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Community and Q&A

Using Sprayed Closed Cell Foam in Bathroom

cohnaudio | Posted in Energy Efficiency and Durability on

I live in Westchester, NY. The exterior of my house is vertical Cedar siding (with stain) over Plywood sheathing, which I believe has house wrap over it. I am redoing 2 bathrooms. My plan was to blow closed cell foam into the exterior walls to cut down on air movement and give me more insulation (the walls are 2 x 4) The banthroom used to get quite cold. I was planning to put blue board or paperless wallboard over the non-shower walls. In the shower (one of the walls of which is an external wall), I was planning on using cement board and the Schluter Kerdi and shower system. Not sure what insulation, but was thinking Owens Corning 705 or something like that to cut down on the noise.

So, some questions:

1. My assumption is that the foam would be perfect for the bathroom as it would cut down on moisture movement (I would not add another vapor barrier) and whatever moisture got in from the outside could dry to the outside. Do you guys think that is correct? If not< what insulation would you suggest – as I mentioned, the walls are only 2x4s. I am putting in a large Fantech (4xl) wall mounted exhaust fan, which has a rating of about 190 cfm. This will be on a timer. 2. As I am using the Kerdi in the shower, I assumed that I should not put on another vapor barrier and should put in an insulation that allows any moisture that does get in to move to the outside wall (on 3 of the shower walls that would be internal wallboard and on one of the walls, that would be the outside wall). My assumption was based on the notion that the Kerdi is a vapor barrier and to put foam in behind the Kerdi could prevent the moisture from exiting. If people think otherwise, I would like to know, as it will be much easier to just have the foam guys spray each wall (plus more insulation and I assume the closed cell foam is a pretty good sound dampener. If people think the 705 and sprayed foam would not be good, what would be good in the shower? 3. What should I use for the ceiling of the shower. In the past, we have just used blue board with some loose fiberglass batt over it. The ceiling will not be tiled, so no Kerdi will go over the ceiling. Does it make sense to use paperless wallboard? What type of insulation? The ceiling of the shower opens to another floor As I plan to have the foam guys come by the end of this week, any thoughts would be greatly appreciated. Thanks in advance

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  1. GBA Editor
    Martin Holladay | | #1

    Closed-cell spray foam is fine -- even in your shower wall. Just be sure that the studs are dry -- I assume there is no reason they shouldn't be dry -- when you install the the Kerdi system.

    Moisture-resistant drywall is fine for the shower ceiling.

  2. cohnaudio | | #2


    Thank you very much for your prompt response. I'm curious though why the combination of the kerdi and the CC foam wouldn't be like having two vapor barriers, which I have read is a no no. This insulation/moisture stuff is very confusing. Every time I think I have a grasp, I find I don't. Can you just explain re the moisture movement in both walls. I guess that in the regular walls, the moisture from the inside won't go past the foam, so it will dry to the inside and the moisture from the outside will go into the ply and dry to the outside, correct?

  3. GBA Editor
    Martin Holladay | | #3

    In order to have a moisture problem, you need (1) a source of moisture, and (2) some type of problem -- usually condensation (which requires a cold surface). I don't know of any cold surfaces in this case -- so why are you worried?

    1. charliexu | | #13

      HI Martin,

      THis is an very old thread and hopefully you'll get a chance to see my reply/question:
      I have the same concern over Kerdi Board + CC Spray Foam wall assembly.
      Condensation is not an issue but if any vapor get in the space between kerdi and spray foam, will mold grow on the studs overtime? Since moisture has no way to dry.
      Thank you.

  4. cohnaudio | | #4


    wouldn't the outside wall in the winter be the cold surface?

  5. GBA Editor
    Martin Holladay | | #5

    Yes, your wall sheathing will be cold. But if you fill your stud bays with closed-cell spray foam, which is air-impermeable and vapor-impermeable, there is no way that interior moisture is going to reach your cold sheathing.

  6. cohnaudio | | #6

    Got it. Thanks. I saw in one article that CCF shouldn't be used with OSB because of OSB's problems with vapor movement, but otherwise (i.e. with Plywood sheathing, it seems that CCF could be used in virtually any enviroment, is that right?

  7. GBA Editor
    Martin Holladay | | #7

    You wrote, "I saw in one article that CCF shouldn't be used with OSB because of OSB's problems with vapor movement." I'm not sure what you mean. Are you referring to the fact that OSB isn't very vapor permeable?

    In the situation under discussion, the sheathing (whether OSB or plywood) is able to dry to the exterior, so I'm not sure why the vapor permeance of the sheathing is relevant.

  8. cohnaudio | | #8

    Just curious under what conditions CCF should not be used

  9. wjrobinson | | #9

    Richard, ccf sprayed to underside of a roof is one place where I say plywood preferred over OSB and even wonder if pressure treated plywood would be most suitable. OSB for wall sheathing is failing where foam under stucco exteriors are prevalent. Sidings that are well vented and rainscreened do fine over OSB along with vinyl siding systems .

    1. user-7005706 | | #10

      I'm replying on an old thread hoping to get an answer. My situation in an addition is what AJ described above (stucco over 1.5in rigid foam outside of wall). I'll be using fiberglass batt's inside cavities so the wall should dry inward. My question is, should I still use fiberglass insulation in the shower walls with the exterior foam? Should I do CCF? Or better to leave without anything?

      1. Expert Member
        Dana Dorsett | | #11

        Climate zone and foam thickness matters in terms of what the rest of the wall assembly needs, but the bathroom walls get insulated in the same way as the other walls in the house unless it's sauna or steam bath.

        For most applications ccSPF between studs is a waste, offering very little additional thermal performance, and has the potential for creating moisture problems depending on climate and the rest of the material stack up of the wall.

        1. user-7005706 | | #12

          That makes a lot of sense, Thank you. I will probably just do unfaced fiberglass if my inspector allows it so that the wall can dry inward

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