We're redoing the master bath. I'm wondering if and how I should insulate the interior wall. Bedroom is kept at 60F. Bathroom will be warm (radiant floor) and humid even if we have and run vent fan.

Should I put insulation in wall? Standard fiberglass with vapor barrier on bathroom side? Foam board? Fiberglass w/o vapor barrier?

1 Answer 1


Given what you know about how you will heat the rooms differentially, insulation is good. I'd go with mineral wool (or fiberglass) since some sound isolation is often desirable between bedroom and bathroom and foam is not great at that application.

If you will be doing the bathroom walls with a moisture barrier no additional vapor barrier on the insulation is called for.

  • Seems reasonable but note that this is the opposite of the answer given here: diy.stackexchange.com/questions/63994/…
    – mankoff
    Apr 22, 2015 at 11:24
  • @mankoff - read more carefully.
    – Ecnerwal
    Apr 22, 2015 at 11:35
  • "if doing walls with vapor barrier" - meaning if I do not, I should do vapor barrier on the insulation, correct? The other answer seems to say not to do vapor barrier in the wall. I've read both, and again carefully now, but I think I'm still missing something...
    – mankoff
    Apr 22, 2015 at 11:57
  • 1
    No! No vapor barrier! Anywhere! Ever! They are just a terrible idea. Do NOT put a dedicated plastic vapor barrier behind your drywall; not in a bathroom, not in a living room, not anywhere! I do not want it, Sam I Am!
    – iLikeDirt
    Apr 22, 2015 at 14:09
  • What about with an exterior wall? No vapor barrier there either?
    – mankoff
    Apr 23, 2015 at 13:17

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