Cavity wall insulation is an efficient way to improve an existing home with an empty or insufficiently insulated cavity. The space between the inner and outer wall is filled with insulation material to reduce heat loss through the external walls and improve indoor comfort.
Nu-Isoleren is a recognised installer and uses bonded Isomo EPS beads for technically suitable cavity walls within the officially approved ATG 3170 system. The insulation is injected through small holes in the mortar joints, so major interior demolition is normally not required.
A cavity wall consists of an inner and outer wall separated by a cavity. In many homes this space is uninsulated or only partly insulated.
Nu-Isoleren fills a suitable cavity with EPS beads injected from the exterior, improving insulation without opening internal walls.
Many existing homes already have cavity walls, but the space between the inner and outer leaves may be empty or only partly insulated. In this situation, the existing cavity can often be retrofitted with cavity wall insulation without opening the internal walls.
Small holes are drilled in the mortar joints and bonded EPS beads are injected into the cavity. The beads spread throughout the available space and form a stable insulation layer.
Before insulating an existing cavity wall, we always check whether the construction is technically suitable. This includes the cavity width, contamination, existing insulation, the condition of the exterior wall and any moisture issues.
Correctly installed cavity wall insulation can reduce heat loss and improve indoor comfort.
Besides reducing heat loss, many households notice the comfort improvement: internal wall surfaces feel less cold and temperature differences are reduced. The outside appearance is largely preserved because drilling takes place in the mortar joints and the openings are made good afterwards.
Not every cavity wall can be insulated without a prior inspection. We check the width and condition of the cavity, the exterior wall, mortar joints, contamination, existing insulation and any moisture issues.
Mortar debris, construction waste or old insulation may prevent new insulation from spreading correctly throughout the cavity. In some cases, the cavity must therefore be cleaned or the existing cavity wall insulation removed first.
A preliminary inspection allows us to determine whether the cavity is suitable for EPS beads and whether any preparation is required.
Nu-Isoleren uses bonded EPS beads from Isomo for cavity wall insulation. The expanded polystyrene beads are injected into the cavity together with a bonding agent. Once the liquid in the bonding agent has evaporated, the beads lose their mobility and form a stable, bonded insulation layer inside the cavity. This installation principle is also covered by the ATG 3170 technical approval.
EPS is lightweight, dimensionally stable and absorbs virtually no water. This helps it retain its shape, stability and insulating performance even where some moisture may be present in the cavity.
For many technically suitable existing cavity walls, EPS beads are therefore a strong solution for retrofit insulation. Before installation, we always inspect the condition and width of the cavity, possible contamination, existing insulation and the exterior wall.
Want to compare EPS beads with mineral wool and PUR? Read our guide to cavity wall insulation materials.
With cavity wall insulation, the insulation material itself is not the only important factor: the complete system and the way it is installed are equally important. Nu-Isoleren works as an approved installer with an ATG-certified ISOMOPEARLS system, registered under ATG 3170.
This means that the insulation system has been technically assessed and must be installed in accordance with defined requirements covering aspects such as the material, preparation of the cavity and installation process. A proper inspection and correct installation therefore remain essential for a durable result.
An ATG approval covers more than just the name of an insulation material. It describes the complete system and the conditions under which its stated performance applies. Installation by approved installers is also part of this framework.
Anyone wishing to verify the technical characteristics, application requirements and system performance can consult the official technical approval under reference ATG 3170.
Correct cavity wall insulation starts with assessing the property rather than simply choosing an insulation material. As an experienced insulation company, we therefore inspect the cavity and exterior wall before recommending a solution.
Nu-Isoleren uses bonded Isomo EPS beads within the ATG 3170 system. Our installers carry out the work in a controlled manner and carefully finish the drilling holes afterwards.
This gives you one company for inspection, advice, installation and finishing.
The exact process depends on the property.
After the inspection we determine the drilling pattern so the beads can spread evenly throughout the wall. Filling is carried out systematically in zones. The drill holes are then closed with matching mortar and the treated areas are checked.
We assess the cavity and façade.
Small filling holes are made in mortar joints.
The beads are injected in a controlled way.
The drilling holes are carefully closed again. We do everything possible to match the original colour of the mortar joints as closely as possible, keeping the intervention visually discreet.
At Nu-Isoleren, the indicative cost of cavity wall insulation using bonded EPS beads is generally €21 to €25 per m² excluding VAT, for a technically suitable property.
For 100 m² of cavity walls, this represents an estimated budget of €2,100 to €2,500 excluding VAT.
The final cost depends on the total wall area, cavity width and condition, accessibility and any preparatory work required. We therefore assess the existing cavity before recommending insulation.
For further information about pricing, VAT, available support and potential savings, read our detailed guide to cavity wall insulation costs in 2026.
For an accurate price, request a quotation tailored to your home.
Grants or other support may be available for cavity wall insulation, depending on the region, the property and the applicable conditions.
As eligibility and amounts can change, check our current information on insulation grants in Belgium, including separate information for Flanders, Brussels and Wallonia.
Explore real cavity wall insulation projects completed by Nu-Isoleren using bonded Isomo EPS beads. Our teams have insulated existing homes in Mechelen and Zaffelare. In Zele, our installers worked systematically across a large side façade, insulating the wall section by section.
View our projects and on-site photographs to see how cavity wall insulation is carried out in practice.
Answers about suitability, material, pricing and installation.
In most cases, the cavity walls of an average home are fully insulated within a single working day, without any demolition work or inconvenience.
Yes. Many existing cavity walls that are empty or insufficiently insulated can be retrofitted. Before installation, we always check the cavity width and condition, possible contamination, any existing insulation and the condition of the exterior wall to determine whether the cavity is technically suitable.
Small holes are drilled in the mortar joints of the exterior wall. Bonded EPS beads are then injected into the cavity in a controlled manner. Once the cavity has been filled, the openings are carefully closed again.
At Nu-Isoleren, cavity wall insulation generally costs around €21 to €25 per m² excluding VAT. The total price mainly depends on the wall area, accessibility, the condition of the cavity and any preparatory work that may be required. For 100 m² of cavity wall, this corresponds to approximately €2,100 to €2,500 excluding VAT. Before preparing the final quotation, we inspect the cavity to check whether it is technically suitable and whether contamination or existing insulation is present.
For many technically suitable existing cavity walls, bonded EPS beads are a very effective choice. Nu-Isoleren therefore mainly uses bonded EPS beads when retrofitting existing cavity walls. The beads are blown into the cavity together with a bonding agent and can distribute around wall ties and other obstructions. Other cavity insulation materials include mineral wool and insulation foams. There is no single material that is automatically best for every property, which is why the cavity width, contamination, existing insulation and condition of the external wall must first be checked.
Common materials for retrofitting existing cavity walls include EPS beads, mineral wool and insulation foams such as PUR. EPS beads are blown into the cavity with a bonding agent, while mineral wool is installed as loose fibres. PUR is injected as a foam that expands inside the cavity. Each system has different thermal properties and installation requirements. For suitable existing cavity walls, Nu-Isoleren mainly works with bonded EPS beads after first inspecting the wall and cavity.
The main risks generally result from an unsuitable wall, poor preparation or incorrect installation rather than cavity wall insulation itself. A cavity containing large amounts of mortar debris, a cracked or frost-damaged external wall, a highly vapour-resistant façade finish or a wall exposed to severe driving rain may require additional measures first. Existing insulation should also be inspected. A technical assessment before installation is therefore an important part of a correct cavity wall insulation project.
For an existing cavity wall to be retrofitted with insulation, the cavity should be at least 5 cm wide according to Belgian technical guidance. Width alone is not enough: the cavity must also be sufficiently clear of mortar deposits, rubble and other obstructions so that the insulation can spread evenly. The external wall, mortar joints, wall ties, existing insulation and possible moisture problems should also be inspected before the cavity is filled.
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