Home generalInterior wall insulation in the old building - instructions and costs

Interior wall insulation in the old building - instructions and costs

  • What you should know in advance
    • Building physics problems
    • remedies
    • Special case half-timbered house
  • Material and preparation
  • Instructions: Inner wall insulation with calcium silicate boards
  • Prices and costs
  • Tips for quick readers

Not all old buildings are the usual facade insulation feasible. In some cases, interior wall insulation may be the only possible solution for insulating a building from heat loss. How interior insulation is made, and what to look for, this article reveals in detail.

An insulation of the inner wall is basically the worse option - but in some cases unavoidable. This is especially true for historic buildings and listed buildings, where the façade should or must be preserved in its original condition. The building physics disadvantages and risks of interior wall insulation can be minimized to a certain extent, but you can never completely exclude them. If you are aware of that, you can get down to business.

What you should know in advance

Building physics problems

... in the interior wall insulation

The biggest problem is the temperature gradient between the outer wall and the inner insulation. At low temperatures, the outer wall cools down to the outside temperature, the inner insulation prevents the inner surface of the outer wall (which is covered by the insulation) from being heated up by the space heating. If the temperature drops below the dew point, condensation forms, which can permanently damage the outer wall and can also lead to mold formation between the wall and the insulation or even mold on the insulating material. This happens all the more, the higher the relative humidity in both areas (especially in old buildings with already existing moisture problems).

temperature gradient


... in the design of the insulation

In essence, one can consider the following points:

  • careful installation of vapor barriers
  • Use of highly alkaline building materials (high pH, ​​such as calcium silicate boards) to prevent mold growth
  • Use of building materials with the highest possible water storage capacity (again calcium silicate boards)
  • sensible selection of the possible and achievable insulation effect (in many cases the EnEV values ​​are not realistically achievable, one has to live with that)
  • Thorough examination of possible thermal bridges (wall and ceiling connections, cavities, etc.) and planning of the remedy from the outset
  • Appraisal and planning of the wall construction by a construction expert

In addition to the planning of the wall construction, the existing windows (thermal bridge danger, connection problems for vapor barriers) should be examined.
In addition, the moisture exposure to which the outer walls are exposed from the outside (driving rain) should be avoided as far as possible with suitable measures (facade protection, enlargement of the roof overhang, etc.).

In a complete renovation, it is also worthwhile to think about the relationship between heating effort, used heating technology and thermal insulation. For example, infrared heaters can effectively and permanently heat the surface temperatures of exterior walls, creating a completely different starting point for interior wall insulation.

Special case half-timbered house

Half-timbered houses, in particular those with externally visible trusses, represent a special feature. On the one hand, two different materials meet here - namely the timber of the truss and the material for backfilling - and traditional half-timbered houses are in their original "material mix" and the construction technique used very cleverly laid out. Moisture can be continuously removed (even from the interior), the wood can dry sufficiently without being rotten.

The use of vapor-tight interior wall insulation is the death for every half-timbered house! The building physics conditions that were originally created with care, are thus overridden and are no longer effective. In this way, moisture damage is inevitable within a very short time, especially in internal insulation where the sd value (water vapor diffusion resistance) is more than 2 m.

If a half-timbered house still requires interior wall insulation, it is always necessary to resort to suitable materials and suitable techniques (such as lightweight clay and reed or, under certain conditions, wood fiber boards). However, as this is always dependent on the respective circumstances, we can not address the specific situation and the differences in Fachwerk in this context. All instructions and instructions below do not apply to historic half-timbered buildings!

Material and preparation

You can use calcium silicate boards in full bonding for a well-effective and relatively reliable interior wall insulation for old buildings. The special properties of calcium silicate boards make them particularly suitable. A vapor barrier is not needed.

Full gluing is therefore necessary to avoid rear ventilation (an air space between the insulation board and the wall) in any case. Otherwise, there could be a thermal bridge and condensation moisture, which causes long-term damage.

You need this:

  • Calcium silicate plates in the desired thickness
  • Glue
  • Smoothing lime for filling
  • Deep ground for priming
  • Saw for cutting
  • notched trowel
  • spatula
  • Material for additional insulation in thermal bridges (typically insulating wedges)
  • no vapor barrier!

Tip: A planning of the insulation together with a construction expert, the calculation of the dew points, and the definition of possibly problematic places which need an additional insulation is in any case absolutely recommendable.

Instructions: Inner wall insulation with calcium silicate boards

1. Carry out planning
Determine desired insulation value and perform calculations. Determine the wall surface to be insulated. Take sufficient consideration of waste. The number of square meters determined is the basis for the purchase of materials (slabs, glue, ice lime, underground).

2. Identify problem areas
Find thermal bridges, cavities, bumps and other problematic areas. Determine suitable additional insulation measures and procure suitable material.

3. Substrate preparation
The wall surface to which the insulation is to be applied must be clean, dry, free from grease and even. Additional requirements for the adhesive force of the adhesive may apply (observe the manufacturer's instructions on the packaging!)

4. Start insulation
Apply adhesive with the notched trowel and press on the plates. Pay attention to the narrowest possible joints between the plates. Cut plates if necessary. Allow the adhesive to dry in accordance with the manufacturer's instructions.

5. Additional insulation measures
The additionally required insulation measures (insert insulating wedges, insulate cavities, etc.) at the problematic places.

6. Fill joints
Smooth the joints with the smoothing lime. There must be no cracks or unfilled areas. If necessary, sand briefly.

7. Apply deep foundation
Apply deep enough several times as instructed on the package. Calcium silicate boards should always be considered as a "strong absorbent substrate".

8. Finish the wall
Coat with suitable mineral plaster, or choose other wall treatment. Every wallcovering has to be impermeable. For wallpaper only thin paper wallpapers are suitable.

Prices and costs

You can also use well-finished interior wall insulation packages for self-insulation. Some DIY stores offer all the required material for a complete price per square meter (Attention, take into account waste also here!). The prices typically start at around 80 euros per square meter.

Otherwise, you have to expect for 5 cm thick insulation panels made of calcium silicate between about 40 and 60 EUR per m², in addition there are the required glue, the smoothing lime and a primer for the wall (Tiefgrund). The costs of these materials are consumption-dependent and can therefore be quite different in individual cases.
The prices for craftsmanship depend, among other things, strongly on the cost of the property, they start at around 40 - 50 EUR per m² (net). In individual cases, however, significantly higher prices are possible. It is best to get several offers and compare them thoroughly.

Tips for quick readers

  • Interior wall insulation only if absolutely necessary
  • plan carefully (thermal bridges, windows, complete planning during comprehensive renovation)
  • Assessment by construction experts recommended
  • Fully bonded calcium silicate boards are ideal for use in old buildings (confirmed by many studies)
  • Complete price around 80 EUR per m²
  • Interior wall insulation in historic timber framing always has to be specially planned!
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