Is Fiberglass Insulation Flammable? All you Need to Know!

Fiberglass insulation is probably the most commonly used material for insulating homes and buildings, be it for thermal or acoustic insulation. But it’s also extremely important to take into account how this insulation material behaves when exposed to fire.

In this article, I will be going over how well Fiberglass insulation does in terms of flammability, if it’s fireproof or not, how all of this is measured (flame spread index), and compare it to other similar materials such as Rockwool, Spray foam, Cellulose, etc., (in terms of flammability), and a lot more.

So, without any further ado, let’s get started!

Is Fiberglass Insulation Fireproof?

Fiberglass is considered to be a non-flammable material that has a flame spread index of 25 and which can withstand temperatures of up to 500°C (1000F) for extended periods of time without melting or even catching on fire. Be careful with bats that have an adhesive backing since this will burn rather quickly.

It’s worth mentioning, however, that no matter what insulation material you use, if the fire is strong enough and burns for long enough, at some point it will catch on fire, and Fiberglass insulation will melt (since it’s made from glass) and it might also catch on fire once it reaches a certain temperature threshold (generally around 500°C).

What is Fiberglass?

Fiberglass insulation, or properly named glass wool (fiberglass is actually the one used for boats, etc., which is rigid), is an insulation material made from fibers of glass arranged into a texture similar to wool.

The process traps many small pockets of air between the glass, and these small air pockets result in high thermal and acoustical insulation properties.

Glass wool generally comes in the form of rolls or in slabs, with different thermal and mechanical properties, but it is also produced as a material that can be sprayed or applied in place, making it quite versatile.

On the back, you can generally see an adhesive aluminum foil or paper, which serves as a vapor barrier and also to keep the fibers in place (it’s worth noting, however, that additional vapor barriers are generally needed).

Since it’s such a commonly used material for insulating a home, the batts you can get generally have the size to fit in standard spacing studs in walls, which makes installation quite simple, but if it doesn’t fit where you need it to, it’s quite easy to cut (just make sure that it fits snugly since any gap left unsealed will let heat and sound through).

How a material’s flammability is measured: Flame Spread Index

I will now be talking about the flame spread index and what it represents, as well as the smoke development index, what the ideal ratings for both are, and more.

The flame spread index is determined by the distance that a flame travels along a test substrate in a specific timeframe to determine its propensity to burn and how rapidly it can spread flames.

It’s divided into these 3 classes:

ClassFlame Spread Index
  1. Class A fire ratings indicate a flame spread rating between zero and 25. Materials that fall into Class A or Class 1 include; brick, gypsum wallboard, and fiber cement exterior materials. These materials do not burn well and are very unlikely to contribute fuel to a fire.
  2. With a Class B or Class 2 fire rating, the flame spread rating would fall between 26 and 75. This rating is typical for slower-burning whole wood materials, such as planks that are in the same form as they were when they were cut from the tree.
  3. A Class C or Class 3 fire rating has a flame spread rating between 76 and 200, which incorporates building materials like plywood, fiberboard, and hardboard siding panels, as well as any of the faster burning whole woods.

What is a good flame spread rating?

The lower the Flame Spread index the better, since this means that the flame won’t travel and spread as rapidly. Class A materials, which have a rating of 0-25, do not burn well and are very unlikely to contribute fuel to a fire, which makes them the safest ones.

Fiberglass insulation is a Class-A material, meaning that it won’t contribute fuel to a fire.

Smoke Development

Smoke-developed index (abbreviated SDI) is a measure of the concentration of smoke a material emits as it burns. Similar to how the Flame Spread Index is measured, it is based on an arbitrary scale that goes from 0 to 100, where a value of 0 represents extremely low smoke emission (asbestos-cement board, for example), and where a value of 100 represents the highest smoke emissions (red oak wood).

Fiberglass, or Glass Wool, since it’s essentially made out of glass, its emissions are very low, but it will still emit some smoke.

Here’s a table with the flame spread and smoke development ratings of the most popular insulation materials:

MaterialFlame Spread RatingSmoke Development
Mineral Wool (Rockwool)2550
Spray Foam75450
Mass Loaded Vinyl25250

Important: Fiberglass will still catch on fire (eventually)

Even though Glass Wool is made out of tiny glass fibers, if the fire is hot enough and burning for enough time, it will eventually catch on fire and burn.

Not only that, but it will start melting as well, although it needs to be exposed to really high temperatures for extended amounts of time for this to happen.

When comparing it to spray foam, for example, even though spray foam has added chemicals to give it flame-retardant qualities, it will still burn a lot faster and way more violently, causing fire to spread much quicker.

Better Alternative

If you’re already thinking about using fiberglass insulation, then I’d suggest going with Rockwool, or mineral wool, instead since its insulation capabilities are slightly better and it can withstand much higher temperatures (over 1000°C without catching on fire whereas Fiberglass starts burning as slightly over 500°C).

I have an article comparing glass wool to mineral wool that you should read where I go into their acoustic insulation properties, their cost, and a lot more.

But overall, Mineral wool insulation is the better option, especially when it comes to fireproofing your place.


Fiberglass insulation is an excellent sound and thermal insulator, and while it may be better than spray foam, Cellulose, etc., especially as far as flammability goes, it still gets beaten by mineral wool.

I would recommend going for mineral wool any day of the week, but Fiberglass insulation is a great alternative to it and it’s also a Class-A fire-retardant material, meaning that it won’t help the fire spread.

Last Updated on August 29, 2022 by Facundo

Scroll to Top