What is a Cavity Wall?

Cavity Wall Insulation can be a great home improvement if it is installed correctly and to well-maintained properties with clear cavity walls.

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What is a Cavity Wall?

Since the late 1930’s the process of building cavity walls was adopted as standard. The reason for this change of construction from the historic solid wall process was to prevent moisture/water penetrating through the outer skin of the building to the inner skin of the building. Unfortunately, this step change in buildings construction didn't substantially change the heat flow characteristics of the external walls.

The cavity (air space) between the external building structure ensures that any water entering the cavity simply drains down to the footings below the damp proof course without crossing to the inner structure. However, since cavity wall construction was incepted, properties have been identified as having minor and sometimes major debris/obstruction blockages in the air space (cavity).

These obstructions in the cavity can be caused by poor building practices such as not removing mortar droppings during construction. This allows blockages to build up in the cavity; sometimes debris is caused by subsequent building alterations such as window/door replacement or the fitting of balanced flue/vents all of which can lead to moisture penetration problems by transmission of the moisture across the blockages to the inner wall construction.

What is Cavity Wall Insulation?

Cavity wall insulation is used to reduce heat loss through a cavity wall by filling the air space with material that inhibits heat transfer. This immobilises the air within the cavity (air is still the actual insulator), preventing convection, and can substantially reduce heating costs. Cavity Wall Insulation is installed by drilling a set pattern of holes into the mortar joints on the outside of the property, and insulation is injected into the cavity. The insulation is gravity fed from the bottom up.

There are three main types of retrofit insulation that has been used across the years:

Urea-Formaldehyde Foam was one of the very early products used as cavity wall insulation. A liquid based resin was mixed on-site and injected into the cavity walls by drilling small low-level holes into the outer façade, the resin would fill upwards and fill the cavity walls (and anything else that hadn’t been sealed in the process!).


When Urea-Formaldehyde Foam Insulation was injected into old properties, it was installed as a mixture of formaldehyde resin, adhesive and compressed air. The foam tended to solidify in just a few hours and then cure over the following week or so. The problem is that this type foam shrinks as it cures – and shrinks significantly. Often this issue only came to light when the homeowner revealed the cavity (by replacing windows etc.) but it meant that there were areas across the wall surface where there was no longer any insulation, resulting in cold/thermal bridging.


The other issues with urea-formaldehyde foam insulation is that it also breaks down as it gets older, changing from a solid foam mass to a crumbly powder. This normally means the insulation slips down with the cavity leading to the same sort of issues associated with shrinking – cold spots across the wall area where there is no insulation.

The final issue with the urea-formaldehyde cavity wall insulation was the release of formaldehyde as it broke down –  it is believed that formaldehyde can to lead to a variety of adverse health effects impacting the eyes, nose, and respiratory system.

This is no longer used in the UK as cavity wall insulation as new technologies have been developed that propose far better thermal values, easier to install and lasts longer and less detrimental health.

Below are some images taken inside a cavity showing how the insulation has broken down and needed to be extracted.

The most commonly used insulation for cavity wall applications. Loose insulation fibres are blown into the cavity under pressure and expand out to fill the space from a pre-determined drilling pattern. The fibres are said to be coated in silicone or very early fibre products, coated in formaldehyde to provide the product with properties to reduce moisture retention. With UK weather conditions becoming more adverse and in some cases, poor building maintenance can expose the insulation to higher levels of moisture ingress therefore saturating the insulation.


The below images are examples of saturated insulation that has been poorly installed leaving voids within the breached material. The only way to remedy the problem is to extract the insulation and clean the cavity walls of significant debris and rubble.

The first image also shows signs of a corroded wall tie that may be as a direct result of the wet insulation being left for a prolonged period before being extracted and remedied.

EPS Bead (Expanded Polystyrene) is known as one of the most technologically advanced cavity wall insulation products in the market. It is installed in a very similar way to fibre/wool insulation by using a pre-determined drilling pattern and each hole is injected with the bead under compressed air. The bead (upon injection) is coated with a bonding agent so the beadS stick together inside the cavity.

By the way it is injected, the beads typically fill the cavity wall very well and often pose less problems reminiscent of other cavity wall insulation materials.

However, the cavity walls must be clear of debris and rubble prior to installation and the building fabric must be in good condition.

Below are some images taken of poorly installed bead that resulted in cold spots (thermal  bridging) that left the property with damp problems on the inner walls.

This short clip shows the effectiveness of EPS-Bead Insulation against high volumes of water:

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