I have lived in my house since new over 15 years ago, it is a 3 bed detached house on a close of mainly all detached houses. I saw alot of these houses being built around us for a couple of years after we moved into ours. The builders put foam blocks into the wall cavities as the walls went up for insulation. Lately I have noticed some of the houses having cavity wall insulation foam being injected. Is this necessary considering what is already in place?