Fireblocking for basement walls

[john6221]

So I thought I understood fireblocking but I've managed to confuse myself.

I'm finishing a space in my basement. I've used 2" foam against the concrete block wall. I've also insulated the rim joist with foam, and spray-foamed the sill plate area/top of block.

My question is, how do I frame a wall against rigid foam and have proper fireblocking?

What confuses me is whether there has to be a block "behind" the foam, contacting the concrete block.

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[paddedhat]

Well, I guess if you really want to do it to the strictest of fire code standards, you could install a 3/4" or better wood "cap" on the top of the stud wall plate extending to the masonry. Other options would be a wider top plate. Neither of these options is practical at this point. You could build the stud wall and add solid blocks vertically above the top plate.  This would typically be short (14-1/2") pieces of the same material as the floors joist, typically 2x10, toenailed in place.  The object is to get a tight fit on the fireblocks, if possible.  If not, larger gaps could be filled with the orange fire block spray foam, smaller ones can be caulked with fire rated caulk, which is typically a red, gritty, fire rated material that is available in standard tubes at the bog box stores.  Or, like 99% of installations like this, you could just ignore the issue. I'm not advocating that you do so, but it's a detail that is typically ignored, even by a lot of inspectors.

[john6221]

Thanks paddedhat. I guess what was confusing me was whether I had to break the continuous layer of foam. I have a continuous layer starting with XPS on the rim joist, then spray foam over the mudsill/gasket/top of block, then XPS on the wall. I couldn't understand if the fireblocking had to break this continuous layer and "cap" the XPS foam on the wall by touching the concrete.

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[Midwest]

I did the same thing you did and my inspector didn't bring it up (and neither did I). 

[john6221]

I did the same thing you did and my inspector didn't bring it up (and neither did I).

Great--that's good to know.

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[john6221]

Follow up for those interested: my building inspector came out to chat. He said that as long as I use regular fireblocking techniques I don't have to cut through the foam. He said that even though the foam isn't fireproof, it does count as enclosing the vertical space separately from the horizontal space above.

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[paddedhat]

Follow up for those interested: my building inspector came out to chat. He said that as long as I use regular fireblocking techniques I don't have to cut through the foam. He said that even though the foam isn't fireproof, it does count as enclosing the vertical space separately from the horizontal space above.

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Makes sense, since you are essentially extending the top plate of the wall with wood blocking, so there is no code issue, the wall is viewed as extending to the subfloor above.  As a general note, some older houses were "ballooned framed" where the stud walls sat on the top of the foundation plate, and extended 18' to the top plate of the second story wall. They would often have zero fireblocking  at all, so it was possible to drop a weight, on a string, from a hole cut in the top of the second floor outside wall straight down to the basement. These places can be an extreme fire hazard, and even more so, if you would have a flammable foam board on the interior of the basement walls, with no proper firestop at the top.