Pnagle , you did well for asking the company to get the confirmation in writing. And they seem to take their job serious, so for € 1500 you would get a good deal.As Lucret has said the investment would pay for itself by the fuel saved on heating. A glass- or mineral wool filling is a tricky thing. Cavity walls are build to keep the outside moisture -the horizontal/diagonal rain- away from the interior part of the wall. Fibrous material can act like a wick, like a sponge, soak up the water and then sagg. Not only would gaps in the insulation pattern occour but once the moisture is trapped it's only slow to evaporate back when the weather(the outside wall) is dry again. Polystyrene beads do not sack, they take up very little water and , if fixed /bonded with a mineral based glue (not an organic glue!), they won't rot either.Even when dipped under water polystyrene keeps it air content and therefore is insulating. Life vests are filled with polystyrene, I have never seen a life vest or raft filled with glassfibre (smiley) .