Hi, I have a Q about the proper way to air seal & insulate around a one-piece 36x48 shower alcove in new construction. On the good side, I remembered to get the shower into the bathroom before framing prevented it from fitting through the doors. On the maybe (?) not-so-good side, the shower got installed (screwed to framing) in the stall's alcove and is right up against the studs (no drywall, moisture barrier, etc.).

Two walls of the alcove are 2x4 & back up to master bed/closet area.

The third wall of the shower alcove is a 2x6 that backs up to a stairwell that leads from outside down to the basement. The basement & band joist will be insulated on the inside (likely with rigid foam). So technically this 2x6 wall is in 'the envelope', but still I think will tend to be chilly (on the basement stairwell side).

Here are my questions: 
 
For the 2x6 alcove wall that backs up to the basement stairwell; planned stackup: (back of shower stall, damp spray cellulose, 1" of XPS & drywall (fire code) before reaching the basement egress stairwell.  Does this wall assembly, behind a shower stall sound OK? Should I be thinking about some type of moisture barrier?

Or, Should the shower stall be removed and framing adjusted to allow for drywall (the moisture resistant type) in the alcove walls, top plate to bottom plate? Or is it OK to just have the shower stall up against the stud framing as currently installed?

Thanks!

Normal practice and most manufacturer's installation instructions have the one-piece tub/shower enclosures attached directly to the wood framing.

Thanks arkie6, I suspected that was a typical install ... but I am wondering if, considering the whole ADA-thing and air sealing in general, there were any energy efficient upgrades I should think about with respect to the shower install. For example, this link shows 'sheet material' behind a tub. Would the same be considered necessary for a one-piece shower stall (in an alcove)? Where as this example shows 6-mil poly between the shower stall and studs. But in many other places I've read poly is a no-no and not done any longer except in extremely cold climates (I'm in a mixed humid climate).

So, if the questions in my original post are too site specific to be answered on a forum, how about the more general question of whether or not a typical (against studs) one-piece shower install is acceptable or even goes so far as to meet best practice? If it is not acceptable, what would be?

Thanks

Posted By BabyBldr on 11 Dec 2011 08:34 PM
. . .
Or, Should the shower stall be removed and framing adjusted to allow for drywall (the moisture resistant type) in the alcove walls, top plate to bottom plate? Or is it OK to just have the shower stall up against the stud framing as currently installed? 
. . .

This may be too late to help BabyBldr, but the most recent briefings on this practice discourages using "moisture resistant type of drywall" or any drywall behind a tub or tub/shower combo.  (I know.  Many of us have used it for years.)  Cement products like Hardie backer panel should be used instead.  Hardie has two thickness of this type of panel - one for the walls and one for the floor.  Use the thicker panel on the walls since it clear spans across the studs.

Posted By Alton on 15 Dec 2011 07:33 AM

This may be too late to help ...

Ha Ha Alton - no it is NOT too late. We are progressing very slowly as I'm working alone during the week and my husband is part-time on weekends. We have finished 95% of the plumbing rough-in, the ducting for the HRV (except for my job of mastic-ing it), and now turning toward the electrical work. I'm getting quotes on insulation (cellulose) and all the while looking-for/tending-to air sealing issues - the shower stall being one of them.

Thanks for the tip on the newer practices, I will keep it in mind if I decide to pull the shower stall out and install something against the studs. I checked the mfg's website (Maax Aker one-piece shower 36x48) and it mentions nothing special, installing it just against the framing as Arkie66 stated above. So maybe we'll just block (air seal) the drain hole in the sub floor and calling it a day.

Thanks

If this is a one-piece molded fiberglass or acrylic shower, I don't really see the need for any backer board or tile membrane between the shower and studs. Especially considering this shower is not against any exterior wall. If one side was against an exterior wall, I might consider a sheet of 1/2" plywood over the studs to cover the insulation and provide an air barrier.

If it leaks water isn't going to shoot sideways into your wall, it'll come out the trap cut-out unless you've air sealed the trap cut-out in which case it will lay up in there and slowly rot out your floor and wall plate.

Did you air seal your stud bays? You should have. If you did, blower door testing would have little to no airflow out of the trap cut-out since your exterior wall is your air barrier. If you didn't, pull the tub and put a layer of drywall or osb or plywood, or I guess Hardi if you're a paranoid type, (schluter over hardi if your paranoid and like wasting money, schluter over drywall if your slightly less paranoid but still enjoy wasting money) air seal to the stud plane and reset the tub.

If you block just the trap cut-out you'll find that leaking air finding the next path of least resistance, since your not stopping the air on your thermal boundary which should be your air barrier plane as well. You'll still pull cold air around your tub and it'll go into interior walls and through wiring and plumbing holes and out the next easiest crack it can find.

Basically, if I was running a blower door and had a whole house CFM leakage reading and put a pressure pan over the tub trap hole and took a CFM reading of the amount of air being pulled through that opening, and then air sealed that tub trap hole, the whole house blower door reduction would not equal the CFM that the tub trap was leaking prior to air sealing.

In effect, air sealing the tub trap is a band aid solution suitable for home performance retrofits when you have few other options.