Construction sealants are typically elastomeric: silicones, polyurethanes, silyl-terminated polymers and polysulfides that cure to a rubber able to stretch and recover. They are specified under ASTM C920 by type (single or multi-component), grade (non-sag or pourable), class (movement) and use. A sealant differs from an adhesive in that its main job is sealing and flexing rather than carrying structural load.
Spec says a class 50 sealant on the precast and a class 25 on the windows, don't mix them up on the ticket.
Read the full C920 designation on the data sheet (type, grade, class and use), because two sealants with the same class can be approved for different substrates.
Also heard as
- elastomeric sealant
- joint sealant
Do not mix it up with
- Caulk: Caulk usually means a low-movement product for small gaps; sealant implies a high-performance elastomer rated for movement.
Related terms
Caulk
Everyday word for any gun-applied gap filler; in the trade, a lower-movement material than a true sealant.
Silicone
A sealant with a silicon-oxygen backbone, prized for UV resistance and high movement capability.
Polyurethane sealant
A tough, paintable elastomeric sealant used widely on concrete, masonry and traffic joints.
Movement capability
How far a sealant can stretch and compress, as a percentage of joint width, while keeping its seal.