I want to insulate internally. Battens on walls vertically + cellotex boards in between. Silver tape the gaps to keep airtight. Then plasterboard over top. I was going to use CLS battens as its cheap. 63mm depth. But insulation boards are either 50, 60, 70mm etc. 1) Which thickness should i use for boards? 60mm? 2) Or should I try and get 50mm rough sawn timbers and use 50mm celotex boards. 3) Any recommendation on insulations boards would useful too, there seems to be so many - Celotex, Kingspan, Ecotherm etc. 4) Is this a reasonable place to buy online? I did notice some places on ebay too which sell 'seconds'. https://www.insulationexpress.co.uk/celotex-ga4000-insulation-board (House is 1950 detached with cavity walls. 1930's style of house. But both inner and out leaf are red brick which makes it generally cold).