Despite their common use and other similarities, Silicone and Polyurethane sealants are the two most commonly used. They each have different ways of getting the job done. These sealants allow for up to 50% movement and are used to fill gaps, keep water and air out, to allow for expansion and contraction of materials, and to enhance aesthetics.
Regardless of similarities, these sealants also have significant differences that make them each suitable for different types of applications. In comparison here are some differences that may help you decide on which one to use.
Silicone vs. Polyurethane
Silicone is best at drying quickly. Polyurethane remains wet or "tacky" for up to 24 hours after the silicone has dried. Silicone is not recommended in high traffic areas, as it does not hold up to abrasion as well as polyurethane does. Silicone works best on metal, glass and tile, while Polyurethane works well on natural products such as wood, because it dries hard. The hard drying makes Polyurethane more difficult to apply as it hardens when out of the tube, whereas Silicone is much easier to apply and cures softer. Silicone is the better sealant for colder temperatures, the cold does not affect the application and will remain flexible. Polyurethane, dries quicker, and becomes even harder to apply in colder temperatures and does not make a permanent bond when cold.
Deciding on the Best Sealant
However, don't overlook the most important factor to consider when choosing the proper sealant for the job: Silicone Sealant has a 20+ year lifespan and Polyurethane Sealant has a 5-10 year lifespan.
Both sealant's are capable of sealing your project and providing the waterproofing desired. The question you need to ask is, would you rather seal your building two to three times over a twenty year period or just once?