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Community and Q&A

Fire Barrier

itserich | Posted in Building Code Questions on

I am installing rigid foam and am considering fire barriers.

Based on this chart, it appears the following are currently accepted fire barriers in occupied space:

Wood Panel / Particle Board / Plywood – 15/32″
Drywall – 1/2″
Steel Base Metal – .016 (?)
Aluminum Base Metal – .032 (?)
Mineral Fiber Insulation – 2″

Thermax insulation is also apparently an approved barrier in attics and crawl spaces, but not occupied spaces, so perhaps it is not as good as the other options.

Two questions. First, is there an overview of the barriers, as to what works best at preventing the spread of fire? Are there any other options?

Second, is steel or aluminum used indoor?This will be a large area, not a small area such as only the rim joists. Drywall is heavy, don’t know the weight of the wood.


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  1. GBA Editor
    Martin Holladay | | #1

    Most building codes, including the International Residential Code (IRC), require spray foam insulation (including foam installed on the interior of basement walls) to be covered with a 15-minute thermal barrier. However, if the spray foam is located in a crawlspace or an attic “where entry is made only for service of utilities,” the code permits the installation of a less stringent covering: an ignition barrier rather than a 15-minute thermal barrier. In sections R314.5.3 and R314.5.4, the IRC defines an ignition barrier as one of six permissible materials: 1 ½-inch-thick mineral fiber insulation; ¼-inch-thick wood structural panels (e.g., plywood); 3/8-inch particleboard; ¼-inch-thick hardboard; 3/8-inch-thick gypsum board; or corrosion-resistant steel having a base metal thickness of 0.016 inch. Presumably, code officials also permit the installation of thicker versions of any of the six listed materials.

  2. Expert Member
    Dana Dorsett | | #2

    " First, is there an overview of the barriers, as to what works best at preventing the spread of fire?"

    In rough order:

    Wood will eventually ignite and sustain flame on it's own

    Gypsum with paper facers can spread fire once the paper lights off, but has low fuel content on it's own.

    Aluminum & steel have fairly high ignition temperatures so your house is already toast if they light off.

    Rock wool will still be intact long after the house has gone up in smoke- it won't melt fall apart or burn even at VERY high temperatures (temperatures well above which fiberglass would melt.)

    I'm surprised that a mere 2" of rock wool qualifies though- I had always remembered 3".

  3. itserich | | #3

    Thank you Dana and Martin.

    Metal is epxensive and hard to work with (I think). Wool sounds like a great barrier but not rigid enough to leave exposed on the interior.

    So will probably choose between Drywall Type X or wood. I have not worked with drywall but it seems to be messy.

  4. Ryan_SLC | | #4

    I know this is a very old post, but it is the #1 google search if you look up "fire thermal barrier hardboard." I was searching hardboard as being a thermal barrier option but gave up after no seeing how to fire assembly protect seams without studs. This was after trying to track down a paper less drywall to find out that cement board is not a replacement because cement board transmits heat so great it passes foam heat rating. I gave up on this too. intumescent paint is mentioned a lot, but from what I can find, most if not all is not spec'd to rigid foam, only spray foam. Paint is out.

    To update the item on mineral wool talk in this thread, I think many have since found it's 1.5" in code for crawl space in the US over foam (both spray or rigid have E84 issues for smoke and flame spread) which is both the requirements for ignition barrier OR thermal barrier.

    If you go to Rockwool's Techinical Bulletin from 2018, it shows that their different products have different thermal barrier fire ratings.
    -Batt (friction fit between studs) is minimum 3". This is probably why "Safe 'n' Sound" and others like it (JM has a safe and sound version) are only 3". Though the less than standard 3.5" is also acoustical purpose too. But it has to be 3" to be part of the "Safe" part.
    -"Comfortboard" 80 is at 1.5" in crawl spaces in the US but only 2" everywhere else (less thick than batt).

    I assume Comforboard lower thickness to meet a fire thermal barrier is because Comfortboard being greater density. My local inspector would not allow batt/AFB to be blanketed without studs and then studs needs drywall for fire assembly. But ComforBoard is assembly for direct attachment without studs, so that was okay. Great. Now I don't need studs as Comfortboard install is detailed to direct contact to wall, though I have rigid foam on the wall. Point being, studs are not part are the fire assembly for Comfortboard.

    Interestingly, 1/2" and 1" Comforboard are a product but the lower fire ignition barrier requirement starts at 1.5" and is also the thermal barrier minim in crawl space. There is something to that--it's odd that 1" isn't ignition barrier but 1.5" is both ignition and thermal.

    What I have found as a score for safety, costs, and R value for others looking to protect their crawl space rigid foam is that 1" #8 density (better than #6) Comfortboard is very cheap (like $8 a board vs $17 for 1.5"). When using two layers, you reach the 2" required outside of crawl space fire thermal barrier thickness, but it is still cheaper than the 1.5" or 2" Comfortboard options. Stacking is in Rockwool literature, so it can be pointed to as correct to a curious inspector.

    So stacking 1" Rockwool 80 is the least cost option in mineral wool and gives you above code fire assembly ignition and thermal barrier in US crawl space code, 1/2 greater than minimum, and a slight bump in R value. And you are at standard for fire thermal barrier in living space areas. Win, win, win.

    This seems like possibly the best solution for many circumstances for those covering their rigid foam assuming low humidity in your crawl space. Standard fire protection of non crawl space ratings, additional 8 R value, and keeps drywall/stud walls out of the crawl space that I am finding has difficulties detailing to true fire assembly on rim joists and bottom plate.

    Take care

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