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.

I recessed a medicine cabinet that was 'supposed' to be wall mounted, just framed for it and shimmed plumb and screwed into predrilled and countersunk holes, similar to window installation. Then I used joint compound over the screws and painted it. Then I drywalled up to the edge of it and caulked around it then painted to finish it out. Inside of my cabinet was white so it was easy to just paint white where needed, would be harder with a wood finish, possibly just leave the fasteners visible. Has to be installed during framing and take thickness of the drywall into account when recessing.

I did a little thinking on this. Light socket to left of mirror- it is attached to a stud on the right side of the little electrical box. I am going to replace it with a 3 capacity box, putting the socket in the spot on far left, then patch/cover the other two. This will allow me to move the mirror down a few inches to be centered on vanity. I will still have room between it and framed the mirror. Lights- I will then be able to center the lights on the vanity/mirror, not on the sinks. But the original hollywood light had one wire in middle of fixture. To install two lights I will need to split that and run through the studs to get to new opening for new fixtures. I was not thrilled about patching drywall to do this. But...... if I remove the mirror ( which is not glued as originally thought) I can open up the wall behind the mirror, do the split, bore through studs, run new wires up to the new fixtures holes. Then I patch the wall which will be covered by the mirror so it doesn't have to be perfect!!!! It is a lot of work but will be worth it in the end. Can't wait to finish and post pics.

Just wanted to give an update on this since so many of you had ideas on what the issue could be. Since the last post we just last week had the drywall removed, new drywall installed, a new cabinet and hooked everything back up. It was under the builders impression that the issue was still caused by the constant fan. However, shortly after the last post we discovered that our Septic Tank was never turned on. It began overflowing into the backyard, which is right behind the side of the home with the moisture issue. We were concerned about why the septic pump alarm never went off and that was because it was never connected by the electrician. We moved in last November so it was off for that whole time. We never noticed back up toilets or any odors or even the toilets making bubbly noises. We got that hooked up correctly and it started working. Last night when I was cleaning my sons room I noticed the edge of the wall on the hardwood flooring had water stains that were solid black. We have the Chesapeake Bay Gray Fog Engineered Hardwood flooring thru-out our whole first floor except for the wet areas where we have tile. The black spots we noticed are the same spots that had high moisture readings when we noticed the mold issue. The builder said the two issues werent related and that we would deal with the moisture after we took care of the mold issue. Since I have looked into all of these possible solutions, my only idea of what could have caused all this is the flashing on the home. It looked correct when they removed the brick on the side of the home but it has to have gaps in places that allowed water to leak in. the water spots are about every two feet away from each other in my youngest sons room and we found one spot in the dining area which is on the front of the home and then in the bedroom next to the bathroom that had the mold. Thank you for all of your help researching and figuring it all out.