3M 7010305974 3M™ Fire Barrier Block B258, Maroon, 2.36 in x 5.12 in x 8 in, 12/Case

The B258 firestop block is rated up to 2 hours. That covers typical 1 and 2 hour rated wall and floor assemblies where cable, conduit or pipe penetrations have to be sealed back to the assembly's rating.

Each block measures 2.36 in (60.0 mm) by 5.12 in (130.0 mm) by 8.0 in (203.2 mm), and they're meant to be stacked and packed to fill medium to large through-penetrations in walls and floors. You size the job by how many blocks it takes to fully pack the opening around the penetrants, not by one block dimension. They ship 12 to a case.

That's the main reason to use the B258. The blocks come out and go back in, and can be repaired and reused, so an IT or facilities crew can add a cable run and re-pack the opening instead of cutting out cured sealant and starting over. For data closets and riser penetrations that get revisited, that saves the labor every time.

Blocks make sense where the opening is medium to large and the cable fill is going to change. The B258 is a pre-formed foam block you pack into the opening, so there's no cure time and the penetration is usable right away, and the blocks can be pulled, re-cut and reused when a new cable run goes through. Sealant or putty is the better call on small single-penetrant openings where nothing will be added later.

No. The B258 blocks install with no special tools. Crews cut and friction-fit them around the penetrants and pack the opening tight. That's the practical reason electrical and low-voltage contractors use them on retrofit work where a caulk gun and cure window would hold up the inspection.

Order it as 3M 7010305974, the Fire Barrier Block B258 in maroon, supplied 12 blocks per case. Country of origin is Germany.

The B258 is built to hold a tight firestop seal against fire, smoke and toxic gas moving through the penetration. That matters in stair enclosures, corridors and smoke barriers where the assembly is doing smoke-containment duty, not just fire resistance. The pack has to be tight and continuous around every penetrant for that seal to hold.

Yes, the block takes paint over a primer. Out of the box it's maroon, which is the usual firestop color cue inspectors look for, so on exposed walls confirm your AHJ doesn't want the firestop left visible before you paint it out.

It's aimed at commercial construction, and 3M also lists it across appliance, construction, general industrial and transportation work. Typical use is sealing wall and floor through-penetrations in a rated assembly, which is why it sits in most electrical contractors' firestop kits rather than only in plant maintenance stock.