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Is Loctite Pro spray foam truly “fire rated” for outlet boxes?

michaelbluejay | Posted in Green Products and Materials on

I’m sorry if I’m asking you to bite the hand that feeds you, but I just got email marketing from GBA about Loctite Gaps & Cracks Foam which claims to be “fire rated”.  However:

(1) Clicking through on the ad to the Home Depot listing, their marketing graphic which lists the applicable uses conspicuously avoids mentioning “inside electrical outlet boxes”.

(2) An article on Life Safety Services (which I can’t link to because I know from experience will cause my post to not go through) says “[F]ireblock foam found in hardware stores is actually flammable and does little to prevent the spread of a fire and smoke,” and that products should pass unmodified ASTM 814, though the Home Depot listing says the product passes ASTM 84 (which doesn’t seem to be a typo, 84 is a different kind of fire test).

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  1. Chris_in_NC | | #1

    What is the intended application for spray foam inside electrical outlet boxes? Wouldn't sealing be done on the outside of the box?

    1. michaelbluejay | | #3

      Let's say the drywall is already installed and you want to air-seal the wire penetrations into the box.

  2. Expert Member
    BILL WICHERS | | #2

    Electric code precludes filling the boxes, so it doesn't matter if a product is "fire rated" or not. Regardless of the rating of the material, you can't use it to fill up the empty space in an electric box.

    What I typically recommend to air seal holes in an electrical box is the red silicone high-temperature caulk (NOT intumescent fire stop caulk!). but even then, I only recommend it to smear into holes NOT to fill up the inside of the box!


    1. michaelbluejay | | #6

      Thank you. High-temp caulk had been my conclusion but when I got the GBA ad about the "fire rated" Loctite foam I thought I'd ask about it.

      I was going to ask you why you recommended against intumescent caulk, but I found another one of your posts which says it's not as high-temp as proper high-temp caulk.

      1. Chris_in_NC | | #7

        Intumescent is meant to expand when it gets hot. There are non-intumescent firestop (or fireblock? I can never remember the nuances) caulks as well; I think 3M makes a gray one. Not sure if that's needed just to seal box penetrations though.

      2. Expert Member
        BILL WICHERS | | #8

        Chris is right, intumescent fire stop caulk is intended to expand when heated, so that it will make a better air seal during a fire. This type of caulk is used to help limit the spread of smoke and fire in commercial buildings where wires and pipes go through fire rated wall (and floor/ceiling) assemblies. Fire codes are much more stringent in commercial buildings than they are in residential structures.

        In an electrical box, you don’t want expansion, you just want a sealant that won’t get damaged by a hot wire. This is a different application from fire stop caulk. I use a red silicone product 3M makes for use around hot things, and it’s rated up to something like 500-600+ degrees F (I forget the exact rating). It’s a good electrical insulator and it can handle high temperatures, so it works for this application. I still consider it best practice to seal the exterior part of the box and not the interior though.

        Note that if you use the white fiberglass “hard boxes”, you have a lot fewer holes to worry about when air sealing. The fiberglass boxes don’t have lots of open holes the way the metal and plastic boxes do. They’re sturdier than the plastic boxes too.


  3. monkeyman9 | | #4

    My recommendation is to use silicone if you are sealing from the inside of the electrical box. When I have a wall cavity open, I have used spray foam on the back side. But now I'm questioning that a little.

    So I ran a test. I took glob of normal great stuff, the glob of fire-rated great stuff, and a glob of the two part class A fire rated foam I sprayed in my attic on the ductwork. Night and day difference. I couldn't tell any difference in how the fire rated or normal great stuff burned. It just burned. Looking at rhe MSDS It may just be coloring that's different. The class A two-part closed cell would smolder a little and go out. I kept trying to hold the blow torch to it to keep it going.

    All I can tell you is if you have a fire, the great stuff is going to burn until it's gone. The other stuff would have to be pretty hot to keep going or it's going to just go out.

    1. michaelbluejay | | #5

      "Fireblock" foam isn't even supposed to resist fire, or burning. The code calls for "fire blocking", which is drywall or wood to block air flow which facilitates fires, and "fireblock" foam is for sealing that kind of blocking.

      I found one spray foam that *does* resist burning, but it's not a canned product, and you probably have to order an industrial amount. They did a video of a test like yours, where they showed that the "fire block" foam burns just as readily as regular foam:

      The Loctite product I asked about in the ad is "fire RATED", not "fire block", so I thought it might have a higher burn point. The spec we're looking for is apparently ASTM 814E (not "MODIFIED ASTM 814E"), which makes sure they won't ignite at 240°F, before the studs will.

      The manufacturers are cagey about the anti-burn properties of their products; they don't really want us to understand what their products are good for and not good for.

      1. monkeyman9 | | #9

        I may jusy have to get a can and test jt out.

  4. Expert Member
    Deleted | | #10


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