Expansion Joints and Their Role in Waterproofing

Keeping water where it belongs
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Sponsored by Inpro
Presented By Peter J. Arsenault, FAIA, NCARB, LEED AP
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Applications and Location

The nature of the building will often dictate the number, type, and location of expansion joints needed. A few of the most common locations where expansion joints are found detailed as follows.

  • Building additions use joints to segregate the addition from the original structure.
  • Long buildings will need a joint approximately every 200 linear feet or so as determined by the structural engineer.
  • L-shaped building footprints can often require separation of the two wings at the central knuckle.
  • Any dramatic changes in height may require an expansion joint to allow movement both vertically and horizontally.

Typical locations for expansion joints are based on the building size, geometry, and three-dimensional makeup.

Typically, the joints need to run continuously through all adjacent planes to fully separate building sections and allow independent movement. This means that any given project scope could include interior joints, exterior joints, or both in things like walls, roofs, floors, building veneers, soffits, parking decks, patios, roofing systems, etc.

Interior versus Exterior Applications

It is easy to assume that exterior expansion joints are subjected to more harsh conditions than interior, but this may not be the case, particularly for joints that cross floors. Interior joints on floors are subject to loading from foot traffic, light maintenance vehicles, or more. Therefore, when determining load requirements, consider what type of traffic will take place. All joint covers are engineered to accommodate a fairly wide range of typical building loads, but it may be important to dig a little deeper to ensure the best cover plate solution is selected. For example, does the owner utilize equipment such as scissors lifts for lighting changes in high-ceiling areas? If it is a hospital, will mobile x-ray machines or gurneys with patients be passing over the joint and therefore require an extremely smooth transition? It is usually best to ensure the worst-case scenarios are considered when specifying systems. A slightly higher front-end cost may alleviate an ongoing facility maintenance headache for years to come.

Form and Appearance

The importance of the form and appearance of covered expansion joints usually depends on the type of spaces where the joints occur and the adjacent finishes. For example, there may be different design criteria for back-of-house conditions than for public corridors or high-end spaces. Depending on the aesthetic being sought and corresponding budget, there are a range of options available to meet different needs. It should never be assumed that one solution works for all expansion joints.

With the above considerations taken into account, we next turn to the different options in the selection of expansion joint systems.

Types of Expansion Joints

There are a number of different types of expansion joints that can be worked into specifications and construction drawings to help assure any building project is properly allowed to move, remains weather and moisture tight, and protects life safety. Each has different characteristics that can make it better suited for certain applications compared to others. To remain water resistive, they need to be detailed and installed properly too. Some of the common types and their typical applications are discussed as follows.

Foam Seals

In certain applications, the use of foam seals in expansion joints provides a solid seal against the elements and moisture protection. Foams can also provide acoustic and insulation properties. As a general rule of thumb, limiting foam seals to applications with a joint width of no more than 8 inches (200 millimeters) or smaller is good practice. Use of foams for expansion joints larger than 8 inches leads to two things: 1) exceeding the foam’s performance characteristics, including possible sagging of the foam seals in vertical applications due the weight of wider sizes, and 2) exponentially higher costs compared to other expansion joint cover solutions.

Fire-rated foam seals are also available and suitable for 6-inch and smaller gaps and conditions where abuse is not likely. These systems are comprised of open-cell polyurethane foam impregnated with a fire-retardant material. These foams can be faced with colored silicone to match a desired decor or design aesthetic. Fire-rated foams are usually lab tested in concrete and cement-board wall conditions (not drywall).

Foam seals come in two basic types: open-cell (left) and closed-cell (right) foam, each
with distinctive properties and characteristics.

When selecting an appropriately sized foam, there are two fundamental types to consider, open-cell or closed-cell foam seals. Open-cell foams provide some breathability and allow for flow-through of water and vapor making them best suited for vertical applications. Like many exterior veneer systems, if moisture becomes trapped in a wall cavity, building systems allow the moisture to wick out. This is a good quality and a major focus to eliminate potential mold issues in vertical applications.

By comparison, closed-cell foams are absolutely watertight and do not allow moisture or bulk water to enter the body of the foam. Closed-cell foams can also be heat-welded at all seams and changes in direction. This is critical for watertight runs, as the seams have the same movement capability as the foam without reliance on adhesives that eventually wear out. They are best suited to horizontal applications where moisture could remain trapped and water penetration cannot be allowed. Closed cell can also be utilized on below-grade vertical applications as support and closure to positive side waterproofing at expansion joints. These are tougher to compress but can be placed under tension (or expand) well.

Whether open- or closed-cell foam is used, it is important to recognize that there is a difference between seals that are manufactured as a layered product and those that are monolithic. Layered foam seals are produced by laminating multiple ½-inch layers of foam together. This is a less costly manufacturing process but can result in a product with reduced life cycle due to the vulnerability of delamination of the various layers from each other or susceptibility to splitting from shear movement. A superior alternative to the layered seal is the monolithic-pour foam seal. As the name implies, by producing a seal in a single pour of the foam material, there are no layers to delaminate. Batch-to-batch variation inherent in layered foam seals is also avoided. While in a compressed state all foams look the same, an argument can be made that specifications calling for foam seals made with “monolithic manufacturing methods” will avoid product failures and claims down the road.

Monolithic foam seals for expansion joints, shown at the bottom of this photo, provide long-term, durable, and waterproof solutions. Laminated foam seals, shown at the top of this photo, are subject to delamination and failure when used in an expansion joint system.

There is one other consideration for foam seals, namely the use of wax impregnation in the foam. Heavy wax-impregnated foams that help keep joints watertight were in use for about 50 years. Some consider the addition of copious amounts of wax as old technology, which is true. However, it is generally regarded that a 2–3 percent wax impregnation is the best alternative since it drastically increases the hydrophobic properties of the body of the foam and extends the seal’s lifespan. Without it, acrylic-only foam can act just like a sponge, meaning that it will soak up and store water. In addition, plain foam assumes an unrealistic expectation of perfect installation of the silicone face and field perimeter caulk seals to keep the foam protected. If the face silicone seal itself is damaged—say, by the tip of a caulk gun jammed between the foam and wall or deck material—leaks will occur. With wax impregnation, the foam seal will remain watertight even if the silicone face seal is compromised because wax does not dry out.

Overall, architects should look closely at the construction of any foam seals and ask questions of the manufacturer as to the makeup of any foam seal being considered.


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Originally published in Architectural Record
Originally published in July 2020