Interior Wall Insulation In Old Buildings – Instructions And Costs

Interior Wall Insulation In Old Buildings – Instructions And Costs

The usual facade insulation is not feasible for all old buildings. In individual cases, interior wall insulation can be the only possible solution to insulate a building against heat loss. This article reveals in detail how interior wall insulation is made and what to look out for.

Insulating the interior wall is generally the worse option – but unavoidable in individual cases. This applies above all to historical buildings and buildings under monument protection, where the facade should or must be preserved in its original condition. The physical disadvantages and risks of interior wall insulation can be minimized up to a certain point, but they can never be completely ruled out. Once you are aware of this, you can get to work.

What you should know beforehand

structural problems

… for interior wall insulation

The biggest problem is clearly the temperature gradient between the outer wall and the inner insulation. At low temperatures, the outer wall cools down to the outside temperature, the internal insulation prevents the inner surface of the outer wall (which is covered by the insulation) from being heated by the space heating. If the dew point is not reached here, condensation will form, which can permanently damage the outer wall and also lead to the formation of mold between the wall and the insulation or even to mold on the insulation material. This happens all the more, the higher the relative humidity is in both areas (especially in old buildings with already existing moisture problems).

temperature gradient

remedial actions

… when designing the insulation

Essentially, the following points can be considered:

  • careful installation of vapor barriers
  • Use of strongly alkaline building materials (high pH value, e.g. with calcium silicate panels) to avoid mold growth
  • Use of building materials with the highest possible water storage capacity (also again calcium silicate boards)
  • sensible selection of the possible and achievable insulation effect (in many cases the EnEV values ​​cannot be realistically achieved, you have to live with that)
  • Thorough investigation for possible thermal bridges (wall and ceiling connections, cavities, etc.) and planning the remedy from the outset
  • Assessment and planning of the wall construction by a building expert

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

In the case of a complete renovation, it is also worth thinking about the relationship between the heating effort, the heating technology used and the thermal insulation. Infrared heaters can, for example, effectively and permanently heat up the surface temperatures of external walls and thus create a completely different starting point for internal wall insulation.

Special case half-timbered house

Half-timbered houses, especially those with half-timbered structures visible from the outside, are a special feature. On the one hand, two different materials meet here – namely the wood of the half-timbered house and the material for the filling – and traditionally built half-timbered houses are also original in their “material mix” and the construction technology used very cleverly laid out. Moisture can be continuously removed (also from the interior), the wood can dry sufficiently without rotting.

The use of vapor-tight interior wall insulation is the death for every half-timbered house! The physical building conditions, which were originally created with care, are thus overridden and are no longer effective. This means that moisture damage is unavoidable within a very short time, especially with interior insulation where the sd value (water vapor diffusion resistance) is over 2 m.

If you still want to insulate the inside of a half-timbered house, you always have to use suitable materials and suitable techniques (e.g. light clay and reeds, or under certain conditions also fibreboard). However, since this always depends on the respective circumstances, we cannot go into the special situation and the differences in timber framing in this context. All of the instructions and information below do not apply to historical half-timbered buildings!

materials and preparation

You can use fully bonded calcium silicate panels for highly effective and structurally relatively safe interior wall insulation in old buildings. The special properties of calcium silicate boards make them particularly suitable for this. A vapor barrier is not required.

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

What you need:

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

Tip: Planning the insulation together with a building expert, calculating the dew points and identifying potentially problematic areas that need additional insulation is definitely recommended.

Instructions: Interior wall insulation with calcium silicate panels

1. Carry out planning
Determine the desired insulation value and carry out calculations. Determine the wall area to be insulated. Sufficient allowance for waste. The determined number of square meters is the basis for the purchase of material (boards, adhesive, smoothing lime, deep ground).

2. Identify problem areas Find
thermal bridges, cavities, bumps, and other problem 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 of grease, and level. Additional requirements for the adhesive strength of the adhesive may apply (observe the manufacturer’s instructions on the packaging!)

4. Begin
the insulation Apply the adhesive to the entire surface with a notched trowel and press the boards firmly. Ensure that the joints between the panels are as narrow as possible. Saw boards to size if necessary. Allow the glue to dry according to the manufacturer’s instructions.

5. Additional insulation
measures Carry out the additionally required insulation measures (use insulation wedges, insulate cavities, etc.) at the problematic areas.

Filling the joints Fill the joints neatly with the smoothing lime. There must be no cracks or unfilled areas. If necessary, grind briefly.

7. Apply Tiefgrund
According to the instructions on the package, apply Tiefgrund generously several times. Calcium silicate panels are always to be regarded as a “highly absorbent substrate”.

8. Finish the wall
Plaster with a suitable mineral plaster or choose another wall treatment. All wall cladding must be open to diffusion. For wallpaper, only thin paper wallpaper is suitable.

prices and costs

You can also use ready-made interior wall insulation “packages” to lay the insulation yourself. Some hardware stores offer all the material you need at a total price per square meter (note: take waste into account here too!). The prices for this typically start at around 80 euros per square meter.

Otherwise, you have to calculate between around 40 and 60 EUR per m² for 5 cm thick insulation boards made of calcium silicate, plus the required adhesive, smoothing lime, and a primer for the wall (deep primer). The costs for these materials depend on consumption and can therefore vary greatly in individual cases.
The prices for craftsman services depend, among other things, on the effort involved in the property, they start at around EUR 40 – 50 per m² (net). In individual cases, however, significantly higher prices are also possible. It is best to get several offers and compare them thoroughly.

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Tips for speed readers

  • Interior wall insulation only when absolutely necessary
  • plan carefully (thermal bridges, windows, complete planning for extensive renovation)
  • Appraisal by building experts recommended
  • Fully bonded calcium silicate panels are ideal for use in old buildings (confirmed by many studies)
  • Complete price is around 80 EUR per m²
  • Interior wall insulation in historic half-timbered houses must always be planned separately!