OSB on soffits

NE78FJ40

Does it matter which side of the soffit, outside or duct side, that you install the OSB on? I've seen pictures with it installed on both sides. What side should it be installed on to pass fireblocking requirements?

Thanks

ccarlisle

Looks like I'm only of the thirty others who don't really understand your question and so can't really post a reply...Pics!

HooKooDooKu

As a guess... I think the OP is trying to build a soffit with 2x lumber and OSB sheets. the question is whether the OSB sheets should be on the same side of the 2x lumber as the ducts or on the room side of the 2x lumber.


As for an answer to this assumed question, every picture I've ever seen has the OSB sheeting on the outside, not the duct side. The whole purpose of this type of soffit is to hide the duct work, not protect it from fire.

Based on my understanding of fire block, I think you might be required to do something every 10' within the soffit to retard the spread of a fire. But that can be done by stuffing fiborglass insulation inside the soffit between the OSB and ductwork.

ccarlisle

Oh...so you're in the basement then? We have a different name for what you are trying to hide.

Most of us, I think, were on the roof.

Language difference, I reckon.

AndrewF

I was thinking roof too. If you are building a sofit in the basement, why not use fire rated sheetrock?

__________________
-Andrew
DIY hobbiest

HooKooDooKu

I think the idea here is that you have to structurally build the soffit first, then sheet rock over it. The issue of fireblocking (again, I think) is coming from the idea that a soffit is going to create a conceled space.

Basically, one of the ideas on how to build a soffit is to attach 2x lumber on the ceiling on either side of the duct work. Then hang OSB from the lumber to form the sides of the soffit. Next, run 2x lunber under the ducts between the pieces of OSB. The OSB makes for simple walls, and all the 2x lumber gives you something to attach drywall to.

The OPs question appears to be asking if the OSB should be on the "outside" of the 2x framing or the inside of the 2x framing. Because I've always seen it on the outside, I never considered doing on the inside. However, I can see where doing in on the inside means you could have the OSB up against the air duct so that you avoid creating "concealed space" to fire block, and you still have the 2x lumber to hang drywall from.

NE78FJ40

Sorry I wasn't more clear in my explanation. I was in a hurry.

This is for the soffits around the ducting in a basement.

Does the OSB installed on the soffits have anything to do with fireblocking?

I framed the soffit with all 2X4's and wasn't sure if I still needed to install the OSB on it.

HooKooDooKu

I've only read about using OSB in a soffit for constructing the soffit.

Otherwise, as I understand it, you don't have to do any fire blocking because the duct work itself should already be insulated, and the soffit shouldn't be creating a continuous air space that could allow fire to spread (assuming the 2x4 lumber is already against the duct work rather than creating a tunnel around the ducts).

ccarlisle

Why I got confused - and I may speak for others - is that up here, our 'soffits' relate to roof overhangs, and we call what you call a basement "soffit" something like a 'bulkhead', or something like that. Just couldn't figure out what a duct was doing in the roof. It is odd, too that 30 visits prompted no replies, so I figured something wasn't right.

But, I learned something, having gone to look up 'soffit' in the dictionary