Active Standard: C1280 - 13a
Rationale: Exterior-installed gypsum sheathings with tightly butted panel edges dimensionally expand and contract due to ambient thermal and moisture variations while in service. One or both of these ambient service conditions occur regularly in gypsum sheathing under most claddings, but especially under metal panel (high heat) and cement plaster claddings during initial installation (while wet and curing), conditions which are exacerbated with water-vapor permeable water-resistive barrier membranes over the sheathing. If gypsum sheathing cannot accommodate these induced movements, the sheathing as a cladding substrate may become unstable and negatively affect the overlaying cladding performance (i.e. cause cement plaster cracks). The Gypsum Association and national gypsum panel manufacturers published material expansion/contraction performance data (coefficients of expansion) for gypsum sheathing panels include: Hygrometric: 6.5x10^-6in./in. per %RH (plywood is 0.0002 in./in. per 10% change in RH expansion from oven dry to fiber saturation ) and Thermal: 9.3x10^-6 in./in/ per degreeF (plywood is 3.4x10^-6 in./in./ per degreeF ). Under service conditions where both thermal and hygrometric conditions occur simultaneously, a mere 50% rise in ambient RH and 50 degreeF rise in ambient temperature, can result in expansion over each 10 lineal feet of gypsum panel sheathing calcualted to 0.095 in. (for plywood in these conditions it is 0.140 in.). If gypsum sheathing control joints are located at 30 ft lengths as the current C1280 text allows, the induced expansion movements accumulate and by calculation are a localized 0.284 in. This magnitude of induced substrate movement may negatively affect the performance of overlaying claddings, especially with code-allowed water vapor permeable water-resistive barrier membranes that allow the panels to expand in a moist environment. These are gypsum panel movements, so the current wording placing the burden on the exterior cladding requirement to determine gypsum sheathing movement joint location is not applicable. Where each panel can be installed to allow to expansion individually, the movement will not accumulate beyond the panel to an excessive amount. The wood structural panel industry (and ICC building code requirement) has solved this same issue by requiring 1/8 in. panel edge gaps at all panel edges under cement plaster claddings. Note that wood structural panels are also installed with fasteners in a restrained condition similar to gypsum sheathing panels so the approach has valid precedent. Considering field construction tolerances, the proposed 0.125 in. panel edge gap provides a 30% safety factor for performance and consistency with the wood structural panel industry requirement, to not confuse installers. Dry sheet and liquid-applied WRB membrane systems can accommodate these small substrate gaps. The panel installation process can be performed in the field with simple temporary spacers between panels as they are installed, and this condition is easily inspectable for quality control. The current C1280 language also alludes to building expansion joints (building construction joint) which perform a different function than sheathing control jointsthe proposed ballot item language addresses this with new language regarding expansion joint assemblies.
Keywordsexterior walls; gypsum; gypsum panel products; sheathing;
Draft Under Development