Bathroom Insulation w/wo vapor barrier

Discussion in 'Shower & Bathtub Forum & Blog' started by Pete Bridwell, May 26, 2021.

  1. Pete Bridwell

    Pete Bridwell New Member

    Joined:
    May 26, 2021
    Location:
    Tennessee
    We're remodeling our master bath and would like to sound proof the interior walls with insulation. All the walls will be covered in moisture proof drywall, except the shower, which is getting cement board and tile. I've researched insulation with and without vapor barriers and get a mix of theories.
    The ceiling is green board with kraft-faced fiberglass insulation above it (kraft side down). No plans to make any changes there.
    The exterior wall is already filled with unfaced fiberglass with a vapor barrier loosely stapled over it. Would it be safe to assume that I can do the same on the 3 interior walls, including behind the cement board in the shower?
    Since structure and location seems to drive a lot of the information I've seen, I live in middle Tennessee on the Kentucky boarder, and it's a brick home, with rigid foamboard on the brick side of the existing fiberglass insulation.
    Thanks in advance.

    -Pete
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    There isn't much point to a loosely stapled vapor barrier, since it's also serving the function of an air barrier that allows to the fiber insulate to operate at spec. If the polyethylene vapor barrier has gaps top/bottom/sides or major holes in it it's not even that great a vapor barrier, since it's allowing convection forces to move air from one side to the other. Small air leaks can move MASSIVELY more moisture through a wall than vapor diffusion alone.

    Best practices would be to AIR SEAL THE SHEATHING on the exterior prior to insulating, and install a tight fitting "smart" vapor retarder such as 2-mil nylon (eg Certainteed MemBrain) on the interior side of the studs & cement board, detailed as an air barrier as per the instructions. Asphalted kraft facers are also smart vapor retarders, but can never be made truly air tight.

    To air seal the sheathing (probably OSB or plywood under your exterior foam?) seal the full perimeter of each stud bay to the sheathing using polyurethane caulk (or purpose made products like ProPink, etc.), and seal any seams in the sheathing with housewrap tape, reinforced with a 1/8" layer of duct mastic to 1/2" or so beyond the tape edges to ensure good adhesion over time. Be sure to put a bead of caulk between any doubled-up framing such as top plates, jack studs, etc and where the bottom plate meets the subfloor.

    If there is no structural sheathing, use foam board construction adhesive to seal the foam board to the framing on all edges of every stud bay, and reinforce any seams between framing using housewrap tape + duct mastic.

    Unless the exterior foam has a foil or vinyl facer it's not a true vapor barrier until it's at least 1.5" thick (for XPS- pink, blue, green, sometimes gray) to even hit class-II vapor retardency (=< 1 US perm). Six mil polyethylene runs about -0.05 perms, foil facers even lower still, making them true vapor barriers (= <0.1 US perms). Anything under 5 perms on the exterior does a pretty good job of protecting the structural wood from the severe moisture drives of sun on damp brick, while still allowing some drying capacity toward the exterior. But installing 6 mil polyethylene on the interior traps that moisture in the studwall, with no drying toward the interior.

    In a TN climate in an air conditioned house this can lead to condensation running down the exterior side of the polyethylene whenever the outdoor dew point is above the indoor temperature even without the moisture drive of the brick, but with a brick clad structure it's pretty important to preserve some drying capacity toward the interior, ergo the prescriptive for using smart vapor retarders. When the air entrained in the insulation or behind the cement board is fairly dry (<40% RH) the smart vapor retarder runs about 1 perm or less, but when the moisture levels are high it become vapor open, allowing moisture to pass relatively freely, preventing moisture from becoming trapped in the stud bay.

    Yes,with smart vapor retarders there will be some moisture transfer from the interior side into the wall assemblies after showers or when the bathroom humidity is high enough for condensation on the walls & mirrors, but those events are transient, limited to a few hours per day- assuming this is a house and not a public bath house. As long as the vapor retarder is AIR tight the amount of moisture getting in to the walls via diffusion is minimal.
     
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  4. Pete Bridwell

    Pete Bridwell New Member

    Joined:
    May 26, 2021
    Location:
    Tennessee
    Thanks Dana. That was very helpful and gives me some stuff to look into.
    Do you think I need to have the same considerations for the 3 interior walls?
     
  5. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    If the interior partition wall has a warm humid indoor swimming pool room or hot tub on the other side those considerations matter- it has to be designed. Otherwise not so much.

    Interior partition walls between the bath and normal conditioned space on the other side can/should be left relatively vapor open to maximize drying capacity for the intermittent high moisture drives of shower. Standard interior latex paint is fine, but vinyl or foil wall paper can present a slight risk of mold inside the walls. In most homes that is not a significant hazard unless the shower is running several hours per day every day.

    Moisture drives from bathrooms can be mitigated by use of timer switches or occupancy sensor switches on bath fans, or bath fans under de-humidistat control (Panasonic has a few models with humidity sensor options.)
     
  6. Pete Bridwell

    Pete Bridwell New Member

    Joined:
    May 26, 2021
    Location:
    Tennessee
    Perfect, thanks. It’s just a residential master bath with the interior walls against normal, conditioned spaces. The bathroom has 2 light/fan combos, one over the toilet and the other in the shower, with the one in the shower being on a timer.
    Thanks again!
     
  7. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Yer welcome!

    In my area most of the big box stores used to keep smaller rolls of Certainteed MemBrain in stock, but like many things in Pandemic land it seems to be in short supply, with no promises for delivery dates- you can't even order it from the orange online(!) even though it's still on their website. The local blue box store doesn't have it in stock, but it can be ordered online for shipment to the store, albeit with a "Hurry, low stock" warning.

    There is currently at least one seller (located in San Diego) on the big popular auction site selling 100" x 50' rolls, which is probably enough for your project.
     
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