Why Sheetrock? Physical properties and Fire Resistance.

With my recent post “The World’s most expensive Sheetrock” came some questions on fire resistance and the requirements for fire separation in a garage.

The main purpose of the assembly in the garage is to slow down a fire and to prevent migration to the home and attic space for a certain period of time. The principle material used for this is gypsum board most commonly called sheetrock or drywall.

We have to understand that garages are much more likely to carry items that can contribute to a fire (a fire load) that is most likely not going to be stored in the home such as, gasoline, propane tanks, paint thinners, tools and vehicles (that have gasoline and oil  in them). Add to the fact that an attached garage typically is in close proximity to many common ignition sources, such as clothes dryers, hot water heaters, electrical motors, receptacles and electrical panels etc…

Now lets look at some thermal transfer properties (conductance) of a few common materials; Wool- 0.07, Gypsum board- 0.17, Silver- 429 (Thermal conductivity units are W/(m K) per the SI/Metric system). So the higher the number the better the material can conduct heat. As you can see gypsum has a fairly low conductance property.

When we process gypsum into a board assembly it takes on another very important principle, when exposed to high temperatures it produces a reaction called the endothermic process.  Endothermic is defined as a reaction or process, accompanied by or requiring the absorption of heat.

The endothermic process in this case describes a process or reaction in which the system ( our gypsum board assembly), absorbs energy from its surroundings in the form of heat thus providing protection to the materials behind it.

“Commercial gypsum boards consist mainly of the crystalline mineral calcium sulphate combined with ca. 21% by weight chemically bound water, known as calcium sulphate dehydrate (CaSO4. 2H2O). In many cases, gypsum boards usually contain an amount of absorbed free water (≤ 4%)

When gypsum boards are heated to temperatures above 80 degrees C, the chemically bound water dissociates from the crystal lattice and, together with the free moisture, evaporates. This process, known as “dehydration” of gypsum, and takes place at temperatures between ~80 degree C and ~250 degrees C.

During the dehydration, a large amount of heat is absorbed and the heat transfer through the gypsum board is practically delayed until the process is completed.”

So in lay terms,  gypsum plasterboards are capable of slowing the penetration of fire and heat transfer through walls and floors due to the release of water into steam (the endothermic gypsum dehydration process) that takes place when high temperatures are present.

From the Gypsum Fire Resistance Design Manual(2009)-

“Gypsum is approximately 21 percent by weight chemically combined water which greatly contributes to its effectiveness as a fire resistive barrier. When gypsum board or gypsum plaster is exposed to fire, the water is slowly released as steam, effectively retarding heat transmission (Figure 1). It can, in a sense, be compared to what happens when a blow torch is turned on a block of ice. Although the ice is being melted, one can hold a hand on the opposite side without being burned. Even though the ice gets very thin it effectively blocks the transfer of the intense heat and one's hand would not be burned until the ice is melted. When gypsum-protected wood or steel structural members are exposed to a fire, the chemically combined water (being released as steam) acts as a thermal barrier until this slow process, known as calcination, is completed. The temperature directly behind the plane of calcination is only slightly higher than that of boiling water (212°F), which is significantly lower than the temperature at which steel begins losing strength or wood ignites. Once calcination is complete, the in-place calcined gypsum continues to act as a barrier protecting the underlying structural members from direct exposure to flames.”

Link to document-

http://www.ncwhomeinspections.com/system/files/userfiles/org_gypsum_GA-600-09.pdf

So the next time you look at your sheetrock give it a nod for what it can do during a fire event. This is why it is important that be installed properly and where required.

“Twenty years from now you will be more disappointed by the things that you didn’t do than by the ones you did do.”

Mark Twain

NCW Home Inspections, LLC  is a Licensed Washington State Home Inspection service located in Wenatchee Washington serving Chelan County, Douglas County, Kittitas County, Okanogan County and Grant County Washington and the cities of Wenatchee, Leavenworth, Cashmere, Oroville, Cle Elum, East Wenatchee, Quincy and many more…  

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