Cavity Wall Insulation: How to Implement It Correctly

In construction, there are various technologies related to the use of thermal insulation, which are mainly distinguished by the positioning of the insulation: externally, internally, or in cavity.

The position of the insulation affects the distribution of temperature and the saturation pressure of water vapor in the wall, with the related consequences concerning thermal and hygrometric well-being.

To always have the values of the conductivities of the materials at hand and not have to consult the technical data sheets every time, you can make use of a professional software for energy simulation, equipped with a catalog of materials with their conductivities. It helps you calculate the transmittances of complex structures and stratigraphies by simply selecting the materials or layers that compose them.

Cavity wall insulation – TerMus PLUS software

What is meant by cavity wall insulation

The term cavity refers to the empty space of varying thickness that, especially in buildings that are not very recent, is found between the external cladding and the internal walls of the perimeter: the air contained in this space limits thermal exchange with the outside, thus taking on the role of thermal insulator.

The cavity can be of two types: closed or ventilated.

Cavity wall insulation represents an effective solution to improve the energy efficiency of the vertical opaque walls of a building.

This technique, although less known compared to the external insulation system, if correctly executed, significantly contributes to reducing thermal losses and increasing living comfort.

With cavity wall insulation, we refer to that design solution which consists of inserting an insulating material into the empty space between two walls, precisely defined as a cavity. The walls that delimit this space can vary in thickness and construction characteristics.

Thanks to the flexibility of materials and the method of intervention, cavity wall insulation can be applied both in new buildings and, in many cases, in existing buildings. Double-wall constructions are, in fact, common even in buildings from the last century, often made with two layers of bricks.

Thermal insulation must be carried out with particular care and attention, as inadequate insulation, improperly installed, or a wall not treated with the right precautions, reduces the energy performance of the entire building. It can, in fact, lead to the worsening of thermal bridges and create mold even in parts of the construction where it was not previously present.

If neglected or improperly addressed, thermal bridges can cause conditions of living discomfort, manifested by the formation of condensation and mold, as well as an increase in energy and economic losses for heating or cooling the internal spaces. Therefore, the analysis of thermal bridges constitutes an essential step for an accurate assessment of the energy performance of a building.

In order to avoid compromising the energy efficiency performance of the property and to prevent the risk of mold formation at points where the thermal bridge is present, you can use the thermal bridge calculation software: design structures identical to real conditions and immediately obtain highly professional results.

Insulated cavity wall column – TerMus BRIDGE software

Types of Insulation for Cavity Walls

Various materials can be used as insulation within cavity walls, each with specific technical and application characteristics. Here are the main ones:

• mineral wool: mineral wool is a generic term that includes both glass wool and rock wool, commonly used for their low thermal conductivity and sound insulation capacity;
• polyurethane foams: these materials expand to fill the cavity, creating a continuous barrier. They have a high insulating capacity but must be handled carefully to avoid environmental risks;
• cellulose fiber: made from recycled paper treated with fire retardants, it offers good thermal insulation and is eco-friendly;
• perlite and vermiculite: inorganic minerals that provide insulation, fire resistance, and do not absorb moisture. They are suitable for filling narrow cavities;
• reflective materials: reflective aluminum sheets or other metallic materials used to reduce radiant heat.

Empty Cavity Wall

When designing a double wall with an empty cavity, it is always advisable to ensure a minimum level of ventilation in the cavity to avoid moisture accumulation due to condensation or weather infiltration, which can result from design or construction errors.

For this purpose, it is generally sufficient to provide small openings at the bottom of the wall. Such openings can be created by installing metal grilles in place of a brick every two meters or by leaving some vertical joints without mortar in the first row of bricks (approximately one every three bricks).

It is important to consider the position of the insulation for this type of structure.

When the insulating panel is placed on the inner side of the outer wall, it is preferred to maintain an air gap of a few centimeters inward, which ensures protection against potential water infiltration and improves the overall thermal resistance of the wall.

In the case where the insulation is instead placed on the outer side of the inner wall, it is essential that the material has adequate vapor permeability and moisture resistance, as interstitial condensation can compromise its performance. Many insulations tend to absorb moisture, which can lead to a rapid degradation of their insulating capacity.

It is therefore advisable to create ventilated cavities, allowing the insulation to dry and reducing the risk of mold and fungus growth. Ventilation is achieved through appropriate openings on the outer surface, while the thermal insulation properties mainly depend on the quality of the inner surface and the insulating layer.

To address the issue of thermal bridges, common in frame structures, it is possible to extend the outer surface and the insulating layer so that they cover columns and floors, thus improving the continuity of insulation and reducing heat loss.

Advantages and Disadvantages of Cavity Wall Insulation

This type of wall primarily presents the following advantages:

• the load-bearing wall inside has, or is generally designed to have, high thermal inertia that accumulates and retains heat, ensuring excellent insulation and thermal comfort;
• the cavity allows the use of a wide variety of materials, including loose materials (cellulose fibers, perlite granules, expanded vermiculite, cork granules, etc.) that are not suitable for external application;
• the characteristics of the brick material allow the (external) visible wall to withstand, for a long time, the freeze-thaw cycles that may occur during the life of the building;
• the wall and the greater weight of the visible brick guarantee a very high sound insulation capacity (about 50 decibels). This increases by 0.7 decibels for every increase of 10 kg/m2 in the weight of the wall.

Like any solution, cavity wall insulation has some limitations that require careful evaluation before choosing the most suitable insulating system. Firstly, the presence of the cavity increases the overall thickness of the walls, which can be problematic in buildings where space is already very limited. In the case of existing buildings, moreover, the dimensions and configuration of the cavity may not be ideal for effective insulation, thus reducing its performance. Finally, in some contexts, installation costs may be higher than those of other insulating techniques.

How to Achieve Cavity Wall Insulation in New Buildings

Implementing cavity wall insulation in a new building is quite simple. Already in the design phase, the structures and the main construction nodes are studied considering the insertion of insulation in this way. Indeed, double wall structures are prepared that can be made with various materials such as brick, concrete, or wood. To achieve the best thermal performance, it is advisable to use two layers of wall with different thicknesses and masses, carefully considering the overall behavior of the structure. The choice of insulating materials is wide and flexible, as the insulation is installed directly during the construction of the wall itself.

How to Achieve Cavity Wall Insulation in Existing Buildings

In the case of existing buildings, it is necessary to resort to the blow-in technique. The blow-in technique for empty cavity walls involves inserting loose insulating material into the space between the perimeter walls of a building. This intervention is carried out by making a series of spaced holes 80-100 cm apart, about 30 cm from the floor and ceiling, on the outer or inner side of the wall. Blow-in insulation is used not only in wall cavities but also to insulate attics and cavities such as shafts and joints.

A disadvantage of this technique is that it does not provide visual control over the precise placement of the material within the wall, making it difficult to ensure uniform and compact distribution, without voids or accumulations.

Moreover, this technique can negatively influence thermal bridges, even promoting the formation of thermal losses at the floors.

However, blow-in insulation may be the only possible intervention and presents significant advantages in terms of low costs and speed of installation compared to other insulation solutions.