Question: Is it acceptable to use interior paper-faced gypsum board in exterior applications if it is covered with a weather-resistant barrier, such as housewrap?

Answer: No. According to the general explanatory notes found in the GA-600 Fire Resistance and Sound Control Design Manual, labeling a gypsum system as an “interior partition” for example does not automatically limit it to interior use. The interior partition designation primarily refers to the context in which the system was originally tested or is most commonly used and not a strict limitation of its use.

A good example of this is UL Design U305. Though technically categorized as an interior partition, variations of this system are widely and successfully used in exterior wall assemblies. In fact, many homes incorporate U305-based assemblies as their exterior walls.

That said, adapting an “interior” system for exterior use requires careful material substitution to maintain code compliance and performance integrity. In particular, the type of gypsum panel used on the exterior side is critical. Standard paper-faced interior drywall is not suitable for exterior conditions. Therefore, in exterior applications, the paper-faced gypsum board must be replaced with a code-compliant, exterior-rated gypsum sheathing such as glass mat-faced panels.

For guidance on which gypsum products are approved for use in UL-classified systems (including when adapting systems for exterior exposure), professionals should consult GA-605-2024 Gypsum Panel Products for Use in UL Classified Systems. This document provides a comprehensive list of gypsum panels that have been tested and approved for various applications within UL assemblies, ensuring both fire resistance and durability.

Therefore, while a system may be categorized for interior use and can be adapted for exterior applications, the appropriate materials must be substituted so that all code and performance requirements are met. Proper product selection, particularly regarding exterior sheathing, is essential to ensure the system performs as intended and remains compliant with building codes and UL classifications.

For more specific requirements, referencing GA-600-2024 or GA-605-2024 directly is best practice. The earlier version, GA-600-2021, and GA-605-2024 are available to download free of charge from the Gypsum Association’s bookstore.

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