How to insulate under uPVC cladding

[thespaceman]

I'm going to replace T&G wood cladding with uPVC on the outside of my house. I suspect there will be minimal insulation under the boards as it was built in the late 60s

The boards are nailed to a frame and on the other side (inside) there is plasterboard.

I would like to fill the void with foam board (the type with foil either side) which is inserted between the frame uprights. Should I leave a gap either between the plasterboard and foam board or cladding and foam board?

Should I also fit some form of polythene sheet between the cladding and foam board to resist moisture?

Any advice welcome.

 

[noseall]

you could fully fill the void with rigid board insulation then fix a breather membrane to the studs prior to fixing the upvc clad.

no need for vent's.

 

[thespaceman]

Ok thanks. So no need for gaps then.