Internal insulation: studwork filled with blown-in insulation

The external wall is the largest component of a building. Consequently, heat loss through this component is also significant. It can account for up to 40 per cent of total heating energy consumption. Depending on structural and individual factors, there are several ways to improve a house’s thermal insulation through wall insulation: Walls can be insulated from the outside using an external thermal insulation composite system (ETICS), within a rear-ventilated façade or, for example, within the air or cavity space (core insulation).

However, if the walls do not have air gaps 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), they can be insulated from the inside. Internal wall insulation can also be carried out bit by bit (room by room), as your own schedule allows.

There are also several options for internal insulation:

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

For example, if you wish to install internal insulation in a half-timbered house, you 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 fully plastered over. For internal insulation within a cladding system, either suitable board insulation materials can be used (internal insulation with boards – please note: the boards must be bonded over their entire surface to the external wall; there must be no gaps in the adhesive layer!). Alternatively, blow-in insulation systems can be used. As most of the work can be carried out by the homeowner themselves, this insulation method is extremely cost-effective.

How is it insulated?

For internal wall insulation, a cladding shell is installed in front of the inner surface of the external walls; this serves to straighten out crooked walls and also acts as a service layer capable of accommodating pipes and cables. 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 surface of the new cladding – to prevent them from freezing in winter.

To construct the cladding, wooden studs or metal stud profiles are first positioned vertically in front of the external wall to create an air gap of approx. 8 to 16 cm. If metal studs are used, they must be thermally decoupled from the external wall using strips of insulation material. In the next step, the studs are clad with OSB boards on the room-facing side. Any new electrical wiring that may be required can also be installed within the cavity thus created. The electrical cables should be secured to the vertical timber studs. Important: This work must only be carried out by qualified electricians!

A specialist blow-in insulation contractor then creates blow-in openings in the upper part of the OSB boards and fills the cavities with suitable blow-in insulation materials. The openings are then sealed airtight with an adhesive pad. The OSB boards can then be finished on the room-facing side with plasterboard, clay-based systems or similar materials.

Good to know: When installing internal insulation, ensure that all connections to adjacent building elements such as internal walls, window and door openings are airtight. Also: Adjacent structural elements (internal walls, ceilings, etc.) should also be insulated – to a width of at least 30 cm – as otherwise there is a risk of mould due to thermal bridging.
Window reveals should be finished with calcium silicate boards, as these are impervious to moisture and prevent mould from forming. (Window reveal insulation with CaSi boards)

You can find qualified tradespeople here

Insulation thickness

The greatest possible insulation thickness should be chosen, but it must be at least 10 cm. If the insulation thickness is less than 10 cm, the blow-in process is difficult and the energy savings are too low. It is possible (and has already been carried out successfully) to install up to 20 cm of internal insulation. It should be noted, however, that as the insulation thickness increases, the usable floor space decreases. The insulation thickness can also vary from room to room (thinner in small rooms, thicker in large rooms).

Which insulation materials are suitable?

Capillary-active materials such as wood fibre, blown-in cellulose insulation or flax/hemp are particularly suitable for internal insulation. All insulation materials used must also be installed in a way that prevents settling (SC 0 ≤ 1%).

Condensation may form at the interface between the insulation material and the external wall. Therefore, only capillary-active insulation materials such as cellulose or wood fibre insulation should be used, as these can absorb, store and then release moisture back into the indoor air via capillary action.

When using boards made from plant- or animal-based fibres, the MVVTB stipulates: “With regard to resistance to mould, the insulation products must be classified as Class 0. Alternatively, it must be demonstrated by hygrothermal simulation that there is no risk of mould growth.”

If installed on the room side, either through the vapour-tight bonding of the OSB cladding or a separate, continuous vapour barrier, blow-in insulation made from glass wool or rock wool with a higher thermal insulation value may also be used.

Please note: When installing internal insulation, care must be taken to ensure there are no air gaps in order to prevent condensation and mould growth. In addition, the ceilings and adjoining internal walls (where they meet the external walls) should be fitted with insulation wedges (preferably made of calcium silicate for half-timbered walls).

Particular care must be taken when installing internal insulation in half-timbered buildings, especially around the beam ends of timber-beamed ceilings. This work should not be carried out by yourself, but by trained and experienced specialist contractors.

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 inside of the external walls, a comfortable indoor environment 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 preparatory work (fitting the cladding) can be carried out by the homeowner, and assistance can also be provided during the blow-in process. This significantly reduces insulation costs.
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; a level surface for installing pipes (no need to cut grooves).

Download document:

Fördermittel - Bundesförderung effiziente Gebäude .pdf