Architectural Record BE - Building Enclosure

A New Era of Exterior Sealants

New technologies allow sealants to provide better adhesion to the wide range of building materials used in modern construction, and they are longer lasting for improved durability
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Sponsored by OSI
By Andrew A. Hunt

Classifications and Sealant Attributes

To better understand how to evaluate and specify the appropriate sealant, let’s take a look at the basic attributes, characteristics, and classifications of sealant products.

Attributes of Sealant

The three main attributes of sealants include flexibility, durability, and adhesion. Flexibility refers to the hardness and joint movement of the sealant. Durability refers to the sealant’s ability to withstand heat, aging and discoloration, cracking, and chalking. Adhesion refers to how well the sealant adheres or sticks to the building materials. To allow for good adhesion, areas must be cleaned prior to applying caulk or sealant. Remove any old sealant, caulk, or paint using a putty knife, screwdriver, solvent, and brush. Most sealants require the area to be completely dry for best adhesion results and to avoid moisture being sealed into the space. The material must have adequate curing conditions and be left to dry for the required time. Not properly following the manufacturer’s instructions can cause a sealant to crack or age much faster. Some sealants dry clear, while others are colored to match adjoining building materials.

A man applying sealant on a wall.

Before applying sealants, all surfaces should be clean, dry, and free of all contaminants, such as old caulk, dust, grease, and any other materials that could hinder adhesion.

The combination of three attributes—flexibility, durability, and adhesion—is what makes sealants so effective in sealing the building envelope. These attributes work in unison to make the sealant strong and effective.

There are multiple kinds of joints that can be found between building materials and many different types of building materials with varying adhesive qualities. The four main joint types are fillet joint, control joint, dynamic joint, and bedding joint. We’ll discuss the proper way to fill each of these a bit later in the course. But first, it’s important to determine what kind of material should be used when sealing openings and joints.

Classification Standards

To help builders and homeowners determine which sealants are best for a given application, industry associations provide performance standards to classify and rate sealants. Classifications are driven by ASTM C920, which is the Standard Specification for Elastomeric Joint Sealants that are cured single- or multi-component cold-applied elastomeric sealants. Using this standard, sealants are tested in the following categories: staining and color change of single- or multi-component joint sealants, flow properties, indentation hardness, tack-free time, adhesion and cohesion of sealants under cyclic movement, effects of accelerated weathering, adhesion and peel, extrusion rate, effects of heat aging, weight loss, cracking, chalking, and durability of sealants after being immersed in liquid.

The ability to withstand weathering, heat aging, cracking, and chalking are measures of performance of the sealant when exposed to the elements. Measurements around staining and color change are performed on light, porous materials. When selecting a sealant, it’s best to pick one that meets or exceeds all categories of the standards.

Following are the specifics on how the ASTM C920 standard classifies sealants by type, use, grade, and class.


The two types of sealants are Type S (single component) and Type M (multi-component). Single-component sealants are easy to use, require no mixing, and less labor. However, they have longer cure times and the materials are more expensive. Multi-component sealants contain two or more components that are chemically cured. They have a faster cure time and a more consistent performance, but mixing the components requires extra labor and can create quality control issues.


The ASTM C920 standard also rates items based on their usage related to exposure and their usage related to material. T stands for “traffic” and is given to sealants that are used in joints that experience vehicular or pedestrian traffic such as door jams. NT stands for “non-traffic” and is used in horizontal joints that are not exposed to traffic, and for joints in walls, windows, etc., I stands for “immersible” and is given to sealants that can be used in areas that may be submersed in water.

The ratings based on usage in relation to material are as follows: M is for sealants that can be used in contact with mortar. G is glass. A is for aluminum. O is for sealants that can be used in contact with all materials other than mortar, glass, and aluminum. Sealants can have multiple materials listed in the rating.


The ASTM C920 standard grades consist of Grade P for pourable or self-leveling sealants, and Grade NS for no-sag sealants. Grade P sealants are used for horizontal applications and generally contain sealants for traffic use. Grade NS sealants are used for both traffic and non-traffic conditions, mainly for vertical joints and sloping horizontal joints.


After being tested using the ASTM C920 standard, sealants are classed in one of five categories: ASTM Class 12, ASTM Class 25, ASTM Class 35, ASTM Class 50, and ASTM Class 100. As the classification number increases, the movement capability of the sealant also increases.

Class 12 sealants are capable of handling movement with either contraction or expansion of 12.5 percent of the original joint width. Class 12 sealants are great for joints with little to no movement.

Class 25 sealants can handle contraction or expansion movement up to 25 percent of the original joint width and are designed for use with joints that have a moderate amount of movement.

Class 35 sealants can handle contraction or expansion movement up to 35 percent of the original joint width and are designed for use with joints that have a moderate amount of movement.

Class 50 sealants can handle contraction or expansion movement up to 50 percent of the original joint width and are designed for use with joints that have a moderate amount of movement, typically in building facade systems and glazing applications.

Class 50/100 sealants can handle contraction movement up to 50 percent and expansion up to 100 percent. These sealants are designed for a lot of movement and are used in building facade systems in areas that experience high winds or seismic movements.


All of these characteristics and attributes can be included in a product’s rating. Here’s an example of how an ASTM C920 rating would appear on a sealant spec sheet:

Type, S Grade NS, Class 50, Use, NT, M, A, and O. This means that the sealant is single component; a no-sag sealant; has 50 percent expansion/compression; can be used in non-traffic applications; and works with mortar, aluminum, and other materials


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Originally published in Architectural Record