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I'm renovating my bathroom, and I have to rip out the entire bathtub and surround, so I decided to add insulation to both the external wall and the internal long edge of the bathtub surround - see diagram:

see diagram.

My question is, do I need to add a moisture barrier inside the walls on either the external or internal walls? I am using Schluter Kerdi board for the tub surround, I read that because this is waterproof the moisture barrier is not required - however I would probably add it if in doubt just to be 100% safe, but I wouldn't want to introduce a new problem (for example, moisture being trapped between the two waterproof surfaces.

EDIT: My external walls are not currently insulated, and I would be adding insulation to the internal wall really just for soundproofing.

  • If you actually take baths (as opposed to having a bathtub but only using it to shower) it's well worth insulating around the whole tub so it cools off more slowly. – Ecnerwal Feb 7 at 16:16
  • Is the goal to avoid rapid expansion/contraction? I would definitely consider doing this if it prolonged the life of the bathtub installation. – Toby Feb 7 at 16:19
  • My goal is wasting less hot water keeping the tub warm while I soak. Though if you are using a plastic tub, having something bedding it to the floor (old school = plaster, these days often spray foam, but do remember to half-fill the tub with water before doing that) will keep the bottom from flexing - feels cheap, also leads to cracking. – Ecnerwal Feb 7 at 16:23
  • That's a great tip! The bathtub we're replacing creaks when we step into it. Many thanks for the recommendations. – Toby Feb 7 at 16:28
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Kerdi is waterproof so you do not add additional barriers because as you stated, you don't want to trap moisture. Just make sure to follow all the directions and tips for successfully installing the Kerdi board.

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  • "Beware the vapor barrier sandwich." – Mazura Feb 7 at 7:05

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