Hi, I'm currently building a 2-storey extension to my cottage. Up to first floor height the inner-leaf has been built using 7N blocks as it forms part of a retaining wall on 3 sides. However as we build up past the first floor I have a choice of using thermolite blocks or 3.5N medium blocks. Can anyone tell me which route to choose as I have to order the materials to crack on with the project. I understand thermolites have a better insulation factor but feel that the 3.5N medium blocks are more robust. Please help, many thanks