Homeowners: Sealant is Not a Substitute for Flashing

The final advice from The Ins and Outs of Caulking deals with flashing. Modern sealants are capable of impressive performance in controlled laboratory testing and also in well-executed joints on buildings. As indicated previously, however, sealant joints in residential construction are usually executed by individuals with limited training and cannot be expected to function indefinitely. Sealant joint failure can occur without obvious indication.


Water staining on the back surface of hardboard siding on a test building. Leakage, although modest, occurred in 29 months of service and was determined to be through sealant joints. The sealant used in this installation met industry requirements. None of the joints showed outward indication of failure. Subsequent laboratory testing indicated that the window unit, although not of the highest quality, was not the leak source. Sealant joints were a combination of bedded, butt, and fillet. At fillet joints, sealant adhesion at joint root was a likely contributor to failure. The building was in Florida where conditions at joint installation were hot and humid. These conditions would accelerate cure of the polyurethane sealant used. “Skinning” of the sealant before tooling was therefore also a plausible contributor to joint failure.

Sealant joints should thus not be considered as acceptable substitutes for formed metal flashing. Well-executed formed metal flashing makes use of an exceptionally dependable force of nature: gravity. Water management by drainage is a time-proven strategy in the construction of wood buildings. For example, at horizontal joints in plywood panel siding, metal Z-flashing or shingle-lap joints are effective; sealant joints are not an acceptable substitute. Where there are drainage paths, it is important that caulk, if used, not block them.

Remember, good caulking can protect your doors and windows if applied properly. This report, available free for download on the FPL website, can tell you all you need to know.

Caulking: Application Conditions and Shelf Life

What other things do homeowners need to keep in mind when caulking their doors and windows? The Ins and Outs of Caulking reminds us of two important factors: the conditions when the work is done and the shelf life for the materials.

thermometer2The range of acceptable application temperatures will be indicated by the sealant manufacturer. Emulsion (“latex”) sealants cannot be applied below about 40°F (4.4°C). Silicone sealants typically have the widest acceptable range of application temperatures. Surface frost, however, remains a concern when temperature at application is below freezing. High temperatures will accelerate cure time. In some cases acceleration can be excessive, and adhesion to substrates will be compromised if the sealant “skins” before tooling can be accomplished.

Sealant should not be applied during or immediately after a rain. Freshly installed joints usually can withstand modest rain exposure, provided that the sealant has first “skinned” and provided that raindrops do not forcibly impact the sealant. Latex sealants are more likely to be deformed by rain exposure shortly after installation than are other sealant types. It is good practice to refrain from installing sealant joints if there is a threat of rain within 3 to 6 hours. The acceptable time “window” will depend on ambient temperature and sealant type.

Shelf life is also important with caulk. Do not use caulk that has been stored for excessive periods. The caulk manufacturer may indicate shelf life on the product packaging. If it is difficult to force the caulk from the tube at normal temperatures, the shelf life has probably been exceeded. However, easy dispensing of caulk from the tube does not necessarily indicate product freshness. Some caulks that have exceeded their shelf life may be pumped easily from the tube, but will fail to cure. Latex caulk that has been frozen in storage should be discarded.


More Tips on Caulking: Tooling Helps Adhesion to Substrates

When caulking, what can help sealants adhere to substrates before the sealant cures? Again, The Ins and Outs of Caulking has the answers for homeowners, telling them that tooling assures uniform sealant contact with each of the substrates and works air bubbles from the sealant. Tooling usually results in a more aesthetically pleasing joint as well. What tools?


As the photo shows, homeowners have some sophisticated choices for tools: fingers. Big box stores sell caulking tools, although a plastic spoon is usually satisfactory.

Emulsion acrylic sealants are the easiest to tool. Water is usually used as a tooling liquid (lubricant), and excessive adhesion to the wetted tooling device is rarely a problem. Polyurethane sealants often adhere mightily to tooling devices and thus usually require a lubricant on the tooling device. Some manufacturers of polyurethane sealants allow water or soap solution for tooling; others prescribe proprietary tooling lubricants. Silicone sealant manufacturers usually recommend dry tooling, although some may recommend a tooling lubricant. With some sealants, a tooling lubricant requires that the joint be completely filled prior to tooling.

Lubricant on the surface of just-tooled sealant or on substrate surfaces may interfere with sealant cohesion or adhesion, respectively, if additional sealant is placed in the joint. Because latex acrylic sealants use water as both a suspension agent and a tooling lubricant, they are relatively immune to this problem. Experienced professional tradespersons rarely try to tool underfilled joints.

In contrast, relatively inexperienced homeowners can reasonably be expected to sometimes make (and have to correct for) this error. The purpose of tooling is to improve adhesion between the sealant and the sides of the joint. Tooling will coincidentally push sealant back into the joint. One of the functions of backing material is to prevent excessive depth of sealant in tooled joints.


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.


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.


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.


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.


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.