We live in a semi D . The front and side walls are cavity walls, The back wall is built with Cavity Brick ( I think) & dry lined approx 13" deep , I have checked wall vents . Is it worth my while upgrading insulation in the cavity walls, and not improving insulation on back wall , as external insualtion is very expensive, i hear .
Any extra insulation is worth doing imho. I'm actually thinking of getting cavity wall insulated on the front of my house as their seems to be a lot of air getting in from that area. I'm thinking of the white beads of polystyrene or whatever its called being pumped in.