Internal insulation using loose-fill insulation and lightweight concrete blocks

The external wall is the largest component of a building. Consequently, heat loss through this component is also significant. Depending on structural and individual factors, there are several ways to improve a house’s thermal insulation by insulating the walls: Walls can be insulated from the outside using an external thermal insulation composite system (ETICS), within a rear-ventilated façade (LINK) or, for example, within the air or cavity space (core insulation).

However, if the walls do not have air or cavity spaces, the building is a listed building, is a half-timbered house, cannot be insulated from the outside due to other town planning considerations, or external insulation is not desired (e.g. in the case of a flat owners’ association), the walls can be insulated from the inside. Internal wall insulation can also be carried out in stages.

There are also several options for internal insulation:

Internal insulation using insulation boards or rolls within a cladding system
Internal insulation with bonded boards
Internal insulation using blow-in insulation within a cladding system, as described here:

For example, anyone wishing to install internal insulation in a half-timbered house must first ensure that driving rain cannot penetrate the external half-timbered wall. Furthermore, the half-timbered beams on the inside of the wall must be plastered over. For internal insulation within a cladding system, either suitable board insulation materials can be used (internal insulation with boards). Alternatively, blow-in insulation systems can be employed. As many of the tasks involved can be carried out by the homeowner themselves, this insulation method is also cost-effective.

How is it insulated?

For internal wall insulation, a cladding shell is erected on the inside of the external walls; this serves to straighten out crooked walls and also acts as a service duct, allowing pipes and cables to be routed through it. Bear in mind that pipes carrying liquids must be laid on the warm side – i.e. on the inside, in front of or as close as possible to the inner face of the new cladding – to prevent them from freezing in winter.

To construct the facing, lightweight concrete blocks are placed at a distance of at least 10 cm from the external wall. Any new electrical wiring that may be required can also be installed in the cavity created in this way. Important: This work must only be carried out by qualified electricians!

The cladding is then filled with SLS20. These blocks serve as a load-bearing layer and provide additional insulation. Finally, an interior plaster is applied once more to the aerated concrete blocks to finish the wall and ensure an aesthetically pleasing surface.

Good to know: When installing internal insulation, ensure that all joints with adjoining building elements – such as internal walls, window and door openings – are airtight.

Window reveals should be finished with calcium silicate boards, as these are impervious to moisture and mould is virtually impossible to form. (Window reveal insulation with CaSi boards)

Insulation thickness

The greatest possible insulation thickness should be chosen, but it must be at least 10 cm. It should be noted that as the insulation thickness increases, the usable floor area decreases.

All the benefits at a glance

Improved thermal insulation: If the wall’s so-called heat transfer coefficient (U-value) was previously up to 1.8 W/m²K (watts per square metre per kelvin), this can be reduced to as low as 0.2 W/m²K (with insulation approximately 20 cm thick) as a result of this measure.

Lower heating costs: If all rooms in the building are fitted with internal insulation, this measure can reduce the building’s heating costs by up to 25 per cent.

Further benefits:

Due to higher temperatures on the inner surfaces of the external walls, a comfortable indoor climate is achieved even at lower air temperatures. This therefore saves energy not only by eliminating heat loss, but also by reducing the overall heating requirement.
Draughts are eliminated
No condensation build-up when installed correctly; the risk of mould is reduced
CO₂ emissions are reduced
The work can also be carried out by the homeowner
The work can be carried out room by room

Many cost-saving side effects (so-called ‘incidental costs’), such as: straight walls simplify floor laying; very rapid construction progress, as, amongst other things, no drying breaks are required; Installation level for pipes (no need to mill slots)

Funding: If the insulation work is carried out by the homeowner themselves, it is only eligible for funding from the BAFA if the material costs exceed €2,000. The grant must be applied for by an energy adviser. Alternatively, you can claim tax relief; in this case, you will need not only an invoice from a specialist contractor but also a specialist contractor’s declaration confirming that the relevant U-values have been achieved.

Can I carry out the insulation work myself? Yes

Insulation materials: SLS20, aerated concrete as interior cladding

Download document:

Fördermittel - Bundesförderung effiziente Gebäude .pdf