BY GREGORY HAVEL
During recent months, fire service publications and Web site articles have expressed concern about Hi-Impact 1 brand impact-resistant wallboard. Concerns include difficulty in breaching it for firefighter rescue and the combustibility of the Lexan 2 (polycarbonate plastic) laminated to the back of sheets of this type of wallboard.
My part-time job as a construction company safety director allows me to observe closely developments in building materials and construction methods. The Hi-Impact brand wallboard is only one of many brands of specialty drywall board that are classified as “impact resistant.”
Drywall board (also called gypsum board and wallboard) was developed in the 1930s and is basically a gypsum product in sheet form with heavy paper facings to give it shape and abrasion resis-tance. It became popular during the building boom during and after World War II, replacing traditional lath-and-plaster because of its comparatively low cost, even though it is less durable if not installed properly. Now, it is also replacing traditional masonry fire division walls in new construction and renovation, also because of its comparatively low cost. Further developments by the gypsum industry in making this product impact-resistant are making it the new “material of choice” by architects for applications previously reserved for masonry. In addition to costing less and requiring less labor to install than masonry or traditional plaster, it is also significantly lighter in weight, thus allowing building designers to further reduce the strength and cost of structural members.
Early refinements of the original drywall board included
• Tapered edges to allow for seamless installation using paper tape and gypsum joint compound;
• Different surface textures for application of different finishes like paint, plaster, and ceramic tile;
• Sheet lead backing, for shielding X-ray and radiation sources in hospitals, clinics, and other medical facilities (lead melts at 621°F; a fire involving a partition with lead-backed drywall can result in molten metal on the floor and lead vapors in the smoke); and
• Additives and glues for special applications, such as the water-resistant board used in bathrooms and other damp locations, and for use as a fire-rated assembly (fire-code) board.
Gypsum is hydrous calcium sulfate, with the chemical formula CaSO4-2H2O, and is a naturally occurring mineral. This means that it is calcium sulfate (plaster of paris) combined at the molecular level with water of crystallization, about 20 percent of the gypsum by weight. After gypsum is mined, it is crushed and ground to the fineness of flour; heated to drive off its water, making plaster of paris; recombined with precise amounts of water and other ingredients; and pressed between two sheets of heavy paper to create drywall board. Traditional plaster is a similar material, except that the plaster of paris and other dry ingredients are mixed with water and other ingredients at the construction site and applied by hand in layers over wood or expanded metal lath.
Drywall board is naturally fire resistant because of the large amount of water contained within the gypsum molecules, about 20 percent by weight. A large amount of heat is required to drive off this water. As the water is driven from the gypsum during a structural fire, it slows the spread of the fire at the same time the gypsum is breaking down chemically and structurally. Recall that when overhauling burned-out rooms, areas of drywall or plaster that were heavily heat-damaged are pulled much more easily than areas of the same materials with little or no heat damage. This is the reason that regulations require replacement of drywall in fire-damaged rooms, although sometimes wallboard that looks undamaged is left in place when it should not be.
Because of its fire-resistant qualities, gypsum drywall board is commonly used in wall and ceiling assemblies with one- and two-hour ratings (UL Fire-Rated Assemblies U495 and V416). Special drywall assemblies have received higher ratings, but these are not common. The fire-code types of gypsum board have additives that make them even more fire resistant than common wallboard.
IMPACT RESISTANCE METHODS
Within the past few years, rising construction costs have led to the development of impact-resistant drywall board. The technology is new enough that patents and copyrights still apply, so each of the manufacturers has taken a different approach. Most use the fire-code type of gypsum as a base, because the places in which this more expensive board will be used are likely to require fire-rated partitions, including schools, hospitals, clinics, and apartment buildings. Sheets of this wallboard may be of unusual colors and will have the manufacturer’s brand name and information prominently displayed on the ends and face of the sheets when delivered. Identifying these specialty products becomes nearly impossible after they are installed and finished (photo 1).
Heavy paper backing. The most basic type of impact resistance comes from an extra layer of heavy paper backing. This provides some additional impact resistance with little extra cost. Several manufacturers use this method for their basic impact-resistant products.
Fiber reinforcement. In this more impact-resistant type of wallboard, cellulose or glass fibers are mixed with the fire-code gypsum before it is formed into board. This type usually has the standard paper facings but may be of a different color. Several manufacturers use this method. USG Fiberock VHI Abuse-Resistant3 is fiber-reinforced with a fiberglass mat laminated to the back side of the drywall.
Fiberglass mat. This type uses fiberglass mat imbedded in one or both faces. This type may not be faced with paper and can be used for exterior building sheathing under stucco, stone, brick, and other finishes and in damp locations. Georgia-Pacific sells its paperless product under the brand names DensGlass Gold 4, DensArmor , DensShield , and ToughRock . Since it is paperless and completely inorganic, it is also used where mold and insect resistance are needed.
Cement board. This type may be fiber-reinforced Portland cement or a fiber-reinforced mixture of Portland cement and gypsum. These boards have no paper facing and can be used for exterior sheathing and as underlay or backing for ceramic floor and wall tile systems, depending on the brand and finish. Cement board is also prepared and finished for use as exterior siding and roofing slate. Some types can also withstand high temperatures. Today’s cement boards are descended from an asbestos-reinforced cement product from the 1950s that was marketed under several brand names, including Transite 5. Today’s cement board usually contains cellulose or glass fibers, although in older buildings it probably still contains asbestos fiber.
Lexan backing. The Hi-Impact brand of impact-resistant wallboard manufactured by NGC uses a thin sheet of Lexan (polycarbonate plastic) backing laminated to a paper-faced fiber-reinforced fire-code gypsum board. Impact resistance increases as the thickness of the Lexan sheet increases. Although polycarbonate plastic will burn at high temperatures, it does not easily support combustion-it will smolder and melt in a fire and may burn as a combustible liquid.
Polycarbonate is the same plastic used in many SCBA face piece lenses and in some fire helmets. At room temperature, it tends to deform rather than break. With a tensile strength of 9,000 psi (ASTM 638) and a flexural strength of 14,200 psi (ASTM 790), this stuff is tough. It has a melting point of 310°F.6 Polycarbonate is made by reacting bisphenyl-A with sodium hydroxide (UN 1823). The resulting salt is then reacted with phosgene (UN 1076) to produce polycarbonate.7
All of these impact-resistant wallboards are approved for use in fire-rated wall assemblies. As in standard fire-rated walls, a layer of 5⁄8-inch impact-resistant board on each side of 2 4 studs will achieve a one-hour rating, if properly installed. A base layer of 5⁄8-inch impact-resistant board on each side of the wall, with an additional layer of 5⁄8-inch fire code board over it, will achieve a two-hour rating, if properly installed, and could be nearly as difficult to breach as a masonry wall. Proper construction of a two-hour-rated wall is as follows:
1. Assemble 20-gauge, 35⁄8-inch steel studs and floor and ceiling runners with Type S (drywall) screws, with the studdings on 16-inch centers.
2. Attach vertically sheets of 5⁄8-inch impact-resistant fire-code drywall board to both sides of the studding assembly with 11⁄4-inch Type S screws spaced on eight-inch centers along the edges and on 12-inch centers in the center of each board.
3. Attach vertically sheets of 5⁄8-inch fire-code drywall board to both sides of the wall with joints staggered from the base layer. Use 15⁄8-inch Type S screws spaced on 12-inch centers along the edges and on 16-inch centers in the center of the board.
4. Tape all joints. Countersink and cover all screw heads. Caulk the perimeter joints with a fire-rated material approved for this application.
One- and two-hour rated walls can also be assembled using wood studs.
Some of these impact-resistant wallboards can be cut like plain drywall: Score one paper facing with a razor knife, snap the board on the score, and cut the other paper facing. Others, such as the fiber-reinforced and cement boards, may break only after scoring with carbide hand tools or may require cutting with power tools.
Although the impact-resistant drywall board weighs less and costs significantly less than traditional types of construction for heavy-use areas like schools, hospitals, clinics, and shopping malls, it is not likely to be common in residential use. Its material cost is significantly higher (15 to 50 percent) than ordinary drywall of the same thickness, which performs acceptably in most residential applications.
EFFECT ON THE FIRE SERVICE
What effect will the use of these specialty drywall boards have on the fire service? It will
• lighten frame construction that is already considered “lightweight” by permitting greater security with lighter and less expensive materials,
• reduce the amount of time we have for interior fire attack when the fire has gone beyond room and contents because of the lighter-weight construction,
• make a wallboard fire division wall that we once could breach easily as significant an obstacle as a masonry wall when rescuing trapped firefighters or occupants, and
• make us more dependent on using thermal imaging cameras to assist in locating hot spots and fire victims and on power tools to cut small inspection openings instead of spending energy and resources in opening difficult walls completely to check for fire extension.
We can offset the effect of impact-resis-tant wallboard by
• promoting the installation of automatic fire sprinkler systems and fire alarm systems with smoke detectors in these buildings;
• promoting fire safety and fire prevention through aggressive public education programs;
• including building owners, engineers, architects, and municipal planners in our public education programs. This may be the most effective way to promote installation of fire sprinkler and fire alarms systems in new buildings;
• continuing to train and educate our firefighters to recognize the building construction types and methods and to understand the implications each has on interior firefighting;
• preplanning our buildings and following up with company-level tours or inspections to acquaint our firefighters with the buildings for which they are responsible and to reinforce the training and education we provide on building construction; and
• including in our preplanning tours buildings being remodeled and those under construction so that we can document where impact-resistant products are used. ■
Endnotes
1. Hi-Impact® is a registered trademark of National Gypsum Company (NGC).
2. Lexan® is a registered trademark of the GE Plastics division of General Electric.
3. USG Fiberock® is a registered trademark of United States Gypsum.
4. DensGlass Gold®, DensArmor®, DensShield®, and ToughRock® are registered trademarks of G-P Gypsum, a Georgia-Pacific Company.
5. Transite® was a registered trademark of Johns-Manville Corporation.
6. Data from www.sdplastics.com (San Diego Plastics, Inc., National City, California).
7. Data from http://www.psrc.usm.edu/macrog/pc.htm (Department of Polymer Science, University of Southern Mississippi, Hattiesburg).
■ GREGORY HAVEL is deputy chief and training officer of the Burlington (WI) Fire Department and a 27-year veteran of the fire service. He is a Wisconsin-certified fire instructor II and fire officer II, an adjunct instructor in fire service programs at Gateway Technical College, and safety director for the Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College and has 20 years of experience in facilities management and building construction.
