Basement fireblocking along joists parallel to wall

[dsmith060]

I'm trying to plan ahead and prepare my vertical fireblocking before attaching my top plate as I frame my basement walls. Basically ripping a sheet of plywood into strips that will butt up to the sill plate and nail into the joists above, then I can just nail my top plate through the fireblock. I've read that this approach is much easier than trying to double back and fire block after the walls are up.

So for the walls in which the floor joists above run perpendicular to the wall, this is straightforward enough as I'm essentially "extending" the rim joist space. However, when I try to take the same approach along the walls where the joists run parallel, the nearest joist to the wall lines up exactly along my chalk line. So in this case I'm closing in my rim joist space for good.

I'm wondering if, A. this is still a sound approach for these parallel-to-joist walls, and B. if it is, is there anything more I should do as far as insulation goes before closing in this space?

I've included a couple images to help illustrate. Thanks in advance for any help!

 

[Breplum]

3/4" plywood as per your illustration, I don't think is qualified as fire block.
I'm pretty sure it has to be solid lumber 2x (nominal 1-1/2").

 

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[wwhitney]

3/4" plywood is allowed under the IRC as a fireblock, with the joints backed by a second layer of 3/4" plywood (splice plate):

https://up.codes/viewer/michigan/mi-residential-code-2015/chapter/3/building-planning#R302.11

Also looks like the mineral wool batts alone may be sufficient, if you fill the entire rim joist.

Are you planning to finish the basement ceiling, or leave it open?

Cheers, Wayne

 

[dsmith060]

Thanks for the feedback! Yup, I confirmed 3/4" plywood can be used in my township, so no problem there. I hadn't really given mineral wool a lot of thought; plywood seemed to be the most cost effective option (one sheet gets me 48' of fireblock), but that might be the way to go, at least for the those sections of wall running parallel to the joists.

I'm thinking we'll have to finish the ceiling, more for sound-proofing purposes than anything else. We'd prefer to leave it open, honestly, but we're in a ranch with two young kids right upstairs so I think closing it up is the best bet. (Planning on using the "soundproof" roxul and drywall for that.) Does drywalling the ceiling vs. leaving it open change the thinking on just using mineral wool batts as fireblock in those sections?

 

[dsmith060]

On a semi-related note, since this section runs into a corner... any bright ideas on better insulating this space? I'm not sure what use the joist on the 45-degree angle serves, but each corner of the basement is done this way with regular fiberglass batts shoved in there.

It's too small of a space to get in there with any rigid foam, but I want to at least remove the fiberglass batts and replace with mineral wool for better moisture resistance. Other than just going to town with a can of spray foam are there any other good options?

 

[wwhitney]

Does drywalling the ceiling vs. leaving it open change the thinking on just using mineral wool batts as fireblock in those sections?

Mostly I was thinking that if you are closing off all the other joist bays with drywall, then it's not worth thinking about whether there would be a way to avoid closing off the rim joist bay.

If you are drywalling the ceiling for sound purposes, consider using resilient channel or drywall with an integral visco-elastic damping layer.

On a semi-related note, since this section runs into a corner... any bright ideas on better insulating this space?

So the right side is a common joist, the left side is a perpendicular block, and the middle is a block at a 45 degree angle? Does the angled block have beveled ends to meet the framing tightl? I would say the diagonal block is not that useful, definitely not if there are gaps at the ends.

Probably not worth removing, so I can't think of anything better than stuffing the triangular spaces very well. For a fixed volume, the denser you stuff in batt insulation, the (slightly) better the R-value. Not sure about sound blocking, though

Cheers, Wayne