December 2, 2019
Q: Can you explain the concept behind the wall assemblies described by UL Fire Resistance Wall Design Nos. U336, U347, U366, U373, U375 and U388?
A: This series of designs is intended for use as townhouse or duplex separations as required by Sections R302.2.2 and R302.3 of the 2018 International Residential Code (IRC), and as a fire wall when required by Section 706 of the 2018 International Building Code (IBC).
These designs consist of two basic sections; an area separation wall, and a protected wall on one or both sides of the area separation wall. The area separation wall consists of two layers of 1 in. thick gypsum liner boards, secured through the use of floor, ceiling and intermediate horizontal C channels, and vertical H channels. This section of the wall provides the majority of the fire performance of the wall assembly. However, by itself, the area separation wall alone does not meet the heat transmission protection requirements of UL 263/ASTM E119 (test by which fire-resistance ratings are developed). The protected wall(s), consisting of wood or steel framing and one layer of gypsum board, provide structural integrity for the area separation wall and provides additional fire protection, allowing the overall assembly to meet the heat transmission requirements of the standard. In most applications the protected walls are constructed in a manner representative of non-fire-resistance-rated construction
Q: Are these wall assemblies rated for fire exposure from either side?
A: The UL Designs covering these wall assemblies typically include two configurations; a symmetrical configuration and a non-symmetrical configuration. When constructed in the non-symmetrical configuration, the assembly is rated for fire exposure from the area separation wall side only. Should a fire develop on the area separation wall side, the area separation wall resists the passage of fire while the protected wall provides the structural integrity and an additional level of resistance to heat transfer. When constructed in the symmetrical configuration, the wall assembly is rated for fire exposure from either side. Should a fire develop on either side, the protected wall, being non-rated, will fail leaving the area separation wall exposed. The combination of the area separation wall and the protected wall on the opposite side will meet the requirements for a 2 hour fire-resistance rating.
Q: Do these wall assemblies allow the use of plywood, OSB or similar facing?
A: Each of these area separation walls include various optional construction methods. These options include:
- Using plywood, oriented strand board (OSB), or wall and partition facings and accessories on the studs of the protected walls in lieu of gypsum board
- Using insulation in the stud cavities in lieu of gypsum board on the studs of the protected walls
- Using 6-inch gypsum board batten strips over the framing on the area separation wall in lieu of gypsum board on the studs of the protected walls
- Using plywood sheathing or OSB applied directly to the areas separation wall in the non-symmetrical configurations.
This series of designs has become a common method of meeting the IRC requirements for a duplex or townhouse separation or for a fire wall separation required by the IBC. There designs can be located using UL Product iQTM.
For more information on area separation walls, visit our Architect Services page.