Selecting caulk

Caulk fills and seals narrow and consistent joints between building materials. Selecting the best caulk requires recognition of the substrate materials (the materials bordering the joint), the gap size, weather exposure, and joint movement from temperature changes.
Common caulks, like acrylic latex and siliconized acrylic latex work well for indoor gaps less than 3/8 inch (10 mm) between wood and other common building materials. Caulk has a limited shelf life, so buy your caulk from a supplier who sells a lot of caulk.
Silicone, polyurethane, and butyl offer superior durability, adhesion, and flexibility compared to less expensive acrylic latex caulks. When in doubt about selection, especially when replacing failed caulk, use a high-performance caulk that the manufacturer specifies for adhesion and flexibility to your particular substrates.
Suppliers sell caulks and other sealants in a wide variety of formulations. Therefore, read the specifications carefully, especially when choosing a sealants for joints between different materials. Unfortunately, many available caulks defy classification because they mix the ingredients of the well-known caulks and adopt a unique name. For these difficult-to-classify formulations, read the specifications carefully before purchasing.

Ask these questions about your caulking needs. Then, consult the comparison table for some preliminary advice.

  • Is the substrate porous like concrete and wood or non-porous like glass or metal?
  • How experienced are the installers at working with caulks that are difficult to apply and clean up?
  • How much movement do you anticipate in the joint? This depends on material and temperature variation.
  • Will you paint the joint?
  • How exposed is the joint to water?
  • How exposed is the joint to heat and UV radiation?
  • How exposed is the joint to abrasion?

Installing caulk

Preparation and installation are just as important as selecting the right caulk. Exterior joints, designed to seal out air and water, must tolerate moisture, ultraviolet light, joint movement, and air pressure. These joints may require careful preparation, to include cleaning, priming, packing, and tooling the caulk after installation.
Porous materials like wood and masonry usually need scraping, wire-brushing, and dusting. Priming both sides of the gap is sometimes necessary, depending on the materials and caulking formulation. Non-porous materials, like aluminum and steel, usually require cleaning with a solvent. Use rubbing alcohol and water for common dirt and methyl ethyl ketone for oil and grease.
Polyethylene-foam rod or bond-breaker tape — can prevent three-sided adhesion. Three-sided adhesion may tear the caulking bead when the joint moves. Tool the caulk with a caulking tool if necessary. Various tools create various shapes to provide flexibility, strength, and appearance. An hour-glass shape and slender profile that optimizes the caulking bead’s flexibility. Un-tooled beads provide more strength.
Joint failure is a major problem in new construction. For challenging joints in important locations, designers should specify the materials and preparation of joints. Then supervisors should enforce these specifications during installation.

For more information

Selecting an Exterior Caulk by JLC Online

Air Sealing Buildings Short Course (7.0 CEUs)

Residential Energy, Chapter 3