I have been redecorating the rooms in our 1925 non cavity wall brick built semi, and taking the opportunity to remove/re-plaster, as well as insulating the outer wall with 25mm kingspan. To prevent losing too much room space did not leave a void, only the natural gaps from the unevenness of the wall. Seems to have helped, but would it have been much better to have a 25mm airgap? Also we have a huge end wall which the stairs go up, so next job is that wall, what would be better 15mm with airgap/25mm no air gap? or to make it as easy what effect would 15mm no airgap produce in insulation value?