Insulation advice

[pajw]

I'm after some advice about insulating a timber frame.

The timber frame is on the first floor and is three sides of a bathroom.

Timber frame is made from 140x50mm timber and was built in the 1980's. The outside of the frame there is cement render over a layer of ply and there is a waterproof membrane between the ply and frame. Everything is watertight and bone dry.

I recently removed the plasterboard inside the frame as some of it was damaged removing the ceramic tiles and i also needed to reroute pipework and electrics.

Previously there was about 100mm of rockwool inside the voids in the wall and i'm going to put 100mm of Kingspan insulation board in the voids while I have the plasterboard off.

My question is should i push the PIR board right into the void up to the  membrane/ply or leave an air gap. The gap wouldn't be ventilated.

My plan was to push the PIR right in and seal the edges with expanding foam, that would leave a 40mm void for pipes etc. I would then fit a vapour barrier and moisture resistant plasterboard on the inside of the frame. Does this sound OK. I know there will be thermal bridging through the timber frame but I'm hoping that will be negligible and am ignoring it.

Thanks