We had cavity wall insulation fitted last year and got a government grant toward the cost of the work. Two of my friends had mentioned in passing that they had the work done, and it appeared to be Ok, so we went ahead. First of all a surveyor came to check that the cavity was in good condition and not blocked by any rubble etc, and then the man came to put in the insulation. First of all he drilled holes in the outside wall of the house (which was the inside wall of our two porches). Then he had a blower to put the mineral wool into the cavity. The mineral wool looks like light feathery cotton wool – we found this out as there was a gap in our interior wall in the loft we didn’t know about and some mineral wool blew through. Luckily we noticed right away and the man fitted a barrier and continued. He then filled in the holes with cement. Our house walls are white and the holes looked like little beige spots, not really noticeable except to the purist. Purist hubby went round with a little brush on a stick and painted them white again. We have a built-out bedroom that has walls on three sides. The cavity wall insulation has made a marked difference to the heat retention in the room. It is south facing so warms up during the day, and now the room remains warm for most of the night too.