We're going to blow in fiberglass insulation to improve the exterior walls we haven't had an opportunity to upgrade—after ten years of remodeling projects we've modernized about 60% of the walls.

The walls we've stripped to studs have batts of rock wool, a black fibrous material made of "fiberized" rock and steel slag. The house is typical construction for the 1950s one-story ranch houses in the Pacific Northwest (North America) but also includes a "luxury upgrade" (for the time) of this rock wool. The markings on the facing says either R-5 or R-7 (one or the other thickness was used in various portions of the house). They are nominally 1.5 to 2 inches thick. (After removing the drywall, it is a no-brainer to remove that sickly-looking insulation and install modern R-13 fiberglass.)

We had one contractor bid $1400 for the job, but we would have to do prep (remove furniture and fixtures) and finish the job (they'll put plastic plugs in the drywall holes but we need to clean their mess, then mud, texture, and paint. I asked if they would remove the rock wool: they said "No. We blow beside it which provides extra insulation."

What are the arguments for and against removal of old insulation?

  • I don't understand. How would you remove the rock wool if the wall isn't open?
    – isherwood
    Jul 30, 2018 at 18:56
  • @isherwood: Well, a hole has to be drilled for blowing it, so why wouldn't one use that to evacuate the old stuff?
    – wallyk
    Jul 30, 2018 at 20:10
  • I don't think that would work as you expect. The holes for filling are about 2-1/2" in diameter. You won't empty a wall through those holes without some impressive vacuum equipment. If it was feasible the contractor would offer it.
    – isherwood
    Jul 30, 2018 at 20:24

2 Answers 2


I can’t think of any good reason to keep the existing insulation in the wall.

Blowing in insulation in walls that are already partially insulated will not produce a uniform layer of blown in insulation. In fact, there is a good chance the old rock-wool insulation has sagged in the wall cavity and will block the installation of the blown in fiberglass insulation uniformly to the stud space.

The option to remove the wallboard will allow the removal of the original insulation in order to provide a uniform layer of insulation. In 1950 the stud depth is 3 5/8” (rather than the current 5.5” depth now) so you want it to be as good as possible to get the maximum R-value possible with the limited stud depth.

Also, be careful of the old electrical wiring that it’s not disturbed in either option.

However, in either case you’ll want to test the 1950’s wallboard for asbestos.

  • "Better late than never", right? That nasty old insulation did not insulate all that well. Additionally, the transmitted sound difference was profound and most unexpected! We could no longer hear that pathetic dog barking a few houses away all night. Now our bedroom seems to be very secure.
    – wallyk
    Dec 11, 2022 at 21:25

If not opening the walls except for the hole to blow the new insulation in I would leave it because of the cost and labor to remove it. If you were opening the walls then I would remove the rock wool and use bats or rolled insulation as its r value is higher or based on the home of mine that I remodeled in the Pacific Northwest I replaced the rock wool in the walls and used it in the attic. Rock wool is a good insulation but they did not use 4" it was 2" so I went from R7 to R15 in the walls and was able to add 2 layers of the rock wool in the attic this did reduce my winter heating bill and really made a big difference in the summer keeping the home much cooler so the ceiling did not feel like it was heated with cable heat our power for AC dropped by over 50% but it was a lot of work, I stripped each room because it had been a rental and with 20? coats of paint and bad DIY patch jobs I felt it was easier and looked better to re sheetrock, if your sheetrock is in good shape they fill the void in the stud bay so you end up with a similar amount of insulation.

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