If you determine that you have a hollow block house you should always check to make sure the top of your cavity is fully closed. If it is not this results in a level of thermal looping that makes a nonsense of a U-value calculation and definitely results in heat loss and much poorer airtightness. Many people are not aware of this and are not taking the proper precautions, resulting in an under performing external insulation system. All good installers should be able to make you aware of best practice.