One of the questions on the PDD practice exam - wall type (sorry for posting again!)
One of the questions on the PDD practice exam is unclear to me.
The scenario involved Type II-B construction and bearing walls which should be constructed of noncombustible materials. First of all, I selected metal stud walls (Walls C or D).
My questions are as follows:
- Between Wall C and Wall D, what makes Wall C the correct choice?
What specific detail or assembly component qualifies Wall C as compliant with Type II-B construction? - In Wall C, gypsum sheathing is part of the bearing wall assembly. But as far as I know, gypsum is not noncombustible material. I wonder if gypsum sheathing is acceptable to be Type II-B?
- Assuming the construction type were upgraded to Type II-A, which requires a 1-hour fire rating for bearing walls, how would the wall be detailed? I guess
Type ‘X’ gypsum board could be placed on both inboard and outboard of a metal stud wall to meet the 1-hour rating. Please correct me if I am wrong.
Thank you!
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I assume this is the question and relevant exhibit from the mock exam:
1. Wall C is compliant as it includes non-combustible material (gyp sheathing vs plywood) and includes rigid insulation to maintain code requirements for metal stud walls
2. Gypsum is non-combustible (USG article, section on Fire Safety here)
3. UL has a wall assembly search page to browse through different wall types, assemblies, etc. that you can browse and become more familiar with assemblies overall for different fire resistance requirements and materials
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Also, I wander if it is a good idea to place the Weather Barrier to the outside of the Rigid Insulation? I would buy it, if it is a rainscreen product! Even if it is a rain screen materials, you would need a substrate in front of Rigid Insulation to fasten to it, such as plywood, DenseGlass, ...etc.
Another question, does the insulation in the rain screen assembly need to breathe?
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@abruno168 – Thank you very much for the detailed explanation. Wall D is indeed non-compliant, as both the wood studs and plywood sheathing are combustible. It's also helpful to know that rigid insulation is required for metal stud walls in accordance with code.
@hussein1986 – Good point. I’ve seen several details—where an air barrier or weather barrier is placed between the sheathing and the rigid insulation, especially when there's a rainscreen cavity.
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