Sand and cement in my house's case. Rubbish insulation, utterly impermeable - combined that means the dew point (where water vapour condenses) is within the brick. The freezing point is too, and bricks are not exactly flexible when saturated with expanding and contracting substances.

Some insulating panels are also dreadful, but on the whole a semi-competent installer will do the thing right. The problem is, some aren't even that good - we're talking a minimum bar of an NVQ level 2, and that's if people don't decide to just.. go self employed and do it without training. No regulation at all, and there can't really be any until it is a licensed occupation.