After the Marshall Fire swept through Boulder County in December 2021 and destroyed more than 1,000 homes, many Front Range homeowners started asking a new question about their walls and attics. If a fire starts in the garage or the kitchen, will the insulation feed it or slow it down? Product labels do not make the answer easy. Words like fire-resistant, flame retardant, and Class A show up on many bags, even though the products behave very differently once a fire grows.
No insulation is fireproof in the sense of being immune to fire, but two types come close. Mineral wool and unfaced fiberglass are non-combustible, which means they will not burn. Mineral wool holds up past 2,000 degrees Fahrenheit, while fiberglass melts at around 1,200 degrees. Cellulose treated with borates and most foam products only slow a fire. They resist the first spread of flame, yet they can still burn once a fire grows, which is why code requires drywall over spray foam and foam board.
For a Denver home, the useful question is where fire performance matters most and which product fits each spot. This guide explains the tests behind the labels, compares each insulation type, and shows where code calls for extra protection.
Which Insulation Types Are Non-combustible in a Denver Home?
Building codes use a standard furnace test, run at about 1,380 degrees Fahrenheit, to decide what counts as noncombustible. A material passes when it does not burn or add heat to the furnace. Only unfaced fiberglass and mineral wool pass that test without help from added chemicals, according to a CertainTeed course on insulation and fire.
Mineral wool is spun from melted basalt rock and slag, so it starts out as stone. It keeps its shape well above 2,000 degrees and is often used as a fire block between floors. Unfaced fiberglass is spun from glass and will not burn either, but it softens and melts at a lower temperature. The weak point with fiberglass is the facing, and whether fiberglass insulation is flammable depends mostly on the kraft paper glued to many batts.
Insulation Types That Only Slow a Fire Instead of Stopping It
Several common products carry a Class A flame spread rating, the same label mineral wool earns. That rating comes from a tunnel test that only measures how fast flame moves across the surface. It does not show whether the material will burn once a fire grows.
Cellulose Treated With Borates
Cellulose is made from recycled paper, so it is combustible at its core. Makers add borate salts that slow ignition and help the fibers char instead of flame. The Insulation Institute notes that borate treatment does not make cellulose noncombustible, and loose fill can smolder if a heat source sits against it. Borates also deter pests, which is why pest-resistant insulation in Denver often uses the same treatment.
Spray Foam and Rigid Foam Board
Foam insulation is plastic, and plastics burn. Spray foam can ignite at around 700 degrees Fahrenheit, according to Metal Construction News, and it gives off thick, toxic smoke. The International Residential Code requires foam in living space to sit behind a thermal barrier, usually half-inch drywall, rated to protect it for 15 minutes. Attics and crawl spaces entered only for service can use a lighter ignition barrier instead. Exposed foam is one of the first things a Denver home inspector checks in the attic.
Kraft and Foil Facings on Fiberglass Batts
The glass fibers in a faced batt do not burn, but the kraft paper does. Manufacturers state that kraft facing must not be left exposed, so it belongs behind drywall. Some foil-faced batts are rated for exposed use, and the label will say so.
Fire Performance of Common Insulation Types Compared
The table sums up how each product behaves when heat builds. Noncombustible means the material will not burn at all. Slow fire means it resists early flame spread but can still burn.
Insulation
Fire behavior
Heat it can take
Code covering
Mineral wool
Noncombustible
Above 2,000 degrees
None when unfaced
Unfaced fiberglass
Noncombustible
Melts near 1,200 degrees
None when unfaced
Kraft-faced fiberglass
Facing burns
Paper ignites early
Drywall over facing
Borate cellulose
Slows fire, can smolder
Chars, then burns
Clearance from heat
Spray foam
Slows fire, then burns
Ignites near 700 degrees
15-minute barrier
Rigid foam board
Slows fire, then burns
Melts or chars early
15-minute barrier
How Fire-Rated Walls and Ceilings Work in Denver Homes
A fire rating, such as one hour, belongs to a whole wall or ceiling tested in a large furnace. The rating comes from the drywall, framing, and insulation working together. Insulation alone does not carry an hour rating, not even mineral wool. Two places in a typical home depend on these rated assemblies.
Garage Walls and Ceilings Next to Living Space
Code requires half-inch drywall on the garage side of walls shared with the house. When a bedroom or bonus room sits above the garage, the ceiling needs 5/8-inch Type X drywall. Mineral wool in those cavities adds margin, since it stays in place longer if the drywall fails.
Shared Walls in Denver Townhomes and Duplexes
Walls between attached homes need a fire rating, often one hour, under the International Residential Code. Unfaced fiberglass or mineral wool fits these walls because neither one adds fuel. Facings and foam need extra care in a rated wall, so check the tested design before you swap products.
Choosing Fire-Resistant Insulation for Your Denver Home
Fire performance should guide the product in a few spots: walls and ceilings next to the garage, walls around a furnace or water heater, and attics in older homes with aging wiring. Plans for insulating historic Denver attics should start with a wiring check, since knob-and-tube wires and hot recessed lights need clearance from every insulation type. Furnace flues need a metal dam that keeps insulation back as well. Blown fiberglass is noncombustible, which makes blown-in Denver insulation a safe pick for many attics.
Fire is not the only factor. Mineral wool also blocks more sound than fiberglass and sheds water, which helps in basements. Spray foam seals air leaks better than any batt, so it still belongs in rim joists and some attics once it is covered. Choosing the best ceiling insulation for a Denver home means weighing fire safety with R-value and cost, and attic insulation built for cold climates has to handle January nights as well. Wildfire is a separate risk, since embers enter through vents and gaps, and ember-resistant vent screens do more there than insulation.
How Grizzly Insulation CO Handles Fire Safety in Denver Attics
Grizzly Insulation CO checks for fire risks before adding any insulation. Our team looks for knob-and-tube wiring, recessed lights that are not rated for contact, and flues that need metal dams. In many attics we use blown-in insulation in Denver that adds no fuel near wiring. For homes with rodent problems, we install borate-treated insulation for pest prevention, which also resists flame spread.
When a project calls for spray foam, Grizzly Insulation CO names the thermal barrier or ignition barrier in the written plan. Skipping that step is a costly spray foam mistake that can fail an inspection. Our team is Building Performance Institute certified and insured.
Conclusion
Only mineral wool and unfaced fiberglass are noncombustible. Mineral wool handles the most heat, and fiberglass gives the same will-not-burn trait at a lower cost. Borate cellulose and foam products only slow a fire, and foam must stay behind drywall in living space. Fire ratings belong to whole walls and ceilings, so the drywall matters as much as the insulation.
No material is immune to every fire, but mineral wool comes closest. It will not burn and holds its shape above 2,000 degrees Fahrenheit. Unfaced fiberglass also will not burn, though it melts at a lower temperature.
Yes, when it has proper clearance from heat sources. The borate treatment slows flame spread and deters pests. It is still combustible, so keep it away from flues, old wiring, and lights that are not rated for contact.
Yes. In living space, code requires a 15-minute thermal barrier, usually half-inch drywall. Attics and crawl spaces entered only for service can use a lighter ignition barrier. Your installer should name the covering in the written plan.
Often yes. It will not burn and stays in place longer if the drywall fails. The drywall still does the main fire job, so the wall needs the code-required gypsum either way. Grizzly Insulation CO often recommends it for that wall.
Not on its own. Wildfires usually enter a home through embers that slip in through vents and gaps. Ember-resistant vent screens and cleared space around the house matter more, though noncombustible insulation adds a layer of safety inside the walls.
Fire-Resistant Insulation: Which Types Are Truly Fireproof & Which Only Slow a Fire
After the Marshall Fire swept through Boulder County in December 2021 and destroyed more than 1,000 homes, many Front Range homeowners started asking a new question about their walls and attics. If a fire starts in the garage or the kitchen, will the insulation feed it or slow it down? Product labels do not make the answer easy. Words like fire-resistant, flame retardant, and Class A show up on many bags, even though the products behave very differently once a fire grows.
No insulation is fireproof in the sense of being immune to fire, but two types come close. Mineral wool and unfaced fiberglass are non-combustible, which means they will not burn. Mineral wool holds up past 2,000 degrees Fahrenheit, while fiberglass melts at around 1,200 degrees. Cellulose treated with borates and most foam products only slow a fire. They resist the first spread of flame, yet they can still burn once a fire grows, which is why code requires drywall over spray foam and foam board.
For a Denver home, the useful question is where fire performance matters most and which product fits each spot. This guide explains the tests behind the labels, compares each insulation type, and shows where code calls for extra protection.
Which Insulation Types Are Non-combustible in a Denver Home?
Building codes use a standard furnace test, run at about 1,380 degrees Fahrenheit, to decide what counts as noncombustible. A material passes when it does not burn or add heat to the furnace. Only unfaced fiberglass and mineral wool pass that test without help from added chemicals, according to a CertainTeed course on insulation and fire.
Mineral wool is spun from melted basalt rock and slag, so it starts out as stone. It keeps its shape well above 2,000 degrees and is often used as a fire block between floors. Unfaced fiberglass is spun from glass and will not burn either, but it softens and melts at a lower temperature. The weak point with fiberglass is the facing, and whether fiberglass insulation is flammable depends mostly on the kraft paper glued to many batts.
Insulation Types That Only Slow a Fire Instead of Stopping It
Several common products carry a Class A flame spread rating, the same label mineral wool earns. That rating comes from a tunnel test that only measures how fast flame moves across the surface. It does not show whether the material will burn once a fire grows.
Cellulose Treated With Borates
Cellulose is made from recycled paper, so it is combustible at its core. Makers add borate salts that slow ignition and help the fibers char instead of flame. The Insulation Institute notes that borate treatment does not make cellulose noncombustible, and loose fill can smolder if a heat source sits against it. Borates also deter pests, which is why pest-resistant insulation in Denver often uses the same treatment.
Spray Foam and Rigid Foam Board
Foam insulation is plastic, and plastics burn. Spray foam can ignite at around 700 degrees Fahrenheit, according to Metal Construction News, and it gives off thick, toxic smoke. The International Residential Code requires foam in living space to sit behind a thermal barrier, usually half-inch drywall, rated to protect it for 15 minutes. Attics and crawl spaces entered only for service can use a lighter ignition barrier instead. Exposed foam is one of the first things a Denver home inspector checks in the attic.
Kraft and Foil Facings on Fiberglass Batts
The glass fibers in a faced batt do not burn, but the kraft paper does. Manufacturers state that kraft facing must not be left exposed, so it belongs behind drywall. Some foil-faced batts are rated for exposed use, and the label will say so.
Fire Performance of Common Insulation Types Compared
The table sums up how each product behaves when heat builds. Noncombustible means the material will not burn at all. Slow fire means it resists early flame spread but can still burn.
How Fire-Rated Walls and Ceilings Work in Denver Homes
A fire rating, such as one hour, belongs to a whole wall or ceiling tested in a large furnace. The rating comes from the drywall, framing, and insulation working together. Insulation alone does not carry an hour rating, not even mineral wool. Two places in a typical home depend on these rated assemblies.
Garage Walls and Ceilings Next to Living Space
Code requires half-inch drywall on the garage side of walls shared with the house. When a bedroom or bonus room sits above the garage, the ceiling needs 5/8-inch Type X drywall. Mineral wool in those cavities adds margin, since it stays in place longer if the drywall fails.
Shared Walls in Denver Townhomes and Duplexes
Walls between attached homes need a fire rating, often one hour, under the International Residential Code. Unfaced fiberglass or mineral wool fits these walls because neither one adds fuel. Facings and foam need extra care in a rated wall, so check the tested design before you swap products.
Choosing Fire-Resistant Insulation for Your Denver Home
Fire performance should guide the product in a few spots: walls and ceilings next to the garage, walls around a furnace or water heater, and attics in older homes with aging wiring. Plans for insulating historic Denver attics should start with a wiring check, since knob-and-tube wires and hot recessed lights need clearance from every insulation type. Furnace flues need a metal dam that keeps insulation back as well. Blown fiberglass is noncombustible, which makes blown-in Denver insulation a safe pick for many attics.
Fire is not the only factor. Mineral wool also blocks more sound than fiberglass and sheds water, which helps in basements. Spray foam seals air leaks better than any batt, so it still belongs in rim joists and some attics once it is covered. Choosing the best ceiling insulation for a Denver home means weighing fire safety with R-value and cost, and attic insulation built for cold climates has to handle January nights as well. Wildfire is a separate risk, since embers enter through vents and gaps, and ember-resistant vent screens do more there than insulation.
Where foam makes sense, pick the right foam insulation for a Denver home and a certified spray foam contractor. Poorly mixed foam can shrink or pull away from framing, and those warning signs of failed spray foam leave gaps behind the drywall. Older houses also carry spray foam risks in older homes that call for a careful plan.
How Grizzly Insulation CO Handles Fire Safety in Denver Attics
Grizzly Insulation CO checks for fire risks before adding any insulation. Our team looks for knob-and-tube wiring, recessed lights that are not rated for contact, and flues that need metal dams. In many attics we use blown-in insulation in Denver that adds no fuel near wiring. For homes with rodent problems, we install borate-treated insulation for pest prevention, which also resists flame spread.
When a project calls for spray foam, Grizzly Insulation CO names the thermal barrier or ignition barrier in the written plan. Skipping that step is a costly spray foam mistake that can fail an inspection. Our team is Building Performance Institute certified and insured.
Conclusion
Only mineral wool and unfaced fiberglass are noncombustible. Mineral wool handles the most heat, and fiberglass gives the same will-not-burn trait at a lower cost. Borate cellulose and foam products only slow a fire, and foam must stay behind drywall in living space. Fire ratings belong to whole walls and ceilings, so the drywall matters as much as the insulation.
Frequently Asked Questions
No material is immune to every fire, but mineral wool comes closest. It will not burn and holds its shape above 2,000 degrees Fahrenheit. Unfaced fiberglass also will not burn, though it melts at a lower temperature.
Yes, when it has proper clearance from heat sources. The borate treatment slows flame spread and deters pests. It is still combustible, so keep it away from flues, old wiring, and lights that are not rated for contact.
Yes. In living space, code requires a 15-minute thermal barrier, usually half-inch drywall. Attics and crawl spaces entered only for service can use a lighter ignition barrier. Your installer should name the covering in the written plan.
Often yes. It will not burn and stays in place longer if the drywall fails. The drywall still does the main fire job, so the wall needs the code-required gypsum either way. Grizzly Insulation CO often recommends it for that wall.
Not on its own. Wildfires usually enter a home through embers that slip in through vents and gaps. Ember-resistant vent screens and cleared space around the house matter more, though noncombustible insulation adds a layer of safety inside the walls.