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Bathroom remodel, question on wall construction and shower area

pumafeet10 | Posted in General Questions on

Hello All,

I am currently remodeling a bathroom and need to replace my fiber glass tub and shower surround, i will be replacing it with another fiberglass/acrylic solid surface material but really had the question on how to properly build the walls in the shower area?

I will have a 2′ section above and around the the shower and wasnt sure the best way or materials to do this, do i drywall with a moisture resistant product, what about vapor barrier, and air sealing?

Thanks the help and im sure there is more info needed but thought i would start here?

Thank you again

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  1. Randy_Williams | | #1

    Hi David,
    Drywall is typically installed above the shower. The flange that attaches the shower to the framing can make a bulge in the drywall. Most drywall installers will drywall to the flange, and then fill the remaining space with drywall compound. This could be an air sealing problem if the shower in on an exterior wall. You may need to add a air barrier before you install the shower. I would check with your code official for air sealing/vapor retarder requirements in your area.

  2. GBA Editor
    Martin Holladay | | #2

    If all three walls around the tub/shower unit are interior partitions, there is nothing wrong with fastening the flanges of the tub/shower unit directly to the studs. Follow the installation instructions provided by the tub/shower manufacturer.

    The only possible problem occurs when one wall is an insulated exterior wall. When that's the case, you need to have a durable rigid air barrier on the interior side of the studs before you install the tub/shower unit (unless the stud bays are insulated with spray polyurethane foam). Thermoply is the traditional air barrier for this application, but moisture-resistant drywall (with taped seams, of course) will also work.

  3. pumafeet10 | | #3

    one of the walls is an exterior wall and was looking at codes and i believe it states i need some sort of vapor barrier, and have been reading endlessly on here that I really dont need it. If it is overkill that is fine, if its going to cause moisture problems and mold, then im not sure what to do. Im a bit gun shy bc the house was a flip, not by me, and some corners were seriously cut and I want to pretty much over do it. I live in zone 5 with a house built in the 50's, so im always fighting some kind of gremlin. I guess im looking for someone to spell out exactly what i should use from the studs inward, as in do i need a vapor barrier, air barrier, what kind of drywall, and paint? I think i know some of the answers but other outside opinions are very helpful.Thanks for taking the time

  4. GBA Editor
    Martin Holladay | | #4

    In Climate Zone 5, building codes require the use of an interior vapor retarder. The traditional choice was 6-mil polyethylene, which is still used on occasion in bathrooms, which have above-average outward vapor drive during the winter. Other perfectly acceptable approaches are the use of vapor-retarder paint on the drywall, or kraft facing on a fiberglass batt, or a smart vapor retarder like MemBrain.

    If you are installing a one-piece fiberglass tub-shower unit, and if your bathroom has an exhaust fan, there is no reason to believe that you will have moisture problems in your walls. Choose one of the vapor retarders I have listed, and don't worry.

    That said, an interior air barrier is very important. So make sure that your air barrier (ThermoPly or drywall) is installed well, without air leaks.

  5. Randy_Williams | | #5

    I use 6-mil poly behind my showers, only because the codes in my area require it. Both the craft face or MemBrain will also work well. The air tight drywall approach will work well if your attaching the shower to the surface of the drywall. I have seen increased air leakage during a blower door test in the area around tubs and showers, mostly in older homes. The air leakage is usually due to unsealed plumbing or electrical chases leaving the building envelope, such as a vent pipe installed through the attic. Some leakage can also be from poorly insulated and sealed exterior walls. If it was my job, I would air seal any gaps or holes from plumbing or electrical in the top plate if the bathroom has an attic above. I may address the bottom plate, depending on what is below. Conditioned basement is not a problem. Vented crawl space will need air sealing. Big holes cut for the plumbing trap are hard to seal. I would typically insulate with fiberglass and use 6-mil poly sealing the edges with acoustical caulking. If using closed cell spray foam, omit the poly. Install the shower and finish the drywall above. Make sure the bath fan is moving enough air. I've seen many fans just making noise, not moving air. A easy test is to use a piece of toilet paper, if it sticks to the fan while its running, your moving at least some air. Hope this helps.

  6. bennettg | | #6

    If the hole for the drain/trap is open to semi- or un-conditioned space, it would be good to seal that up as well. If you didn't air-seal this and the wall, picture air traveling up through the hole around the drain up the wall through the insulation, and out through the cracks around the top plate.

    Sealing any exterior wall up prior to placing the tub/shower makes it easier to get tight, vs detailing all the way round the shower stall.

    Stopping the air movement is the key to keeping water vapor out of your walls. A quarter-sized hole can move a lot of air, and with it, a lot of water vapor.

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