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I have been carrying out a bit of DIY in my bathroom, in a new wall which was put up with my loft conversion which happened of the winter. Inside this new wall, I've found a significant quantity of insulation (rockwall or something of the like).

Clearly this was put in for a reason. But this is in a wall separating my bathroom from my internal staircase to the loft, so there is unlikely to be much of a thermal gradient between these two rooms.

Can anyone tell me why this was put in? Thermal insulation seems irrelevant, unless maybe it's to increase the fire redundancy of the partition wall. Only other idea I had was sound insulation, but wouldn't expect it to have that much effect.

I'm in the UK in case that helps anyone.

It was probably installed to reduce sound transmission. It's common to see materials that are more dense than fiberglass used as acoustic insulation, but fiberglass is used as well.

The sounds of water running and being flushed, drawers being closed, and other... human activities is often something folks wish to reduce in areas designed for serenity, such as a loft.