There are lots and lots of how-to's out there on how to put rigid foam against a concrete basement wall before putting up 2 X 4 framing. Seal the joints, etc…doesn't seem to complicated. On to framing…it also seems easy (and stupid not to) to get the required fire blocking at the top…below your joists…in the vertical direction. What I'm really scratching my head over, and I haven't see ONE SINGLE EXAMPLE of how to do this correctly in all of the so-called "Internet How-to's"…is how do you achieve horizontal fire blocking every 10 feet when you've got rigid foam between the concrete and the framing? Picture this…concrete walls covered with rigid foam floor to top of concrete wall. Now throw up your framing. Let's say I'm using 1" foam (southern climate zone, above grade). As I interpret the rule, you need a framing member DIRECTLY against the concrete…so do I rip a 2 X 6 to 4.5"…carve out 1.5" wide of foam at that spot? Doesn't make sense…not supposed to put lumber against concrete, and I'm pretty sure you can't have PT every 10 ft in an interior space. Yes, I did check with the AHJ, and the answer was "Yes, you need fire blocking and no, rigid foam or fire-retardant great stuff does not qualify" which makes sense…that stuff melts if your hot breath gets on it. Would would help me get my head around this would be a example photo..which I've spent hours searching for.
Basement – Horizontal fire blocking when using rigid foam in basement
basementfire-hazardframinginsulation
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Best Answer
You don't need to interrupt the foam insulation with fireblocking, but if there's a gap between the foam insulation and your framed wall, then you'll need fireblocking across the gap every 10 feet.
IBC 2015, 718.2:
While 718.2.2 designates a "concealed wall space" as requiring fireblocking, 718.2 (quoted above) prescribes the nature of this fireblocking: It interrupts "concealed draft openings." Building your framed wall tight to the foam will avoid any draft openings, but if you've got a gap between the framed wall and your foam, then that's a draft opening that will need a fireblock every 10 feet.