Subject: Using Gypsum Wallboard for Plenums
Jan. 5, 2000
The use of gypsum board systems to fire-protect sheet-metal ducts and plenums has been an accepted practice of this industry for many years, and has an excellent track record. We support the use of gypsum board for the fire protection of these assemblies.
However, a potential problem exists when the sheet metal is removed from a design, leaving just the gypsum board system to serve as the duct or plenum. This decision often comes either from the cost-cutting measures of value engineering or from an engineered design that specifies a gypsum board plenum. Properly engineered systems are the responsibility of the designer and should be constructed in strict accordance with the engineered design to assure optimum performance. The purpose of this technical bulletin is to make you aware of the risks of making random modifications to engineered systems and of arbitrarily substituting gypsum board for sheet metal in ducts or plenums designed to be constructed of or lined with sheet metal.
Gypsum Association, ASTM, and NFPA recommendations for gypsum board in air handling systems recommend against the use of gypsum board where it will be exposed to temperatures in excess of 125 degrees for periods of time sufficient to cause calcination. Gypsum board may begin to calcine after short-term exposure to temperatures at or above 150 degrees F. Additional recommendations call for the air stream contacting the gypsum board to be above the dew-point temperature. Removal of the sheet metal or other duct material from gypsum-protected engineered ducts and plenums can create conditions under which the gypsum board can become either wet or calcined from exposure to temperatures outside of the recommended service temperature range. Continuous exposure to temperatures below the dew-point temperature can cause condensation of moisture, thereby creating conditions conducive to the growth of fungi and damage to the gypsum board face paper.
Gypsum board plenums must be tightly constructed, with all joints, penetrations, and connections sealed to prevent failure of pressurization tests. Expansion and seismic relief joints must be designed into the system and be properly installed to prevent leakage and loosening due to seismic activity and thermal expansion.
We are aware of lawsuits resulting from problems created when the “as built” condition varies from the engineered condition of the original design, and recognize the potential for additional problems when industry recommendations are not followed. The Drywall Information Trust Fund does not want to see any of its contractors or the design community exposed to unnecessary risk.
If you have further questions, feel free to call US any time at (408) 518-8187.