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Community and Q&A

Insulating interior basement wall that has integral cast iron plumbing

steveheller | Posted in General Questions on

I’m finishing my basement in Portland, OR (Zone 4, marine) with 1-2″ of foil-faced EPS foamboard (depending on the particular wall of the foundation, see below), seams taped with foil tape and edges treated with Great Stuff Pro gaps and cracks spray foam, building a standard 2×4 stud wall directly inside the foamboard, adding R-13 mineral wool batts (Portland requires R=15 for finished basement walls), then finishing with drywall; however, I have a trouble spot I’d appreciate suggestions on how to handle.

A previous owner added a bathroom and ran the waste stack down through a joist bay and into the top of the foundation wall such that about 4.5″ of the cast iron stack protrudes directly from the foundation wall–as in, the foundation was repaired by encasing part of the pipe in concrete. I used 2″ of foamboard on this particular wall so as to allow the stud wall to stand proud of the pipe and allow for good drywall installation, but I’m unsure what to do to insulate the pipe itself. I was thinking cut and cobble foamboard to cover concrete wall around the pipe, but wasn’t sure of a next step.

Should I cover the front (closest to drywall) of the pipe with mineral wool batts, leave it open, or try to make a foamboard enclosure around it? I figure the pipe could be a condensing surface because it is in direct contact with the foundation wall, but I would appreciate advice from more experienced folks. Thanks!


  1. Expert Member
    Peter Engle | | #1

    I wouldn't worry about it. Run the foam up to the sides of the pipe and seal the joint with canned foam. Cover with mineral wool to fill the cavity. The risk of condensation on the pipe is low most of the time. Lower when it is carrying room-temperature or warmer water.

    1. steveheller | | #2

      Excellent. Definitely the easiest option. Thanks for your advice, Peter!

  2. gusfhb | | #3

    They make a pipe insulation with a white coating that looks reasonable and would take care of any condensation

  3. Expert Member
    Peter Engle | | #4

    Not sure about Keith's product specifically, but not a bad idea in general. There are self-adhesive pipe insulation products that would work. They are generally thin, flexible, and covered with a plastic film on at least one side. They probably only have an R-value of about .25-.5, but they would give just a bit of extra condensation control at a very low cost. Sold at the big box stores.

  4. Expert Member
    BILL WICHERS | | #5

    I would paint the cast iron pipe with several coats of an epoxy paint prior to insulating the pipe. This epoxy paint will help protect the pipe from corrosion from any mositure that may accumulate over time. The paint is basically cheap insurance.

    Commerical rigid pipe insulation is a type of rigid fiberglass formed into two halves that fit around pipes, and is finished with a normally snap-on white plastic outer layer. There is also a type with a white exterior that is bonded to the foam, this exterior looks like white paper with a mesh reinforcement. Either type should be available through commerical mechanical supply houses. This type of insulation is commonly used on hot water heat pipes and chilled water lines in larger commerical buildings.

    I don't think you'll have any problems just boxing over that pipe with rigid foam, but I would paint it first as I described above. Just be sure to use one of the epoxy paints and not regular paint.


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