Is Celotex insulation flammable? (An overview)

This article will answer the following question: “Is Celotex insulation flammable?”. We will discuss the flammability of Celotex polyisocyanurate (PIR) insulation products.

Is Celotex insulation flammable?

Yes, Celotex polyisocyanurate (PIR) insulation products can be considered flammable. According to the manufacturer, the product (although has fire retardants in its formula) is combustible and must be installed using aluminum foil to protect against an eventual fire.

What is Celotex?

Celotex is a brand. It produces many insulation materials.

This article will provide information about the characteristics of Celotex polyisocyanurate (PIR) insulation products, especially considering fire performance. 

Celotex PIR insulation is rigid plastic foam, sandwiched between facings. The foam is made by mixing the compounds methylene diphenylmethane diisocyanate (MDI) with a polyol, a flame retardant.

Other facings (foil-paper composites and/ or plasterboard) are combined with the board during the manufacturing process.

The thermal conductivity of Celotex PIR insulation materials is 0.022 W/m.K for the following materials: TB4000, GA4000, CW4000, and XR4000.

For Thermaclass, although, it is 0.021 W/m.K.

How do insulation systems work?

Most insulations in buildings are for thermal purposes, but there’s also acoustic insulation, fire insulation, and impact insulation. Some materials serve more than one finality.

Nowadays, many thermal insulations are also firing insulators, which combines business with pleasure.

A good insulator is a material that covers the greatest area possible where heat exchange can happen between the inside and out. This effect is possible by using materials that prevent the heat flux, which is normally achieved by mixing air in the insulator.

How does heat flux happen?

Heat is a kind of energy. Each kind of energy has its own way to dissipate or flow.

In general, heat always goes from the hottest region to the coldest. But this is only valid for the heat itself.

Heat always required a means to go through. The closer molecules are to one another, the easier find their way through, which is why we use metals to craft kettles.

The air, although, is made of gases. Gas molecules take some time to pass the heat energy to each other because they can hardly touch each other. 

Some insulation materials are filled with trapped air, so even when the heat gets through it, the air molecules struggle to pass the energy forward. 

As we saw from the equation, the temperature difference is not enough for something to get colder or hotter, heat flow is also required. This is why we feel colder when the wind touches us, even though the temperature is the same with or without the wind.

Fire performance

According to the manufacturer, Celotex PIR insulations were tested according to a European standard procedure that’s common for all construction products, called BS EN 13501-1.

The products achieve a classification of E (the scale goes from A to F, in which A1 is the highest classification and F, the lowest).

This means that in an eventual fire, the insulation material would contribute to the overall fire load. The combustion would unleash toxic gases and fumes.

Due to its flammable nature, PIR insulation products have restricted uses. They must not be used in external facades of buildings, and there are also many height restrictions that vary according to local and national standards.

If you plan on using a Celotex product, it’s better to check for specific safety information first, consult the certificates, and understand its details, as well as legal regulations.

Celotex PIR is a combustible material that can serve as fuel in a fire. When used, it must be installed accordingly to lower fire risks.

PIR insulation boards must be kept away from any sources of ignition at all times. Cutting, drilling, and handling it may release dust particles, which can be dangerous for the worker if inhaled, besides being very flammable.

When working with Celotex PIR, all personal safety equipment must be used, especially dust masks, gloves, and safety glasses. The room must be well-ventilated to avoid the formation of flammable mists.

If the product is exposed to weathering or left outside, it will deteriorate and cannot be used anymore. Otherwise, the flammability danger may be even greater.

The manufacturer also states that Celotex PIR insulation boards and joints must be installed covering the edges with aluminum foil tape to provide fire protection and to optimize resistance to moisture transmission. 

Combustible or flammable?

Flammable is usually a term used to denote a compound that’s capable of igniting easily and burning quickly.

Combustible, although, is simply something that can catch fire.

These are general definitions. To correctly assess the information about how flammable a substance is, much more context and research are required. 

We must understand which specific product are we dealing with and in which scenario it’s normally applied.

There are many scenarios in which the same compound could behave differently when it comes to combustion.

What is fire?

Fire is the thermal decomposition of carbon molecules, usually organic matter.

Organic matter is something that, at least at some point, originated from life. Some beings, especially plants, have the ability to capture carbon dioxide in the air, and water, to produce complex compounds. They are the so-called producers.

When a fire happens, the organic matter is forced to decompose until originates the same compounds it had in the beginning: carbon dioxide and water.

Fire is the visual part of a combustion reaction. It is the energy from inside the chemical bondings being unleashed in the form of light and heat. 

There are three ways in which the thermal decomposition of organic matter happens: oxidation, pyrolysis, and evaporation/ vaporization.

All three happen at once during a fire. How fire will look depends on how these three events take place. 

For example, in a blue fire from an oven, oxidation is almost the only thing that happens, because the fuel is already vaporized (it is a gas), and its chemical composition is simple (the carbon chains are small), so pyrolysis can’t be seen.

In a fireplace, although, we can see many more yellow and red flames, smoke, fumes, and all kinds of byproducts from incomplete combustion. This happens because the chemical reactions are much more complex since the fuel has a very different composition.

When wood burns, the solid material is constantly yielding to a vapor-like state and suffers all kinds of breakdowns before finally reacting with oxygen (combustion). All these chemical reactions lead to many byproducts, many of which are toxic to us.

We can say that wherever there’s smoke there’s fire. And wherever there’s smoke, there’re toxic fumes.

Can Celotex insulations catch fire?

Yes, Celotex PIR insulation products can catch fire, and will probably do if direct flames are applied to them.

Celotex PIR insulation can contribute to the overall fire load.

The installation of these Celotex materials is determinative of how flammable they will be. You can check specifics at this link.

Can Celotex insulation become a fire hazard?

Celotex PIR insulation products can become a fire hazard, especially if installed incorrectly. 

Even though Celotex PIR formulas are made with fire-retardant compounds, the products can still burn if there’s enough source of heat and/or flames around.

There are also restrictions on their use. For example, they must not be installed outside. The product may get damaged and more inclined to ignite.


Celotex PIR insulation can be considered flammable. The flammability trials the products had gone through showed that they have a low fire-resistance score (“E”, in a scale from A to F), which means that they can burn considerably easily.

Frequently Asked Questions (FAQS): Is Celotex insulation flammable?

Is celotex waterproof?

Celotex PIR boards themselves are not exactly waterproof, but they present resistance against moisture. Although, they are not installed alone.

Also, Celotex PIR products must be installed using aluminum foil tape to cover their edges and joints. If well installed, the resistance to moisture is high because the weak spots are protected with aluminum.

Is celotex fireproof?

The following Celotex PIR products received a fire retardancy rating of E (a considerably low rating, in a scale from A to F): TB4000, GA4000, XR4000, CW4000 Thermaclass Cavity Wall 21.

In a fire, these products are considered to contribute to the overall fire load. Also, the combustion products may unleash toxic gases like carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx) and hydrogen cyanide (HCN).

is Celotex breathable?

Thin particles of Celotex PIR must not be inhaled. The manufacturer states that dust masks must be worn at all times when dealing with the boards, especially during the installation, because small particles can become airborne.


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