The Ins and Outs 

of Caulking

Charles Carll

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

General

Technical

Report

FPL–GTR–169
May 2006

Carll, Charles. 2006. The ins and outs of caulking. General Technical 

Report FPL-GTR-169. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory. 8 p.

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Abstract

Effective water management involves understanding the 

roles of siding, trim, fenestration units, flashing, and (where 

appropriate) caulk seals and how these entities interface 

with each other. Where caulk seals are used, their geometry 

and dimensions are important to joint performance, as is the 

care with which they are executed. In selection of caulking 

material, consideration should be given to compatibility 

with substrates, to ambient conditions at application and 

during cure, and to the skill level of the applicator. Where 

and when caulk seals are used, they should not be assumed 

to remain functional indefinitely.

Keywords: caulk, sealant, water management, flashing

Contents

Page

Introduction..........................................................................1

Caulk or Sealant? .................................................................3

Substrate–Sealant Compatibility..........................................3

Sealant Adhesion..................................................................4

Butt and Fillet Joints............................................................4

Joint Dimensions Matter......................................................5

Tooling Helps Adhesion to Substrates.................................6

Application Conditions........................................................6

Shelf Life .............................................................................6

Sealant Is Not a Substitute for Flashing...............................6

References............................................................................7

Sources of Additional Information.......................................7
The Ins and Outs of Caulking

Charles Carll, Research Forest Products Technologist 

Forest Products Laboratory, Madison, Wisconsin 

Introduction

Caulk is sometimes used in residential construction to inhibit rainwater intrusion where wall cladding interfaces 

with windows and doors (fenestration units) and is commonly used where utilities (such as pipes, vent hoods, and 

electrical conduit) penetrate the wall. Prior to development 

of modern caulks, caulk typically was not used at the interfaces of wood siding and fenestration units. Although not 

assumed to be watertight when subjected to windblown rain, 

these interfaces nevertheless usually did not leak noticeably. 

Fenestration units were designed to shed water at sills and 

drip it beyond the exterior of the cladding (siding) system. 

Siding pieces were tightly fitted to jamb casings (Figs. 1A, 

1B) and were shingle-lapped with sills (Fig. 2). At window 

heads, siding pieces were shingle-lapped with either head 

flashing or drip cap (Fig. 3); in some cases both drip cap and 

head flashing were used.

In contemporary residential construction, windows may not 

have protruding sills that collect water and shed it to the 

exterior of the siding. Contemporary metal and vinyl siding systems are designed to allow for drainage of water that 

penetrates between the siding and fenestration units. These 

systems incorporate termination and transition accessories 

that work with concealed flexible flashing materials (installed around the perimeters of fenestration units); they are 

generally intended to function without caulk joints between 

siding and fenestration units or fenestration unit trim 

(Figs. 4A, 4B). Other contemporary cladding systems may, 

however, rely on caulk joints at cladding system–

fenestration interfaces and cladding-to-trim joints (Fig. 5).

Caulk joints cannot be expected to remain functional indefinitely; leakage will eventually occur. Sealant joint failure 

can occur by adhesive failure between the caulk and either 

of the substrates between which a seal is desired. Cohesive 

failure or chemical degradation of the caulk can also cause 

sealant joints to fail.1 Of these possible types of failure, 

adhesive failure between caulk and substrate is the most 

common failure mode. ASTM C 1193, Standard Guide for 

Use of Joint Sealants (ASTM 2005), provides extensive 

guidance for execution of sealant joints. The guide was developed for use in commercial construction, where caulking 

is a specialized trade. Many of the principles outlined in the 

1An example of chemical degradation in service is “reversion,” 

which sometimes occurs with certain polyurethane sealants. Reversion is the term used when the cured sealant “reverts” to a tacky 

state; in extreme cases, reverted sealant can flow from joints under 

the influence of gravity.

Figure 1A—Painted redcedar bevel lap 

siding butted tightly to window jamb 

casing without caulk. Photograph taken 

summer 2005 on ranch-style home constructed in 1960. 

Figure 1B—Painted redcedar lap siding 

butting tightly against a window casing 

without caulk. Photograph taken summer 2005 on a single-story home built 

in 1940. 
General Technical Report FPL–GTR–169

2

Figure 2—Wood siding shingle-lapped with an outwardly sloped wood window sill without caulk. Photograph taken in summer 2005 on a 11⁄2-story home 

constructed in 1916. 

Figure 3—Wood drip cap, atop window head casing, 

installed in shingle-lap fashion with redcedar trim 

and without caulk. In this example, there is no metal 

head flashing over the wood drip cap and behind the 

siding. Water intrusion between the drip cap and the 

siding could be expected if this detail were exposed 

to significant wind-blown rain. A modest 0.3 m (12 in.) 

roof overhang on this single-story home evidently 

provided adequate shelter from wind-blown rain. This 

is the same window shown in Figure 1B. 

Figure 4A—Vinyl siding terminating in a 

J-channel receptor along window jamb 

trim.

Figure 4B—Interface of vinyl siding 

J-channel receptor and aluminum 

window jamb trim. Outward thumb 

pressure against the J-channel reveals 

that there is no caulking between the 

J-channel and the jamb trim. This is 

consistent with installation guidance 

available from the trade association for 

vinyl siding. This installation was about 

a decade old when the photo was taken 

and had performed acceptably. 
The Ins and Outs of Caulking

3

guide are, however, also applicable to sealant joints in residential construction.

Caulk or Sealant?

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 solventborne 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. 

Caulks that meet the requirements of ASTM C 920, Standard Specification for Elastomeric Joint Sealants, are 

termed “sealants.” ASTM C 920 was originally developed 

for sealants used in commercial construction; it covers a 

wide variety of sealants, some of which (such as polysulfides and two-component polyurethanes) are not used in 

residential construction. Conformance with C 920 requires, 

among other things, adequate performance in test procedures that evaluate adhesion (to specified substrates) and 

cohesion under cyclic movement. Standard C 920 also 

specifies resistance to cracking at elevated temperature, at 

cold temperature, and after exposure to ultraviolet radiation. 

Conformance with ASTM C 920 generally is an indication 

of superior performance, although a simple statement of 

conformance to the standard does not fully describe performance. The standard recognizes five different classes, categorized by the degree of movement between substrates that 

the sealant can tolerate. Classes range from a Class 100/50, 

indicating that adhesion and cohesion meet acceptance 

criteria at 100% elongation and at 50% compression, to a 

Class 121⁄2, indicating that adhesion and cohesion meet 

acceptance criteria at 12.5% elongation and 12.5% 

compression. 

ASTM C 920 (indirectly) specifies limits on maximum allowable shrinkage of sealant during extended cure. Latex 

acrylic caulks usually shrink enough during extended cure 

that they will not meet this particular requirement for conformance with C 920. 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, ranging from low-cost products that may incorporate 

vinyl rather than acrylic polymers and that meet none of the 

performance requirements of C 920 to products that meet all 

except the shrinkage limitations of C 920 Class 25. Latex 

acrylic products are in many ways the easiest of the sealants 

to use. In residential building, where the knowledge and 

skill of applicators may not be 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.

In this report, the terms “caulk” and “sealant” are used more 

or less interchangeably, as often occurs in the building industry, even though there is a rational basis for distinction 

between the terms. As indicated previously, the term “sealant” implies superior performance. 

Substrate–Sealant Compatibility

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. 

Chemical components of sealants with the potential to damage substrates are most commonly either organic solvents or 

acidic constituents. Organic solvents used in most sealants 

are not sufficiently aggressive to harm most substrates. The 

Figure 5—Joint between embossed oriented strandboard (OSB) lap siding and embossed OSB gable-rake 

trim. The siding is installed without a siding termination 

channel. The joint is gapped and sealed with caulk, as 

recommended by the siding manufacturer. Caulk seals 

were just under 4 years old when photo was taken and 

were apparently functional. 
4

General Technical Report FPL–GTR–169

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. 

Sealant Adhesion

Nothing adheres well to a dirty surface. In addition, new 

and apparently clean metal components may have oils in 

their surfaces left from manufacturing processes. Likewise, 

the extrusions of vinyl, vinyl-clad wood, and wood–

plastic composite windows may have residual extrusion die 

lubricants on their surfaces. Satisfactory sealant adhesion 

requires removal of such contaminants. Wiping with a clean 

rag moistened with mineral spirits is an effective method of 

removing surface oils and die lubricants. This can, however, 

pose health and fire risks if done carelessly, and residual 

mineral spirits that do not fully evaporate before caulk is 

applied may compromise adhesion. Substituting denatured 

alcohol for mineral spirits generally poses fewer health 

risks, and alcohol’s relatively rapid evaporation rate is 

more likely to leave a clean dry surface. Organic solvents, 

while generally effective at removing organic surface 

contaminants such as oils, may not be compatible with all 

substrates; this is an additional reason that justifies caution 

in their use. Surfaces contaminated with dirt, airborne dust, 

and mud usually are most effectively cleaned with a wellrinsed water-wetted rag. Rinsing the rag in a detergent solution can aid in surface cleaning, but if this is done, residual 

detergent left on surfaces will interfere with caulk adhesion. 

If use of detergent solution is deemed necessary to obtain 

adequate surface cleaning, it must be followed by thorough 

rinsing with a water-wetted rag. Adequacy of rinsing can be 

difficult to ascertain; therefore use of water and a non-abrasive nylon cleaning pad should be attempted before resorting to use of detergent solution. Because porous surfaces 

are generally absorptive and thus difficult to adequately 

rinse, use of detergent solution on them as in preparation for 

caulking is not recommended.

At application, surfaces must also be free of ice or frost. At 

below-freezing temperatures, frost may accumulate on surfaces from an applicator’s breath; this is among the reasons 

that manufacturers, as discussed below, commonly restrict 

application temperature.

Butt and Fillet Joints

ASTM Standard C 717, Standard Terminology for Building Seals and Sealants, 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 (Fig. 

6). A fillet sealant joint is a joint in which sealant is applied over (not into) the intersection between surfaces that 

are approximately perpendicular to each other (Fig. 7). In a 

well-executed butt joint, the sealant does not adhere to any 

rigid material at the back of the joint. In a well-executed fillet sealant joint, the sealant is not adhered 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 

Figure 6—Cross-sectional sketches of butt 

sealant joints. Virtually identical sketches 

can be found in ASTM C 717-05, Standard 

Terminology of Building Seals and Sealants.

a. face to edge

joint 

substrate edge 

sealant 

b. edge to edge

joint 

sealant backing substrate edge 

sealant 

sealant backing 
5

The Ins and Outs of Caulking

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. 

Joint Dimensions Matter

In narrow joints, a given amount of differential movement 

between substrates translates into relatively large strain rates 

in the sealant. ASTM C 1193 makes no recognition of sealant joints narrower than 6 mm (0.25 in.). Figure 8 shows a 

6-mm- (0.25-in.-) wide perimeter butt sealant joint around 

a residential window, which was in accord with the window 

manufacturer’s installation instructions.

ASTM C 1193 states that acceptable sealant joint depth 

varies with joint width and sealant type. A generic rule for 

joints up to 13 mm (0.5 in.) 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. With butt joints (Fig. 6), 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 Figure 6 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 (discussed 

in the following section) results in surface concavity that 

provides in part for the hourglass 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 midwidth can be instructive.

Figure 7—Cross-sectional sketches of fillet 

sealant joints. Virtually identical sketches can 

be found in ASTM C 717-05, Standard Terminology of Building Seals and Sealants.

Figure 8—A perimeter sealant joint around a contemporary flanged window. The joint is of appropriate width at 

6 mm (0.25 in.). The joint had been in service for roughly 

3 years when photo was taken. 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). 

bond breaker 

joint 

substrate face

sealant 

substrate face

sealant backing 

joint 

substrate face

sealant 

substrate face
6

General Technical Report FPL–GTR–169

Tooling Helps Adhesion to 

Substrates

Adhesion to substrates is improved by tooling the sealant 

before it cures, because 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. In residential construction, sealant tooling is most commonly done with a finger, although 

use of a tooling device, such as a plastic spoon, is usually 

more satisfactory. Professional caulkers in commercial construction commonly fabricate their own tooling sticks and 

spoons. 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. 

As indicated above, 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.

Application Conditions

The range of acceptable application temperatures will be 

indicated by the sealant manufacturer. Emulsion (“latex”) 

sealants cannot be applied below about 4.4°C (40°F). 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 h. The acceptable time window will depend on ambient temperature and 

sealant type.

Shelf Life

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.

Sealant Is Not a Substitute for 

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 

Figure 9—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 the requirements of ASTM C 920. 

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. 
7

The Ins and Outs of Caulking

function indefinitely. Sealant joint failure can occur without 

obvious indication (Fig. 9). 

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 (Fig. 10) or shingle-lap joints (Fig. 11) 

are effective; sealant joints are not an acceptable substitute. 

Where there are drainage paths, it is important that caulk, if 

used, not block them (Figs. 12, 13A, 13B). 

References

ASTM. 2005a. ASTM C 717. Standard terminology of 

building seals and sealants. West Conshohocken, PA: ASTM 

International.

ASTM. 2005b. ASTM C 920. Standard specification for 

elastomeric joint sealants. West Conshohocken, PA: ASTM 

International.

ASTM. 2005c. ASTM C 1193. Standard guide for use of 

joint sealants. West Conshohocken, PA: ASTM 

International. 

Sources of Additional Information

Jackson, D. 1997. Choosing high-performance caulks. Journal of Light Construction, October 1997.

Pontolilo, B. 2004. Making sense of caulks and sealants. 

Fine Homebuilding #162, May 2004. 

Figure 10—Metal Z-flashing at a horizontal joint in plywood panel siding. The second-story (upper) plywood 

panels should have been installed approximately 3 mm 

(1/8 in.) higher than in this installation. This would have 

provided more adequate drainage space between their 

lower edges and the horizontal leg of the Z-flashing (a 

paint bridge between the siding and the Z-flashing can 

be seen at the left-most groove in the photo). Well executed Z-flashing (without caulk) has proven effective at 

joints like the one shown here.

Figure 11—A shingle lap horizontal joint in plywood 

panel siding. At the time this photo was taken, the joint 

shown had performed adequately for in excess of four 

decades with no maintenance.

Figure 12—Transition between a contemporary finned 

window (below) and OSB horizontal lap siding (above). 

The window shown has been field-mulled (multiple units 

joined side-to-side). A metal head flashing is used that 

shingle-laps behind the siding above and covers the 

end of the mull cap. Caulk is not used between the head 

flashing and the siding, as it would block drainage off 

the flashing.
8

General Technical Report FPL–GTR–169

Figure 13B—Extensive jamb decay in a contemporary 

exterior entry door. The decay was in all likelihood exacerbated by the short length of fillet joint that blocked 

drainage at the outside lower edge of the jamb.

Figure 13A—A failed fillet caulk joint between the ribbed 

aluminum sill and the pine jamb and jamb casing of a 

contemporary exterior door. The ribs retard drainage off 

the aluminum sill. The short length of fillet that runs parallel to the ribs also retards sill drainage and does so at 

a critical location.