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.
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.
Going back to The Ins and Outs of Caulking, we learn that 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 (windows and doors). 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. Other contemporary cladding systems may, however, rely on caulk joints at cladding system–fenestration interfaces and cladding-to-trim joints.
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.
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. An 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.
Of these possible types of failure, adhesive failure between caulk and substrate is most common failure mode. ASTM C1193, Standard Guide for use of Joint Sealants, 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 guide are, however, also applicable to sealant joints in residential construction.
Are your windows and doors caulked? With winter approaching, the homeowner may be thinking about this task. The Ins and Outs of Caulking by the late Charles Carll will tell you everything you need to know.
Painted redcedar lap siding butting tightly against a window casing without caulk. Photograph taken summer 2005 on a single-story home built in 1940.
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 wasn’t 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 and were shingle-lapped with sills with head flashing or drip cap at heads or both.
Different styles of buildings call for different techniques in caulking, as the photos below show.
Wood siding shingle-lapped with an outwardly sloped wood window sill without caulk. Photograph taken in summer 2005 on a 1½-story home constructed in 1916.
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 inch) roof overhang on this single-story home evidently provided adequate shelter from wind-blown rain. This is the same window shown in the second photo on this post.