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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.

  • I have this exact same question. I'm floored nobody answered it. It seems to be something every basement finishing DIYer should be learning about, but the answer is nowhere to be found. This must be the best kept secret in the industry. Since you had this issue in October 2016, I hope you eventually found out the answer. If you have time, please write back what your solution was. Thanks. – Lakey Aug 1 '17 at 0:43
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Yes “fireblocking” is required in COMBUSTIBLE construction. It’s required: 1) at 10’ intervals (both vertically and horizontally) in concealed spaces, 2) between floor lines, and 3) between top flood and attics. (See ICC 718.2.)

Fireblocking can be: 1) 2” (nominal) thick lumber, 2) two layers of 1” (nominal) lumber with laps staggered, 3) 3/4” plywood, 4) 3/4” particle board, 5) 1/2” gypsum board, 6) 1/4” cement based millboard, 7) batts or blankets of mineral wool insulation as long as it’s securely fastened to stay in place, or 8) cellulose insulation. (See ICC 718.2.1.)

The ICC 718.2.1.2 allows fiberglass insulation too, as long as it completely fills all the voids and is unfaced.

If you use wood (lumber, plywood or particleboard) it must be pressure treated or naturally durable wood if it touches concrete or masonry, with some exceptions, see below.


Actually, the common notion that ALL wood must be treated if it’s in contact with concrete or masonry is incorrect. I did some checking and found the following:

The ICC Code Section 2304.11 Protection against Decay and Termites covers this requirement. What the Code says is that protection against decay and termites, “shall be by naturally durable or preservative-treated wood.” (See ICC 2304.11.1.) I’m not sure what “naturally durable” wood is...and I’m sure it’s NOT cedar or redwood, which is probably the most durable WOOD for exterior use, where I live. (Please don’t lecture me on Hardie lap siding, etc. It’s not wood...it’s composite wood”.)

So, in your case, “naturally durable wood or preservative-treated wood” is only required where framing and furring strips are attached directly on the interior of exterior concrete or masonry foundation walls below grade. (See ICC 2304.11.2.3.)

This is extremely interesting because it says “ATTACHED” and it says, “interior side of EXTERIOR concrete or masonry walls BELOW grade.” My understanding of this is your fireblocking would NOT need to be preservative-treated if it’s 1) not attached, or 2) not on an exterior wall, or 3) it’s not below the exterior grade. I learned something today.

  • IBC 718.2 states that "fireblocking shall be installed to cut off concealed draft openings." Rigid insulation built tight to a foundation wall exposes no draft opening, so no fireblocking is necessary. If you read 718.2.2 to 718.2.7, you'll notice that every scenario involves potential air void that drafts can traverse without interruption. – popham Oct 19 '18 at 18:17
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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:

In combustible construction, fireblocking shall be installed to cut off concealed draft openings (both vertical and horizontal) and shall form an effective barrier between floors, between a top story and a roof or attic space. Fireblocking shall be installed in the locations specified in Sections 718.2.2 through 718.2.7.

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.

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It seems to depend on your building department's interpretation of whether foam is a "concealed space" or not.

Here is another discussion about this exact question: http://www.greenbuildingadvisor.com/community/forum/general-questions/39458/basement-wall-fire-blocking-material

  • Could you provide an expanded summary of the linked discussion? Links rot, sometimes faster than wet particleboard... – ThreePhaseEel Aug 11 '17 at 23:22
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The link posted doesn't answer the dilemma: non-treated wood against concrete. What I ended up using was Hardi-trim (1/2) thick, which matched the thickness of the XPS I was using, then a stud in front of the Hardi.

  • So to close the circle on this, the inspector asked for, but did not want to reinspect, that I use the Fire Block Sealant where the XPS, Hardy, and 2X4 met. The explanation was to restrict air (gas) flow between horizontal spaces. Since I had a case of the stuff to seal the wiring and plumbing penetrations, this was no problem. A picture would explain this better, but what I did was essentially the same as caulking siding where it meets trim. Yes, 50$ worth of sealant, but compliant. If I'm still asleep by the time the XPS behind roxul insulation ignites, I'm not waking up....ever...... – tpcolson Jun 8 '18 at 20:16
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In the following video, it is implied his local building code requires fire blocking to go all the way to the concrete. He places a vertical piece of treated plywood between every other sheet of XPS (so every 8 ft.) and seals it with fire block foam sealant.

https://www.youtube.com/watch?v=8-Ec_atkhQs

This is the only video I've found that does this. Everything else I've seen covers the entire wall with XPS, as you suggested. I'm waiting to hear from my local building department to find out if this is required for me or not.

  • What did your building department say? – mackstann Jun 21 at 22:52

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