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I was replacing some damaged siding and decided to replace the old dirty insulation at the same time. After pulling out the insulation I noticed that the Romex running to two outlets had some brown sticky stuff on it. Any ideas what it is and why it's there? I'm including a picture of one of the the wires.

Went back and found a piece of the old insulation and took a picture. It looks like tar paper on the inside. That's probably it. Thank you.

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    I've heard rumors of wire heating due to metal staples being too tight, but have never seen any evidence of it. This to me looks like something somebody might squeeze into that box. It looks like the cable might be damaged just at the bottom of the photo, was there a staple removed from that location. If it were my house, I'd likely replace this section of cable, and investigate potential overcurrent situations on the circuit.
    – Tester101
    Jul 5, 2014 at 12:09
  • @Tester101 If that were the case it would be worse where the staples are. It is obvious that it is just some stuff on the surface as it didn't work its way into the lettering of the cable. I would still consider replacing it as may be damaging the insulation.
    – Brad Gilbert
    Jul 5, 2014 at 12:22
  • @BradGilbert That may be true. The whole metal staple, heating wire thing is just an urban legend to me at this point. Not sure what it would look like. It's hard to tell from a photo whether or not the marking is only on the surface. Maybe a razor and a gentle touch, could determine the depth of the marking.
    – Tester101
    Jul 5, 2014 at 12:34
  • It looks like there's a bit of water damage, what does the insulation look like? was there any marking, damage, etc. on the insulation in this area?
    – Tester101
    Jul 5, 2014 at 12:45

3 Answers 3

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That is tar from the old insulation, that's why it is sticky. I don't know why or how it happens, but old insulation had tar in the paper, and it is very common.

It is not scorching or melting from overheating.

Take a utility knife and try to gently scrape some of the black away. You should see it's just on the surface. It's nothing to worry about.

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    That's why I said GENTLY. I am not telling him to cut into it.
    – Speedy Petey
    Jul 5, 2014 at 12:58
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    Use paint thinner to confirm what it is, that will get past all the concern. It is as Speedy says.
    – Jack
    Jul 5, 2014 at 13:03
  • The tar often was used as an adhesive for the fiberglass batt to stick the paper to it. It will stain the Romex sheath.
    – Fiasco Labs
    Jul 5, 2014 at 16:07
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I’m an electrican and I have people call me all the time saying there house is gonna burn down thinking the wire is causing the black burnt like marks but it’s from old insulation that has tar on the inside and you see this mostly in workshops,ten buildings,attics,or ten roofed buildings heat is the cause of the tar to melt and seep threw I hope this helps!

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Looks like heat. You say it is sticky so maybe it is residue from the insulation. The cable can get hot if you are running a heavy load on it. If the receptacle is a long way from the circuit breaker the breaker might not trip with an overload.
You can check for heating by plugging in the normal load and then check to see if the cable is hot. Loose connections at your receptacle can cause heat damage to the cables.

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  • Given its location, it could also be heat from outdoors.
    – wallyk
    Jul 5, 2014 at 19:02
  • I don't think that heat from "outdoors" is going to cause insulation to turn black, do you?
    – The Evil Greebo
    Jul 8, 2014 at 12:01

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