Type of insulation for new shower. Vapor barrier?

Discussion in 'Shower & bathtub Forum & Blog' started by RobertZ, Mar 6, 2012.

  1. RobertZ

    RobertZ New Member

    Messages:
    14
    Location:
    Southern California
    Hello everyone.

    I am getting ready to tile up a new shower in our master bath. The shower will have two exterior walls and one interior wall and I will be hanging 1/2" cement board and applying Redgard before tiling.

    I had a few questions I was hoping to get some help with:

    1. What type of insulation should I use behind the cement board? Our house is about 40 years old and there is no insulation currently installed on the exterior walls.
    2. Do I need to install a vapor barrier in the shower area? I live in Orange County California and there was no vapor barrier previously installed.

    Thanks for the help!

    Rob
  2. RobertZ

    RobertZ New Member

    Messages:
    14
    Location:
    Southern California
    Hey guys. I was hoping to do this project this weekend. Any help would be greatly apprecaited.

    Thanks again

    Rob
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    I'd check out www.johnbridge.com. Some of this depends on what's on the outside and how it was built. Redgard will prevent moisture from getting into the wall from the shower, but if the wall isn't tight there could be a fair amount getting inside it from either outside or other areas. Need to avoid condensation in there with no way for it to dry out.
  4. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

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    post(s) deleted by John Whipple
    Last edited: Mar 17, 2014
  5. RobertZ

    RobertZ New Member

    Messages:
    14
    Location:
    Southern California
    Thanks for the insight gentelmen.

    JW,
    I am using a pre made shower pan for this project.

    To give a bit more info, this is a stucco home that has the black tar paper on the exterior of the walls. If I'm not mistaken, if I use a vapour barrier won't that create a sandwhich in the walls for condensation? For that same reason, my batt insulation should be without face, correct?

    I'm leaning towards installing faceless batts, no barrier, and using Redgard for the cbu. What do you guys think?

    Thanks again for this help. This is an awesome site!

    Rob
  6. Dana

    Dana In the trades

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    Redgard is a vapor retarder- if you apply it correctly you needn't, indeed SHOULDN'T put up any other vapor barrier in the stackup.

    Or you could skip the and put up something like Kerdi between the cement board and the studs. This has been covered here before.

    The type of siding may put limitations on the type of insulation used. If this house has NO insulation it's worth doing the whole house, not just the part behind the shower.

    As for insulation types the food chain runs:

    Unfaced high density "cathedral ceiling" batts are far superior to any lower density batts (R15, if 2x4.)

    Blown higher density fiber (cellulose @ 2.5lbs+ or Spider/Optima @ 1.8ls) is superior to any batt solution.

    Beyond that it starts getting expensive/exotic with spray foams, poured foams, non-expanding injection foams, etc.

    The value leader tends to be cellulose, but there is potentially a problem under stucco/brick unless there's a cavity between the sheathing and the masonry vented at top & bottom, as well as decent overhangs on the eaves etc. to keep rain-splash off the cladding. IN SoCal you can probably use Optima or Spider under unvented stucco in most applications, but non-expanding injection foam would be better (at several times the cost) from an ability to limit moisture absorption into the studs. Cellulose is fine (even preferable) under well vented masonry, or with vinyl/wood/fiber-cement, but under unvented masonry it can potentially load up with significant moisture during rainy periods and take a long time (much longer than with the new-school fiberglass) to dry.

    The tar paper under stucco is not a true vapor barrier, but rather runs ~ 4-5 perms if one layer, 2 3-perms when doubled up, but the drying direction for the is still to the exterior (and must be with the low-perm layers on the interior.) Batts with kraft facers are OK as long as the facer is on the interior of your stackup. Kraft-facers are 0.4 perms, and you're not trapping any "mold food' other than the facer itself between the facer and your interior vapor retarder. If there's even 1/4" of space between the exterior wood sheathing and the stucco and it's vented top/bottom, you're fine.
    Last edited: Mar 12, 2012
  7. RobertZ

    RobertZ New Member

    Messages:
    14
    Location:
    Southern California
    Hi Dana,

    Thanks for the help. Now I know how my set up will go.

    Thanks again everyone!
  8. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

    Messages:
    3,787
    Location:
    North Vancouver, BC
    post(s) deleted by John Whipple
    Last edited: Mar 17, 2014
  9. Dana

    Dana In the trades

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    I put the link to the prior thread to point to the more detailed answer.

    You need at least something that behaves as a capillary break between tile-on-cement-board and the studs (or any hygroscopic insulation) to ensure that water doesn't wick into the wood/insulation, but in a SoCal climate it can be fairly vapor-open to let the air-conditioning dry the stud bays. (You'd need a real vapor retarder in colder climates though.)

    It's OK to be vapor-tight on the interior as long as there's a ventilated gap for the stucco to dry into. The 2-layer felt is probably between 1-2 perms, which is enough to protect the studs from the extreme moisture drives that occur when the sun hits wet stucco, but if there's no convection to purge the moisture from the cavity it would need to dry toward the interior, and a strong interior side vapor retarder like 6 mil poly there could be a problem.

    I wouldn't advise putting 6-mil poly anywhere in an Orange County CA wall stackup without a serious reason to do so, and particularly not on the interior side under a stucco cladding- this is a cooling dominated climate with a moisture-reservoir siding. Condensation on the poly under brick & stucco buildings creates summertime wet-insulation and wet stud-plate rot problems even in climates like Ottawa or Winnpeg (but they're harder to deal with there, due to the high wintertime moisture drives from the interior.)

    It's probably not a disaster if he did or didn't drop in poly sheeting. No house is perfect, yet most of them seem to survive. In Orange County the climate (including summertime dew points) is dry enough to get away with all sorts of things that simply wouldn't fly in Florida or Ontario or Manitoba.
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