Wall insulation comes in various types, depending what kind of wall you want to insulate. There is:
A cavity wall is made up of two walls with a gap in the middle. Usually, one wall is made of brick and the other is made of concrete. The gap between the two walls means that air runs through it lowering the temperature in your home and making it more difficult to heat. Cavity wall insulation is applied by filling the gap in the wall cavity with a suitable insulating material. Most houses which were built after the 1920's will more than likely have cavity walls.
If your home was built before the 1920s then it is likely to have solid walls. Therefore, the walls do not have cavities. The walls will usually be made of brick or stone. The absence of cavities does not, however, mean they are an effective form of insulation as a solid wall can be just as ineffective as a cavity wall.
If you want to see if your home has a cavity or a solid wall you can usually tell if you look at the brickwork outside your home. If you can see full-length bricks with half-length bricks around the middle of the wall this is a good sign that you have a solid wall. Another way to find out is to measure your wall from a door or window. If it is thinner than 30cm it is probably a solid wall. It is possible to insulate a solid wall with internal or external insulation.
Internal Wall Insulation can be done by applying boards or rolls of insulation to the walls. The installation will mean that your floor space is slightly reduced because of the extra thickness of the walls. The brickwork will also have to be prepared beforehand including re-plastering any existing damage to the walls before the insulation is fitted. If the walls are unsuitable for boarding for any reason, then a stud wall would have to be built. This would further decrease the size of the room.