I’ve had the process done on three occasions in three houses. The loft insulation works; sadly I have logged the difference and found really good savings can be made. In most cases, a 25 per cent saving in energy use is to be had. However, cavity wall insulation is rubbish; it has minimal effect on the heat losses from your house - again I have logged it.
Let’s look at why this is. The problem is that the lagging applicator (I refuse to call him an engineer) never actually fills the cavities because he knows you can’t easily check his work. The only way to see is to either drill loads of holes yourself and look inside the cavity with a bore scope or measure the inside wall temperature with a thermometer and find the cold spots... yes I’ve done it.
The problem lies in the method of application. A man drills 22mm holes all over the outside of your house and using a reverse vacuum cleaner pumps the cavity’s full of insulation. The lagging doesn’t go very far into the cavity unless it is very wide and has no debris in it. So it doesn’t work if the house is old - brilliant.
This weekend I was in the loft spaces lagging the last of my house, from up there I could see into the cavities, in my case only 100mm wide. See the picture above for how effective the cavity wall insulation has been.
There is nothing in the cavity at all except cold, fast moving air. The lagging has only entered up to 250mm from the point it was sprayed in. Annoyingly, the holes are over 1,000mm apart.
So Mr Huhne and chums: If you insist on giving all the money to the insulation fitters, maybe you might want to check they are doing a good job.