An elastomeric sealant is a flexible, rubber-like material used to fill joints and gaps in buildings, infrastructure, and other structures where movement is expected. Unlike rigid fillers that crack when surfaces shift, elastomeric sealants stretch and compress with the joint, then return to their original shape. They’re the standard choice for sealing expansion joints in concrete, gaps around windows and doors, connections between dissimilar building materials, and anywhere that seasonal temperature swings cause materials to expand and contract.
What Makes a Sealant “Elastomeric”
The word “elastomeric” comes from “elastomer,” meaning a polymer with rubber-like elasticity. These sealants are built from long-chain synthetic polymers that can stretch well beyond their resting length and snap back without permanent deformation. The most common polymer bases are silicone, polyurethane, and polysulfide, though formulations also use butyl rubber, neoprene, and newer hybrid chemistries. Each polymer gives the sealant different characteristics in terms of flexibility, durability, and chemical resistance.
The key property that sets elastomeric sealants apart from ordinary caulk is movement capability. Standard caulk handles less than 12.5% joint movement, making it suitable only for gaps that barely shift. Elastomeric sealants exceed that threshold, with many rated for 25% or 35% movement and high-performance silicone sealants reaching 50% movement capability. That means a sealant rated at 50% in a one-inch joint can accommodate half an inch of total expansion and contraction without losing its seal.
Common Types and Their Strengths
Silicone sealants are the most weather-resistant option. They tolerate UV exposure, extreme temperatures, and moisture without breaking down, and they stand out for high-temperature resistance that other elastomeric types can’t match. Most non-silicone elastomeric sealants are limited to service temperatures between roughly 150°F and 200°F. Silicone also stays flexible in cold weather, which matters in climates with harsh winters. The tradeoff is that silicone can’t be painted over easily.
Polyurethane sealants offer strong adhesion and are paintable, making them popular for exterior building joints where appearance matters. They bond well to concrete, wood, and metal. However, they can degrade faster under prolonged UV exposure compared to silicone, so they sometimes need a topcoat for long-term outdoor use.
Polysulfide sealants resist fuels, solvents, and chemicals, which makes them common in industrial settings, fuel tanks, and airport pavements. They’re also used in below-grade applications where contact with soil and groundwater is constant. Hybrid formulations that combine polysulfide with epoxy resin offer even greater chemical resistance.
A newer category, silyl-terminated polyether (sometimes called MS polymer), has gained attention because it combines many of the best properties of silicone and polyurethane without the health concerns tied to certain chemical components. The European Union recently restricted the use of diisocyanates, compounds found in some polyurethane formulations, due to their link to occupational asthma. This has accelerated development of hybrid sealants that avoid those ingredients while maintaining high performance.
How Elastomeric Sealants Cure
Most single-component elastomeric sealants cure by reacting with moisture in the air. Silicone sealants, for example, undergo a condensation reaction: the polymer chains cross-link when they encounter atmospheric humidity, gradually transforming from a paste into a flexible rubber. This is why humidity levels affect cure time. At 77°F and 50% relative humidity, a typical silicone bead becomes tack-free in 10 to 15 minutes and fully cures in 10 to 12 hours.
Two-component sealants work differently. You mix a base with a curing agent just before application, and the chemical reaction begins immediately regardless of humidity. These are common in large-scale commercial projects where controlled cure times and deeper joint fills are needed. Application temperature matters for both types. Many elastomeric sealants can be applied in conditions ranging from 10°F to 120°F, though adhesion and curing performance are best in moderate temperatures.
Where Elastomeric Sealants Are Used
These sealants bond aggressively to a wide range of construction materials: concrete, masonry, aluminum, galvanized steel, wood, vinyl, fiberglass-reinforced plastic, and many types of coated metal. That versatility makes them the go-to choice for joints between different materials, where each side of the gap expands at a different rate.
Typical applications include expansion joints in concrete slabs and bridge decks, perimeter seals around window and door frames, curtain wall joints on commercial buildings, and weatherproofing connections in metal roofing systems. They’re also used in below-grade waterproofing, highway pavement joints, and parking structures where the sealant must handle both movement and water exposure.
How Sealants Are Classified
The construction industry classifies elastomeric sealants under ASTM C920, a standard specification that sorts them by several characteristics. Type refers to whether the sealant is single-component (ready to use from the tube) or multi-component (requires mixing). Grade indicates whether it’s a pourable, self-leveling formula for horizontal joints or a non-sag formula for vertical and overhead work.
The class rating is the most practically important designation. It defines the percentage of joint movement the sealant can handle repeatedly without failing. Classes range from 12.5 (the minimum to qualify as elastomeric) up through 25, 35, 50, and 100/50. A Class 25 sealant tolerates 25% expansion and 25% compression. A Class 100/50 sealant can stretch to double its width and compress to half, making it suitable for the most demanding structural joints. Use designations then specify whether the sealant is approved for traffic-bearing surfaces, non-traffic joints, immersion in water, or other specific conditions.
How Long They Last
Durability varies significantly by chemistry, installation quality, and climate. Research in the construction sector has found that 50% of sealants fail within 10 years of installation, and 95% fail within 20 years. That might sound discouraging, but it reflects the full range of products and conditions, including budget formulations and poor installation practices.
The leading cause of sealant failure is cyclic movement, the repeated stretching and compressing that happens with temperature changes. This matters more than UV exposure or rain alone. A sealant installed at the wrong temperature can be pre-stressed from day one: if you apply it on a hot day when the joint is at its narrowest, the sealant has to stretch further during winter contraction than it was designed for. That’s why manufacturers specify installation at moderate temperatures close to the annual average for the location.
Silicone sealants generally last the longest in exterior applications, often exceeding 20 years when properly installed. Polyurethane and polysulfide sealants typically fall in the 10 to 20 year range depending on exposure. Climate plays a major role: sealants in regions with wide temperature swings and intense UV radiation degrade faster than those in mild, stable climates.
Elastomeric Sealant vs. Caulk
People often use “caulk” and “sealant” interchangeably, but they’re functionally different products. Caulk is a rigid or semi-rigid filler designed for static joints, gaps that don’t move much. Acrylic latex caulk, the most common type, handles less than 12.5% joint movement. It works well for interior trim, baseboards, and crown molding where gaps stay consistent.
Elastomeric sealants pick up where caulk leaves off. Their higher movement capability, stronger adhesion to diverse substrates, and superior weather resistance make them necessary for any joint that will expand, contract, or vibrate over time. The cost difference reflects this: elastomeric sealants typically cost more per tube than basic caulk, but using caulk in a moving joint virtually guarantees premature cracking and water intrusion. For exterior work, expansion joints, or any gap between materials that move at different rates, an elastomeric sealant is the appropriate choice.