Fire Resistant Sealant Guide: Silicone vs Acrylic

Fire resistant sealant is an essential part of many passive fire protection systems, helping maintain fire compartmentation where walls, floors and other fire-resisting elements contain joints, gaps or service penetrations.

But not every fire-rated sealant performs in the same way.

One of the most common choices is between acrylic fire resistant sealant and silicone fire resistant sealant. Both can form part of tested fire-stopping systems, but their flexibility, weather resistance, application and intended use can be very different.

So, which should you choose?

The quick answer is that acrylic fire resistant sealants are generally best suited to internal joints and many service-penetration applications, while silicone fire resistant sealants are particularly useful where greater long-term flexibility, joint movement or weather resistance is required.

However, the correct product must always be selected according to the complete tested fire-stopping system rather than simply choosing a sealant because it carries a particular fire-resistance claim.

In this guide, we explain the difference between silicone and acrylic fire sealants, what to look for when specifying them and some of the leading Nullifire fire-stopping products available from National Insulation Supplies.

What Is a Fire Resistant Sealant?

A fire resistant sealant, also commonly called a fire-rated sealant, firestop sealant, fire stopping mastic or fire-resistant mastic, is a product used to seal certain joints, gaps and openings within fire-resisting construction.

Its purpose is to help maintain the performance of the fire-separating element when a joint or penetration would otherwise create a path through which fire, smoke or hot gases could spread.

Fire-resistant sealants are commonly used in areas including compartment walls, compartment floors, plasterboard systems, concrete and masonry construction, joints between different building elements and around suitable service penetrations.

In England, Approved Document B states that joints between fire-separating elements should be fire-stopped and that openings for services through fire-resisting elements should be kept as small as practicable and appropriately fire-stopped. It also highlights the use of proprietary fire-stopping systems that have been demonstrated by testing for the relevant application.

That final point is particularly important.

A sealant should never be considered in isolation. The substrate, backing material, joint dimensions, wall or floor construction, service type and installation depth can all influence the tested performance of a fire-stopping system.

What Is an Acrylic Fire Resistant Sealant?

An acrylic fire resistant sealant is generally a water-based sealant designed to cure into a firm but flexible seal.

Fire-resistant acrylic products are commonly specified for internal passive fire protection because they can provide a convenient combination of fire, smoke, acoustic and air-sealing performance.

A good example is Nullifire FS702 Intumastic Fire Resistant Acrylic Sealant, which is designed for linear joints and suitable service penetrations.

Nullifire states that FS702 is a water-based acrylic sealant designed to cure into a firm but flexible fire seal. It has been tested to EN 1366-3 and EN 1366-4, with systems providing up to four hours of fire resistance depending on the tested configuration. It is suitable for applications involving flexible walls, rigid walls and rigid floors and can also form part of the Nullifire FB750 Intubatt system.

What Is a Silicone Fire Resistant Sealant?

A silicone fire resistant sealant uses silicone technology rather than a water-based acrylic formulation.

The key advantage is generally flexibility.

Silicone sealants are commonly selected for linear joints where movement, weather exposure or long-term elasticity is an important consideration.

Nullifire FS703 Fire Resistant Silicone Sealant is a low-modulus, neutral-cure silicone designed to reinstate the fire performance of compartment walls and floors at tested linear joints.

Nullifire specifies FS703 for a variety of static and movement-joint applications. The product offers UV and weather resistance, superior elasticity and a maximum allowable deformation of 25% according to ISO 11600, while tested systems can provide up to four hours of fire resistance.

This makes silicone particularly useful where a construction joint is expected to move during the life of the building.

Silicone vs Acrylic Fire Resistant Sealant: Quick Comparison

Feature	Acrylic Fire Resistant Sealant	Silicone Fire Resistant Sealant
Typical use	Internal joints and suitable service penetrations	Linear and movement joints
Flexibility	Good	Excellent
Long-term joint movement	Application dependent	Particularly well suited
Weather resistance	Usually more suited to protected/internal applications	Strong weather and UV performance
Ease of application	Excellent	Excellent
Paint finishing	Often more convenient	Product/paint compatibility should be checked
Acoustic sealing	Often excellent	System dependent
Service penetrations	Suitable products available	Not automatically suitable for every penetration
External exposure	Product dependent	Often the preferred option
Best example	Nullifire FS702	Nullifire FS703

Which Is Better: Silicone or Acrylic Fire Resistant Sealant?

Neither is universally better.

The right choice depends on what you are trying to fire-stop.

For a relatively stable internal joint, acrylic can be a practical and cost-effective choice. For a joint expected to move repeatedly or one exposed to external environmental conditions, a suitable silicone fire-resistant sealant may be the better option.

If the opening contains combustible services, however, neither a standard acrylic joint sealant nor a silicone movement sealant should automatically be assumed to be suitable. A reactive intumescent product or a different tested penetration-sealing system may be required.

This is why the service, substrate and tested installation detail must be identified before selecting the sealant.

Best Acrylic Fire Resistant Sealant: Nullifire FS702 Intumastic

Nullifire FS702 Intumastic Fire Resistant Acrylic Sealant

For internal fire-stopping projects, Nullifire FS702 is one of the most versatile options in the National Insulation Supplies range.

FS702 is a water-based acrylic sealant intended for both linear joints and suitable service penetrations.

According to Nullifire, tested applications can provide up to four hours of fire resistance, with testing to EN 1366-3 and EN 1366-4. It is suitable for flexible walls and rigid walls and floors and is also designed for use with systems including Nullifire FB750 Intubatt.

FS702 also offers acoustic and smoke-sealing performance and can be used around specified non-combustible services, cables and rock-fibre-insulated non-combustible pipes when installed within an appropriate tested detail.

When Should You Choose FS702?

FS702 is particularly worth considering when the project involves internal fire-stopping, linear joints, certain service penetrations, plasterboard or rigid wall systems, acoustic sealing or use alongside compatible Nullifire fire-stopping products.

Its acrylic formulation also makes it a practical option where easy application and finishing are priorities.

BUY NOW - NULLIFIRE FS702 INTUMASTIC FIRE RESISTANT ACRYLIC SEALANT

Best Silicone Fire Resistant Sealant: Nullifire FS703

Nullifire FS703 Fire Resistant Silicone Sealant

Where movement and flexibility are more important, Nullifire FS703 is the standout option.

FS703 is a low-modulus neutral-cure silicone sealant developed specifically for fire-resistant linear joint applications.

Depending on the tested installation, it can provide up to four hours of fire resistance and has been tested in linear gaps from 10mm to 220mm.

FS703 also offers UV and weather resistance and a maximum allowable deformation of 25%, making it particularly useful where the joint cannot be treated as completely static.

When Should You Choose FS703?

FS703 is particularly appropriate for suitable movement joints, linear joints between tested substrates, locations exposed to weather or UV, and projects where long-term elasticity is important.

It can also be used alongside Nullifire FJ203 Fire Resistant Rope within tested movement-joint configurations.

BUY NOW - NULLIFIRE FS703 FIRE RESISTANT SILICONE SEALANT

What About Service Penetrations and Plastic Pipes?

This is where choosing fire-resistant sealant becomes more complicated.

If a compartment wall or floor has been penetrated by a combustible pipe, cable system or other service, the required solution may need to do considerably more than simply seal the visible gap.

When some combustible services are exposed to fire, they can soften, melt or burn away, potentially leaving an opening through the fire-resisting element.

For these applications, a high-expansion intumescent sealant may be appropriate where supported by the tested system.

Best High-Expansion Intumescent Option: Nullifire FS709 HP

Nullifire FS709 HP Intumescent Sealant

Nullifire FS709 HP is not simply another acrylic-versus-silicone alternative.

It is a high-expansion graphite intumescent sealant designed to react when exposed to fire.

The product expands rapidly under fire conditions and is designed to reinstate the fire performance of compartment walls and floors breached by specified services.

Nullifire has tested FS709 with a broad range of applications, including combustible pipes, insulated non-combustible pipes, cables, cable bundles, cable trays, cable ladders and other services.

Tested systems can provide up to four hours of fire resistance, and the product can also be used within compatible systems including FB750 Intubatt, FP170 Intucollar, FB770 Intudeck and FR230 Intucompound.

When Should You Consider FS709?

FS709 should be considered where a tested specification requires a reactive intumescent seal around suitable service penetrations, particularly where combustible components may disappear or deform during a fire.

It is very different from selecting an everyday flexible linear-joint sealant, so the correct tested installation detail should always be confirmed first.

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FS702 vs FS703 vs FS709: Which Nullifire Sealant Do I Need?

Product	Technology	Best Suited To	Up to 4-Hour Fire Resistance*	Movement/Reaction
Nullifire FS702	Water-based acrylic	Internal joints and tested service penetrations	Yes	Firm but flexible
Nullifire FS703	Neutral-cure silicone	Linear and movement joints	Yes	High flexibility
Nullifire FS709 HP	Graphite intumescent	Tested service penetrations and reactive fire stopping	Yes	Expands when heated

*Maximum performance depends on the complete tested system, supporting construction, dimensions, service type and installation configuration.

How Do You Choose the Correct Fire Resistant Sealant?

The first question should not be “which sealant has the highest fire rating?”

Instead, establish the exact application.

For example, a 120-minute requirement for a static joint between masonry elements is a very different specification from a 120-minute requirement around a plastic pipe passing through a plasterboard compartment wall.

1. Identify What You Are Sealing

Determine whether the opening is a linear construction joint, movement joint, cable penetration, metal pipe penetration, combustible pipe penetration or part of another fire-stopping system.

This immediately narrows down which products can be considered.

2. Check the Required Fire Resistance

The project specification may require 30, 60, 90, 120 minutes or another level of fire resistance.

Products such as FS702, FS703 and FS709 can achieve high levels of fire resistance in appropriate tested systems, but “up to four hours” does not mean every installation automatically achieves 240 minutes.

The specific tested detail must support the required performance.

3. Consider Joint Movement

Movement is one of the clearest reasons to choose one sealant technology over another.

Where a joint is expected to move, the seal must accommodate that movement without cracking, separating from the substrate or otherwise compromising the tested detail.

For demanding movement-joint applications, a silicone product such as FS703 can be particularly useful.

4. Consider Internal or External Exposure

An internally protected joint and a weather-exposed external joint face very different conditions.

Moisture, UV, temperature changes and repeated movement should all be considered.

FS703 specifically offers UV and weather resistance, making it particularly relevant where environmental exposure is part of the specification.

5. Check the Substrate

Fire-stopping test evidence is linked to particular supporting constructions and substrates.

A product tested in rigid concrete construction cannot automatically be assumed to offer identical performance in every flexible plasterboard wall.

Always check that the test evidence covers the actual construction being used.

6. Check the Joint Width and Seal Depth

Fire-resistant sealant is not simply applied until a gap looks full.

The dimensions of the joint and the depth of sealant can be critical to performance.

Backing materials such as mineral wool or fire-resistant rope may also form an essential part of the tested system.

Nullifire specifically advises following tested sealant dimensions for movement joints because using too much or too little material can affect how the joint accommodates movement.

7. Check the Service Type

Where services penetrate a fire-resisting wall or floor, identify exactly what is passing through it.

Plastic pipe, steel pipe, insulated steel pipe, cable bundles, cable trays and ducts may all require different fire-stopping details.

Do not substitute one penetration type for another unless permitted by the applicable test evidence or certification.

Fire Resistance vs Reaction to Fire: What Is the Difference?

These two terms are often confused.

Fire resistance describes how a building element or system performs when subjected to fire for a period of time. Depending on the test and classification, this can include criteria such as integrity and insulation.

Reaction to fire, by comparison, describes how an individual material contributes to the development and spread of fire.

A product's reaction-to-fire classification therefore should not be confused with the fire-resistance period achieved by a complete tested joint or penetration-sealing system.

This distinction is important when comparing passive fire protection products.

Why Tested Fire-Stopping Systems Matter

A fire-stopping product does not work independently of the construction around it.

Its performance may depend on the wall or floor thickness, joint dimensions, service diameter, backing material, annular gap, seal depth, insulation type and other installation conditions.

For that reason, choosing a product simply because its packaging states “four-hour fire resistance” can be misleading.

UK fire-safety guidance recognises the importance of appropriately tested fire-stopping and sealing systems, and manufacturers such as Nullifire provide technical data sheets, declarations of performance and detailed installation information covering their products.

For safety-critical applications, the installation should follow the relevant tested detail and project fire strategy.

Common Fire Sealant Installation Mistakes

Even a high-quality fire-resistant sealant can perform incorrectly if the installation does not follow the tested system.

Common problems include using an unsuitable sealant for the service or joint, applying the incorrect seal depth, failing to install specified backing materials, installing onto dirty or unsuitable substrates, substituting components from different systems without supporting evidence, ignoring movement requirements and assuming that every application receives the maximum stated fire-resistance period.

Fire stopping is a system, not simply a tube of sealant.

Is Fire Resistant Sealant the Same as Intumescent Sealant?

Not necessarily.

“Fire resistant sealant” is a broad term covering products designed for passive fire-protection applications.

Some fire sealants are intumescent, meaning they react to heat by expanding. Others achieve their function without relying on major expansion.

FS703, for example, is a flexible fire-resistant silicone sealant intended primarily for linear joints.

FS709 HP, by contrast, is specifically a high-expansion graphite intumescent sealant that reacts when exposed to fire.

Choosing between the two depends on what the tested system is required to do.

Is Fire Resistant Silicone Better Than Fire Resistant Acrylic?

For movement and weather resistance, silicone often has an advantage.

For many protected internal joints and suitable service penetrations, acrylic can offer an excellent balance of performance, application and value.

Therefore, the better question is:

Which sealant technology has been tested and specified for this particular joint or penetration?

That approach is much safer than choosing solely by material type.

Can Fire Resistant Sealant Be Used Around Pipes?

Yes, certain fire-resistant sealants can be used around specified pipe penetrations, but the correct product depends on the pipe.

Metal pipes, insulated metal pipes and combustible plastic pipes behave differently in a fire and may therefore require very different fire-stopping systems.

FS702 has tested service-penetration applications, while the reactive FS709 HP has been tested across a wide range of combustible and insulated service types.

Always check the relevant manufacturer's test evidence before installation.

Can Fire Resistant Sealant Be Used Around Cables?

Yes, appropriate tested fire-stopping sealants can form part of cable penetration systems.

The permitted cable type, bundle size, supporting construction, aperture size and seal arrangement must match the tested system.

For larger or mixed-service openings, a coated batt or compound system may also be required rather than relying on sealant alone.

Does Fire Resistant Sealant Stop Smoke?

Many fire-stopping sealants are also designed to help limit smoke or air leakage when correctly installed.

For example, Nullifire describes FS702 as providing acoustic and smoke sealing as part of its performance characteristics.

However, the required smoke-control or air-leakage performance should still be checked against the relevant product data and project specification.

Can You Paint Over Fire Resistant Sealant?

This depends on the sealant.

Acrylic products are generally more convenient where a painted finish is required, while silicone sealants can be more difficult to coat successfully.

Always check the sealant manufacturer's technical guidance and paint compatibility rather than assuming a particular paint system will adhere correctly.

How Long Does Fire Resistant Sealant Last?

There is no single lifespan that applies to every fire-stopping seal.

Service life can be affected by movement, moisture, environmental exposure, substrate condition, damage, building alterations and the quality of the original installation.

Fire-stopping should also be considered whenever services are added, removed or altered because later building work can compromise previously completed compartmentation.

Fire Resistant Sealant for External Applications

For external or weather-exposed linear joints, a fire-resistant silicone such as FS703 will often warrant consideration because of its UV and weather resistance.

However, exterior use does not remove the requirement to confirm that the actual joint construction falls within the product's tested or assessed field of application.

Weather resistance and fire resistance are separate performance requirements, and both need to be satisfied.

Fire Resistant Sealant for Internal Applications

For protected internal conditions, acrylic sealants such as FS702 can offer a versatile solution.

They are particularly useful where the specification includes linear joints, suitable service penetrations, acoustic sealing or integration with other compatible passive fire-protection products.

For penetrations involving combustible services, however, a reactive product such as FS709 HP may be required instead.

Which Fire Resistant Sealant Should You Buy?

For a straightforward comparison:

Your Application	Product to Consider
Internal linear joint	Nullifire FS702
Suitable internal service penetration	Nullifire FS702, subject to tested detail
High-movement linear joint	Nullifire FS703
Weather or UV-exposed fire-rated joint	Nullifire FS703, subject to tested detail
Combustible service penetration	Nullifire FS709 HP, subject to tested detail
Mixed or larger service opening	Check the complete Nullifire fire-stopping system
Unsure of specification	Speak to National Insulation Supplies before ordering

Shop Fire Resistant Sealants at National Insulation Supplies

National Insulation Supplies supplies a growing range of Nullifire passive fire protection products for commercial, residential and specialist construction projects across the UK.

Whether you need an acrylic firestop sealant for internal works, a flexible silicone solution for movement joints or a reactive intumescent sealant for service penetrations, choosing the correct tested system is essential.

For larger quantities or projects requiring multiple passive fire protection products, you can also contact National Insulation Supplies for assistance with product selection and bulk quotations.

Conclusion

Choosing between acrylic and silicone fire resistant sealant becomes much easier once the application is properly understood.

For many protected internal joints and tested service penetrations, Nullifire FS702 acrylic sealant provides a versatile solution.

Where flexibility, repeated movement, UV exposure or weather resistance are greater priorities, Nullifire FS703 silicone sealant is likely to be the more appropriate option.

For certain combustible pipes, cables and other service penetrations, a reactive product such as Nullifire FS709 HP intumescent sealant may instead be required.

The most important rule is not to select fire-stopping products by fire rating alone. Check the supporting construction, service type, gap dimensions, movement, environmental conditions and the manufacturer's tested installation detail.

National Insulation Supplies stocks Nullifire fire-stopping products alongside a comprehensive range of fire protection insulation, cavity barriers and passive fire-protection systems, with nationwide UK delivery and support for projects of all sizes.

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Frequently Asked Questions

1. What is the best fire resistant sealant?

There is no single best fire resistant sealant for every application. Acrylic sealants such as Nullifire FS702 are particularly versatile for internal joints and tested service penetrations, while silicone products such as FS703 are better suited to applications requiring high flexibility, movement capability or weather resistance. Reactive intumescent sealants such as FS709 HP are designed for different applications, including tested service penetrations.

2. What is the difference between silicone and acrylic fire sealant?

Acrylic fire sealant is generally water-based and is commonly used for internal joints and suitable service penetrations. Silicone fire sealant offers greater long-term elasticity and is particularly useful for movement joints and applications requiring weather or UV resistance.

3. Is silicone fire resistant?

Standard silicone should not automatically be treated as a fire-stopping product. A purpose-designed fire-resistant silicone such as Nullifire FS703 must be used within an appropriate tested fire-stopping application.

4. Is acrylic sealant fire resistant?

Standard acrylic sealant is not necessarily fire resistant. Fire-stopping requires a specifically tested product such as Nullifire FS702 and installation in accordance with the appropriate system detail.

5. What fire resistant sealant should I use around plastic pipes?

Combustible plastic pipes often require a reactive intumescent fire-stopping solution capable of closing the opening created as the pipe deforms or melts. Nullifire FS709 HP has been tested for various combustible pipe applications, but the exact pipe, wall or floor construction and dimensions must match the relevant tested system.

6. What is intumescent sealant?

Intumescent sealant contains materials that react when exposed to heat, typically expanding to help close or protect an opening. It is commonly used in passive fire protection systems involving certain service penetrations.

7. Is intumescent sealant the same as silicone?

No. A product such as Nullifire FS709 HP is a high-expansion graphite intumescent sealant, while Nullifire FS703 is a neutral-cure fire-resistant silicone. They are designed for different types of application.

8. How many hours of fire resistance does fire sealant provide?

The answer depends on the complete tested construction. Some Nullifire FS702, FS703 and FS709 systems can provide up to four hours of fire resistance, but this should never be interpreted as meaning every installation automatically achieves four hours.

9. Can fire resistant sealant be used outdoors?

Some products can. Nullifire FS703 offers UV and weather resistance and is particularly relevant for suitable external or weather-exposed linear joints. The fire-stopping detail still needs to be covered by the relevant product evidence.

10. Can I use normal silicone instead of fire-rated silicone?

No. Where a fire-resisting seal is required, the selected product and complete installation need appropriate evidence of performance for the application. General-purpose silicone should not be substituted for a specified fire-resistant product.

11. Should fire sealant be installed by a professional?

Fire stopping is safety-critical work. Installers need to understand the required tested system, substrates, joint dimensions, penetration type, backing materials and installation requirements. Complex or regulated projects should use appropriately competent fire-stopping contractors.