You can use fiberglass batts in the wall, and cover with 6 mill plastic for a vapor barrier. You can use Tyvek tape - or any tape that is made for taping plastic sheeting - they have it in the asile with the insulation products.

First, the tuck tape. Look for "sheathing tape". It's usually sold in the box stores, sometimes it's red colored. You could use Tyvek tape as well.

The vapor barrier is a bit tougher to diagnose.

Your exterior foam, "expanded polystyrene" is usually quite leaky with regards to moisture transmission and is not a strong vapor retarder. "Extruded polystyrene" is a much tighter product, and it could indeed be a vapor barrier, probably a Class II barrier.

If the exterior foam is indeed continuous on the walls and well detailed, then it might be best to omit the interior poly and just use the Roxul.

Your walls may take in some vapor over time, but the vapor should also transmiss back into the bathroom during drying cycles.

The biggest problems occur when you have two barriers and one or both are poorly detailed. Over time moisture gets in but has difficulty getting out. Thus the "sandwich" that you referenced.

The "imperfect" in your system is the exterior foam. I'm a proponent of foam, but only if it is the right type and the proper thickness.

With your exterior foam probably being a Class II on the exterior side of your wall, your best bet is probably to omit the interior poly and simply use other methods on the interior walls.

You can use something like Hydroban in your shower (if the shower is on an exterior wall), it's a waterproof barrier but does allow vapor to transmiss through it.

For the remainder of your walls, simply use two coats of latex paint over your interior drywall. Latex paint is classified as a Class III vapor retarder. As such, it'll slow moisture vapor transmission that wants to get into your wall, but if any does, it'll allow your wall cavities to dry out during a drying cycle.

It's an imperfect solution for an imperfect situation. I'd be completely confident in my reply were your exterior foam thicker than it is and if I knew it was well-detailed. My concern is the interior side of the foam reaching dew point temperatures in winter and any moisture vapor within the framing cavity condensing on the interior surface of the foam, between the foam and the sheathing. That may not be a problem. But it is a consideration, especially since I tend to go towards the conservative side with an "over the internet diagnosis" such as this one.

I'd certainly do more research if you can, you might get better information than what I'm offering.

You are addressing moisture generation within the bathroom. That's good.

Other stuff:

1) I do sometimes insulate interior walls, Roxul is good for that.

2) A simple yet effective sound deadener behind your washer and dryer is nothing more than a second layer of drywall over the first. 5/8" is better than 1/2" due to the added mass.

Beyond that you can go fancier:
2a) sandwich a 1/2" sheet of homasote between the two layers of drywall.
2b) Use RC clips and metal channel between the studs and the drywall.
2c) Use green glue between the two layers of drywall.
2d) If you do research you'll probably see references to mass loaded vinyl, but I usually use that only when doing fully integrated sound deadening room "systems". I think you'd do better (and save money) with just using drywall.

Thanks for the input Geoffrey_B and Mongoct.

Mongo with regards to your comments, my guess is that the Sears installation crew would not have done a "detailed" job, but that is only a guess. I don't intend to offend anyone but I'm thinking that it would be too much time for most big box type installers to worry about.

I'll have the shower on the interior wall with a cast iron pan. Shower walls will be Hydroban'd, tiled with 12x24 porcelain and Spectralock grouted.

The rest of the room will be painted with BM Bath and Spa latex paint. Exterior wall is only 2x4.

The extruded polystyrene is under the vinyl and up against painted large masonite type siding from the seventies. It has had multiple coats of paint over the years and was in good condition, The masonite was laid on a tar paper product, over 1x8 T&G fir, which is the wood layer that the wall cavity 2x4's are up against.

What I want to do is to not put poly up. I want the walls to dry as able. It just seems that trying to dry to the outside would be difficult. But this is pure speculation on my part as I don't have tools to figure this out. I don't want to damage my structure with doing the wrong thing.

What I want to do is to not put poly up. I want the walls to dry as able. It just seems that trying to dry to the outside would be difficult.

That's the proper choice, your safest bet would be to omit the polyethylene and use unfaced Roxul and latex paint.

Mongoct, thank you for that input. That is what I will do.

What are your thoughts on paperless drywall? Should I consider using paperless sheet rock for these walls? There will be CBU on the shower wall.

"Should I consider using paperless sheet rock for these walls?"

Any time you can get away from cellulose ("food" for mold) it can be a good thing. A few considerations for you to think about if you go that route:

1) If going with paperless sheetrock for mold considerations, don't use paper tape. Use fiberglass mesh tape instead. Paperless drywall is not "mold proof". It simply removes a potential food source, so it's more highly "mold resistant". That added resistance can be an enhancement in bathrooms and laundry rooms, etc.

2) The FG surface can be an irritant. Keep that in mind when handling the sheets. It's not like you'll get airborne fiberglass from hanging the sheets and suck it down your lungs. It's more of a bare skin irritation from your arms brushing up against the FG surface. That's my experience at least.

3) With no paper surface to provide a bit of "surface tension" (so to speak), some folk have complained about overdriving screws when hanging the sheets. If you have difficulty and you do tend to overdrive the screws, you can instead leave the screw heads slightly proud when using a screw gun, then give them the final turn manually. Or use a PL adhesive or liquid nails to adhere the sheets to the studs with just a few screws to hold the sheet while the adhesive sets.

4) The FG sheets do have a fine texture on their surface. Not smooth like paper. So in areas where you tape and mud (panel seams, corners, screw heads), the smoother mud can show up as a textural difference versus the slightly textured FG surface, it's more pronounced if the walls are painted with a non-flat paint, or in a room with low angle raking light.

A lot of folk use a high-build primer. Or they'll dilute regular joint compound with water, then roll it on with a thick nap roller, then use a wide taping knife to remove the excess, essentially skim coating the entire board. "Skim coat" really isn't the correct term as you're not leaving a true veneer of mud on the surface. It's more like you're using the taping knife to almost scrape the board clean and you're just leaving mud in the micro-recesses in the fiberglass surface.

That's another reason some folk use adhesive and fewer screws to hang the sheets on the studs. Fewer screw heads in the field of the board means less mud to fill those holes and fewer areas to get that textural difference.

5) You might find joint compound dries a bit slower on FG board versus paper board, it's due to less water absorption into the board.

I think that's about it! And of course you're probably thinking "it's way to much!" I wrote a lot of "be careful" things, not intended to scare you off, just things to look out for.

"An ounce of prevention...",> so to speak.

My plan was to have someone else hang the drywall. I will ask them what the up-charge would be to hang paperless drywall, if they even do this. I can see that the fiberglass contact could be a problem for installers.

Thank you so much for the detailed information.

I will keep you posted.

Paint as a vapor retarder...who knew?

We put DensArmour Plus (fiberglass gyp) in all our bath remodels, though this last time around it was nigh impossible to find. Everyone around here carries it special order only, large minimum quantities apply. It's not any harder to cut and hang than normal drywall, so I wouldn't think there'd be an upcharge.

Weedyacres, how do you like the DensArmour? Are others happy with the product? What leads you to use it? Is it for the same reason I want to use it, one less thing for mold to grow on?

I thought maybe the fact that one had to wear long sleeves might create an up-charge ;) But seriously, I also read that setting type joint compound was recommended, and to use the setting type compound to skim coat the whole wall too for a better blended wall. I don't know if setting type compound makes anything more expensive, because it is out of the routine.

DensArmour is good stuff. We used it because it's superior technology to greenboard: no paper, good for damp areas. We just used normal joint compound on it, and no skim coat. After primer and paint with a roller with some nap on it, it blends in fine.

We hired out the drywall in most of our remodel of Weedy Acres (had a friend who moonlighted) and he didn't charge any more for DensArmour, nor do anything differently with the finishing.

Of course, given your habits of extensive analysis and planning, you might want to do a test piece to make sure it has the outcome you want. :-)

Weedy thanks very much for the input. And on a lighter note, check out my vanity door vs drawer thread I started tonight.