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Cavity Wall Insulation vs. External Wall Insulation

What is a Cavity Wall?

Developed in the early 20th century, cavity walls were introduced into building practices as a means of alleviating problems with damp.

As opposed to traditional solid wall properties, cavity wall properties were made up of two layers of brick with a gap in the middle of around 20-100mm wide. Cavity walls are therefore much wider than solid brick walls, which is one of the ways you can tell whether your property has cavity walls.

Another telltale sign of whether your property has cavity walls is the appearance of the exposed brickwork. If the bricks have been laid lengthways, you probably have a cavity wall.

What is Cavity Wall Insulation?

Cavity wall insulation is the process of filling the gap between the two layers of bricks with different types of materials. It’s fairly cheap to install and on the average house it only takes a few hours. Cavity wall insulation is cheap, easy to install and can save you plenty on your bills. Most properties in the country that are able to install this form of insulation have already had it done. Worth mentioning, however, is that the amount of insulation that you can install, and therefore the energy saving result, is completely dependent upon the size of the cavity.

Cavity wall insulation materials can range in price and vary according to whether they are being retrofit or newly installed. Modern properties will be built with rigid board insulation already in place. When insulation is retrofitted for an older property, usually people go for a fibreglass type insulation which is pretty cheap, or a polystyrene bead insulation, which is more expensive due to its higher insulating capabilities.

What is a Solid Wall?

A solid brick wall is two bricks wide, with each row of bricks interlocking to form a completely solid 9-inch brick wall. There is no gap between the bricks as with cavity walls, so to insulate it you can either add insulation to the inside or outside of the property (i.e. internal wall insulation or external wall insulation).

As with cavity walls, you can identify a solid wall by looking at the brickwork. If the bricks alternate between a mixture of header (short) and stretcher (long) bricks, you have a solid wall. If the property is rendered, check the width of the wall – if it is less than 260mm, chances are that the wall is solid brick. The vast majority of properties built prior to the 30s will be solid brick.

What is External Wall Insulation?

SWI is more expensive than cavity wall insulation, whether it’s internal or external, the price is more within the region of thousands than hundreds. There is some funding available for external wall insulation but unfortunately, it is very limited.

There is a range of options when it comes to external solid wall insulation, however, EWI boards are typically made of EPS, Mineral Wool, Phenolic .The insulation boards are secured to the wall using adhesive and screw fixings, and a decorative, weather-proof render is then applied on top.

Choosing external wall insulation is probably the best bet when comparing it with internal wall insulation and even cavity wall insulation. This is because you can gain greater energy saving benefits due to the fact that the thickness of the insulation that you install can be as thick as 200mm – you aren’t restricted by the thickness of the cavity or the loss of internal space.

Mineral Wool external wall insulation prior to the application of the render topcoat.

If you have cavity wall insulation already and you’re looking to increase your thermal performance even further by installing external wall insulation, check out our dedicated blog post ‘Can I Install EWI on a Cavity Wall?’


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