Air acts as a natural insulator – in other words, heat does not travel through the air very easily. Most people know that the best way to keep your body warm is to wear several layers of clothing. This is because the layers trap air in between them and keep it still. CWI works in a very similar way. You can also look for high-quality wall insulation products online.
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If you're not sure about the age of your house, another simple way to tell is by looking at the brick pattern of your outside walls. In a typical cavity wall, all of the bricks will look the same, with the long side of the brick visible on the outside. With solid walls, the bricks will be in an alternating long-short pattern.
What if your walls are rendered or the brickwork isn't visible for some other reason? In that case, take a look at the thickness of your exterior walls. You can do this easily by looking at a doorway or a window. If it is more than 260mm thick, chances are you have cavity walls.
Finally, how do you know if a previous owner hasn't already had the CWI installed? You can actually tell by taking a close look at your external walls. If the cavities have already been insulated, you will notice a few small drill holes in the mortar between the bricks – they will have been filled and won't be very obvious, but look closely and you will see them.
If it turns out you have solid walls, or for some other reason your house is not suitable for CWI, then you may want to consider solid wall insulation instead.