Joint Dimensions in Caulking: Yes, They Are Important!

The Ins and Outs of Caulking by the late Charles Carll reminds the homeowner that joint dimensions matter.

In narrow joints, a given amount of differential movement between substrates translates into relatively large strain rates in the sealant.

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A perimeter sealant joint around a contemporary flanged window. The joint is of appropriate width at 0.25 inch. The joint had been in service for roughly 3 years when the photo was taken. The joint is mostly intact, but some adhesion failure is evident at lower right corner. As is common in residential construction, neither bondbreaker tape nor sealant backer was used. Joint failure is likely the result of three-sided adhesion, an unprimed siding edge, and other-than-ideal sealant width–depth ratio (sealant depth exceeding width).

The photo shows a 0.25-inch-wide perimeter butt sealant joint around a residential window, which was in accord with the window manufacturer’s installation instructions.

Industry standards call for sealant joint depth that vary with joint width and sealant type. A generic rule for joints up to 0.5 inches wide is that joint depth should not exceed joint width. Minimum acceptable joint depth varies with the sealant type, and sealant manufacturers rarely if ever provide minimum depth recommendations to retail customers.

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Cross-sectional sketches of butt sealant joints.

With butt joints, some minimum depth dimension at the substrate surfaces is necessary for adequate adhesion. The hourglass shape of the sealant cross section that can be seen in the above graphic is considered desirable, as it provides the greatest possible adhesive-bond area at substrate surfaces and provides a region of relatively low stiffness at mid-width of the joint. Tooling of sealant (to be discussed in an upcoming post) results in surface concavity that provides in part for the hour-glass shape of the sealant cross section. With sealants that shrink during cure, concavity of the cured sealant joint surface is likely to be accentuated, and as a result, sealant depth at joint mid-width may be less than anticipated.

When using sealants that shrink, making some trial joints to identify cured sealant depth at joint mid-width can be instructive.

Butt and Fillet Joints

The Ins and Outs of Caulking defines butt sealant joints and fillet sealant joints. A butt sealant joint is a joint in which sealant is applied between two approximately parallel substrate surfaces that are either edge-to-edge or face-to-edge.

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Cross-sectional sketches of butt sealant joints.

A fillet sealant joint is a joint in which sealant is applied over (not into) the intersection between surfaces are approximately perpendicular to each other.

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Cross-sectional sketches of fillet sealant joints.

In a well-executed butt joint, the sealant does not adhere to any rigid material at the back of the joint nor does it adhere in the root of the joint. If sealant adhesion occurs at the back of a butt joint or in the root of a fillet joint, stress concentrations will occur in the sealant when there is differential movement between substrates. Joint failure will thus be likely, even when a high-performance sealant is used.

To prevent adhesion behind butt joints or in the roots of fillet joints, use non-rigid sealant backers or bond-breaker tapes. In commercial construction, caulking tradespersons are familiar with non-rigid sealant backers and bond-breaker tapes, and part of a tradesperson’s skill involves his or her ability to fit joints with backer or bond breaker (or both) before application of sealant. Unfortunately, residential construction contractors and home owners rarely pay attention to prevention of three-sided adhesion in butt joints or to sealant adhesion at the roots of fillet joints.

Hardware stores and home centers may sell sealant backer rods, but the variety of shapes and sizes is usually limited and virtually none of these retail businesses sell bond-breaker tape. An internet search will typically locate a handful of online merchants that market bond-breaker tapes to the general public. In retail home centers, backer rods are usually stocked with weatherstripping rather than with caulks and sealants.

No professional consensus exists on how long sealant joints in residential construction can be expected to remain functional. Professionals commonly believe, however, that the service life of residential sealant joints is usually shorter than 20 years. Manufacturers’ warranties of multiple decades of sealant joint performance only provide for replacement of the caulking material. Cost of application labor is not covered by the warranties, nor is the cost of repairing damage sustained as a result of a failed sealant joint.  

Substrate–Sealant Compatibility: More Considerations for the Homeowner

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Compatibility of the finish and the substrate is critical.

The Ins and Outs of Caulking states that compatibility between substrates and sealants involves two issues. The first is sealant adhesion to substrates (which may be dissimilar); the second is the potential for uncured sealant to damage the substrates (by chemical action of a component in the uncured sealant).

Silicone sealants are recognized as adhering well to most non-porous substrates. Their effective adhesion to porous substrates like wood or masonry may, however, require the use of primers. Polyurethane and latex acrylic sealants are generally recognized as adhering relatively well to porous substrates like wood and wood-based products without the use of primers. Sealant primers are rarely if ever available at hardware stores and home centers. A quality primer paint, suitable for the (porous) substrate, usually improves sealant adhesion. If end and edge cuts on siding or trim surfaces are primed before the siding or trim is installed, the priming is significantly more effective.

The chemical components of sealants with the potential to damage substrates are most commonly either organic solvents or acidic constituents. The organic solvents used in most sealants are not sufficiently aggressive to harm most substrates. The smell of vinegar in uncured silicone sealant indicates the presence of an acidic constituent. Most substrates are unaffected by such acidic-cure silicone sealants, but a few are. Installation instructions for components such as fenestration units may indicate if sealant use is recommended, and if so, what type of sealant to use. Use of a different type of sealant than recommended by the manufacturer may result in chemically induced damage to the component. Some silicone sealants are unsuitable for certain porous substrates because they can leach oily materials, resulting in staining.

Caulk or Sealant? Choices for the Homeowner

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The homeowner has a wide variety of choices in sealants and caulks for DIY projects.

Although the term “sealant” implies superior performance, the terms “caulk” and “sealant” are used more or less interchangeably throughout The Ins and Out of Caulking. These terms are also used interchangeably often in the building industry, even though there is a rational basis for distinction between the terms.

Modern caulks incorporate synthetic polymers. Higher performance caulks marketed for residential construction incorporate one of four different types of synthetic polymer: silicone, polyurethane, emulsified (latex) acrylic, or solvent-borne block copolymer. Of these, latex acrylic, silicone, and polyurethane caulks are widely used. Block copolymer caulks are relatively uncommon; they are specialty caulks, usually selected for their clarity, where this attribute is important. Each of the polymer types has its own particular (and in some cases, peculiar) advantages and disadvantages.

Latex acrylic caulks usually shrink during extended cure. Uncured latex acrylic sealant can, however, generally be applied successfully to cured sealant of the same type. A wide variety of emulsified resin (“latex”) caulk is available, and these products are in many ways the easiest of the sealants to use. In residential building, where the knowledge and skill of applicators are not particularly well developed, latex acrylic sealants may provide performance as effectively as silicone or polyurethane sealants even though they do not match the performance of these (usually higher cost) sealants in controlled laboratory testing.

Staining Previously Painted Cedar Siding: Instructions on a Tricky Job

Solid-Color Stains

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Severely weathered wood surface.

When reapplying solid-color stain, wash the surface to remove dirt and mold. If areas have peeled exposing the wood surface, the weathered surface must be removed prior to re-priming the peeled area. The photo shows an extreme case of surface degradation following the failure of the coating. Solid-color stains form films, and like all other film-forming finishes, do not bond properly to weathered wood. It has been well established through several studies that cedar exposed to sunlight for as little as 2 to 3 weeks will not hold film-forming finishes as well as an unweathered surface. It may be possible to remove the damaged wood from small areas by scuff sanding, but this is difficult with shakes and shingles because the surfaces are saw-textured or split. The sanded surface won’t match the unsanded surface. If the previous finish contained lead, do not sand.

The situation shown in the photo is an extremely difficult problem to reconcile. A penetrating finish, such as an oil-based semitransparent stain, cannot be used over the existing finish, and a film-forming finish will not adhere to the weathered surface. It is probably necessary to remove all the finish and the weathered surface by power washing prior to application of the primer and solid-color stain. This may be one of the few instances where careful power washing by a skilled operator is necessary. As with using a garden hose to wash siding, keep the nozzle pointed down to avoid forcing water behind the siding. Paints refinish in the same way as solid-color stains. Lap marks should not be a problem with paint systems.

Careful and judicious work will yield good results for the homeowner.