I’m looking for a cheap improvement to provide a bit of insulation to my downstairs toilet. I’m not after toasty warm, just something better than the igloo it was last winter.

I’m currently re-tiling and have moved an extractor fan from B to position A. In doing so, I’ve discovered it’s only single brick (4.5″ thick) behind the cavity. The A & B walls were originally the exterior walls and the shed was an extension around them it seems.

So, is the best cheapest solution to clad the inside of the shed walls in polystrene sheet and pour some EPS beads into the cavity? I’m not going to clad the inside of the toilet as it’s small enough already.

There’s no electrics in the cavity just copper & plastic water pipes.

I’m not clear on whether you want to include or exclude the shed from your thermal envelope.

Also you seem to be venting the extractor into an enclosed space (the shed).

Excluding the shed.

Yes, I realise I’m venting into the shed but that’s how it was previously and the sheds no where near airtight so I’m sure it’ll be fine.