Insulation in a bathroom serves two completely separate purposes. Thermal insulation on the envelope — the walls that face outside — keeps surfaces warm. That warm surface is what prevents condensation from forming on the wall behind the shower. Acoustic insulation on internal partitions — the walls between the bathroom and the bedrooms, the walls around the toilet stack — keeps sound in and out. These two jobs call for different material decisions and different wall details.
A cold surface behind a shower is the everyday thermal symptom people notice. The wall behind the shower is outside the building envelope. If that wall is not insulated, the interior surface is cold. Humid air from the shower meets that cold surface and condenses. Condensation runs down the wall, soaks into the wall assembly below, and mould starts growing. Putting thermal insulation in that wall keeps the inner surface warm and prevents condensation. This is why insulation on the bathroom envelope is necessary.
An outside wall can be insulated from the inside, but the insulation layer has a cost — it takes up floor space or wall depth. Some walls cannot be insulated from the inside without losing room width. In those cases, the cold surface is accepted, and condensation is managed with ventilation and humidity control rather than with thermal insulation. The choice is made during planning with full knowledge of the trade-off.
Acoustic insulation is a separate job. A bathroom is loud. The toilet flushes, water runs, the exhaust fan runs — all of these are noise. The walls between the bathroom and bedrooms need acoustic treatment to keep that noise out of the bedrooms. This is batt insulation in the wall cavities, around the drain stack, and at penetrations between the bathroom and adjacent rooms. This acoustic work is done while walls are open, and it is almost always worth doing.
A floor over an unheated space — a basement, a crawlspace — feels cold underfoot. The floor assembly does not have insulation. Putting insulation in the cavity below the floor warms the floor surface and changes how the room feels. This is a separate job from the walls. It is specified during planning so the work can be done during construction.
The insulation strategy for a bathroom is really three separate conversations: the envelope (outside walls), the partitions (internal walls), and the floor. Each has its own material, its own performance goal, and its own trade-offs. Most bathrooms only have one conversation happen — usually the envelope, if it is an outside wall — and the other opportunities get missed.