Smoke Seals & Curtain Wall Fire Safety

Fundamentals

What Is a Smoke Seal?

A smoke seal is a passive fire protection material applied to joints, gaps, and interfaces between building assemblies to prevent the migration of smoke, toxic gases, and flames from one compartment to another during a fire event.

In curtain wall systems, the critical zone is the perimeter gap — the void between the exterior glazed façade and the interior floor slab edge. This seemingly minor gap acts as a vertical chimney, enabling fire and smoke to bypass floor-to-floor separation and travel rapidly through the entire building height.

Smoke seals are engineered to react under heat, expanding (intumescing) to close off the gap before flames and toxic gases can breach the compartment boundary.

Curtain wall cross-section showing firestop gap at slab edge

▲ Cross-section showing the perimeter void between curtain wall and floor slab

Code Requirement

Why Smoke Seals Are Mandatory in Curtain Wall Buildings

The International Building Code (IBC) Section 715.4.1 explicitly requires that voids at the intersection of exterior curtain wall assemblies and floor slabs be sealed with an approved material to retard the interior spread of fire and hot gases between stories.

An effective smoke seal system must meet three fundamental performance demands: it must accommodate building movement (thermal expansion, wind sway, seismic drift), maintain structural integrity under fire exposure, and prevent air and gas passage at ambient temperatures.

Failure to install a compliant system exposes occupants to mortal risk, voids insurance, and results in costly remedial works — often requiring façade access after handover.

IBC curtain wall firestop requirement illustration

▲ IBC-mandated perimeter firestop at floor-to-curtain-wall junction

Hilti Solutions

The Hilti Intumescent Paintable Seal Range

Hilti's firestop portfolio for curtain wall applications is the most widely specified and tested range on the market. Each product is engineered to expand under heat — sealing gaps with up to 10× its original volume — while remaining paintable and aesthetically neutral in normal conditions.

Intumescent Sealant

Hilti FS-ONE MAX

Premium water-based intumescent firestop sealant. Highly paintable with a smooth, white finish. Water- and smoke-resistant after curing. Ideal for sealing gaps around penetrations and construction joints in curtain wall assemblies.

Intumescent Paintable UL Listed

Spray Sealant

Hilti CFS-SP WB Joint Spray

Water-based membrane-forming spray specifically developed for curtain wall edge-of-slab applications. Applied over mineral wool backfill using an airless spray pistol. Fully paintable and meets ASTM E2307 movement requirements.

2-Hr Rating Spray Applied Paintable

Pre-formed Barrier

Hilti CFS-EOS QuickSeal

A pre-formed edge-of-slab solution requiring no mineral wool or spray sealants. Installs quickly in all weather conditions. Joint sizes from 1.5″ to 5″. The fastest inspection-ready solution for unitized curtain wall systems.

Pre-formed All-Weather Intertek Listed

Silicone Sealant

Hilti CFS-SIL Silicone Firestop

Fast-curing, highly elastic silicone-based firestop sealant for curtain wall or edge-of-slab applications. Water-resistant — ideal where façade exposure is expected during the building lifecycle. Paintable after cure and rated for high movement capacity.

Silicone Base Water Resistant High Movement

Smoke & Acoustic

Hilti CP 506 Smoke Sealant

Lightweight acrylic-based smoke and acoustic sealant for non-fire-rated assemblies and smoke partitions. Paintable, easy to apply, and provides excellent acoustic insulation. Compliant with ASTM E84, ASTM C834, and ASTM C919. Ideal for interior smoke partition sealing.

Smoke Partition Acoustic ASTM E84

Installation Guide

How to Apply Hilti Intumescent Seal in a Curtain Wall Gap

The following steps cover installation of Hilti CFS-SP WB (spray sealant with mineral wool) — the most common system for stick-built and unitized curtain wall edge-of-slab conditions.

01

Gap Measurement & Preparation

Measure the gap width between the floor slab edge and the back of the curtain wall assembly. Clean all surfaces, removing dust, oil, loose material, and any moisture. The substrate must be structurally sound. Typical gap width for tested systems ranges from 1″ to 5″ depending on the Hilti listed detail.

Tool: Measuring Tape, Wire Brush, Vacuum

02

Install Mineral Wool Batt Backfill

Cut mineral wool (minimum 4 PCF density) to the required depth — typically 4″ to 6″ — and compress it into the gap by at least 25–50% to ensure a tight friction fit. The mineral wool provides the thermal mass barrier and supports the spray sealant above. The top surface of the mineral wool should be recessed to accommodate the required sealant depth.

Tool: Utility Knife, Safety Gloves, PPE

03

Apply Hilti Firestop Spray / Sealant

For CFS-SP WB: Use an airless spray pistol at the recommended pressure to apply an even membrane coat over the mineral wool and adjacent surfaces. The sealant must cover the top surface and overlap onto the floor slab and curtain wall frame by a minimum of 1″ each side. For FS-ONE MAX: Apply with a caulk gun and tool smooth with a wet spatula for a finished surface.

Tool: Airless Spray Gun or Caulk Gun, Spatula

04

Verify Minimum Sealant Thickness

The minimum cured depth of firestop sealant must match the tested UL or Intertek system specification — typically a minimum of ⅝″ to 1″ depth of sealant over the mineral wool. Use a depth gauge to verify before proceeding. Inspect overlap onto adjacent assemblies and confirm no voids or pinholes.

Tool: Depth Gauge, Flashlight

05

Allow Full Cure & Paint Finish

Allow the sealant to fully cure per the technical data sheet (typically 24–72 hours depending on temperature and humidity). Once cured, the sealant can be painted with any standard latex-based architectural paint to match the interior finish. Document the installed system with photos for the Authority Having Jurisdiction (AHJ) inspection record.

Tool: Latex Paint, Roller or Brush

Standards & Certification

How ASTM Ratings Are Assigned

Fire performance ratings for smoke seals and firestop assemblies are determined through rigorous standardized testing. For curtain wall applications, the following ASTM and UL standards are most critical.

Primary Standard

ASTM E2307

The core test standard for perimeter fire barriers at curtain wall/floor intersections. The system is exposed to a standardized fire (ASTM E119 time-temperature curve) from below, measuring how long the assembly prevents flame passage, smoke spread, and heat transfer to the unexposed side.

Critical for curtain wall firestop listing

Fire Exposure Curve

ASTM E119

Establishes the standard time-temperature fire exposure curve used in most fire-resistance tests. A 1-hour rating means the assembly withstands the ASTM E119 fire environment for 60 minutes without failure. 2-hour ratings are typical for high-rise curtain wall conditions.

1-hr and 2-hr ratings most common

Smoke Sealant

ASTM E84

The Steiner Tunnel Test evaluates the surface burning characteristics of smoke sealant materials — specifically flame spread index (FSI) and smoke development index (SDI). Products used in smoke partition applications must achieve Class A (FSI ≤ 25, SDI ≤ 450).

Required for smoke partition sealants

Acoustic Performance

ASTM E90 / C919

For smoke and acoustic sealants like Hilti CP 506, ASTM E90 measures sound transmission loss and C919 defines construction practices for acoustic sealants at joints. Products are assigned an STC (Sound Transmission Class) rating. Relevant where firestop joints also serve acoustically rated assemblies.

Required for acoustic smoke sealants

Hilti Product Ratings Summary

Tested and listed performance of key Hilti curtain wall firestop products.

Product	F-Rating	T-Rating	L-Rating	Test Standard	Listing Body
FS-ONE MAX	Up to 2-Hr	0-Hr / 1-Hr	<1 CFM/ft	ASTM E2307 / E814	UL / cUL
CFS-SP WB Joint Spray	Up to 2-Hr	0-Hr	Qualified	ASTM E2307	UL / Intertek
CFS-EOS QuickSeal	Up to 2-Hr	0-Hr	Qualified	ASTM E2307	Intertek
CFS-SIL Silicone	Up to 2-Hr	0-Hr	Qualified	ASTM E2307	UL
CP 506 Smoke Sealant	Not Rated*	Not Rated*	<1 CFM/ft	ASTM E84 / C919 / E90	UL

*CP 506 is for smoke partitions only — not fire-rated assemblies. Always verify the applicable UL/Intertek system number for the specific joint configuration.

Final Word

The Seal That Cannot Be Skipped

In modern curtain wall construction, smoke seals are not optional details — they are life-safety systems mandated by code, tested to rigorous standards, and verified by third-party agencies. Hilti's intumescent paintable sealants offer the rare combination of fire-tested performance, movement accommodation, aesthetic finish, and a 30-year service life expectancy.

The cost of the right product is negligible compared to the consequence of the wrong one. Specify tested systems. Install to the letter of the listed detail. Protect every floor.

For tested system details, engineering judgments, and project-specific support, visit hilti.com/ej or use the Hilti Firestop Submittal Generator at us.hilti.com

The Invisible Gap That
Puts Lives at Risk

What Is a Smoke Seal?

The Hidden Danger Nobody Talks About

Why Smoke Seals Are Mandatory in Curtain Wall Buildings