Intumescent mastic or intumescent acrylic – what's the difference?

When it comes to firestopping in the construction industry, there is more often than not a big choice to make. Do you use intumescent mastic, or do you use intumescent acrylic? 

Both are designed to stop the spread of fire and both prevent gases and smoke from moving through a building, but which should you be using? 

The short answer to that is very dependent on your project, but you can rest assured that a good mastic or a good acrylic will keep residents safe.  

To help you understand which is better for which scenario, in this article, we will explain the differences between the two.  

In addition, we will highlight what their strengths and weaknesses are and what projects suit each other best. 

What are the benefits of using an intumescent mastic? 

Intumescent mastic is a product that can be very flexible when you choose to use it. For example, if you need to seal a gap that has something of an irregular surface, a mastic would be a great fit.  

They also expand when they are exposed to heat, which makes them very effective as a firestopping solution. In the event of a fire, a mastic will expand to try and protect a door frame or whatever it is being used to protect.  

As a result of their flexibility, a mastic is ideal for use in areas where movement is anticipated. As an example, somewhere where vibrations are rife or somewhere where structural movement could be a possibility. 

They also provide a secure and lasting seal, which is just one reason why they are trusted by a great number of construction professionals.  

Are there any downsides to using an intumescent mastic? 

If aesthetics is something of a priority, then using an intumescent mastic may not be the best decision. The reason for this is that it can be a very messy application. 

Because intumescent mastic is quite thick and paste-like, it can get everywhere and that could be an issue, depending on your project.  

In addition, intumescent mastics aren’t normally clear. This means when they are applied to something, it is likely to be visible. 

Again, if aesthetics aren’t a concern, then this isn’t really a problem. However, in situations where it is a concern, then it is a problem. 

Finally, for whatever reason, intumescent mastics traditionally aren’t as adhesive on vertical surfaces. This could limit their performance somewhat.  

What about the pros of using an intumescent acrylic? 

One of the main pros about intumescent acrylics is that they are very versatile. They can be used to seal up a number of different applications, including pipes, cables and conduits.  

You can also buy intumescent acrylics in clear, which is exceptionally helpful if you’re working on a project and aesthetics is a big factor.  

They are also very easy to apply, with little to no mess. Intumescent acrylics are also generally very good at being resistant to UV light, so they can be used for outdoor applications as well.  

Perhaps the main pro for the acrylic though is the strength of the product itself. Intumescent acrylics are revered for their strength and will make a very strong bond with a variety of surfaces.  

It doesn’t matter what kind of surface it is, either. Whether it’s horizontal or vertical, intumescent acrylics are very consistent.  

And does the intumescent acrylic have any cons? 

Generally speaking, intumescent acrylics are much less flexible than mastics. Their range of movement is much less, so they’re maybe not best suited for areas that will have structural challenges or copious vibrations. As such, they are best suited for service penetrations. 

This is also a factor when it comes to the event of a fire, too. The intumescent acrylic will likely expand to a certain amount, but it won’t expand to the same degree that the mastic will.  

In terms of protecting you from a fire and giving you enough time to escape, intumescent acrylic is still a standout product. But, if it's for a linear gap seal, you may prefer to have something that expands more and can be seen to be doing more.  

Intumescent acrylic is also more expensive than intumescent mastic on average. As an example, a 600ml cartridge of Everbuild Intumescent Mastic has a list price of £4.20. By contrast, a 310ml cartridge of Protecta FR Acrylic will have a list price of either £3.20 or £4.24. 

Which of the two is best for me? 

Both are great products and both do a tremendous job at keeping people safe, so it’s really very much dependent on what your project is.  

If you have a project where aesthetics are a major factor, then the intumescent acrylic will undoubtedly be a better choice. It’s less messy and the option to choose clear ones make a huge difference.  

Equally, if the building’s design shows that there could be a lot of vibration around where you’re looking to apply passive fire protection, then the intumescent mastic is a better choice. 

It is better designed to deal with it, plus mastic is also very flexible, which can play a key role in your decision-making. However, if you’re looking to be neat and tidy, you’ll have problems using intumescent mastic. The other side of the coin is that it is better for linear gap seals.   

Ultimately though – and most importantly – whatever you choose is going to play a huge part in providing residents with safety. Both can protect for up to four hours, so you’ll be in safe hands with either.  

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