteristics. Is the material porous, like wood, or non-porous, like aluminum or vinyl?  Is it coated with a paint, stain or oil? What is the material’s water absorption process?  Will its surface be wet during application? What is the age of the substrate and does it show visible signs of wear or corrosion?  Knowing the answers to these questions can help choose a sealant appropriate for the material it will be applied to.

2. Joint Dimensions

The size of the gap or joint is important in determining which sealants should be used. Also important is the gap width tolerance (movement that a gap between two substrates will allow) and the material’s movement capability. For example, a brick joint that is a ½” wide could have a gap width tolerance of plus or minus ⅛” or more. To prevent sealant failure under expansion and contraction, a flexible sealant that will accommodate the joints movement is required.

A sealant’s expansion and contraction capabilities is defined by one of five primary classes.  The higher the class designation, the more flexible the product.  For example, a Class 25 sealant allows for up to 25% expansion or contraction of the joint without failure. If there are two numbers in the movement class (100/50), the first number represents the expansion and the second number represents the contraction limit.

Class 12.5: 12.5% Expansion or Contraction

Class 25: 25% Expansion or Contraction

Class 35: 35% Expansion or Contraction

Class 50: 50% Expansion or Contraction

Class 100/50: 100% Expansion or 50% Contraction

Joints can move in three ways: expansion, contraction, and shear movement.  These movements are caused by a combination of mechanical stressors like vibrations and radiation and temperature variations resulting from sunlight, seasonal climate shifts or other sources of hot and cold.  Thermal variants can affect the expansion and contraction of building materials differently.  A list of common substrates and their expansion coefficient is provided below.


Different joint construction types require a sealant with certain performance attributes to provide proper, long-lasting performance.  These joints can be formed by similar or dissimilar materials. 

A fillet bead is triangular in shape and is applied to surfaces that form a right angle, for example when siding terminates into trim.

A control joint is assembled when two materials, that are flush, or provide a gap of ¼” or larger, and parallel to each other, form a gap to prevent the transfer of forces across the joint as a result of material movement or dimensional change. This joint requires a backer rod and sealant to prevent three-sided adhesion and eventual joint failure.

A bedding joint provides for a 3/8 sealant bead that is parallel between two bedding planes to bond and seal both surfaces, for example behind a window flange or an entry door brickmold. The sealant for this joint application must not shrink as it cures and needs to be compatible with all materials that it contacts.

Performance Characteristics

A sealant’s performance must complement all joint applications.  Great consideration must be taken to know the sealant’s physical, chemical and durability properties before making a selection. Here is a list of common sealant performance characteristics:

In addition to performance characteristics, sealants can be characterized by their curing mechanism.  There are both physical drying and moisture curing sealants available on the market today.

Physical Drying

Once a physically drying liquid sealant is applied, the water, or solvent, inside begins to evaporate, and the sealant forms a skin. As the sealant continues to dry, more evaporation occurs. During this process, the volume of the sealant is reduced by at least 22% and sometimes as much as 35%. Latex or acrylic and solvent-based thermoplastics are examples of physical drying sealants.

Moisture Curing

A moisture curing sealant reacts with atmospheric moisture after application to initiate a chemical cross-linking of the ingredients. As the ingredients crosslink, the sealant builds internal strength and creates a chemical bond to the substrates. The sealant’s volume does not decrease. Polyurethane, silicone, and hybrid sealants are examples of moisture curing sealants.

There’s a lot that goes into selecting the right sealant, from substrate materials to joint construction and movement to sealant performance attributes. Paying attention to these considerations can help make sure that the integrity of the building envelope is maintained now and into the future.  Below is a list of OSI sealant products and some of the important technical information needed to make the proper sealant selection for your job.