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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. This is one of the ways you can tell whether your property has cavity walls.

Another tell-tale sign of whether your property has cavity walls is the appearance of the exposed brickwork. Brickwork laid lengthways indicates a cavity wall.

Typically, the two skins have always been held together with ties made from metal. These ties used to be made from iron, mild steel, or copper. Such materials were prone to corrosion. Wall tie corrosion was first noticed during the 1960s, and from this point, stainless steel became the material of choice for the manufacture of wall ties. Stainless steel is more resistant to corrosion than the earlier metals. This helps keep the structural integrity of the building intact for longer.

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. However, the amount of insulation is totally dependent on 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. Rigid wall insulation is already in place on most modern properties. Cheap fibreglass insulation is usually the most popular solution when retrofitting properties. Alternatively, polystyrene bead insulation is an option, but it 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. To insulate it you can either add insulation to the inside or outside of the property (i.e. IWI or EWI).

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. Check the width of the wall to see if it rendered. Walls under 260mm are very likely to be solid wall. The vast majority of properties built prior to the 30s will be solid brick.

Photo courtesy of TheGreenAge

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. External wall insulation is unfortunately fairly limited at present.

There is a range of options when it comes to external solid wall insulation. EWI boards are typically made of EPS, Mineral Wool, Phenolic .The insulation boards are secured to the wall using adhesive and screw fixings. 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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