If you put that crap in your walls, you'll have condensation and eventually damp.
The cavity is there for a good reason ;)
i've not got cavity wall insulation in my house, and you can bloody tell when it turns into late autumn/winter. The heating just disappears out of the house. I can get it warm, but it will not stay warm and that racks up the bills.
I've heard mixed opinions about cavity wall insulation, and yours is not uncommon; it will create a damp bridge and cause more problems longer term. But what do i do? it's really quite chilly in my house without the heating. Surely i can increase the efficiency some how...