Reference
That Burning Feeling
Burn Tests on OSB vs. Drywall
By Mark C. Hughes, P.Eng.
© 2005 Sintra Engineering Inc.
Sometimes we get to do some really interesting performance based testing of materials. I thought I would share some results from some testing we conducted for an assignment. We were asked to investigate a fire that had originated in the attached garage of newer home. The issues surrounding this fire were not limited to the determination of the origin or cause, but the concern was why the fire had spread so quickly into the house resulting in a significant amount of damage. The builder had used oriented-strand board (OSB) sheeting instead of drywall to finish the interior of the garage to save on material costs and with the approval of the home owner. In addition, the use of OSB was not contrary to the building code.
The home owner's savings by using OSB were offset by the fire that erupted in the garage and rapidly spread to the house. The use of OSB ended up costing the insurance company significantly more in reconstruction fees to the building and the homeowner lost a significant number of personal effects. The occupants luckily heard noise, noticed smoke and got out of the house just before the fire breeched into the house.
To evaluate the significance of the building material's implications, we set up a burn test to observe the performance of both drywall and oriented strand board (OSB) sheeted wall assemblies when exposed to a fire. One would obviously think that there would be a difference but the results were still surprising.
To test the materials, wall assemblies were constructed using 2'x4' studs, and 3/8" drywall or 3/8" OSB on one side. A controlled flame was directed at the sheeted face of each assembly. Each sheeting material was tested twice, once with the flame directed at a seam in the sheeting and once with the flame at an area away from a seam. The time to breech the wall assembly was recorded for each test.
It took less than five minutes for flames to be visible on the back side of the OSB sheeted wall assembly. The breech time was similar regardless of whether the flame was directed at or away from the seam in the OBS sheeted wall assembly. More importantly, the OBS became a significant secondary fuel after the fire breach. The drywall assembly performed very differently. With the same flame directed at an untaped/mudded seam of the drywall assembly took about 15 minutes to breech. The heat from the flame went through the seam in the wall assembly and then pyrolyzed until ignition the wood framing behind. However, at a point away from the seam, it took over 1 hour for visible flames to be noted on the back side of the drywall. Intellectually, I knew that there was a difference between the materials, however, I did not expect it to be as dramatic as it was.
Going back to our investigation, the majority of damage to residence would have been prevented had the common wall to the garage been constructed of regular drywall rather than OSB. OSB is a combustible material and provides almost no fire resistance, particularly once the wall has breeched.
The Alberta Building Code (ABC) requires that the construction assembly act as a barrier against flame spread, built with appropriate materials in a continuous and approved fashion, but OSB is considered to comply with the code. The common wall must also provide an effective barrier to gas and exhaust fumes. While the construction for our investigation and many new homes that have been built in recent years may technically comply with the requirements of the ABC, the use of OSB in a common wall between a garage and house is a significant fire hazard. This code description leaves too much up for interpretation resulting in materials such as OSB being used in construction assemblies, where traditionally drywall was used.
If I was building a new house with an attached garage, I would ensure I had drywall walls and a drywall ceiling, just for peace of mind. Have a nice summer.
Mark Hughes, P. Eng. is a principal with Sintra Engineering, a Forensic Engineering firm.
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