Core insulation

Core insulation refers to the retrofit insulation of the air gap (core) in double-shell masonry by blowing insulation material into the air layer between the inner and outer shells. It is a minimally invasive insulation method that can be carried out with little effort, without altering the facade, and without significantly inconveniencing the residents. A prerequisite is that the masonry has a sufficiently wide, continuous air layer of at least 4-5 cm.

How does core insulation work?

During core insulation, specialized contractors drill small holes (diameter approx. 35-50 mm) from the outside into the outer shell of the masonry, distributed in a grid pattern across the entire facade. Insulation material-usually mineral wool granules, EPS beads (Styrofoam balls), cellulose flakes, or perlite-is blown or poured through these openings until the entire cavity is completely filled. The drill holes are then sealed with color-matched mortar. The interventions in the facade are barely visible.

Suitable Building Types and Materials

Core insulation is particularly common in buildings from the 1950s to the 1980s that were constructed with double-shell clinker or brick masonry. This construction method is very common in Northern Germany and North Rhine-Westphalia; it is also found in Bavaria and Franconia, though less frequently than solid single-shell masonry. The choice of filling material depends on the width of the cavity, the requirements for thermal and sound insulation, and the moisture conditions. Mineral insulation materials (e.g., mineral wool granules) are particularly recommended in areas exposed to moisture.

Prerequisites and Technical Inspection

Before cavity wall insulation can be installed, the cavity must be carefully inspected. A specialized contractor uses an endoscopic camera to check: Is the cavity continuous and sufficiently wide (at least 4 cm)? Are there any narrowings in the cross-section caused by masonry anchors, spacers, or construction debris? Is there moisture in the cavity? Are the wall shells in good condition? Moisture in the cavity prior to insulation can lead to mold growth in the core area after backfilling-one of the most common sources of failure in improperly installed cavity wall insulation. Insulation may only be installed once the cavity is dry and free of obstructions.

Another point to check is the driving rain exposure zone of the building’s location (classification according to DIN 4108-3). In areas with high driving rain exposure, the outer shell must be sufficiently airtight so that rainwater does not penetrate the cavity through cracks or open joints. In such cases, mineral wool granulate may be preferable to EPS beads, as it is more capillary-active and dissipates moisture more effectively.

Energy Benefits and Cost-Effectiveness

Properly installed cavity wall insulation can significantly improve the wall’s U-value, depending on the cavity width and material, to values of 0.3-0.5 W/(m²K). The measure is significantly more cost-effective (approx. 20-50 €/m²) compared to an external thermal insulation composite system (ETICS) or a rear-ventilated curtain wall and preserves the facade’s appearance. For a single-family home with 120 m² of wall surface to be insulated and a cost of approx. 30 €/m², the total cost amounts to around 3,600 €. Compared to an ETICS, which often costs €80-120/m², core insulation is a very cost-effective alternative.

The savings on heating costs depend on the initial condition. Typical savings are 15-30% of heating costs for the wall surfaces in question. For a single-family home with €1,500 in annual heating costs and 30% wall area, approximately €450 could be saved-with a €3,600 investment, the payback period would be around 8 years. The measure is subsidized by the BAFA and through KfW individual measure programs; the subsidy rate is currently 15% of the investment costs.

Practical Tip for Homeowners in Nuremberg and Franconia

In Nuremberg and the Franconia region, double-shell clinker brickwork is found primarily in housing developments from the 1950s to the 1970s, e.g., in parts of Langwasser, Wetzendorf, or the Nuremberg suburbs. Owners of such buildings should have the technical feasibility of cavity wall insulation assessed-an endoscopic camera inspection of the cavity will provide clarity.

We recommend consulting a certified energy consultant (energy efficiency expert for federal funding) before proceeding with the work. They will not only assess technical feasibility but also submit the funding application to the BAFA and oversee the proper execution of the work. Poorly executed cavity wall insulation can do more harm than good-especially if the cavity has not been thoroughly inspected beforehand. At my-home.de, we are happy to refer you to experienced energy consultants in your region.

Frequently Asked Questions

How can I tell if my house has double-shell masonry?

This can often be determined by looking in the basement or checking the original building plans. A wall thickness of more than 36 cm is also a telltale sign-this indicates a double-shell construction. An energy consultant or expert can provide certainty using a thermographic survey or endoscopy.

Can the cavity insulation be removed later?

No, generally not. Once blown-in insulation material has been installed, it cannot be easily removed. Therefore, a thorough preliminary inspection of the cavity for moisture, damage, or foreign objects (e.g., construction debris) is absolutely necessary before proceeding.

Does core insulation require a permit?

Generally, no. Since the exterior facade is hardly altered visually, a building permit is not required in most states. However, in cases involving historic preservation or special local design regulations, you should still check with the building authority or the historic preservation office beforehand.

Which materials are best suited for cavity wall insulation?

The choice depends on the width of the cavity and the exposure to driving rain at the location. Mineral wool granules are preferable for areas at risk of moisture, as they are capillary-active and do not retain moisture. EPS beads (Styrofoam granules) are well-suited for dry areas and are slightly more affordable. Cellulose flakes are environmentally friendly and provide good sound insulation, but require a completely dry cavity. Perlite is particularly suitable for very narrow cavities, as it fills tight spaces well when used as a loose-fill material. A specialized contractor will select the material based on local conditions.

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