I have a 14 foot basement which is waterproofed. It has a french drain and a heavy duty white plastic covering the entire outside wall. I have half of the basement finished. My question is how do I insulate the side of the basement wall that is concrete? Is it okay to fill the cavities with insulation since the plastic is already there? The plastic is not necessarily a vapor barrier but a way to direct any leaks down to the french drain.

  • 1
    Hello, and welcome to Stack Exchange. A picture or two would really help us understand your question better. – Daniel Griscom Jan 1 '18 at 13:13
  • Is the plastic on the outside of the exterior walls “directing any water down to the french drain”? Do you live in a climate that is primarily in the heating mode the majority of the year? – Lee Sam May 26 '19 at 22:38

Don't fully understand but I see no reason not to install insulation on top of vapor barrier.

  • Hi, Paul. This is a pretty low-quality answer; a few more sentences would really help. ("Don't fully understand" doesn't help...) – Daniel Griscom Dec 31 '17 at 15:39

If the plastic is only there to direct water down, it is not necessarily thick or durable enough to be a reliable vapor barrier. I'd recommend adding another vapor barrier just to be safe, because better safe than sorry. Alternatively use a mold-proof insulation, such as mineral wool.

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