Sealing Out Water
To select the correct product for the job, we must calculate the expected expansion and contraction of the materials being sealed, looking at total movement as well as differences between adjacent materials. If the sealant material cracks but both edges are still fully adhered, the sealant bonded well but did not have enough elasticity.
We should select a sealant product whose elasticity exceeds the calculated movement by a factor of two to four. For example, if the materials are going to move 1/16 inch (0.15 centimeter), the sealant should be able to stretch and compress 1/8 inch (0.3 cm) or more without coming loose from either of its two adhered surfaces.
A major cause of material expansion and contraction is temperature variation. The thermal coefficients of common materials differ greatly, so differential movement is to be expected. For instance, the coefficient of glass is nearly seven times that of wood.
To calculate the expected expansion of a material, multiply the application length by the coefficient for thermal expansion. Multiply this result by delta T — the difference between the highest and lowest temperatures the material will experience in a year. (As a rule of thumb, use a delta T of 120 degrees Fahrenheit or 67 degrees Centigrade). Note that this is not air temperature, but surface temperature. When exposed to the sun, metal surfaces can reach 180 degrees F (82 degrees C).
If we want to minimize the visibility of the sealant joints, we might choose to limit them to 3/8 inch (1 cm) in width, so they would be the same size as the mortar joints in a typical masonry wall.
I prefer to design joints to move about 1/4 of their width so they don't bulge out noticeably in the summer, when building materials are at their most expansive. To limit a joint's movement, divide the total length into smaller spans. If a long joint were divided into, for example, four equal sections, each section would experience one fourth as much movement.
A good polyurethane or silicone sealant will be able to stretch four to seven times its installed width. In a 3/8-inch (1 cm) joint, for example, it could stretch more than 1-1/2 inches (4 cm).
Another major cause of sealant separation is faulty design. Sealant joints should not be confused with expansion joints; rather they should work in concert with them.
Designers need to consider three principal factors related to sealant joints: frequency, location, and geometry. Frequency can depend on primary structure, soil bearing capacity, temperature differential, and so. Joints should be located to minimize cracks in exterior walls. Manufacturers and industry associations publish recommendations for maximum distances between joints.
Location depends on type of material, support conditions, and many other factors. In brick veneer, we typically place vertical control joints where support angles are fastened to the primary structure, at the center of column lines for example. In a tall wall, we often locate horizontal joints at bond beams or where ledger angles are required.
The design goal for finish materials is to minimize cracking that could result from movement related to temperature, vibration, or differential settling. It is also a good idea to place joints where forces are concentrated, such as in line with window or door heads and sills. Another recommended location is where materials or systems change, such as where concrete masonry units (CMU) walls meet floor or roof slabs.
For most materials, I recommend a joint width in the range of 1/2 to 5/8 inches (1.25 to 1.6 cm), but try to avoid widths over 1-1/2 inches (3.75 cm). Using the method above, you can work backwards to determine how long the joints can be.
Joint geometry is the third important design factor. Three common sealant applications are fillet joint, lap joint, and butt joint. A typical butt joint (such as used with glazing) is 5/8 inches (1.59 cm) in width. The depth of the sealant should be about half the width. In no case should there be three-sided adhesion. While some of the better sealants have great adhesive properties, they should be using those properties to stay in place, not to hold window jambs in place or copings to a parapet wall.
It is common to use foam backer rod material at the back side of a joint. The backer rod should be clean and dry when installed. It should be compressed to about 2/3 of its full size in the void. Backer rods should be installed so that the remaining depth for the sealant is about half the width at the center. You should have more depth for bonding at both ends where the two materials are joined.
Use a 5/8 to 3/4 inch (1.5 to 1.75 centimeter) backer rod for a half-inch- (1.27-cm-) wide joint. The depth should be constant. If the depth of the joint is insufficient for backer rod, an alternative is bond-breaker tape. It can be used in shallow joints to ensure that only two-sided adhesion takes place. In most cases, the tape is applied to the back side, to prevent three-sided adhesion.
Poor preparation of materials can be another cause of joint failures. The most common cause of sealant adhesion problems is insufficient preparation of bonding surfaces. Sealants adhere best to dry surfaces, and all surfaces to be in contact with the sealant must be clean — free of dirt, dust, oxidation, oils, and most paints.
Some painted surfaces, such as baked-on enamels or Kynar, have adequate bond. The chosen sealant must be carefully studied to make sure it is right for the materials it will be in contact with. Some sealants will react with solvent-based paints.
It is not advised to leave sealants exposed to the weather in situations where failure will give water an easy pathway behind wall or roof membranes. Sealants can be protected by, for instance, metal cover plates.
I have learned from experience that preconstruction mock-ups are invaluable in assessing sealants and difficult geometries of windows, doors, and rooftop elements. These allow physical testing before designing the final details.
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William L. Walker, AIA is senior project manager at C.T. Hsu + Associates and author of An Illustrated Guide to Building Envelope Design, to be published by McGraw-Hill in August 2007.