I know that this has been come up before. However I have had a serious problem with rats invading my loft, especially in the winter. Sick and tired of putting rat bait in the loft then waiting to see if the little darlings had the decency to go outside to die, I have tried Pestguard PST093 devices.
I have two lofts, both quite large, with a 200mm layer of insulation. Despite this the devices seem to be a complete success. I have heard a rat shuffle around for a short time then silence, showing that it couldn't stand the radiation. The devices have been in about two months now at the worst time of year. So these things work and I recommend them.
Of course I have no connection with Pestguard except as customer. Though they cost over 30 pounds each, and you do need to run a mains supply, I reckon they're worth it. I was spending a fortune on the little blue granules.
Peter Scott