There is a lot of debate about whether to put a vapour retarder (vapour barrier) on an insulated wall behind shower tiles. The code has not historically been specifically clear on this so we get lots of different interpretations. What the code does say clearly is that we should not have two vapour barriers on the same wall where moisture could collect and get trapped between them.
The USG Gypsum Construction Handbook (2000), somewhat the bible in use of gypsum in construction, states on page 127: "Vapour retarders must not be installed between water resistant panels and framing." Note that they do not state this as a "recommendation", but "must not be installed".
Vapour Retarders are supposed to prevent moisture problems in a wall, yet it seems that in the case of a shower, it could cause them. Let's look at the building science that could guide us to avoiding moisture problems caused by trying to avoid moisture problems!
A quick look at a vinyl shower stall or tub surround will show us that the vinyl is totally a vapour barrier in itself, and experience has it that the joints or pipe openings or even the drywall above the vinyl always lets a little, or even a lot of moisture in behind the vinyl. Where there is a vapour barrier just on the other side of the drywall this relatively small, to potentially large quantity of moisture will be totally trapped water that simply increases in wetness and causes eventual breakdowns in the drywall, even in water resistant drywall. This all happens hidden behind the vinyl and the vinyl covers it over so well that we don't know anything about it until there is already real damage in the wall.
A surprising fact about tiled showers and tub surrounds is that the tile field (which includes the grout) is simply not vapour proof and often not water proof. Even if there are no cracks in the grout and the pipe holes are sealed around the pipes, moisture very often accumulates behind the tile. Usually it goes in very slowly but even moisture entering very slowly that has no exit because of a vapour barrier just behind the drywall will eventually cause swelling drywall, rot in wood or even slow drips to the ceiling below. We usually see this first with the bottom row of tiles pushing forward because of swollen drywall.
The reality is that both of these systems can be considered a bit of a vapour retarder on the wall because they certainly slow down vapour trying to get back out of the wall towards the room. You have probably heard that two vapour retarders on the same wall are not recommended nor allowed by code, precisely because of this mechanism of trapping moisture. The code requires one vapour barrier on the warm-in-winter side of any wall insulation. Vinyl sheets or tile fields plus a polyethylene sheet over the insulation constitute two vapour barriers -- and a very high risk of moisture collection. Hence it has been found that imperfect as that tile field may be as a vapour barrier, it is better to let a little moisture migrate into the deeper wall cavity and slowly migrate out the wall, which is what happens when there is no polyethylene vapour retarder directly over the insulation. Even this forgiving assembly can get into trouble with either vinyl or tiles when there are really large cracks feeding the wall with streams of water, not just the migration of vapour.
Shower & Tub Surround wall membranes
Now, all of this may become a mute question as we begin to install waterproof membranes prior to the tiles. Well installed membranes have proven to be totally effective in stopping all moisture movement from a shower area into the wall, insulated or not insulated, and there is even some push to make them code required because they solve so many problems related to water.
These could be elastomeric rubber sheets or liquid applied coatings, covered with a mortar scratch coat to receive the tiles, or specific waterproof tile membranes like the Schluter Kerdi system (that I like so much) that are both waterproof and made to receive the tiles directly with thinset (no scratch coat).
With all these new possibilities the shower or tub surround wall will be totally waterproof before the installation of either tiles or vinyl when they are applied or installed correctly. This will also make it very clear that this is the one and only vapour barrier on the wall where there is insulation behind it. One added bonus to these membranes is that you no longer have to use either cement board or water resistant drywall on shower or tub surround walls as no water can get far enough back into the wall to cause any problems. In fact a close look at this concept shows that there is now very little space to store moisture behind the tiles (just the thickness of the thin-set) and by using thin-set mortars, not mastic type tile adhesive, there is no organic material present to create problems with what moisture might be there. Now moisture can go slowly in and out of the tile filed and not react at all with the wall behind the membrane nor the ceiling below! In fact with a well installed membrane, even water from large cracks in the tile grout or vinyl joints will be contained, controled and directed into the tub or floor drain.
If the builder has already put a vapour barrier over the insulation before the panel boards on the wall, do you need to remove everything to take out that vapour barrier when using a membrane system? The reality is that if your shower membrane installation is perfectly water and vapour tight -- all joints, penetrations etc perfectly sealed -- then it probably doesn't matter because the shower membrane will probably be a better vapour retarder than the one that was put on over the insulation -- staple holes, overlaps, etc. One of our general cold climate building science rules is that we want the surface the most resistant to vapour movement on the warm side of the insulation, and all other vapour resistant surfaces should allow moisture to pass more easily than that first warm side barrier. The exit path towards the cold side should always be more open than the entry path for moisture into the insulated wall. See What is the difference between a vapour barrier and an air barrier?
Of course, any wall that is on a non-insulated partition wall should be kept dry, but has no vapour barrier requirements because both sides of the wall are the same temperature -- hence condensation will not occur as any moisture moves through the wall. In fact without a temperature differential, there will be little driving force for the moisture to go anywhere. Steam showers create their own driving force so here you really do need a complete moisture tight membrane, even over the ceiling.
Keywords: Adhesive, Ceiling, Codes, Condensation, Construction, Cracks, Damage, Drywall, Grout, Installation, Insulation, Joints, Leaking, Membrane, Moisture, Mortar, Pipes, Problems, Rot, rotten, Rotting, Shower, Steam, System, Tiles, Vapour Barrier, Vinyl, Walls, Water, Waterproof, Wood