What is radiant heat?

Where there’s smoke, there’s fire. Where there’s fire, there’s an invisible deadly threat commonly referred to as radiant heat.

When you warm yourself in front of a fireplace, you experience radiant heat firsthand. In small doses, radiant heat can be warm and welcoming, but the amount of radiant heat generated by an uncontrolled fire can be a serious threat to life safety and property.

Here’s what happens. Fire emits electromagnetic radiation that travels in invisible waves through space. When these waves hit a combustible material or a person, the radiant energy is absorbed and converted into heat. When radiant heat is absorbed by a combustible material, the object catches fire when the material’s ignition temperature is reached. Protecting people from radiant heat is vital because exposed individuals quickly feel unbearable pain, followed by second-degree burns, making safe egress impossible.

Radiant heat is typically measured on the non-fire side at a distance of 1.5 to 2 meters. Studies of critical radiation levels show that unbearable human pain occurs at 5 kilowatts per square meter. Spontaneous combustion of wood occurs at 12-13 kilowatts per square meter, and values as low as 7.5 have been reported.

What are the dangers of radiant heat transfer?

Each year lives and property are destroyed by fire — destruction that could be avoided, or certainly minimized, by applying what we know about fire and fire protection. For example, structures that survived the Oakland/Berkeley Hills fire in 1991, which killed 25 people, injured 150 and destroyed more than 3,000 buildings, had several things in common. In addition to having fire-resistive roofing material; large, clear yards; stucco exterior walls; and few overhangs, they had windows that resisted breakage in the face of fire and reduced the transfer of radiant heat. These windows protected a structure’s interior from even the highest heat levels.

Fire Resistive Glazing Blocks Radiant Heat

Fire-resistive glazing is designed to block radiant heat transfer from one building compartment to another and protect people leaving a building through an exit corridor. Click here to view a video summarizing the risks of radiant heat. In order to specify the right type of fire rated glass, it is important to look beyond the fire endurance rating (20/45/60/90/120/180 minutes) and focus instead on the overall performance of the glass. Fire-resistive glass, in combination with fire-resistive framing members, is tested for both fire endurance and its ability to limit a rise in temperature on the surface opposite the fire per ASTM E119.

Fire Protective Glass Does NOT Block Radiant Heat

Fire protective glass is designed to compartmentalize smoke and flames and is subject to application, area and size limitations under the IBC. Fire protective glass is typically used in doors and openings up to 45 minutes and cannot exceed 25% of the total wall area because it does not block radiant heat transmission. These products are also subject to the following test standards:

  • NFPA 252, Standard Methods of Fire Tests of Door Assemblies
  • NFPA 257, Standard on Fire Tests for Window and Glass Block Assemblies
  • UL 9, Standard for Safety Fire Tests of Window Assemblies
  • UL 10B, Fire Tests of Door Assemblies
  • UL 10C, Positive Pressure Fire Tests of Door Assemblies

Click here or the image to the right to see a video demonstrating the effect of radiant heat energy passing through three types of fire rated glass – fire protective ceramic Glazing, fire protective Safety Wired Glass and a specialty tempered heat reflective glazing called SuperLite I-XL, which offers partial radiant heat protection.

Where should fire resistive glazing be used?

Building and fire codes work to prevent the transfer of radiant heat where the objective is to protect lives or property, where uncontrolled radiant heat can impact occupants needing to exit past glass and where the potential for fire spread due to auto-ignition of combustibles is a possibility. Of particular consideration is the speed with which occupants can pass by the glazing, the width of the corridor, and the amount of fuel on the other side of the glazing.

Do labels always show a glazing product’s ability to block radiant heat?

Before the passage of 2012 IBC, sometimes. It’s about to get a lot easier to use the right glazing product in the right application, thanks to new and revised tables in the 2012 IBC. Specifically, Table 716.3, Marking Fire Rated Glazing Assemblies, provides architects, specifiers, installers, plan reviewers, and building officials with an easy reference that shows which performance standard(s) a product meets along with its duration rating. The new marks replace test standard numbers with letter identifications, which are then used in tables for Opening Fire Protection Assemblies, Ratings and Markings (new Table 716.5 revised Table 715.4) and Fire Window Assembly Fire Protection Ratings (Table 716.6). The same marking system was included in the 2012 edition of NFPA 101 Life Safety Code. Read more…

Which products block radiant heat?

Fire resistive glass and frames meet ASTM E119 wall standards.
  • Insulated products, such as SuperLite II-XL, Pyrostop, Pyrobel, Contraflam, Swissflam, etc. — products that are rated 60-minute plus to ASTM E119, limit temperature rise to 250F degrees and reduce radiant heat flux to 0 kilowatts per square meter.
  • SuperLite I-XL – a window and door product that reduces radiant heat flux to one kilowatt per square meter at 60 minutes and is like products widely used in Europe to limit radiant heat flux.
  • Fire Resistive Framing

Which products do NOT limit radiant heat?

  • Wired glass allows radiated flux on the unexposed side of 35-38 kilowatts per square meter at 37 minutes.
  • Ceramics, such as FireLite and Keralite, allow radiant heat flux of 75 kilowatts per square meter at 60 minutes.
  • Hollow Metal Framing