• Welcome to The Building Code Forum

    Your premier resource for building code knowledge.

    This forum remains free to the public thanks to the generous support of our Sawhorse Members and Corporate Sponsors. Their contributions help keep this community thriving and accessible.

    Want enhanced access to expert discussions and exclusive features? Learn more about the benefits here.

    Ready to upgrade? Log in and upgrade now.

UL Rated Assembly vs Non-Combustible or Limited Combustible

F

faviolosky

REGISTERED
Joined
Nov 8, 2022
Messages
3
Location
New Orleans
Hello All. first post here and im hoping you can help shed some light on this issue. We have a kitchen grease duct exhaust that we're in a bit of a pickle with. IBC called out for it to be in a 2hr shaft enclosure and so we provided a UL U301 assembly with 2 layers of gyp either side and wood framing. We were hoping to achieve the 6" clearance around the duct. Somewhere along the lines, our mechanical engineer did not call out for this duct to be fyre wrapped thinking, well we have a 2hr rated enclosure. But the issue came out that the SFM reviewed the kitchen hood/duct and the shaft wall is not approved. IMC calls out for a "non-combustible" enclosure.
Everything we're finding is saying that because of the wood stud, this is a combustible assembly. Now our mechanical engineer is saying that we'd only need the assembly to be "limited combustible" to avoid wrapping the duct and keep the 6" clearance, but the only thing we can find is that the gypsum itself is limited combustible, but we don't know if that is enough to make the assembly "limited-combustible". Any thoughts?

Context, the duct is in place already, the walls have been built and the other alternative is to wrap in place, or remove the duct (wrap) and put it back. The building is a wood framed building.

Thank you all in advance.
 
Sounds like you have to wrap. The clearance to combustible framed shaft walls is 18", 6" is for non-combustible framed.
Depends on the local code but that is from the model IMC. (506.3.11 2018 IMC)
 
2018 IMC definition of a combustible assembly: "Wall, floor, ceiling or other assembly constructed of one or more component materials that are
not defined as noncombustible." Wood is a combustible material, and so long as the shaft is constructed with wood framing then the GWB isn't going to bring you into compliance with 208 IMC 506.3.11.1.

My local code even includes the following addition: "Interpretation: Gypsum wallboard installed on a combustible substrate or on wood studs does not cause the wall to be considered as a noncombustible assembly, and the 18 inch minimum clearance still applies. The classification of combustible and noncombustible materials is not changed by the use of fire-retardant-treated wood products or fire rated (Type “X”) gypsum wallboard."

I agree with @redeyedfly that you will likely need to wrap the duct.
 
Yeah. We are going to wrap the duct. I think we were thrown on a rabbit chase when the "limited-combustible" was thrown into the mix.
 
You must log in or register to reply here.
Back
Top