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Safe to use caulk to air seal around wires coming into the outlet box through knockout holes?

GuyintheSouth | Posted in General Questions on


I have very drafty electrical outlet boxes and switches that I have started to seal. 
I have used great stuff (blue can less expanding) poly foam around the box where it meets the drywall and then trimmed the excess to make it as flush as possible. 
Also, I used the straw that comes with the foam to reach the back of the outlet box and sprayed some foam by placing the straw a in between the electrical wire and the knockout hole so it fills on the outside of the box. Then I clean and trimmed any excess foam that inevitably expanded inside the box so that there is no foam inside the electrical box. 
Since spraying behind the wires through the knockout holes still left gaps there, I squeezed some DAP dynaflex 230 to completely surround where the wires come in through the knockout holes. 
Basically I ended up with sealed outlet boxes, but I am a little worried that I might have not used the right products. 
Dap dynaflex 230 is not flammable according to DAP cust service and can be cleaned with water. It is not silicone but has silicone strength and it might not be latex either, but am I in the clear by just knowing it is not flammable?

This is the end result. Please let me know if this is ok. Thanks

Note: I know there is a fire resistant caulk I could have used, but I have already done about 5 outlets this way and will do the others with the fire resistant caulk, but in reality I want to know if using the DAP caulk is just fine and not to worry about. Thanks a lot for your insight.!

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  1. GBA Editor
    Martin Holladay | | #1

    The electrical code forbids you from filling the air in the box with spray foam. You've done a pretty good job of keeping the foam out of the box. As long as you don't let the foam encroach on the volume of the box, everything should be OK.

    Don't worry about the caulk.

  2. GuyintheSouth | | #2

    thank you for replying.

  3. Expert Member
    BILL WICHERS | | #3

    Having done lots and lots of work sealing wire and cable penetrations in a former life as a datacom cabling installer, I agree with Martin — you should be ok with what you’ve done. The only thing I’d do differently is to use red silicone fire-rated caulk (it’s sold for use on flue pipes and the like).

    If you have any boxes you can access from behind, you can use the orange fire stop spray foam. There is also a product out there that is sold as small square sheets of reddish clay-like material. It’s a putty that can be squished around a box from behind to seal the box and wall. The material is commonly used in fire rated wall assemblies and for soundproofing.

    The number one rule is to keep the material you use from getting into the box too much. Electrical boxes are rated for volume and that volume determines how many wires and splices can fit. You also don’t want the foam sealing up the electrical connectors inside the box.


  4. GuyintheSouth | | #4

    I hear you Zephyr7 and you know, I did buy 3m fire caulk that I tried using and it was a pain to use and squeeze in there from the from. There is no way to access these outlets from the back unfortunately.
    I made sure there is no silicone on the electrical connectors, only on the holes in the back. I did a couple with the 3 fire caulk though, but majority with either GE silicone and the DAP polymer caulk.
    Assuming that there was a spark or a fire at one outlet, would the silicone actually contribute to the fire? I did a small burn test on a squeezed bead of silicone and the dap polymer and they both burn, but you have to expose them to a direct flame like I did with a lighter. Is that what happens in an outlet that starts a fire? The heated wiring ignites the plastic sheeting on the cables and then there is flame?
    Thanks for any insight.

    1. Expert Member
      BILL WICHERS | | #5

      Which type of fire rated caulk did you use? There are two main types: intumescent, which expands when exposed to heat/fire. It’s usually only rated up to a few hundred degrees or so. Intumescent caulk is intended to keep fires from propagating by sealing any gaps when a fire gets going. The other kind of fire rated caulk is intended to stay put and NOT do anything when there is a fire. This type is more like regular silicone, but it’s rated up to 600+ degrees. It’s about $10-12 per tube. You want to be using the high-temperature kind for your application, not the intumescent kind.

      Silicone isn’t really a fire hazard. The insulation on wiring isn’t either, it’s supposed to be self extinguishing (that’s what the “vertical flame test” tests for). Electrical arcs are VERY hot though, and just about everything burns at those kinds of temperatures. Many plastics are not so much fire proof as they are fire resistant, and the self extinguishing properties are supposed to keep a fire from spreading. As soon as you take your flame away, or at least quickly after you remove the flame, the fire should go out on any wire insulation used inside walls. The wire should NOT support flame, it should self extinguish.

      If you’re really concerned about fires starting in faulty receptacles, here are some tips:
      Don’t use the cheapie receptacles in the box stores, the less than $1 kind. Use the “spec grade” receptacles which are for commerical applications. In box stores, the spec grade parts are sold in individual boxes (usually) for a few dollars each. They are FAR better quality and hold plugs much more tightly. They are safer and will stay safe much longer. I replaced every receptacle in my house with spec grade receptacles within the first few months after moving in.

      Never use the poke-in backwire holes on the back of receptacles. These are nortriously poor connections. I think they should be banned in the code, but they’re not. Wrap the wire around the screw and tighten the screw. Even better, and very common on the spec-grade receptacles, is the clamp arrangement where the wire goes into a small slot in the back of the receptacle and the screw tightens a clamp that bites the wire. These are excellent, and easy to use.

      Don’t waste money on “hospital grade” devices if you see them. Some people think they have better ground connections but that’s not what makes them “hospital grade”. Hostpital grade receptacles just use plastics in their faces (usually nylon) that are resistant to the cleaning and sterilizing chemical solutions hospitals use. They are otherwise pretty much the same as spec grade devices.


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