Cavity Wall Insulation (Blown-In)

Filling the air gap in a cavity wall with loose insulation granulate (mineral wool, cellulose, perlite) — no change to the facade appearance.

How does blown-in cavity wall insulation work?

Many buildings from the 1950s to 1970s were built with cavity walls: a load-bearing inner leaf and an outer facing leaf are separated by a 6 to 10 cm wide air gap. This cavity can be filled with insulation retrospectively. For this, holes are drilled in a grid pattern in the outer leaf and the cavity is filled under pressure with loose mineral wool, cellulose, perlite or EPS granulate until it is completely filled without gaps. The drill holes are then closed and colour-matched. The insulation materials have thermal conductivities of 0.035 to 0.045 W/(m·K) and lower the wall's U-value from about 1.2 to 0.4 to 0.5 W/(m²·K). Before execution, the cavity is inspected by endoscope for width, mortar bridges, moisture and the condition of the wall ties. The work is completed without scaffolding in one to two days.

Which buildings benefit from blown-in cavity insulation?

Typical buildings are detached houses and apartment buildings from the 1950s to 1970s with cavity walls and an air gap of at least 4 cm. Because the facade appearance does not change, the measure can also be used with fair-faced masonry, in conservation zones and in heritage-compatible refurbishments. It is not suitable for solid single-leaf masonry, for cavities under 4 cm, or where there are moisture problems in the outer leaf, which must be remedied first. Where external wall insulation with U-values below 0.20 W/(m²·K) is possible and desired, this is the better solution in energy terms, but considerably more expensive.

What does blown-in cavity insulation actually deliver?

Heating energy savings are 15 to 25 %. For a 1960s detached house with 180 m² of cavity wall facade, the saving is around 10,700 kWh of natural gas per year, equivalent to about 2.2 t of CO2. At a cost of 35 to 70 €/m² and a payback period of 3 to 8 years over a 40-year service life, blown-in cavity insulation is one of the most economical insulation measures of all. The achievable U-values are limited by the cavity width, but the ratio of effort to saving is hard to beat.

Energy carrier

Erdgas, Heizöl, Pellet, Fernwärme → Erdgas, Heizöl, Pellet, Fernwärme

Savings potential

15-25 %

Worked example

A 1960s detached house has 180 m² of cavity wall facade with an uninsulated air gap. The wall's U-value is 1.2 W/(m²·K). An endoscope inspection confirms a continuous, dry cavity of around 8 cm. After blowing in insulation granulate, the wall achieves 0.45 W/(m²·K). The house is heated by gas at 90 % efficiency, and the location has 3,500 heating degree days. With a correction factor of 0.85, the calculation is: 180 × (1.2 − 0.45) × 3,500 × 0.024 / 0.9 × 0.85 = 10,710 kWh/a. The house thus saves around 10,700 kWh of natural gas per year and about 2.2 t of CO2. At a cost of about 6,300 to 12,600 €, payback is a matter of a few years, and the facade appearance remains unchanged.

Investment & payback

35-70 €/m² · Service life: 40 a · Payback: 3-8 (eine der wirtschaftlichsten Dämm-Maßnahmen) a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

How much does blown-in cavity wall insulation cost?

Blown-in cavity insulation costs 35 to 70 €/m² of facade area including endoscope inspection, drilling, filling and closing the drill holes. For a detached house with 180 m² of facade, that is about 6,300 to 12,600 €. Since no scaffolding and no surface work are needed, the price is a fraction of external wall insulation with ETICS, which costs 150 to 220 €/m².

How do you tell whether a house has cavity walls?

Cavity walls are typical of buildings from the 1950s to 1970s and can be recognised by a wall thickness of 30 cm or more combined with a thin facing or clinker brick leaf. An endoscope inspection gives certainty: through a small drill hole, the cavity is checked for width, mortar residue, moisture and wall ties. Building records and features such as window reveals or ventilation openings at the plinth also provide clues to the wall construction.

Which insulation materials are suitable for blown-in cavity insulation?

Loose insulation materials that can be blown in under pressure are used for cavity insulation: mineral wool flakes, cellulose fibres, perlite and water-repellent EPS granulate. Thermal conductivity is 0.035 to 0.045 W/(m·K). A water-repellent insulation material is important so that driving rain penetrating the outer leaf does not reach the inner leaf. ÖNORM B 6010 governs the requirements for blown-in insulation materials.

Is blown-in cavity insulation worthwhile compared with external wall insulation?

In energy terms, blown-in cavity insulation achieves less, with U-values of 0.4 to 0.5 W/(m²·K), than ETICS at 0.15 to 0.20 W/(m²·K). Economically, however, it is hard to beat: at 35 to 70 €/m², it pays back in 3 to 8 years, whereas ETICS at 150 to 220 €/m² takes 15 to 30 years. Where the cavity exists, blown-in insulation is therefore almost always the first step.

Can blown-in cavity insulation cause damp problems?

Not with professional execution. The air gap in a cavity wall was originally intended to drain away penetrating driving rain. If it is filled with water-repellent insulation material, this function is retained. Existing moisture damage in the outer leaf, defective joints or cracks are critical; these must be remedied before insulating. WTA Guideline 6-5 describes the requirements and the necessary preliminary endoscope inspection.

Related measures

Standards & sources

Metering points in the EDM Toolbox

Erfolg primär durch witterungsbereinigten Heizenergie-Vergleich messbar. Optional: Oberflächentemperatur Innenwand vor/nach zeigt Komfortgewinn.

All measures