Which insulation materials are suitable? For core insulation or blow-in insulation in external walls, only hydrophobic (water-repellent), non-settling core insulation materials in accordance with DIN 4108-10 (glass wool or rock wool, polystyrene beads, poured polyurethane foam or Cavipor) or those with building authority approval.

Suitable insulation materials for core insulation are:

• Stone wool blow-in flakes
• Glass wool blow-in flakes
• Polystyrene granules
• Recycled polyurethane granules
• Lightweight silicate foam granules
• PUR cast foam
• INSUTE-25 pro
• Insu-Core
• Aminotop++
• bonded polystyrene beads 

For cavity wall insulation with air gaps between 2.5 and 4 cm wide, particularly fine blow-in insulation materials such as ‘SLS 20’ lightweight silicate foam granules, Insute-25 pro, polystyrene beads or poured polyurethane foam. However, as in narrow cavity layers (25 or 30 mm), mortar bridges usually connect the two wall faces over a large area, the insulating effect of this measure is severely limited due to the formation of thermal bridges at these points.

Legal requirements (insulation materials): 

The relevant building regulations approvals issued by the Institute for Building Technology (Berlin) stipulate that core insulation materials are only considered insulation materials if they are installed by specialist contractors who have been thoroughly trained by the respective manufacturer in the installation of the product and are included on a list maintained by the manufacturer. If such training and listing have not taken place, the materials are not considered insulation materials within the meaning of the approval, but merely fillers. The manufacturer’s warranty becomes void, and strictly speaking, public funding is not possible – if funding is nevertheless claimed, this constitutes, strictly speaking, subsidy fraud! It therefore follows that DIY installation is not advisable. In the event of any structural damage, the homeowner is left to deal with it alone and cannot expect any support from the manufacturers. This can even lead to insurance issues in the event of damage (fire, water) and result in the insurance cover being voided. 

With FVED members, the homeowner can rest assured that all legal and building physics aspects of retrofitting core insulation are understood and applied.

Please note: 

Blown-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 eventually be destroyed, or mould would develop on the internal wall surface.

What needs to be taken into account during the planning stage?
If, for example, windows or doors are also to be replaced as part of a larger renovation project, this should be carried out before the core insulation work begins. However, in the case of fibrous, mineral-based products and foam-based products, this is also possible after the core insulation has been installed.  
In the case of red clinker brickwork, particularly in northern Germany, if the bricks 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 as far as the windows and the window reveals are usually 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 Is

cavity wall insulation also possible if there is already insulation material (usually rock wool boards, possibly also polystyrene boards) in the cavity? Yes!

Please note: 

if the interior plaster is loose (you can tell this by the fact that it sounds ‘hollow’ when tapped), installing fibrous insulation materials (glass wool, rock wool) may cause the plaster to flake off. The reason for this is that fibrous insulation materials must be installed using a small amount of air pressure to ensure the cavity is filled seamlessly and without the risk of settling. In this case, the use of polystyrene beads or other free-flowing products is recommended. These do not build up pressure, so flaking of the plaster cannot occur. Should this happen nevertheless, it does not constitute a defect for which the installation company is liable.

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 set there are: duration of the condensation period 3 months, temperature difference between inside and outside 25 K (i.e. a constant -5 °C outside and +20 °C inside for 3 months). And a relative humidity of 50 per cent inside the building. These values are unrealistic. If one uses reasonably plausible values (3 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 verification of condensation resulting from water vapour diffusion is not required.

If using the free UBAKUS tool, it should be borne in mind that the default boundary conditions are completely unrealistic or open to interpretation. They should be set to realistic values.

Thermal bridges:

When retrofitting core insulation to double-skin masonry, thermal bridges are unavoidable. These are:

1. all window reveals
2. Concrete window lintels
3. Concrete slabs (ground floor, upper floor ceiling)
4. cantilevered balconies/canopies over front doors
5. cross-laid bricks (bricks laid crosswise, which were commonly used before the introduction of steel anchors to connect and stabilise the two masonry shells)
6. windows/doors bricked up at a later date
7. Mortar that was scraped into the cavity during the construction phase and is located at the base of the cavity.

These (localised) thermal bridges do not have a negative effect (e.g. through increased condensation); on the contrary, they are in some cases even partially heated (to a limited extent) by adjacent, insulated components on the inside.

If no condensation was observed at the aforementioned thermal bridges prior to the installation of core insulation, this will not be the case afterwards either, as the surface temperature of the thermal bridges rises due to the heat flow from the insulated building elements. However, it is important to adapt ventilation practices to the changed situation.

It is, however, perfectly acceptable and recommended to install calcium silicate boards in the window reveals (the coldest part of a room is where the internal window reveal meets the window frame) as a preventative measure against mould. See: This can also be done as a DIY project.

A best-practice example from Kiel: here, a Kiel-based company insulated 100 buildings ‘in series’ within a month. The residents are reaping the full benefits of the heating cost savings of €250–320 per year, as the owner, KiWoG, has refrained from increasing the rent. 

Mice/martens:

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

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 approximately 1.4 W/(m²K) (watts per square metre per kelvin) , the insulation work can reduce this to approximately 0.37 W/(m²K) with a joint width of 8 cm. This represents a thermal improvement in the external wall component of approximately 70 per cent.

In relation to the whole house, energy savings of 15–25 per cent can be achieved, depending on the thickness of the insulation and the lambda value of the material used. In a retrofit core insulation project involving 100 houses in Kiel, a measured saving in heating energy of 21 per cent was achieved. See below: A claimed saving of more than 25 per cent is misleading, as it is unachievable.
 

Lower heating costs: 

with cavity wall insulation, depending on the specific circumstances, energy, heating cost or CO₂ savings of between 15 per cent and a maximum of 25 per cent (relative to the whole house) can be achieved. Claims of savings of up to 40 per cent are misleading and cannot be justified. See below:
In Kiel, 100 houses on Rendsburger Landstraße were retrofitted with core insulation ‘in series’, i.e. within a single month, by one company. Gas consumption was measured before and after the work. This resulted in an average heating energy saving of 21 per cent for the houses.

Further benefits:

• Comfort is improved due to higher temperatures on the inner surfaces 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
• CO₂ emissions from fossil-fuelled heating systems are reduced by 15% to 25% (depending on the specific situation)
• In-wall insulation can be retrofitted quickly and cost-effectively and pays for itself within a few years; it is also eligible for government grants
• Core insulation can usually be carried out in a single day
• The appearance of the façade is preserved. This is particularly important for brick façades in northern Germany (urban planning considerations, and in some cases, listed building status)
• Core insulation – like other insulation methods – is an important prerequisite for the cost-effective use of a heat pump.

Costs/Cost-effectiveness: 

Depending on the size of the building, the location, the thickness of the cavity and the material used, the cost of cavity wall insulation is approximately €15–30 per m². Retrofitting cavity wall insulation pays for itself quickly – the payback period is between 4 and 6 years (taking government grants into account).
 

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Legal information (Building Energy Act): 

Notwithstanding the statutory requirement that a U-value of 0.24 W/m²K must be achieved for retrofitted external insulation (e.g. ETICS or curtain wall), in the case of retrofitted cavity wall insulation, the Building Energy Act (Building Energy Act) is met if the cavity is completely filled with a core insulation material (even if, as a rule, U-values of only 0.35 to 0.5 W/m²K can then be achieved). 

Subsidies: 

Only blow-in insulation materials with a design thermal conductivity value (‘lambda value’) of 0.035 W/mK or better (i.e. lower) are eligible for funding. Retrofitted core insulation is eligible for a tax credit of 20 per cent or, under the BEG, the Federal Subsidy Scheme for Efficient Buildings (BEG), by up to 20 per cent (as part of an individual renovation roadmap (iSFP) – 15 per cent without an iSFP) of the total costs in the form of an investment grant.

The BEG funding 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 residential unit, and at 600,000 euros per building. To receive BAFA funding, you must engage a qualified energy consultant before work begins. An energy consultant is not required for the tax incentive.

Can I install the insulation myself? 

No, retrofitting core insulation requires specialist equipment and technical expertise and must be carried out by specialist blow-in insulation contractors. By assisting with the insulation work (setting up the site, feeding the insulation material into the blow-in machine, sealing the injection holes, and clearing up the site), you can significantly reduce the costs involved. Ask your specialist contractor! Unfortunately, there are some building materials suppliers who hire out blow-in machines and equipment to end customers and sell the material. Whilst this is legally permissible, insulation installed by a DIY enthusiast does not constitute ‘insulation’ within the meaning of the GEG (German Energy Conservation Act), but is merely a filler material. Furthermore, it should be noted that, in the event of damage, neither the building materials supplier nor the manufacturer will honour the warranty. In addition, you will not be eligible for government grants.

History/Building Physics/Problems: 

Since the mid-19th century, the construction method known as ‘double-shell masonry’ has become established, particularly in Northern Europe. Why? Because in this region, rain often comes from the front (‘driving rain’) and soaks through the walls. If the wall is divided into two layers, the inner wall remains dry, even if the outer wall is soaked. Contrary to popular belief, therefore, the cavity was not invented for insulation purposes (thermal insulation was not a consideration in the 19th century), but solely for reasons of moisture protection (and thus also to prevent mould). Since the 1960s – starting with perlite – the retrofitting of a hydrophobic core insulation material has become standard practice. Since then, several million buildings across Europe (particularly in the Netherlands, including through ) have been retrofitted with core insulation – largely without causing any structural damage whatsoever. As only core insulation materials that are hydrophobic (i.e. water-repellent) may be used, moisture transfer from the outer shell to the inner shell is ruled out. Moisture or mould problems arising from retrofitted core insulation are rare (and occur only when, for example, resistance to driving rain has not been tested and guaranteed), as well as in buildings insulated with UF foam (urea-formaldehyde foam). A number of cases of damage have been recorded here. However, the product is no longer approved in Germany and may therefore no longer be installed.