Is cellulose insulation flammable? (A Complete Guide)

This article will answer the following question: “Is cellulose insulation flammable?”. This guide will help you understand if cellulose insulation products are flammable, in which conditions they might ignite and how safe they are for your purposes.

Is cellulose insulation flammable?

No. But at the same time, it depends. Cellulose insulation can catch fire, but it’s not flammable if installed according to manufacturer instructions. These insulation products often have fire-retardant compounds added, but can still ignite under some circumstances.

What is cellulose?

Cellulose is the most abundant natural organic material. It’s harvested from green plants that have the ability to create fibers.

For example, 90% of cotton and 50% of wood are composed of cellulose. Hence why they’re so used in paper-making, and in the fashion industry. 

The source of cellulose often comes from plants, including sources of wood, cotton, and hemp. It can also come from the recycling of paper.

Cellulose is the name of the molecule that’s a precursor to all commercial applications under the name of cellulose. The molecule is a polymer made of thousands of glucose molecule units linked together.

Cellulose insulation

Cellulose insulation is plant fiber used normally in roofs, attics, walls, and other cavities to provide thermal and noise insulation.

Historically, many insulation materials have been used such as newspaper, cardboard, cotton, straw, sawdust, hemp, and corncob. 

Modern insulation forms began during the 50s, by adding fire-retardant compounds to the combustible cellulose.

By then, contractors, retailers, and consumers struggle to find good insulation materials. There was a growth in the number of cellulose manufacturers, each claiming their products were better than the others, but people could only take their word for it.

In the 70s, the Federal Consumer Products Safety Commission passed a safety standard that must be followed by the fabricants. Ever since then, every cellulose insulation material has had an attribute called R-value.

The R-Value

When applied to the building industry, this attribute helps to measure how well a layer of insulation can resist thermal conduction, to figure out how thermal resistant it is.

The R-value can be given to a certain material, or to a set of compounds assembled together. 

The higher the R-value, the better the insulating properties a material has. It can be calculated through the following expression:

Rvalue= Temperature AreaFlow rate


  • “Temperature” is the difference in the temperature between the two sides of the barrier (in Kelvin scale);
  • “Area” is the exposed surface area of the barrier (in square meters);
  • And the “Flow rate” is the heat flow rate that goes through the layer (in Watts).

What is important in this equation is to understand how the R-value can be higher (which means, better or more suitable for heat insulation).

The more the temperature differs (let’s say, inside and outside a home), the more the heat is inclined to flow to achieve thermal equality. This means that more heat will pass through the insulation. But this depends overall on how the temperature is outside.

The area also can’t change, since it depends on the size of the building. 

The flow rate, although, is something that depends on the insulation material, how it’s assembled, and how the heat is dispersed on other attached materials (e.g other composites, or walls, roofs, etc.).

Cellulose chemistry

Before we proceed to explain if cellulose can become flammable, let’s take a look at what cellulose looks like chemically.

Cellulose is an important structural component of green plant cells. 

Some animals such as ruminants and termites can digest plant cells and retrieve energy from them because they have microorganisms living in a symbiosis that allows them to do so. We humans can’t digest it, which is why we consider it a source of fibers, not energy.

But a cow can get fat by eating it.

The cellulose molecule has the formula (C6H10O5)n. The ‘n’ varies depending on the source of cellulose, it could be worth thousands. This is what a polymer is, a simple unit that repeats itself lots of times. Biopolymers like this receive the name carbohydrates.

As we said, cellulose is made of glucose, but not entirely. A glucose molecule has the formula C6H12O6. If you compare the two formulas it’s possible to see that the cellulose units are glucose molecules, minus a water (H2O) molecule.

This water is lost in the process of crafting cellulose by the plants.

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 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  R-value 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.

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. 

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

Can cellulose insulation catch fire?

Yes, it can catch fire. But it doesn’t mean that’s a big fire hazard.

Most cellulose insulation materials are not made from cellulose alone, they have different fire-retardant materials added that help during an eventual fire.

Of course, cellulose insulation can still burn if direct heat or flames are applied to it, but that can’t happen spontaneously.

Combustion is also associated with the ability of the material to unleash volatile organic compounds into the air, which can create a mist of flammable gases or other suspensions that can ignite easier, and start a bigger fire.

Although, cellulose doesn’t possess this ability under normal conditions. Cellulose insulation requires at least a big amount of heat for it to become combustible, and a source of ignition to become flammable.

If you get a piece of cellulose insulation, shred it, and apply flames direct to it, it will surely burn but that’s something that simply won’t happen under normal conditions.

Overall, cellulose insulations have a good R-value and are safe to use if the instructions from the manufacturer are followed. This includes that the material must be installed by a professional.

If you still think it’s not safe, consider using another kind of insulation. Many insulation types use glass fibers, which are not even combustible. But of course, they are more expensive.

Will cellulose insulation burn easily?

No. Cellulose insulations are normally made to resist fire until a certain point. 

Let’s say somebody left a lit candle on top of the cellulose insulation, in an attic. The insulation will surely start burning at some point, but not as a match or piece of paper, it would take much longer. 

This longer time is considered enough so the firefighters can arrive and stop the fire, or even the place owner. 

At the same time, we must also think about the other things that are up there. What if there are curtains there? Or toilet paper? How about family photos?

It’s just too hard to answer rather cellulose insulation will be the center of a fire or not, but we can assume that it can burn under some conditions. But these conditions would already be hazardous, with or without cellulose insulation.

This flammability test showed that cellulose insulation can burn, but at a slow pace. It also shows another kind of insulator that can’t burn at all.

Is it fire resistant?

Most cellulose insulations have fire-resistant compounds added to them, which diminish the fire hazard a lot. Although, fire can still happen.


Cellulose insulation products are surely combustible, but they’re formulated with other flame-retardant compounds. But there’s no general explanation when it comes to flammability. 

An already fire-hazardous scenario is required so cellulose insulation becomes flammable. Overall, we can say that cellulose insulation can be in the center of a fire, but it wouldn’t happen spontaneously. 

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

Is cellulose insulation better than fiberglass?

It might be. Each cellulose insulation formulation differs from the other, fiberglass too. 

If you wish to decide which one of them are better thermal insulator, we must look at their R-values. Research made by the Ridge National Lab showed that fiberglass may lose up to 50% of its R-value in northern weather.

Analyzing only from the thermal conductivity point of view, Cellulose insulation can be considered better in that case.

is cellulose insulation fire resistant?

Many cellulose insulation materials go through a fire-resistant treatment that makes them safer. They never lose their combustible proprieties, but the delay in fire spreading can is enough to give people time to deal with an eventual fire.

is cellulose insulation toxic?

Overall, it’s not toxic under mild conditions. In an eventual fire, toxic vapors and mists may arise from it.


Salmeia, K. A., Jovic, M., Ragaisiene, A., Rukuiziene, Z., Milasius, R., Mikucioniene, D., & Gaan, S. (2016). Flammability of cellulose-based fibers and the effect of structure of phosphorus compounds on their flame retardancy. Polymers, 8(8), 293.

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