The Two Firestop Lies Your Contractor Keeps Telling You
And the Hilti products that prove them wrong

If you work in fire protection — especially in healthcare — you have heard at least one of these statements from a contractor, a fellow inspector, or maybe even from yourself:

Both of these statements sound reasonable. Both of them are wrong. And both of them are costing building owners money, costing contractors credibility, and — in healthcare — generating K-tag citations that could have been avoided with five minutes of reading.

This article came out of a recent conversation on our team. We were comparing notes on recurring field findings, and improper firestop application on fire-rated walls kept coming up — the same two misconceptions, over and over. Miller Brown, who reviews construction documents and inspects these installations for a living, shared the code reasoning and the product references that finally put both myths to rest. This write-up is the result of that collaboration.

We'll walk through why each myth is wrong, show you the code sections that prove it, and point you to the Hilti products that were designed to solve these exact problems — products some crews are afraid to use because of Myth #1, and products others are using in ways that would make the manufacturer weep because of Myth #2.

Myth #1: "No Foam Allowed in Type I/II Construction"

Where the Myth Comes From

Type I and Type II construction under IBC Table 601 require noncombustible structural elements — steel, concrete, masonry. This is the backbone of every hospital, most medical office buildings, and the majority of large commercial structures you walk into.

IBC §603 lists the exceptions where combustible materials ARE permitted in these building types — fire-retardant-treated wood in certain partitions, blocking and nailers, thermal and acoustical insulation, etc. Somewhere along the way, a contractor or an inspector read §603, didn't see "foam" in the exceptions list, and concluded: no foam, period.

That reading is wrong — and it's wrong because it confuses building elements withfirestop systems.

What the Code Actually Says

Firestop systems are not governed by §603. They are governed by IBC §714, which requires through-penetration firestop systems to be:

• Tested per ASTM E814 or UL 1479
• Installed per the conditions of the UL classification
• Rated with an F-rating (flame passage) and T-rating (temperature rise) equal to or greater than the fire-resistance rating of the assembly being penetrated

Read that again: the code requires a tested system. It does not prescribe the chemistry of the sealant inside that system. If a UL-classified firestop assembly uses intumescent foam as one of its components, and that assembly passes ASTM E814 at the required rating — it satisfies §714. The foam is not a standalone material floating around inside a noncombustible building. It is acomponent of a tested fire-resistance system.

The Products That Prove It

Hilti makes multiple intumescent foam products that carry UL classifications for use in fire-rated assemblies — including Type I and Type II construction:

These are not hardware-store canned foam. These are UL-classified products tested to ASTM E814 in specific configurations documented in the UL Fire Resistance Directory. When you install Hilti CP 660 per system W-L-1290, you are installing a listed firestop system — and the fact that it contains intumescent foam is no more of a code violation than the fact that a fire door contains combustible wood. The system is what matters.

Watch: Intumescent Foam in Action

This Hilti demonstration shows exactly what happens when intumescent firestop foam is exposed to fire — it expands and seals the penetration, which is the entire point of the product:

Hilti firestop foam expanding under heat exposure — this is what a tested system looks like

Field Proof: The Foam That Is NEVER Allowed

Let's be surgical about the boundary of Myth #1, because busting it cuts both ways. Saying "UL-classified intumescent foam IS allowed" does not open the door to any canned foam from the hardware store. Two photos from recent field walks show exactly what the wrong answer looks like — both from fire-rated assemblies, both write-ups on sight.

Above the ceiling on a rated wall: the red material is a listed firestop sealant, installed per a UL system. The orange blobs around the conduits are canned "fireblock" foam — no listed system, no F-rating, no T-rating. Same wall, two products, and only one of them is legal here. The contrast could not be more instructive.

That orange foam is the single most misunderstood can in the industry: Great Stuff Fireblock. The word on the can is fireblock — and fireblocking is a real code concept IBC §718 IRC R302.11, but it lives in combustible (Type V, wood-frame residential) construction, where it slows draft and flame spread through concealed stud and joist cavities. It is a completely different job from firestopping a penetration in a fire-resistance-rated assembly IBC §714. The manufacturer's own literature is honest about this: the product is intended to maintain the continuity of an approved fireblock in residential construction — not for firestop applications in rated assemblies.

Look at the test standards and the gap is obvious. Great Stuff Fireblock is evaluated to ASTM E84 (surface burning — how fast flame spreads across the foam's face) and a modified ASTM E814. A real firestop system passes the actual ASTM E814 / UL 1479 furnace-and-hose-stream test in a specific tested assembly and earns an F-rating, a T-rating, and an XHEZ system number in the UL Fire Resistance Directory. Canned fireblock foam has none of those. Cured polyurethane foam is itself combustible — it burns at temperatures a rated wall is designed to laugh at. The Joint Commission's own standards FAQ says it plainly: expandable polyurethane foam is typically not UL listed for firestop application in a fire-rated wall, and surveyors cite it when they find it.

The installer didn't just use canned foam at the head-of-wall joint — they left the empty can sitting on the top track as evidence. The joint where a rated wall meets the deck above is its own listed-system category (UL 2079 joint systems, IBC §715), with tested movement capability and cycling requirements. A bead of hardware-store foam satisfies none of it.

The second photo fails twice. The head-of-wall joint isn't even a §714 penetration — it's a fire-resistant joint governed by IBC §715 and tested under UL 2079, which adds movement cycling on top of the fire test because the deck above deflects and the wall doesn't. There are entire families of listed head-of-wall systems (mineral wool + spray, intumescent joint sprays, factory-formed joint profiles) engineered for exactly this condition. Canned foam is not one of them, in any color, from any brand.

Myth #2: "Just Use a Putty Pad — It's Rated Material"

What Putty Pads Actually Are

Hilti CP 617 putty pads are UL-classified (category CLIV — Wall Opening Protective Materials) intumescent wraps designed for one specific job: wrapping the exterior surfaces of electrical outlet boxes, junction boxes, and metallic washer/dryer boxes recessed in fire-rated wall assemblies.

When installed correctly — pressed firmly onto ALL exterior surfaces of the box, engaging every channel and contour — they swell in a fire to seal the box opening. They allow reduced horizontal spacing between back-to-back boxes on opposite sides of a rated wall (less than the standard 24-inch separation required by IBC §718.3.1).

That is a brilliant product solving a specific code problem. The issue is what happens next.

The Lazy Shortcut That Breaks Everything

Here's the pattern Miller flagged in our conversation — the one he keeps running into on plan review and field inspections:

An electrician relocates a junction box. The old cutout in the drywall is now an empty hole in a fire-rated wall. The correct fix is straightforward — patch the gypsum board with a matching piece, tape and mud the joints, restore the wall assembly. It takes maybe 30 minutes.

Instead, someone reaches for a CP 617 putty pad sheet, peels off the backing, and smooths it over the hole like a Band-Aid. "It's rated material," they say. And they walk away.

The putty pad material was formulated to bond to the smooth metal surface of an electrical box under compression. It was not formulated to adhere to the paper face of gypsum board in a vertical application with no mechanical fastening. Gravity wins. Thermal cycling helps. The edges lift first, then the center follows.

What you're left with is a fire-rated wall with an unprotected hole, covered by a curling sheet of clay that provides the appearance of compliance to anyone who doesn't push on it.

Field Evidence: Putty Pads Where They Don't Belong

These aren't hypotheticals — they're field photos from real buildings. Each one shows putty pad material used in a configuration it was never designed or tested for.

A putty pad pressed directly over a wall penetration. The blue cable exits below — this is not an electrical box wrap. It's a clay Band-Aid over an opening in a fire-rated wall, with no UL-classified system backing it.

Two angles of the same problem above the ceiling. Left: putty pad blobbed around cables at a fire-rated wall penetration — the drywall is already deteriorating around the edges. Right: the wall is literally stamped "FIRE S… PROTECT ALL" — the crew read the warning and still reached for the wrong product. The putty pad has no structural bond to gypsum board and no UL system number for this application.

Floor penetration through a metal sleeve. Putty pad material has been blobbed over each pipe penetration instead of using a UL-classified through-penetration firestop system. The scattered mineral wool debris suggests someone started with the right idea and gave up. A proper system here would be caulk + mineral wool packing per a UL system detail — not loose clay on top of pipes.

The Right Way vs. The Wrong Way

Watch: Firestop Putty Pads — Proper Application

This video walks through the correct installation of firestop putty pads — every surface of the box, firm pressure, engaging the channels. Compare this to what you see in most buildings:

Proper putty pad application — notice the firm pressure on every box surface

Read the Tag: The 60-Second Check Almost Nobody Runs

Both myths above are about the wrong product going into the hole. This one is about the right-looking paperwork going onto the wall — and it is the failure mode that survives inspections, because the tag is the most trusted and least read object in the building.

A penetration label carries a UL system number. That number is not a serial number or a job reference. It is a coded description of the assembly the system was tested in and the kind of thing allowed to pass through it. Once you can read it, you can check a firestop from six feet away without touching anything.

What the Number Actually Says

UL through-penetration systems (tested to UL 1479, CCN XHEZ) are numbered letter – letters – four digits. The first letter says which assemblies the system covers, the next says what that assembly is built of, and the digits say what is allowed to penetrate it:

The letters matter too. C covers floors or walls, F floors only, W walls only. A is a concrete floor 5 in. or thinner, J a concrete or masonry wall 8 in. or thinner, L a framed wall. So a tag reading C‑AJ‑3021 is telling you: concrete floor or masonry wall, cables through it.

Field Evidence: The Tag That Falls Apart When You Read It

Two core-drilled penetrations in a masonry wall, red Hilti putty in both, a proper “WARNING • DO NOT DISTURB” label between them — system number, hour rating, install date, installer’s initials, all filled in. It looks like the good example. (Contractor identity blurred; the lesson is in the number, not the company.)

This is the same rule as Myth #2, wearing a suit. Rated material in an unrated application is unrated — and a listed system cited for the wrong penetrant is not a listed system either. The putty may well be the right product. The wall may well be the right thickness. But nobody has shown that this combination was ever tested, and the tag that was supposed to prove it is the thing that gives it away.

One honest limit: the number is the part you can check from across the room. Fill depth, mineral wool backing and annular spacing are inside the wall and cannot be judged from a photograph — those come from the detail sheet and, where it applies, ASTM E2174 inspection.

Why Healthcare Gets Hit the Hardest

Hospitals are almost always Type I construction. Medical office buildings are typically Type I-B or Type II-A. Both require noncombustible elements. Both have extensive fire-rated wall assemblies — smoke compartment barriers, corridor walls, hazardous-area separations.

Every one of those rated walls has penetrations. Every penetration needs a firestop. And every firestop is something a CMS surveyor will inspect during an unannounced visit.

Here's what happens when the myths are in play:

• Myth #1 in action: A contractor avoids intumescent foam in a cable tray penetration because "no foam in Type I." Instead they use silicone sealant — which has no intumescent properties and won't seal the opening when the cables burn through. CMS surveyor cites K-0345 (fire barrier maintenance).
• Myth #2 in action: Maintenance relocates an outlet in a smoke-compartment barrier. The old cutout gets a putty pad Band-Aid. Six months later the pad is peeling. CMS surveyor finds an unprotected opening in a smoke barrier. K-0363 (smoke barrier maintenance) plus potential K-0321 (hazardous areas) if the penetration connects to a hazardous space.
• Both myths combined: The facility ends up with an ILSM (Interim Life Safety Measure) assessment requiring fire watches, accelerated inspection rounds, and a corrective action timeline — all because someone used the wrong product in the wrong way to save 30 minutes.

The Firestop Product Decision Tree

Before you reach for a product, answer two questions:

The Bottom Line

Intumescent firestop foam is allowed in Type I and Type II construction when it is part of a UL-classified system. Full stop. If your contractor is afraid of foam, hand them the UL system number and the ASTM E814 test report. The code is on your side.

Putty pads are for electrical boxes. They are not drywall patches, they are not general-purpose firestop material, and they are not a substitute for a 30-minute gypsum-board repair. If you see putty pad material smoothed over a wall opening, that is a deficiency — write it up.

And if your team is installing firestop products without formal training on the UL system details — get them trained. The difference between a compliant firestop and a citation is not the product. It's the application.

With thanks to Miller Brown for raising this issue with our team and sharing the product references and video resources that made this article possible. This is what practitioner collaboration looks like — one conversation, two myths down.