How to Choose the Right Firestop Sealant

• Intumescent — expands when heated (up to ~18×) to crush a melting plastic pipe and seal tightly around cables and conduit. The default for most through-penetrations. (Metacaulk 150+, 1000, 350i)
• Elastomeric — stays flexible and moves with the building; the choice for head-of-wall and other dynamic joints. (Metacaulk 1200 — caulk, self-leveling, and spray grades)
• Silicone — handles high temperatures, high movement, weather and UV; used on exterior and curtain-wall joints. (Metacaulk 835+)
• Economical / latex — budget-friendly and paintable for basic, static metallic penetrations. (RS136)

All are UL Classified and tested to ASTM E814 (UL 1479) for penetrations and ASTM E1966 (UL 2079) for joints, with ratings up to 4 hours.

• General penetrations (cables, conduit, metallic pipe) → an intumescent multi-purpose sealant like MC 150+, which covers more UL systems than any other single product.
• Plastic pipe (PVC/CPVC/ABS) → an intumescent sealant plus, on larger diameters, a collar or wrap strip that crushes the melting pipe.
• Moving or head-of-wall joints → elastomeric MC 1200.
• Exterior / curtain wall / weather exposure → 835+ silicone.
• Walls that need a sound rating → SAS Smoke & Acoustic, which seals smoke and sound together.

1. The wrong UL system. A sealant is only rated as part of a tested assembly. Install it in a UL system that matches your wall type, penetrant and annular space — not just "a firestop caulk." More on UL systems →

2. The wrong depth. Every system specifies a sealant depth and width-to-depth ratio. Use backer rod to set it — too shallow fails, too deep wastes material. It is the cheapest insurance on the job.