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Community and Q&A

Basement shower Rockwool

Dbuzz | Posted in General Questions on

I’m looking to redo our basement bathroom. I live in pgh, PA. The house was built in early 60s. The basement bathroom is an 8’ x 6’ area with painted cinderblock walls and a cement ceiling and floor. (It actually sits right under our front steps – roughly). The entire bathroom sits underground. I’ve  kept a hygrometer in this room for the past year and haven’t seen it go above 60% so it is a relatively dry room surprisingly.

I want to redo this bathroom. I’m in the process now of moving the shower drain which  is all cast iron (as well as toilet and sink drains too – not fun). But I want to know what I can do regarding insulation. I know I will have to insulate. Issue is that I will need to run pex and Romex to fixtures and that will need to be outside of the walls. I was thinking of studding  out the shower with wood studs (~30” x 60”) and putting Rockwool in between the studs and against the cinderblock. Since this is such a small space every inch matters.

Can I put Rockwool against cinderblock? Is that enough R value for the space? Since there will be a shower I know condensation and humidity are a factor. I’m going to put a vent in there as well. Should I put like a 1/2” or 3/4” rigid foam against floor and ceiling? We wanted to tile the floor.

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  1. user-2310254 | | #1


    I'll give you a bump to see if we can generate some replies. FYI. If the humidity is 60% during the winter, that's probably not a good thing. (See for one article on this topic.) Also inexpensive hygrometers are often not very reliable.

    Is there any evidence of water infiltration into the basement? Do you have a sump (and does it run)? Air permeable insulation in the basement can be problematic. Better to go with rigid foam applied to the interior walls. ( provides guidance on this topic.)

  2. Dbuzz | | #2

    Thanks for the bump!
    Sorry, humidity level in basement during winter is around 40% or lower. Just wanted to make the point that even at the most humid/hottest points of the summer, it won’t go above 60%. Also wanted to mention I have two supplies and a return to the basement as well. So it’s conditioned.

    No water infiltration. There was at one point but found out the downspout (right before we moved in) was not properly directed away from the house . Water was running down the side of the house and must have made it way via a crack or something. Water was minimal - maybe an 1/8” at its worst. At any rate, I fixed the downspout and the basement has been drier than ever. (Kept the basement unfinished for a few months during rain to make sure).

    I really can’t lose that much room to do rigid foam behind studs. I need room to install a shower valve and plumbing. I suppose I could do rigid with furring strips and cut into the foam for the valve/pex. Though doesn’t that break the thermal seal that the foam would provide?

  3. Expert Member
    Akos | | #3


    The room is similar to a standard basement. So in your area that would mean around R10 of rigid insulation on the walls. Rockwool won't work in this case because it is air permeable.

    If you are tight on space, you can install 2" of foil facedpolyiso on the walls (if the walls are smooth, these can just be glued in place) and build a 2x3 stud wall in front. From energy savings perspective, you can just leave studs empty, but it doesn't hurt to get some fluffy in there, the R8 attic stuff can work.

    This wall is only slightly thicker overall than a standard 2x4 wall but completely eliminates the chance of moldy insulation down the road.

  4. Dbuzz | | #4

    Thanks Akos.

    Yeah sort of a bummer. Really like that Rockwool stuff. I actually just reconfigured the shower drain to fit the shower base with 3.5” from wall. So with a 2” polyiso and r8 that would put me above the R10 right? Could I get away with a 1” rigid foam (does it have to be foil faced?) with the batt between studs? Can I go unfaced batt?

    Should I also be insulating ceiling? Same thickness? Ceiling is all concrete.

    Also should I be putting pex in the rigid foam (cutting out channels from the rigid) or should I be running outside of it? I don’t think it condensates but the copper connections might?

    Thanks everyone for your comments. I live inside my head a lot - helps to have some input.

    1. Expert Member
      BILL WICHERS | | #5

      2” polyiso is R13, so it would be sufficient by itself. I don’t know what the “R8” is that you’re referring to. 1” of any type of rigid foam would be insufficient here.

      If the ceiling is outside the building envelope, which it probably is if it’s under your stairs (assuming they are stairs to an outdoor porch or similar), then you’d want to insulate that too.

      You need that R10 minimum of rigid foam between the exterior wall and your plumbing. If you only have some PEX supply lines and electrical to run in the walls (no drains), you could use 2x3s flat against the wall to give you a 1.5” chase for mechanicals. This and 1/2” drywall would be your minimum dimension wall for this.


  5. Dbuzz | | #6

    Thanks Bill.

    The R8 was in reference to Akos’s comment earlier - the batt insulation that would be in addition to the rigid foam.

    Yeah it’s all outside of the building envelope. I was thinking the same thing with the 2x3s lying flat. That in addition to the 2” should still satisfy what I measured for the shower base - 3.5”.

    Now I’m likely going to have to move the toilet and sink drain as they are currently too close to the wall to meet code. Was really hoping not to have to reconfigure the whole bathroom but, got to do what makes sense insulation wise.

    Thanks for the input.

    1. Expert Member
      BILL WICHERS | | #7

      If you have to cut some of the slab, you can do small cuts using an angle grinder and a concrete cutting disc. Start by marking where your trench needs to go, then use the angle grinder to cut the lines 1-2 inches deep. Use a rotary hammer or similar with a chisel attachment in hammer-only mode to crunch the concrete you need to remove into chunks. The slits you cut with the angle grinder will ensure you have clean edges for your trench.

      If you need to do a long cut, it’s often better to hire a concrete contractor. Be sure to use proper PPE when working with the concrete as the dust is hazardous. Wet cutting will cut the dust down, but will throw concrete slurry all over so you get to pick a dusty dry mess or a muddy wet mess.


      1. Expert Member
        Akos | | #8

        Definitely go for the wet mess.

        I use an old circular saw with a 7 1/4" diamond blade for this. Make sure you are plugged into a GFCI if you are working around water.

        1. Expert Member
          MALCOLM TAYLOR | | #9

          And wash off the slurry quickly. It can dry almost as hard as it was initially.

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