A fully insulated timber frame building panel system incorporating wall, ceiling and floor panels of varying dimensions which are made up of vertical timbers 20 and horizontal timbers 34 & 36 sheathed both sides 21 & 22 to create a cavity which is filled with expanding polyurethane insulation 27. Breather membrane 24 is then attached to the external face of the panel and heat reflecting membrane 25 to the internal face. Battens 23 & 38 are applied to the internal face of the panel, which is then sheathed in gypsum based boards 32 to form an air gap 29. Extruded polyurethane insulation 26 is fixed to the panel through the breather membrane 24. External cladding 31 is always fixed to the timber frame to create a cavity 30. This external cladding can be of a variety of materials.