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Firestop exhaust collar in Garage (intended use)


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OK, this question isn't about this particular situation, but more about the use of this collar in relationship to drywall in a garage:

Does anyone have a clear understanding of the intended use of this collar?

Is the drywall only supposed to go over the collar, but stay back from the vent pipe?


Is the drywall supposed to go over the collar and seal tightly against the pipe?

Most of the time I see it back from the pipe, and assume that's why the collar is so wide - to permit the drywall to stay back from the pipe and yet provide a seal. But, typically, the drywall doesn't end up laying nicely over the collar - leaving gaps which could permit flames a path.

Opinions?... or better yet, specs?

PS. If you have the proper name of this collar, I'll modify the Subject so the thread can be properly searched in the future.

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Depends on the type of collar.

If it's through a ceiling, I call them a "firestop/ceiling support"; it's a simple piece of metal that provides the minimum 1" clearance around the B vent.

If it's through a wall, they're usually called thimbles.

If there's firestopping and minimum clearance, that should work, but it's ugly.

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It is a thimble. I have never heard of it refered to differently depending on location, but you know how phrases change around the country.

The photo you provided shows the thimble set too far back from the surface of the drywall. The intent of the thimble is to provide the one-inch separation to framing and drywall (an air gap if you will), while still providing a block for flame and smoke spread. Usually there is some tape and mud repair around the thimble to provide a consistent joint around the annulus. Many jurisdictions also require intumescent sealant at the joint between the thimble and drywall.

Download Attachment: icon_adobe.gif Simpson Dura Vent 1998.pdf

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Simpson describes it as firestopping in their document.

They also describe through wall as a thimble. Thimbles normally do not provide firestopping; they're often vented to prevent heat build up in the wall.

Either way, doesn't matter. The idea is firestopping and minimum clearances at the ceiling.

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One thing I find a little bit humorous, is that the drywall (considered to be a firestop in this application) needs to be held back an inch from the outside surface of B Vent... Huh?

That's a part of why I teed up the question - it seems that it shouldn't matter if the drywall is snugged up against the B vent.

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This should clear it up for ya

Download Attachment: icon_adobe.gif b_90_3.pdf

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Yeah, thanks Darren.

Oddly enough, during my time in disaster restoration, I never really saw anything that resembled the surface of drywall actually burning. Many times I worked on projects in which rooms got unbelievably hot. Yet, the drywall came through it, OK.

Due to convection, after these fires, you could see where every stud and nail was behind the drywall, as if you had xray vision. The latent heat in the framing kept the surface of the drywall warmer longer where the studs were, which permitted more soot to be deposited to those surfaces.

Bottom line: drywall actually fares amazingly well in a fire - even one so hot that if you stood up in the room you'd fry. It actually does take a TON of heat before it finally gives up the ghost.

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Mike, I think the concern with the paper is from pyrolysis. I've seen plenty of crispy drywall paper.

Oh, yes, we're in agreement, it will eventually burn, and when it goes, there doesn't seem to be any middle ground. It just falls apart. But, often when I used to do take-offs on these fires, the firefighters were still hanging and would tell me some of the fires I was looking at were so hot that just standing up, in such a room, would instantly melt the skin right off of you and sear your lungs - pretty much instant death. Yet, the drywall did pretty good.

I had never honestly realized the importance of staying low in a fire - alwasy assuming it had more to do with breathable air - not understanding the extreme stratification going on.

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