Which insulation materials are suitable? As the remaining cavity is usually very narrow and the insulation installed retrospectively must not compact the existing insulation, only free-flowing materials are suitable. For core or blow-in insulation in external walls, only hydrophobic (water-repellent) core insulation materials in accordance with DIN 4108-10, application type WZ (glass wool or rock wool), or those with a building authority approval are suitable. All insulation materials used must be installed without settlement (SC0 <=1%).

Suitable insulation materials for the additional insulation of cavity walls are:

• Polystyrene granules
• Lightweight silicate foam granules WLS 040 (unfortunately, no financial assistance available)
• Insute

Please note: Blow-in insulation materials made from renewable raw materials (wood fibre, cellulose, grass, Neptune ball fibres, straw) are not suitable and are also not approved by the building authorities! As these materials are hydrophilic (they absorb and transport water), the masonry would be destroyed over time, or mould would develop on the inner wall.

If the bricks in red clinker masonry are highly absorbent, it is advisable to apply a water-repellent, vapour-permeable treatment (functioning similarly to a ‘Gore-Tex membrane’).

As the cavity does not extend right up to the windows and the window reveals are solid, there are significant thermal bridges here, often resulting in mould growth (‘mould stains’). It is advisable to insulate the internal window reveals with calcium silicate boards. See under 

What needs to be taken into account during the planning stage? If, for example, windows or doors are to be replaced as part of a larger renovation project, this should be done before the core insulation work is carried out.

Condensation: 

Contrary to popular belief, no condensation forms on the inside of the outer shell. When carrying out the relevant calculations using the Glaser method – on which the UBAKUS tool is also based – care must be taken to ensure that the boundary conditions are set to realistic values. The default values are: duration of the condensation period 4 months, temperature difference between inside and outside 25 K (i.e. a constant -5 °C outside and +20 °C inside for 4 months). And inside the building, a relative humidity of 50 per cent. These values are completely unrealistic. If one uses reasonably plausible values (4 months at +5°C outside and 40 per cent relative humidity inside), the calculations show that no condensation occurs. This is also taken into account in the manufacturers’ general building regulations approvals (AbZ), in which the DIBt (German Institute for Building Technology, Berlin) states under section 3.1.2: A mathematical demonstration of condensation resulting from water vapour diffusion is not required

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All the benefits at a glance

Improved thermal insulation: The level of thermal insulation achievable depends on the joint width and the insulation material used. If the so-called thermal transmittance coefficient (U-value) of the external wall was previously approx. 0.7 W/(m²K) (watts per square metre per kelvin), this can be reduced to approximately 0.3 W/(m²K) with a 10 cm joint width (5 cm of existing insulation, 5 cm of additional insulation) as a result of the insulation measures. This represents a thermal improvement in the external wall component of approximately 55 per cent.

Lower heating costs: The building’s heating costs are reduced by up to 10 per cent.

Further benefits:

• Comfort is enhanced due to higher temperatures on the inside of the external walls
• Mould growth is prevented, as the inner surfaces of the external walls become warmer and therefore drier
• Draughts caused by openings and leaks in the wall (e.g. sockets, etc.) are eliminated
• Gaps in the existing insulation are sealed, and the insulating performance of these panels is improved
• CO₂ emissions are reduced
• In-wall insulation can be fitted retrospectively at relatively low cost and pays for itself within a few years
• Core insulation can usually be carried out in a single day
• The appearance of the façade is preserved
• The work is minimally invasive

Mice/martens:

... like to use the cavity spaces to run up and down inside the building structure. Filling these spaces with polystyrene beads or SLS20 permanently eliminates this problem: the resulting environment is extremely unattractive to these animals, so they will avoid it.

Costs/Cost-effectiveness: Depending on the size of the building, its location and the thickness of the cavity, the cost of retrofitting insulation is approximately €10 to €15 per m². Retrofitting additional insulation pays for itself quickly – the payback period is between 7 and 10 years.

Financial support: Only blow-in insulation materials with a thermal conductivity rating of 0.035 W/mK or better (i.e. lower) are eligible for funding.

Retrofitting core insulation is eligible for a tax relief of 20 per cent or, under the BEG (Federal Subsidy Scheme for Efficient Buildings), for a grant of up to 20 per cent (as part of an individual renovation roadmap, iSFP) of the total costs in the form of an investment grant.

The BEG scheme is administered by the Federal Office for Economic Affairs and Export Control (BAFA). The eligible investment amount is capped at 60,000 euros per calendar year and per residential unit, and at 600,000 euros per building. To receive the funding, you must engage a qualified energy adviser before work begins.

Can I carry out the insulation myself? No, retrofitting core insulation requires specialist equipment and technical expertise and must be carried out by specialist blow-in insulation contractors. However, you can offer to help the contractor carrying out the work – for example, with drilling and sealing the holes, or with cleaning and tidying up the building site – which they will certainly be happy to accept and, ideally, offset against the final price.