I didn't know the XPS was classified as semi-impermeable. See here, first link, p.7. Actually, I've never used any vapour barrier under garage floors. For these and a few other questions you've posted, there would have best been recommendations in your building specs from a local P.Eng.* who reviewed the plan. However, I know many/most designers here don't go that extra step. Considering even the cost of a modest home, I think the $500-$600 cost would have been worth it. *mrs. worthy's OBY/GYN is also a P. Eng., but we've never solicited construction information from him!

A vapor barrier protects the warmth and integrity of your home. Below was pasted from the internet. The friend is incorrect in his statement. New homes are very tight due to better barriers and very good sealing windows, but eliminating barriers causes bigger issues. Very tight homes you can leave a window cracked at times. Some newer heating/cooling systems have exchange systems built to pull in fresh air. I personally use poly sheeting as a vapor barrier because it is far superior to faced insulation. Regarding ceilings, check your local building codes, this may vary. But as mentioned below, attics can naturally ventilate as compared with a closed in wall. Through a process called convection, warm air will always move toward colder air, meaning that during the winter the air inside your home is always trying to escape the house and move toward the cold air outside. Warm air also expands, and therefore has the capacity to hold more moisture than cold air -- moisture that is picked up naturally from inside your house. As the air cools, it contracts and loses some of its capacity, so the moisture is given off to cooler surrounding surfaces. The result of all this is that warm, moisture-laden air is constantly on the move from inside to outside. As it leaves the warm interior of the house and enters the walls, attic, and crawl space, it cools off and gives up its moisture, which in turn can saturate the insulation and framing. The more moisture there is in the air and the more differential there is between the inside and outside temperatures, the worse the problem can become -- in the more extreme cases, the insulation is rendered virtually useless and severe rot can take place in structural members. It is the purpose of the vapor barrier to prevent that movement or penetration of moisture from one area to another. Placed between the inside of the house and the insulation, the vapor barrier acts as a physical shield to repel the moisture and keep it from entering enclosed cavities where it can do serious damage. Vapor barriers can be created from a variety of materials. The only requirement is that the material be resistant to moisture, which is indicated by its vapor permeance value, called a "perm" rating. To be an effective vapor barrier, a material needs to have a perm rating of 1.0 or lower. Common vapor barriers used with insulation include plastic sheeting, foil sheeting, and kraft paper that has been treated to achieve the proper perm rating. Vapor Barriers in the Home One of the most important vapor barriers in the home is the one between the wall insulation and the interior side of exterior walls. Because wall cavities are sealed and have no means of allowing accumulated moisture to escape, an effective moisture barrier is crucial in preventing warm, wet air from entering the walls. For wall insulation, foil- or kraft paper-faced insulation is typically used. The insulation is installed between the studs with the vapor barrier facing in toward the house -- the one thing to remember about vapor barriers is that they are always installed between the insulation and heated portion of the house. The "wings" along the edges of the insulation are stapled to the studs to hold the insulation in place -- stapling to the face of the studs instead of the sides creates a more effective vapor barrier. Another alternative for walls -- which is also commonly used in the enclosed cavities of vaulted ceilings where the pressure of warm air trying to escape is even greater -- is to place unfaced insulation into the cavities between framing members and then cover the insulation on the inside with plastic sheeting. The sheeting is stapled to the face of the studs or joists, and overlapped at all seams to prevent moisture penetration. In today's construction, separate vapor barriers are typically not used between the home's interior and a ventilated attic. Most attics are insulated with blown fiberglass, which is relatively porous in comparison to the denser batt insulation used in the walls. The attic also has a continuous flow of air through it that is created by the roof and soffit vents, so moisture from inside the house passes through the insulation and is removed from the attic by the air currents. In areas of high humidity where moisture in the attic could still be a problem, a vapor barrier paint can be applied over the inside ceilings. Another area that typically does not have a vapor barrier is the floor. Most of today's homes are now constructed with subfloors of interlocking tongue-and-groove plywood or other sheet materials -- waterproof resins are used in the making of the sheets, so the subfloor itself becomes an effective vapor barrier that protects the insulation below it. A vapor barrier is used, however over the ground under the house. Typically a layer of 6-mil plastic (1 mil = 1/1000 of an inch) that is laid directly on the dirt floor of the crawl space, the moisture barrier prevents ground moisture from entering the enclosed crawl space and wetting the insulation or the framing.

So now your plasterer is an expert on not just Building Science, but bathroom construction too? The vapor barrier in an exterior wall does work well in new construction. It doesn't just prevent vapor from being driven into your wall assemblies, but it stops air infiltration as well. Moisture can get into the wall even with no air movement. The air movement carries moisture into/through the wall. You'll note many older leaky homes don't have moisture issues that today's homes can have. That's because they are so drafty. Air can whistle into the walls, but it can also whistle right out the other side, so the walls can easily dry. But the leaky wall also carries your warm interior air outside in the winter, and allows hot humid exterior air to be driven into the wall assembly in the summer. That kills your energy use when heating and cooling. So, a properly detailed vapor barrier in an exterior wall will indeed stop air movement, it will also stop moisture vapor from transmissing into your walls. That can increase comfort and save your energy dollars. As to the bathroom...what did you use for a tile backer? Water can get through tile/grout. If your tile backer is standard gypsum board, they GB will eventually soften and decay. If your tile backer is cement board, cement board is porous, water and moisture vapor can pass right through it. That moisture will then hit your vapor barrier and be stopped there. Over time the vapor will travel back into the bathroom, it'll exit the wall through the path it first entered. With your plasterer having removed the vapor barrier, any moisture that gets into the wall assembly will now wet saturate the framing and sheathing. Moist wood in a dark space. Perfect for mold and wood decay. So will you have problems? Maybe yes, maybe no. Depends on how the thermal envelope of your house performs as a whole, and it depends on how your shower performs on its own. But building code does require a vapor barrier of some sort between the shower tile and the framing. That vapor barrier can be 6-mil poly sheeting, or it can be the surface skin of the tile backer board (depending on which board you use), or it can be a topical sheet membrane like Kerdi, or a topical trowel or roll-on membrane that meets the parameters of a vapor barrier.

Just another idea... If expense and sourcing is a concern, do understand that you don't need either RG or HB. This is a tub, and tubs surrounds see water exposure, but not as much as the lower walls of a fully tiled non-tub shower or the tiled pan of a shower floor. Let's say your horizontal tub flange is will be 18" above the floor after the tub is installed. Run a 24" high strip of 6-mil poly horizontally across the bottom of the tub wall. Detail the poly at the sole plate as required. Set the tub. That previously installed piece of poly will now extend 6" above the height of the tub flange. Now run a generous bead of caulk horizontally across that exposed 6" of poly, say 3" above the tub flange. Now hang another sheet of poly horizontally across the wall from the ceiling down to a couple of inches below the tub flange, with the bottom edge hanging into the tub. Detail it at the ceiling, lower down press it into the bead of caulk. Let the excess overlap the tub flange and hang into the tub a few inches. Depending on the style of your tub flange you can just let the poly sit as-is, or you could lift that overhang and run another bead of sealant on the tub flange to seal the poly to the flange. Install your cement board. Cut the excess overhanging poly off at the bottom edge of the cement board with a sharp utility knife. That'll provide you with a continuous VB behind the tub, as well as draping the poly over the tub flange if needed for drainage, and save you the cost and labor of installing the topical membrane.