by Jeff Oertel

This article is prepared for those of us who go to the hardware store and expect to buy a $1.59 tube of caulk to seal a number of conditions. In reality, there are a variety of caulks or sealants, and there are as many misconceptions and misunderstandings concerning the selection and installation of these sealants.

A basic list of sealants and their characteristics includes the following:
Latex: Inexpensive, easy to apply, quick drying, paintable, cleans up with water and non-staining. These sealants should be limited to interior locations only (e.g., drywall, doors) and can be found siliconized for better adhesion.

Silicone: Moderately expensive, elastic and stains slightly. This sealant is often used inappropriately. Silicone should be used at exterior glass and glazing, and in restrooms and kitchens on countertops, fixtures and faucets. Silicone often is used as a sealant and mastic all in one tube because it bonds tightly to certain surfaces. A mildew-resistant product is also available.

Urethane: Moderately expensive, elastic, slightly difficult to apply, slightly toxic and very "sticky." This sealant is the least utilized by laypeople, though it can be used for a wide range of exterior materials. These include masonry, stone, wood, steel, aluminum and plastic with optimum results. A urethane sealant that resists ultraviolet light, resists stains and cleans itself also is available.

Polysulfide: Moderately expensive, difficult to apply and non-staining. This product is seldom preferred relative to other types and applications. However, it is necessary when chemical-resistant or oil-resistant joints are required.

There are several other factors to consider when selecting sealants. Two of the most important are the use of primers and the need for backer rods. Depending on the application, primers may be necessary to ensure a long-term bond between sealant and surface. Manufacturer's recommendations and test areas should be considered when in doubt.

Installing a backer rod is critical to most sealant applications and yet this is seldom performed except on commercial jobs. It is important to understand that the main purpose of a sealant is to bridge a joint and allow for both expansion and contraction. Sealants are capable of moving in two dimensions, but not in every possible direction. Therefore, a bond breaker is required at one plane. See diagram at left. Without this design, cracking or failure of the sealant is likely.

When using high-performance sealants, the range of expansion and contraction is approximately 25 percent each way. Regardless of the sealant type, expect to replace several sealant joints in about 10 years.

There are single- and multiple- component sealants. The single-component variety is most commonly used by non-professionals. These are standard, caulk gun-grade products that take longer to cure in temperatures above freezing.

A final application note: Wear thin, disposable, latex gloves and finish the seal with a finger. This strengthens the bond and looks better.

Jeff Oertel, of Oertel Architects in Minneapolis, MN, specializes in architectural conservation surveys of historic museum structures.