3M Fire Barrier Sealant: Installation, Code Compliance, and What You Need to Know

Most contractors deal with fire-rated assemblies on almost every commercial job. You know the walls need to be fire-rated. You know the floors need to be fire-rated. What doesn't always get the same attention is what happens the moment you cut a hole in one.

Every pipe, conduit, cable tray, and duct that passes through a fire-rated wall or floor assembly creates an opening that can let fire and smoke spread. That's what firestopping is for. And it's not optional.

This guide covers what firestopping actually is, how 3M Fire Barrier Sealant CP 25WB+ works, which codes it satisfies, and how to install it correctly. Whether you're a general contractor, an electrician, a plumber, or a fire protection specialist, this is the stuff you need to know.

Firestopping is the process of sealing openings and joints in fire-rated construction to restore the fire resistance of the assembly after penetrations have been made. In plain terms: you cut a hole for a pipe or conduit, and then you seal that hole so fire can't travel through it.

Fire-rated walls, floors, and ceilings are designed to contain fire and limit its spread for a rated period of time, typically one hour, two hours, or four hours. The moment you cut a penetration through that assembly, you've compromised that rating. Firestopping restores it.

This isn't just best practice. It's code. The International Building Code, International Fire Code, NFPA 101, and the National Electrical Code all require through penetrations in fire-rated assemblies to be firestopped. Failing an inspection because penetrations aren't sealed is an expensive problem to fix after the walls are closed up.

 3M Fire Barrier Sealant CP 25WB+ is a one-component, gun-grade, latex-based intumescent firestop sealant. It's designed to seal through penetrations and blank openings in fire-rated floor, wall, and ceiling assemblies.

It comes in a 10.1 oz cartridge for standard caulking guns and a 20 fl oz sausage for bulk guns. No mixing required. You load it, apply it, and let it cure. The red-brown color makes it easy to spot during inspections.

CP 25WB+ is water-based, which means it cleans up with water, it's paintable, and it's low-odor. Those might seem like small details, but they matter on occupied jobsites or in areas with limited ventilation.

Here's what makes firestop sealant different from regular caulk. When CP 25WB+ is exposed to temperatures above 1,000 degrees F, the intumescent material inside it expands. It expands to at least three times its dried volume.

Think about why that matters. You've got a plastic pipe running through a fire-rated wall. In a fire, that pipe melts and disappears. Without firestopping, you've now got a clear path for fire and smoke to travel through the wall opening. With CP 25WB+, the expanding sealant fills that void as the pipe melts, sealing the opening and maintaining the fire resistance of the assembly.

That's the basic principle of intumescent firestopping. The material is engineered to react to fire exactly when it's needed.

CP 25WB+ is rated to maintain a firestop penetration seal for up to 4 hours. That means an assembly with a 2-hour fire rating, properly firestopped with CP 25WB+, maintains that rating through the rated test period.

This is where a lot of contractors get caught. Knowing you need firestopping isn't enough. The product you use has to be listed and tested for the specific assembly you're working on. Here's what CP 25WB+ is tested and listed to:

CP 25WB+ is listed by UL, Intertek, and FM. When an inspector asks for documentation on your firestopping installation, these listings are what you're referencing.

Now, here's the part that trips up a lot of contractors: it's not enough to use a listed product. You have to install it in accordance with the specific UL or Intertek system that applies to your assembly. Each listed system defines the penetrant type, the size of the opening, the depth of sealant, and the annular space requirements. Installing outside those parameters means the installation isn't covered by the listing, even if you used the right product.

Through penetrations are the most common application for CP 25WB+. A through penetration is any conduit, pipe, cable, duct sleeve, or other item that passes completely through a fire-rated wall or floor.

Electricians and linemen deal with this constantly. Every conduit or bundle of cables that passes through a fire-rated wall or floor assembly needs to be firestopped at the penetration. CP 25WB+ is listed for electrical penetrations under the NEC and IBC. Apply it around the annular space between the conduit and the sleeve or opening to the required depth specified in the relevant UL system.

 Plastic pipes present the biggest challenge in firestopping because they melt in a fire, leaving an open hole. That's exactly what the intumescent sealant is designed to address. For copper, steel, and cast iron pipe, the sealant fills the annular space and prevents smoke and flame from passing around the pipe. For plastic pipe, it fills the void as the pipe collapses.

HVAC ductwork that penetrates fire-rated assemblies requires firestopping at the penetration point. Depending on the duct size and the assembly, you may need sealant, a wrap strip, or a combination. CP 25WB+ is part of the 3M firestopping system and works in conjunction with other 3M fire protection products for large mechanical penetrations.

Cable trays that pass through fire-rated walls are a common issue on commercial and industrial projects. The approach depends on the tray size and fill. For smaller penetrations, CP 25WB+ sealant can be used. For larger cable tray openings, 3M Fire Barrier Mortar is typically the right product.

 A blank opening is a hole in a fire-rated assembly with nothing passing through it. This happens when an opening is cut for a penetration that ends up not being installed, or when an oversized sleeve is used. CP 25WB+ can be used to fill blank openings to restore the fire rating of the assembly.

Construction joints, also called head-of-wall joints, are the gaps between the top of interior partition walls and the deck above. These gaps are intentional. They accommodate building movement, thermal expansion, and structural deflection. But in a fire-rated assembly, they need to be sealed.

CP 25WB+ is listed for certain construction joint applications. Check the 3M system documentation for the specific assembly type, gap size, and installation method that applies to your joint.

Installation is straightforward, but getting it right means following the listed system, not just squeezing sealant into the gap.

Before you pick up the gun, find the applicable UL or Intertek firestop system for your assembly. The system number specifies the wall or floor type, the penetrant type, the opening size, the annular space, and the required sealant depth. 3M's online system search tool and the UL Product iQ database are the standard references. If you're on a job with a specific firestopping spec, the spec will call out the system numbers.

The opening must be clean and dry. Remove any debris, loose concrete, or drywall material from around the penetrant. The annular space (the gap between the penetrant and the edge of the opening) should be within the limits specified in the listed system. If the annular space is too large, backer material may be required before applying sealant.

Load the cartridge or sausage into a standard caulking gun. Apply CP 25WB+ to fill the annular space to the required depth. The sealant needs to make contact with both the penetrant and the surrounding substrate. For deeper applications, you may need to apply in layers.

CP 25WB+ is sag-resistant, which means it stays in place on vertical penetrations without slumping before it cures. That's important for wall applications.

Tool the sealant with a wet finger or spatula to ensure good contact with both surfaces and a finished appearance. CP 25WB+ is paintable once cured, which helps on finished surfaces.

On any job where inspections will happen, document your firestopping installations. Photograph each penetration before and after, note the UL or Intertek system number used, and keep the documentation with your project records. Many general contractors now require firestopping documentation as part of project closeout.

This is one of the most common disputes on commercial jobs. The short answer is: the trade that makes the penetration is typically responsible for firestopping it. Electricians firestop their conduit penetrations.

Plumbers firestop their pipe penetrations. Mechanical contractors firestop their duct penetrations. On some projects, a specialty firestopping subcontractor handles all penetration sealing. Check your spec and your subcontract language early.

One of the practical advantages of CP 25WB+ is that it's re-enterable and repairable. If a penetration needs to be modified after the sealant has cured, you can cut it out, make your change, and reinstall the firestopping. This is a significant advantage over rigid firestopping systems when rework is common. It also means future trades who need to add or modify penetrations in the same opening can do so without tearing out a permanent system.

3M's warranty on CP 25WB+ requires installation by a 3M trained installer. Many commercial projects, especially larger ones, specify that firestopping be installed by a 3M Master Contractor or a certified 3M Trained contractor. If you're bidding jobs where firestopping is a line item, getting your crew certified is worth the investment. It opens doors to jobs that require it and gives the GC confidence in the installation.

The Authority Having Jurisdiction (AHJ) for your project has the final say on whether your firestopping installations pass inspection. Different inspectors look for different things, but they all want to see that you used a listed product, installed it per the listed system, and can document it. Don't close walls before firestopping is inspected if your project requires an inspection signoff.

ProWorkSupply carries 3M fire protection products including Fire Barrier Sealant CP 25WB+ and the broader 3M firestopping line. Available in cartridge and sausage format for job site needs.

Firestopping isn't the most glamorous part of a job, but it's one of the parts that matters most when something goes wrong. Fire-rated assemblies only work as designed if the penetrations through them are properly sealed.

3M Fire Barrier Sealant CP 25WB+ gives you a listed, code-compliant solution that's straightforward to apply, works with standard caulking equipment, and is re-enterable if conditions change. Know the system that applies to your assembly, prep the opening right, apply it to the required depth, and document it.