Modern commercial buildings rely heavily on joint sealants to prevent water damage to the building and contents. While residential buildings use water-shedding techniques like sloped roofs, lap siding, and overlapping flashings, many commercial designs don't, providing little or no barrier to leakage when the joint sealant fails. And there are many points in the design and construction process where bad judgment or bad behavior result in inevitable sealant failure.

In addition to sealing out liquid water, exterior joints are sealed to minimize air infiltration. Interior joints in wet areas are sealed to keep out water, and other interior joints are sealed for appearance and cleanability and to reduce sound transmission through cracks and holes. If none of these considerations is applicable, joint sealers are probably not necessary.

Exterior Uses: If the exterior shell of a building could be built without joints, and cracks wouldn't develop later, joint sealing could be eliminated. However, most modern homogeneous rigid exterior substrates are purposely jointed, to allow movement without damage to the material. The two principal causes of movement are thermal expansion and contraction and seismic movement. Some substrates can be overlapped to allow rainwater to run off while allowing movement -- these usually won't need sealing (e.g. traditional shingling) or will incorporate the seal into the product design (e.g. metal panels with edge joints designed to prevent water infiltration). Other combinations of exterior materials are simply different and, as a result, seldom form a watertight joint without the addition of a sealer.

The principal exterior substrates that are sealed are:

Interior Substrates: Joints indoors don't usually go through the thermal fluctuations that exteriors do, but they are also often jointed for other reasons (e.g. gypsum board and plaster assemblies). Joints in these are usually sealed up to keep dirt out and make them look better. The principal interior substrates that are sealed are:

Note: Although there are many types of joint sealers, this review covers joint sealants only -- pourable or gunnable material of mastic consistency that sticks to each side of a joint.

Avoiding Mistakes and Failures

1. Choose the correct design solution: The life span of even the best sealer materials available is finite -- usually less than the expected building life span. If failure of the joint seal would be very costly, a water-shedding solution might be a better solution.

2. Estimate the actual amount of movement correctly, considering the width of the joint, the distance between joints, and the thermal range. ASTM C 1472 can help. Joint movement is of three types:

3. Choose a sealant product that will withstand the movement expected: Most joint sealer mistakes relate to movement -- misjudging the actual amount of movement or selecting a product that won't withstand the movement expected. Movement capability is the relevant product characteristic.

4. Choose a sealant that will withstand the environmental conditions: The second most common cause of product failure is degradation by water and weather (including indoor wet areas).

5. Specify the sealant product correctly: The two most common ways to specify sealants are a) by listing the manufacturer and brand name(s) of acceptable products, and b) specifying characteristics by description and/or by reference to voluntary standards (some of which are listed in the References section). If both techniques are used for the same product, be sure that they are not contradictory.

6. Specify the scope of sealant work completely: The most common implementation mistake is failure to completely identify the joints to be sealed and the products to be used for each (when more than one product is specified). The extent of sealing work is not always apparent from the drawings (perhaps hardly ever). Sometimes a detail shows the cross-section of the joint and may reference the type of sealer. So, it is commonly necessary to describe the extent of the sealing work in words. This may be placed on the drawings, as notes or a schedule, or may be included in the specification. Regardless of methodology, the important point is that the description fully describe the extent of the work by identifying all the joints to be sealed.

7. Specify execution correctly: Most joint sealers require expert installation, without which failure is likely. Require installers to follow the manufacturer's installation instructions AND specify reputable manufacturers who provide detailed instructions (do-it-yourself products don't usually come with detailed instructions). Do not introduce errors by contradicting manufacturers' instructions in your specifications.

Implementation in Project Specifications

Joint sealers are usually specified in a single section describing products, execution, and administrative requirements:

Concrete pavement joint sealants are sometimes specified in a separate section:

When more than one type of sealer is specified, the drawing notes or a schedule must identify which ones are to be used in which locations. The method used to identify the specific product is less important than explicitly tying the identification to a product in the specification. Some people like to give each specified sealer a Type A, Type B, etc., designation. This technique has the advantage of allowing the exact type of sealer to be changed by changing the specification, without any need to change the drawing/schedule notation.

Some statements that might appear in a sealer schedule:

In some cases, the sealant is to be furnished and installed by the installer of the product to sealed. For instance, the window specification might require the window installer to complete the installation by sealing around the window. In that case, the sealant and the sealing work can be specified in the window section. Alternatively, the sealant product and installation may be cross-referenced from the window section to the joint sealers section. Whenever there are a lot of instances like this on the project, it's more convenient to cross-reference -- that eliminates a lot of repetitive language.

Although installation requirements for sealers should be specified, it is relatively safe to rely on a statement to "install in accordance with manufacturer's instructions". Most manufacturers will not stand behind their product if not installed in accordance with their instructions and recommendations, so start there. ASTM C 1193 covers typical applications in great detail as help to the specifier, but referencing it as a specification requirement is basically useless -- there are too many options to be able to enforce any of them. The free U.S. government design guide listed below should also be helpful in developing execution specifications.

Product Selection Technique

More joint leakage is caused by using the wrong sealant than 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, some people use 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 be sealed with the first selected sealant, identify the factor that makes that sealant unsuitable and look for a substitute.

4. Use as few different 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.

Product 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.

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: Sealants come in two consistencies: 'non-sag' and 'pourable'. Vertically oriented joints require non-sag sealants, so the sealant will not run down out of the joint. Horizontally oriented joints can use either non-sag or pourable sealants but pourable will yield better looking results with less effort. 'Non-sag' is the term used in standards; 'gunnable' is an equivalent term. 'Self-leveling' is a more apt name for 'pourable', since the pouring is not optional -- 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: If the sealant must be immersed in water, not just wetted and later dried, this criterion will rule out many sealants. Only use products that their manufacturers stated 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 that are suitable for continuous immersion, making this one of the few common applications of polysulfide today.

Suitability for Exterior Exposure: This is a combination 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 not subject to rain or freezing temperatures. In another aspect of exterior exposure, silicone sealants tend to 'pick up' atmospheric dust - dust sticks to the surface and is subsequently washed off by rain (usually streaking down the face of the building) - this makes it unsuitable, in some specifiers' eyes, for exterior use.

Suitability for Traffic Exposure: The hardness required to resist puncture and tearing due to grit under traffic is generally in conflict with movement capability, which requires resilience. 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 Width: Most elastomeric sealants are limited to joint width between 1/4 inch (6 mm) and 1-1/4 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 gage 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 7-1/2 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, vinyl. For instance, products tested to ASTM C 920 Use M (for 'Masonry') have acceptable adhesion on the most common porous substrates: 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 out or migrate, especially into a porous substrate. This is a 'pass-fail' judgment, it either stains or not. Elastomeric products tested to ASTM C 920 meet minimum non-staining requirements. Although this is mostly intended to eliminate oil-based putty type calks, most manufacturers also recommend that sealants be tested with the actual stone to be used, as stone porosity varies (and stained marble would be very disappointing, to say the least).

VOC Emission: Volatile compounds evaporate easily into the air under normal conditions; volatile organic compounds that are of concern are those that irritate or impede respiration or that damage the ozone layer. Solvents and refrigerants are the two primary VOC's that occur in construction and which are regulated by law and international convention. Solvents occur in some sealants (see Cure Types). 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 (e.g. OTC).

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 do (except for non-curing applications), but sometimes the cure type is relevant because it affects other characteristics, notably VOC emission and "installation friendliness".

Color: Color options vary by product, by manufacturer, and by quantity. Custom colors usually require a minimum quantity. 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, the manufacturer's technical reps will tell you the relative ranking of their 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 reason butyl and acrylic sealants are still made is that they are less expensive than polyurethanes and silicones. When the performance of the latter are not required, particularly high movement capability or unusual exposure conditions, the former are more cost effective. Other than this consideration, it is usually false economy to scrimp on sealants - 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:

Related Products

Gaskets: Hollow preformed strips, usually rubber, that are overcompressed before insertion into the joint.

Compressible Foam Sealer: Plastic foam strips that are overcompressed before insertion into the joint.

Accessory materials that are typically required for joint sealants include backer rods and bond breaker tape.

Manufactured expansion joint covers are often used instead of sealants for large joints having large movement, such as building expansion joints and seismic movement joints.

Firestopping sealants are required to have a tested fire resistance rating and should be specified in a separate section.

The following types should be specified with the affected work, rather than in the Joint Sealants section:

References and Sources

Design Guides:

Laws and Regulations Relating to VOC's:

Product Specification Standards,

Pavement Sealant Standards,

Installation and Usage Standards,

Principal Manufacturers: