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.

you wouldn't want to sandwich the insulation between two vapor barriers. finding out what the vapor barrier location is for Oaklahoma would be the first step. I think they put theirs to the interior (whereas we put ours to exterior) also the rest of the walls may be uninsulated also. just how we built in the 70's. as I've worked on my house I've added insulation to the walls I've opened. does it help...but not as much as the air sealing has. best of luck

I think it would depend what sort of ties are used. These days the ties are made form stainless steel and are designed to last the life of the building. For a building the age of yours, it is possible that they are made from mild steel, which means they might show signs of corrosion. [I should add that I am not familiar with Spanish building and they could be made of something else altogether]. If done properly, the ties should be installed so that they take moisture away from the inner wall. The type of ties used and the spacing is now governed by Eurocodes and local regulations/national annexes. However, I am not sure if these would apply to modifying an existing structure or only for new walls. How would you get access to the cavity to remove and replace the ties? I'd have thought that would be quite difficult. I think the new ties would need to be embedded in the two skins rather than simply bridged by mortar or plaster, otherwise they would be quite weak. Also, cavity wall insulation is usually done from the outside, so you would drill holes in the exterior wall, insert the insulation and them refill the holes. That would seem easier than trying to make good the interior walls.

Insulating any cavity where interior and exterior walls meet is a best practice that's, well, not typically practiced. Unless you're building a really, really tiny house, the amount of heat transfer at those intersections will be pretty small compared with the home's overall heat transfer. I don't know what the Texas residential code allows, but the International Residential Code (IRC) allows an overall heat transfer approach (UA method) to demonstrate compliance with code requirements. I believe Worthy refers to that in his post above. With the UA approach, your builder wouldn't need to insulate those cavities to comply with code, provided there are offsets elsewhere. I wouldn't push the issue. In my opinion, the more important consideration at the intersection of interior and exterior walls is air leakage which can be addressed by drilling some holes and using spray foam insulation to fill the cavity from the interior side. Since the volume is enclosed, I recommend a low pressure expanding foam and drilling a series of holes. If you buy a kit, I suspect you'll find other places to use spray foam as construction progresses.