A bathroom generates more water vapour per hour than any other room in a house, and it does it in the smallest volume of air. Where that vapour goes when it leaves the room is a building science question, and the answer in an older Central Vancouver Island home is frequently: into the exterior wall assembly, where it cools, condenses and stays.
Why the coast changes the arithmetic. Vapour moves from warm to cold and from high concentration to low. In a Nanaimo winter that means from the inside of a heated bathroom outward through the wall, and the point where it reaches the dew point temperature is where it turns back into liquid. In a cold, dry interior climate the assembly gets long freezing periods and long dry periods. Here, the temperature hovers around a narrow band and the exterior air is already humid, so a wall that gets damp does not get a strong drying season to recover in. Slow, chronic wetting with weak drying is precisely the condition that degrades framing and grows mould, and it is invisible until something else prompts opening the wall.
What older island homes were actually built with. Housing stock across Nanaimo, Ladysmith, Chemainus and Cedar spans many decades, and the wall assemblies vary accordingly. Homes from before the war may have no insulation in the walls at all, with shiplap sheathing and building paper. Post-war houses often have thin batts and no vapour barrier, or a barrier applied only in patches. The 1970s and 1980s brought polyethylene, sometimes installed continuously and sometimes not. None of these assemblies were designed around a modern shower used daily, and none of them anticipated the airtightness that later window and door replacements added — which is the common pattern here: a house made significantly tighter over the years without its ventilation being upgraded to match, so moisture that used to leak away now has nowhere to go.
A vapour barrier only works if it is continuous. A polyethylene sheet with gaps at the top plate, unsealed penetrations for pipes and cables, and staples every 300mm through the middle of it, is not a vapour barrier — it is a partial one, and the moisture simply goes around it. Doing it properly means sealed laps, sealed penetrations at every pipe, cable and box, continuity at the top and bottom plates and at the junction with the ceiling, and attention to the corners where an interior partition meets an exterior wall. Electrical boxes on exterior walls need airtight enclosures. Where the wall assembly or the climate zone suggests a different strategy — a smart vapour retarder that permits drying inward in summer, or exterior insulation that keeps the sheathing above the dew point — that is a design decision made against the specific wall, not a default.
Insulation is not only about heat. Batts in the wall between a bathroom and an unheated space keep the interior surface temperature up, and a warmer interior surface is a surface that condensation does not form on. That is the mechanism worth understanding: mould in the corner of a bathroom's exterior wall is usually a temperature problem before it is a cleaning problem. Insulating the interior partitions is worth doing too, for a different reason — acoustic batts around a bathroom, and around the waste stack in particular, make an enormous difference in a house where the bathroom shares a wall with a bedroom, and costs very little while the walls are open.
Moisture and mould found during a renovation. Opening a bathroom wall sometimes reveals damage: darkened sheathing, degraded framing at the base of a shower wall, insulation that has compacted and stained. Where this is found, the scope is to remove and replace the affected material, establish and correct the source, allow the assembly to dry, and rebuild it with the vapour control and ventilation the original construction lacked. The cause matters more than the cleanup — a wall repaired without correcting the mechanism will be back in the same condition. Where the extent is beyond the scope of a renovation, the honest answer is to say so and bring in the appropriate specialist assessment rather than build over it.
Moisture-resistant construction is a system, not a product. It is the ventilation running long enough to clear the room, the waterproofing behind the tile stopping bulk water, the vapour control stopping diffusion into the wall, the insulation keeping surfaces warm enough not to condense, the paint system resisting what still lands on it, and the materials at the floor line chosen not to wick. Each covers a different mechanism, which is why doing three well and skipping the fourth still produces a problem — and all of them are cheap while the walls are open.