When picking insulation, you want one that has high R-values and is safe. Flammability is an important consideration—after all, nobody wants to install insulation that can catch fire. So, is fiberglass insulation flammable?
Fiberglass insulation cannot catch fire, but at high enough temperatures, it can melt. However, the brown paper that is often attached to batts as a vapor barrier can catch on fire. Blown-in fiberglass is also considered non-flammable, but installation requires specialized equipment.
As with many things in construction, the answer is not a simple yes or no. We’re going to explain why fiberglass insulation does not burn but why it can still contribute to a house fire. We’re also going to discuss the flammability of an alternative to fiberglass insulation.
What Is Fiberglass Insulation?
Fiberglass is made from silica sand mixed with limestone and soda ash. The sand is used to create the glass while the limestone and soda ash lower the melting temperature needed to manufacture it.
Borax and other ingredients such as feldspar, magnesite, and kaolin clay are added to improve the fiberglass’s performance.
After the ingredients are mixed, they must be heated. Fiberglass has to be heated to 2,500 Fahrenheit, which is a higher temperature than most glass. Once the glass is molted, it can be formed into fibers, filaments, or glass wool.
Fiberglass products need other coatings, including lubricants that reduce the product’s abrasive qualities, and an anti-static agent. Then the fiberglass is shaped, and other materials are added, including lubricants.
An additional additive might be an anti-static coating that will also be applied. Finally, the fiberglass can be shaped, packaged, and shipped.
Besides insulation, fiberglass is used throughout the house, including windows, bathtubs, and roofing materials. Your car most likely contains fiberglass, and if you own a boat, it contains fiberglass as well.
Can Fiberglass Insulation Catch Fire?
Since the insulation is made from glass, fiberglass insulation is considered non-flammable and will not catch fire. However, the paper and foil backings can catch on fire. Fiberglass manufacturers can add flame-retardant adhesives and foils to kraft paper backings.
One such product is an FSK shield, which can be used as a vapor barrier as well. By installing an FSK-shield product with the foil side exposed, you will slow the speed at which flame spreads.
Owens Corning EcoTouch Flame Spread is fiberglass insulation that uses a facing that also slows the speed at which flames spread.
Fiberglass Melting Can Cause Problems
Although fiberglass insulation cannot burn, if it reaches a high enough temperature, it can melt. Fiberglass is rated to melt at temperatures above 1,000 degrees Fahrenheit (540 degrees Celsius).
Other insulation materials have a higher temperature range before they melt. Mineral wool and ceramic fiber melt at 2,200 Fahrenheit (1200 Celsius) and vermiculite needs 1,400 degrees Fahrenheit (760 Celsius) to melt.
Why is this important?
A chimney fire exceeds 1,700 degrees, so should you have a chimney fire, then there is a chance the fiberglass can melt.
If fiberglass melts, it could cause several problems. One of them is that the fiberglass begins to spread the heat to other materials that can ignite.
Therefore the International Residency Fire Code requires that there be a one to two-inch space around chimney vents. This requirement is for gas appliances (fireplaces and furnaces). The spacing requirement is in addition to a second requirement—that the chimney vent be a Type B.
A Type B vent has two tubes. The outside tube does not touch the inside tube to prevent heat from spreading. However, because of the danger that melting fiberglass poses, fire codes call for this additional space.
For more information read Insulation Around Furnace Exhaust: Important Do’s And Don’ts
Melting Fiberglass Increases Oxygen Supply
As fiberglass melts, it opens up and releases the trapped gases, including oxygen. That increased oxygen feeds the fire and turns a cavity wall into a convection chamber. Ironically, the non-flammable fiberglass can speed up a fire.
This has been tested in several fire demonstrations. In a structure insulated with fiberglass, the ceiling collapsed after 20 minutes, and the entire building had burned after two hours. In the same test, other insulation materials lasted longer (source).
Flame Spread Is Another Important Fire-Safety Measure
When a fire begins, the speed with which the fire will travel is important. The slower the spread, the more time you have to put out the fire, or if that is not possible, get out.
Are There Any Products That Perform Better Than Fiberglass?
Cellulose is an alternative insulation material. It is made from recycled newspapers. First, the paper is shredded into strips. Next, boric acid is added, primarily as a fire-retardant.
However, it also helps fight mold, wood decay, and pests. The final step is to run it through a “fiberizer” that turns it into the fluffy final product. Additional boric acid is added during this process.
You may hear or read that cellulose is not a safe product to install in a house. Numerous sites, such as DFW Thermal Solutions, will tell you that it is “considered a fire hazard by the Consumer Product Safety Commission.”
However, if you follow their link to NAIMA, you might think it is a neutral site. NAIMA is the North American Insulation Manufacturers Association that represents manufacturers who make fiberglass and mineral wool insulation.
Cellulose Does Not Have Large Air Pockets
Remember the air pockets in fiberglass. Cellulose is denser insulation and does not contain pockets of air. Therefore, a fire that starts in a wall filled with cellulose insulation will not spread as quickly.
Here’s a manufacturer’s video demonstrating how celulose insulation can offer more resistance to fire than fiberglass:
The same tests mentioned earlier, where the ceiling of the fiberglass insulated building collapsed after 20 minutes, took the ceiling insulated with cellulose 70 minutes to collapse.
And instead of burning down in two hours, the cellulose building was still standing after three hours.
Cellulose Has Other Advantages
In addition to having better heat resistance, cellulose has superior insulating properties. In a study at the University of Colorado (source), researchers compared buildings insulated with fiberglass and cellulose and discovered that:
- Cellulose lost less heat overnight.
- The building insulated with cellulose was 36 to 38% tighter than the fiberglass-insulated building.
- Three weeks later, the building insulated with fiberglass had 26.4% more heat than the cellulose-insulated house.
Cellulose is also better able to insulate at lower temperatures. Fiberglass begins to lose R-value as the temperature drops. The large air pockets in fiberglass insulation contribute to heat loss.
If you live in an area where the temperatures drop to the low teens, your insulation could lose up to half of its R-value.
Disadvantages of Cellulose Insulation
If cellulose has superior insulation properties and is less flammable, why isn’t it used more frequently? It comes down to cost and ease of installation.
- Cost. Fiberglass batts cost about half of what blown-in insulation costs. While you can expect to pay 70 to 80 cents per square foot for cellulose, fiberglass batts will run you an average of 30 to 40 cents.
- Ease of installation. Since fiberglass bats can be installed more quickly, the labor costs are much lower as well. Figure around $1 for fiberglass insulation and twice as much for cellulose.
A final disadvantage for a homeowner wanting to retrofit a house is that cellulose can only be blown in, meaning specialized equipment needs to be rented.
The good news is that insulation is the cause of less than .5% of all house fires. Fiberglass insulation is safe; however, you need to be careful when installing it near sources of heat, such as fireplaces and lights. The same is true of cellulose or other insulation products.
As a homeowner, I am constantly experimenting with making the structure of my house more energy-efficient, eliminating pests, and taking on DIY home improvement projects. Over the past two decades, my family has rehabbed houses and contracted new home builds and I’ve learned a lot along the way. I share my hard-learned lessons so that you can save time and money by not repeating my mistakes.