If you’re looking to make your home more energy-efficient, chances are you’ve considered improving your insulation. Properly insulating your home is a great choice because you’ll spend far less on heating your property throughout the year, as less heat will escape. 

Two types of insulation you may be considering are external wall insulation and cavity wall insulation. Here’s everything you need to know about these two systems.

What is external wall insulation?

External wall insulation, also known as EWI, is insulation fitted to the outside of a home, where cavity walls are not present. These tend to be properties of solid wall construction or system-built homes with steel or timber frames. EWI insulation boards are usually made from EPS, mineral wool, phenolic or wood fibre.

What is cavity wall insulation?

Cavity wall insulation, also known as CWI, is a type of insulation inserted into the space inside the walls of a property – the cavity. The cost of these systems can vary depending on the material used for insulation, with cheaper options such as fibreglass and more expensive options such as polystyrene bead available.

External wall vs cavity wall insulation: which type is right for my property?

The external wall insulation vs cavity wall insulation debate can be somewhat misleading, as the type of insulation you choose will depend on the type of walls your property has. If you are unsure what type of walls you have, you may be able to tell by the brickwork. If all the bricks are laid lengthways, it is likely a cavity wall. If the bricks alternate between long and short, then it is likely a solid wall.

For example, if you have cavity walls, then cavity wall insulation will probably be the best option for you. However, if you have a solid wall that you want to insulate, then external wall insulation or internal wall insulation will be your options. On this point, it is worth noting that internal wall insulation requires a lot of preparation and will reduce the interior space of your rooms, whereas external wall insulation will not.

Whilst you can’t install cavity wall insulation if you don’t have a cavity wall, it is possible to install EWI in addition to a cavity wall that already has CWI – it can provide an added layer of protection to the exterior of your property, giving you even greater thermal efficiency.

Which type of insulation is best?

There are some distinctions between the insulation properties of the two systems. External wall insulation tends to offer far greater energy efficiency than cavity wall insulation because the amount of insulation you can use is not limited by the size of the cavity. EWI systems can be as thick as 200mm which provides great thermal efficiency but won’t restrict the space available in the property interior.


The bottom line in the external wall insulation vs cavity wall insulation debate is that EWI provides greater insulation, but your property’s wall type will determine which system you can install. Many cavity wall homes may already have had CWI, so even if your home has cavity walls, you may still be able to have EWI installed to make your home even more energy-efficient.