If you are a firefighter today you should be well aware that fires fought today are not the same as the fires your father fought in yesterdays. There many great programs, tests, case studies and classes out there that brings this to light. If you care about your family then get out of the recliner and learn about the changes in the modern fire environment that we operate in today. However most of the emphasis has been on modern fuel loading and building construction changes. These are extremely important and should be a major focus of your training. I feel that a part of the modern building construction changes that is often overlooked or not considered is energy efficiency.
Our first installment on energy efficiency changes is about OSB.
What is OSB?
Oriented strand board, also known as OSB, is an engineered wood product formed by layering strands (flakes) of wood in specific orientations. In appearance, it may have a rough and variegated surface with the individual strips (around 2.5 by 15 cm (approx. 1 in by 6 in) each) lying unevenly across each other.
Some OSB products today have been married to a radiant barrier.
Studies have shown that radiant barriers can reduce cooling energy consumption by up to 17%, depending on the design of the building, insulation levels and other factors. OSB radiant barrier sheathing can lower peak attic temperatures up to 30°F by reducing summer radiant heat gain in the attic. Since less heat is transferred into living areas through the ceiling, indoor temperatures stay cooler and more comfortable while helping your cooling system operate more efficiently.
- Ideal for use in hot climates
- Reflects up to 97% of radiant heat
- May lower cooling energy consumption by up to 17%*
- ENERGY STAR® qualified
Why it works so well:
Backed with specially designed aluminum foil/craft paper laminate, this OSB sheathing is installed foil side down facing into the attic space. Since aluminum foil is highly reflective, up to 97% of the radiant heat is reflected from entering the attic space.
This is an excellent study that was done by MCDOWELL OWENS ENGINEERING, INC.
Available evidence strongly suggests that these products can in fact provide significant reductions in home energy consumption (U.S. Department of Energy, the Oak Ridge National Laboratory, et al.). Unfortunately, the use of these products also provides some insidious and unintended side effects. The physical and electrical properties of these materials are such that they introduce new and very serious dangers of ignition and fire.
PHYSICAL AND ELECTRICAL PROPERTIES OF RADIANT BARRIER MATERIAL
This is where the real problem shows up. The physical and electrical properties of the radiant barrier foil are such that the material is not only an excellent electrical conductor (Wikipedia); it is also a very viable ignition source. When electrical current flows through it the material not only readily generates heat necessary for ignition, it also becomes the first ignited material!
The primary physical property of the paper we are interested in is the ignition temperature. This value can vary significantly for different types of paper; however, it is virtually always less than 500 degrees F. Measurements in our study showed that the paper ignited when the foil temperature was between 420 and 450 degrees F.
Clearly then, if the radiant barrier foil material is overheated as a result of electrical current flow, the temperature of the material will easily exceed the ignition temperature of the paper backing.
SUMMARY of IMPORTANT PHYSICAL and ELECTRICAL CHARACTERISTICS:
1. CONSTRUCTION: Aluminum Foil bonded to Paper.
2. MELTING TEMPERATURE of the ALUMINUM: >900 degrees F.
3. IGNITION TEMPERATURE of the PAPER: <500 degrees F.
4. SHEET RESISTANCE of the ALUMINUM: 4.50 ohms per square.
5. CURRENT DENSITY for FAST IGNITION: Approx 300 Amps per sq. mm.
SPECIAL NOTES ABOUT THE ELECTRICITY:
This study has shown it is possible to have a fire originate in the radiant barrier if it becomes energized by virtually ANY commonly available electrical source. However, it should also be clear that if there is danger of a fire from “single digit” voltage levels and “tens” of amps, the danger from lightning at
15,000,000 volts and 100,000 amps will be MANY times greater. This is why we feel strongly that the presence of radiant barrier material, as presently manufactured and installed in a structure, greatly increases the risk of a fire in the event that the structure experiences a lightning strike.
SUMMARY of FINDINGS
1. Standard installation methods for roof sheathing with integrated radiant barrier are such that the end result is an overall environment where all of the radiant barrier material and virtually everything metal on and around the roof are electrically connected.
2. In most cases, something in that environment is connected to earth ground. If anything in the roof environment becomes electrically energized (by lightning or any other common source) there is a high probability the current will pass through the barrier material at some point on the way to earth ground.
3. The physical and electrical properties of reflective radiant barrier materials which we tested are such that the material in a structure provides two new and unique hazards relative to fire causation.
(a) When energized by an electrical current the material readily generates temperatures sufficient to ignite MANY materials.
(b) The barrier material itself readily serves as the first ignited material.
Another point to ponder is that if the OSB with radiant barrier keeps out the heat it will also keep in the heat from a fire and cause thermal radiation feedback much faster causing Flashover to happen quicker.
A special thanks to MCDOWELL OWENS ENGINEERING, INC. for all the testing they have done to bring this issue to light for the fire service.
I hope that you have learned of yet another hazard we face in the fire service and that you will share this information to all you know.
Lt. John Shafer