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Community and Q&A

Insulating plaster/lath walls with double wall in a bathroom

mainejim | Posted in Energy Efficiency and Durability on

I have an old Maine house with minimal wall insulation and plaster and lath walls.  I am in the process of adding cellulose to the attic to R60 but wonder about the walls.  

Specifically, in the bathroom since there is one stud bay that I know has vermiculite in the wall but the rest of the bays are empty.  Could I simply build a new interior 2×6 wall and insulate with cellulose and leave the plaster walls alone?   I don’t want to disturb the vermiculite, or pay the cost to get rid of it.  Would this double wall, one insulated, one not, cause any problems?

Thank you!

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  1. Expert Member
    BILL WICHERS | | #1

    Are there any utilities in that wall? If there is plumbing in there, insulating the way you propose will make the plumbing much colder in the winter and will greatly increase the chance of freezing pipes.

    There might be some issues with moisture accumulation on that original plaster wall since it will effectively be near outdoor temperatures just like exterior sheathing would be. I would think if you detail your new wall as if it were an exterior wall you’d probably be ok. I’ll be interested to see other replies to your post here since this is an unusual insulating situation.


  2. mainejim | | #2

    There will be new plumbing vents and drains in the new insulated interior wall, no electric. That is also one of the purposes for the wall is to be able to put plumbing in new wall and not disturb the vermiculite or the balloon framed walls. The balloon frame wall studs, joists, and rafters are so undersized in this old house that I'm hesitant to cut into any wood for plumbing.

  3. tommay | | #3

    Well, I know you said it was an interior wall so can I assume it is an inside wall? 1st floor or second floor? What size piping is going in the wall? A 3" stack or just 1-1/2- 2" drains/vents? If it's an outside wall you could just cover it with 1/2 or 1" foam insulation with reflective vapor barrier first. A 2x4 wall should suffice if no 3" stack is to be installed. If it is an interior wall why not just slap up some 3/8 drywall to secure the old plaster and build your new wall up against that.
    If it's a first floor remodel and piping is currently on an outside wall, now would be the time to move the pipes into an interior wall.

    1. mainejim | | #4

      Just a 2" drain pipe in the new wall, or between the new insulated wall and the old plaster wall.

      It's a 1st floor room and an outside wall. I would love to install plumbing in only an interior wall, but the layout of the bath fixtures would make it impossible.

      1. tommay | | #5

        As mentioned, some rigid foam and / or 1/2-5/8 wallboard will do to secure the old plaster, then 2 x4 wall with insulation, since there shouldn't be any worries of frozen water pipes. Just out of curiosity, you say just a 2" drain / vent.....what type of fixture?......seems like it could be 1-1/2"....unless it's coming from an upstairs shower.

        1. mainejim | | #6

          It is for a first floor washing machine and sink combined with a possible future upstairs bathtub.

          1. tommay | | #7

            So you are looking at three 1-1/2 pipes and H & C water lines for the future tub, which should fit in a 2x4 wall. The other water lines could come through the floor or should be no problem in the wall if insulated.

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