Selecting Products

More joint leakage is caused by using the wrong sealant than is caused by specifying the right sealant badly. There are certainly dozens of brand name products to choose from, possibly hundreds. Some sealant characteristics are critical to performance, others are arbitrary or user preferences.

To simplify the selection and specifying process, try the following technique:

  1. Identify the exterior joints with the most extreme movement and select a sealant for them.
  2. Using the characteristics of the sealant selected, identify any other exterior joints that cannot be sealed with this sealant; assume all others will be sealed with this sealant.
  3. For those joints that cannot use the selected sealant, identify the factor that makes that sealant unsuitable and look for a substitute.
  4. Use as few types of sealants as possible.
  5. Include an entry on the sealant schedule, “All other exterior joints: Use … ” listing the first sealant selected as the default sealant.

Use a similar technique for interior sealants.

Sealant Characteristics

Joint sealants are usually characterized by their material or chemical composition and some major characteristics. The types listed below are the most common, but there are also many unique chemical compositions made by only a few manufacturers.

Pecora introduces 890TRS, which the company claims is the industry's first Class 100, nonstaining, field tintable silicone. The ability of distributors to carry an inventory of 48 standard silicone colors means no more special-order lag times.

Pecora introduces 890TRS, which the company claims is the industry's first Class 100, nonstaining, field tintable silicone. The ability of distributors to carry an inventory of 48 standard silicone colors means no more special-order lag times.

  • polyurethane (one-part, multipart; non-sag, pourable)
  • silicone
  • polysulfide (two-part)
  • butyl rubber (solvent-based)
  • acrylic (solvent-based)
  • acrylic latex (water-based)

    The characteristics listed below are arranged in order of their importance to performance, with the most important first. If you work through the list from the top to the bottom, you'll rule out products that can't do the job more quickly.

    Consistency. Joint sealants come in two consistencies: non-sag and pourable.

    Vertical joints require non-sag sealants so the sealant will not run down out of the joint. (Non-sag is the term used in standards; gunnable is an equivalent term.)

    Horizontal joints can use either non-sag or pourable sealants, but the pourable variety will yield better-looking results with less effort—the sealant is poured into the joint and levels itself under gravity.

    Polyurethane sealants are usually available in both consistencies. Acrylic latex, butyl, solvent-based acrylic, and silicone are non-sag only.

    Continuous immersion durability. In a fountain or other circumstance where the sealant must be in contact with water at all times, use only products that manufacturers state are suitable for continuous immersion or which are tested to ASTM C 1247 with the minimum values specified in ASTM C 920. Polysulfide sealants are among the few varieties that are suitable for continuous immersion.

    Suitability for exterior exposure. The qualifications for exterior exposure consist of low-temperature resistance, UV and ozone resistance, and heat-aging resistance.

    ASTM C 920 sealants are suitable for exterior exposure. Acrylic latex is not suitable for exterior exposure, unless it is not subject to rain or freezing temperatures. Silicone sealants are eminently suitable for exterior exposure from a durability point of view, but they tend to “pick up” atmospheric dust. Because rain can cause the dust to streak down the face of the building, some specifiers do not use silicone for exterior joints.

    Suitability for traffic exposure. Sealants subject to vehicular and pedestrian traffic need to have a certain hardness to resist puncture and tearing. This is generally in conflict with movement capability (see below), which requires the sealant to be resilient. Elastomeric sealants tested to ASTM C 920 Use T (for traffic) are generally suitable for traffic applications, as are other sealants recommended by their manufacturers for this use: polyurethanes (pourable types) and specialty silicone (non-sag). Products that meet other standards written specifically for pavement use are also acceptable.

    Mildew resistance. ASTM C 920 sealants are not tested for mildew resistance. For “bathtub calk,” specify a white silicone sealant specifically manufactured for mildew resistance.

    Acceptable joint widths. Most elastomeric sealants are limited to joint widths between ¼ inch (6 mm) and 1¼ inch (32 mm). Some polyurethanes are designed for wider joints. The problem with narrower joints is that the absolute movement (especially compression) exceeds the capability of the sealant; some polyisobutylene sealants are designed for very narrow joints. Some narrow joints have no movement at all.

    Movement capability. This is the gauge of how much extension and compression the sealant can withstand without either pulling away from the sides of the joint or failing in the body of the sealant. It is measured as a plus/minus percentage of the joint width at the time of installation, tested to ASTM C 719. This test also evaluates adhesion and cohesion as criteria for the movement limits. Movement capability over 7.5 percent rules out latex and butyl sealants. Movement capability over 25 percent is available but not universal in polyurethane and silicone sealants. Movement capability over 50 percent is rare.

    Adhesion. Suitability for a specific substrate usually comes down to adhesion. Minimum adhesion is usually taken for granted, but there are variations—most of which cannot easily be quantified. Polyurethanes generally have the best adhesion, followed by silicones, then butyl and acrylic. Some substrates that may be problematic include masonry, stone, and vinyl. For instance, products tested to ASTM C 920 Use M (for masonry) have acceptable adhesion on the most common porous substrates, such as concrete, masonry, and stone.

    Hardness. Primarily a measure of indentation resistance, hardness is mostly used to judge whether the sealer is suitable for traffic use; minimum and maximum values are specified in ASTM C 920.

    Hardness is also a measure of vandal resistance, but the harder the material, the less movement capability it has. Epoxy joint sealants are used in detention occupancies, as they are hard enough to resist picking with fingernails and plastic spoons; however, they have much lower movement capability.

    Porous substrate staining probability. Sealants can potentially be made of chemical compounds whose component materials might leach or migrate, especially into a porous substrate such as stone. This is a pass-fail judgment—the sealant either stains or it doesn't. Elastomeric products tested to ASTM C 920 meet minimum nonstaining requirements. Although this is mostly intended to eliminate oil-based, putty-type caulks, most manufacturers also recommend that sealants be tested with the actual stone to be used as stone porosity varies.

    Volatile compound emission. Volatile organic compounds (VOCs) evaporate easily into the air under normal conditions; VOCs that are of concern are those that irritate or impede respiration or that damage the ozone layer. Solvents and refrigerants are the two primary VOCs that occur in construction and that are regulated by law and international convention.

    Solvents occur in some sealants. VOC emissions from architectural joint sealants are regulated by states and regional air-management districts. In some cases, independent regional commissions develop rules that are adopted by states.

    Cure type. All joint sealants cure (change from being of toothpaste consistency to being solid) except those that are intended never to cure for some specific reason.

    In principle, we usually don't care how sealants cure, as long as they perform as intended, but sometimes the cure type is relevant because it affects other characteristics, notably VOC emission and installation friendliness:

  • Solvent release sealants cure by the evaporation of solvents, usually referred to as VOCs (most butyls and acrylics, but not acrylic-latex).

  • Water-based sealants cure by evaporation of water (limited to acrylic-latex).

  • Chemically curing sealants cure by the combination of chemical compounds. There are two subtypes:

  • multipart sealants combine two or more synthetic materials just before application;

  • some single-component sealants absorb moisture from the ambient air to combine chemically (usually referred to as “moisture curing”).

  • Noncuring sealants are deliberately designed to never cure; they don't evaporate anything. They are also usually described as nondrying and nonskinning. They stay sticky and malleable and are usually intended for completely concealed moving joints and connections.

    Sikaflex 15lm is a polyurethane sealant that offers the same low modulus and joint movement (+100/-50%) capabilities of a silicone sealant, while offering all of the benefits of a polyurethane, including primer-less adhesion to porous substrates and paintability with acrylic coatings.

    Sikaflex 15lm is a polyurethane sealant that offers the same low modulus and joint movement (+100/-50%) capabilities of a silicone sealant, while offering all of the benefits of a polyurethane, including primer-less adhesion to porous substrates and paintability with acrylic coatings.

    Color. Color options vary by product, by manufacturer, and by quantity. Custom colors usually require a minimum-quantity order. An alternative to integral color is to paint the finished joint. This is only appropriate when the type of paint used will withstand the movement, and the manufacturer says the sealant may be painted.

    Expected service life. Service life is usually used to represent the longevity of the material under the conditions it was designed for. There are no tests for service life; statements of expected service life are usually based on field experience and are seldom published by the manufacturers. If you ask, though, a manufacturer's technical rep will tell you the relative ranking of the company's own sealants and estimated life span. Silicones and polyurethanes typically last 20 years or more. Most other types last less—sometimes a lot less.

    Relative cost. The least expensive sealant is acrylic-latex, followed by butyl and solvent-based acrylic, with polyurethanes and silicones the most expensive. If the higher movement capability or weather resistance of the latter are not required, the former are more cost-effective.

    On the whole, however, it is usually false economy to scrimp on sealants, especially when you consider the cost of labor to remove failed sealant and install a replacement (and the effects of enduring leakage for an extended period before deciding to make the replacement).

    Extreme and unusual exposures. Always verify suitability with the manufacturer. For example:

  • exposure after installation to solvents, petroleum products, oxidizing chemicals;
  • chlorine in water (swimming pools as opposed to fountains);
  • high temperatures, as on exhaust and boiler stacks and other hot equipment; and
  • high expectation of vandalism, as in detention facilities.