I'm having a difficult identifying the type of insulation behind a section of drywall. The home was built in North America around the 1940s. It's undergone several rounds of renovation projects and there's evidence of building materials from the '40s, '60s, '90s, and the early aughts.

I've looked through some photos over at Inspectapedia's insulation identification page. After looking closer at the fiberglass identification page I did see the following image which bears some resemblance to the insulation in question (please the second and third images). However, I have no experience identifying insulation and I'd like the feedback of those who are more experienced.

I plan to install an electrical receptacle on the stud here. I'll need to accommodate for the insulation while doing so in a safe manner. I won't hold anyone liable for opinions shared, and I will use PPE when interacting with the insulation either way.

Photo 1: Inspectapedia fiberglass reference image. Inspectapedia Fiberglass Reference Image

Photo 2: Unidentified greenish/desauturated insulation in home behind drywall. Closeup with flash on. enter image description here

Photo 3: Unidentified greenish/desauturated insulation in home behind drywall. Taken at arms length with flash off. Green-ish insulation at a distance

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    Welcome, and well asked for a first post! Be sure to take the tour while you're at it. You get a free badge! :)
    – FreeMan
    Oct 1, 2021 at 12:53

2 Answers 2


Based on Photo 3 that looks very much like Rockwool insulation. Not sure if that's an actual trademark or just a common trade name, but that's the link to what appears to be an "official" site about it.

As I understand it, this stuff cuts very cleanly - I think you'd use a smooth edged knife (like a utility knife), not a serrated knife (like a drywall knife). You'll want to cut out a hole for the new box to go in, not compress it behind.

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    I'd agree based on the distinct production mark pattern. You don't see that with fiberglass.
    – isherwood
    Oct 1, 2021 at 15:39
  • FreeMan, thanks for taking the time to share your feedback on my question. Your advice on how Rockwool takes well to slicing is much appreciated.
    – DeadBranch
    Oct 2, 2021 at 13:01
  • One follow-up question: I planned to use a one hole metal clamp to secure the NMD90 to the stud. Would you advise slightly pulling the rockwool outwards (without compressing it) to make room for fastening the clamp to the stud? I'm not sure if I should try to minimize the number of times I disturb the rockwool with one big displacement out then back. Or, several smaller disturbances.
    – DeadBranch
    Oct 2, 2021 at 13:06
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    @DeadBranch A) I haven't a clue, and B) that should be a whole new question because I'm likely going to be the only one to ever notice it down here in the comments.
    – FreeMan
    Oct 2, 2021 at 23:56
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    @FreeMan Ah! That makes sense. Thanks for explaining a bit about how things work around here.
    – DeadBranch
    Oct 3, 2021 at 0:45

This looks like mineral wool insulation, and not fiberglass, the pattern on the top of the batting is the giveaway. Generally mineral wool batting between studs is semi-rigid, and not as loose as paper backed fiber glass batting (aka, the pink stuff).

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