General Information on the Insulation of Exterior Walls

The German word „Wand” (=wall) derives from the Old High German term „Want” and originally referred to a woven wattle wall covered with clay. A wall has the function to separate two areas from each other, for example the inside from the outside. The heat insulation of a wall serves to reduce or delay the heat exchange between the areas separated. During the winter the insulation should prevent warmth from escaping from the inside of the building to the outside, whereas during the summer time the intrusion of heat from the outside should be prevented. Below we want to present you different ways of insulating a wall with the help of reed panels.

Requirements for Exterior Walls

On the first of February 2002 the new German Energy Saving Act (EnEV) came into effect and tightened the requirements of the former Thermal Insulation Regulation (WSV). The aim of the EnEV is to fundamentally reduce the energy demand of buildings. This is to be achieved by means of enhanced heat insulation and air tightness, the avoidance of thermal bridges and the implementation of improved installation engineering. Meeting with the requirements of the EnEV is binding for all new buildings as well as for structural alterations on existing buildings covering more than 20% of the structural member surface of the same orientation. The EnEV requires all new exterior walls of heated rooms to reach a U-value of 0.45 W/ m²K. For the additional fitting of insulation layers a U-value of 0.35 W/ (m²K) is required.

The specifications of DIN 4108 (Thermal Protection and Energy Saving in Buildings) have to be met in order to protect the construction from condensate and mould growth.
According to DIN 4108-3 the thermal conduction resistance (R-value) of the wall build-up has to be higher than 1.20 m²K/W and the theoretical amount of condensate should not exceed a certain value (mW,T < 0.5 kg/m² for non-water-absorptive material and mW,T < 1.0kg/m² for water absorptive material. The amount of condensate accumulating has to be calculated with another approved method (as for example the Glaser Method).

Critical comment: The commonly used Glaser Method for the calculation of vapour diffusion has proved its practical value as a simple evaluation method. Nevertheless, it assumes static climatic conditions in summer or winter and its results only rarely correspond to the actual transport of moisture in the building components and to the constantly changing natural conditions.
The Glaser Method also does not take into account the hygroscopic properties of the building materials (e.g. clay), a fact, that can lead to inaccurate results. The arithmetical value describing the amount of condensate to be expected does not necessarily have to correspond to the actual conditions.

Special Requirements for Timbered Houses

Since recently also timbered houses have to fulfil special requirements. We cannot imagine our countryside without the characteristic traditional timbered houses, which are quite often listed buildings. Timbered houses can be insulated in order to meet the requirements of thermal and climate protection and the wishes of the inhabitants for a pleasant indoor environment.
Quite often, an outside insulation, which is a popular choice for other types of houses, is out of question for aesthetical and preservationist reasons. In these cases an inside insulation seems appropriate.

However, especially with inside insulation, it can be difficult to comply with the requirements of the EnEV without running the risk of damaging the wattle and daub. To avoid damages caused by a careless implementation of the EnEV, upon request an exception can be granted.

Section 24 of the EnEV offers the possibility to apply for an exception with the local building authorities if the structure or the appearance of an historical building are endangered. According to §24, the same applies to measures which would cause excessive and unreasonable costs. §25 allows applying for exemptions if the implementation of the necessary measures would cause undue hardship due to unreasonable additional expenses (§25).
Conserving the wattle and daub should have priority over exaggerated insulation measures and the closely connected danger of damages.

In order to prevent the growth of mould and the accumulation of condensate on the inside surfaces the requirements of DIN 4108-3 (Thermal Protection and Energy Saving in Buildings) also apply to the exterior walls of timbered houses. The requirements, however, due to the special constructional properties of timbered houses, can only be met with difficulty or not at all. With a thermal conduction resistance of the wall system R ≤ 1.0 m² K/W and a SD-value of interior plaster and lining between 1.0 m and 2.0 m no mathematical verification of condensate is necessary.

The International Association for Science and Technology of Building Maintenance and Monuments Preservation (WTA) which concerns itself with the preservation of old timbered houses, opposing DIN 4108, calls for a limiting of the thermal conduction resistance of the inside insulation without further tests to ΔRi ≤ 0.8m² K/W.
This equals an insulation of a thickness of 4 – 6 cm. An additional analysis has to be supplied in the case of a thicker insulation. Even thin insulation layers can lead to a considerable reduction in the energy consumption.

In order to prevent damages caused by condensate and to preserve the capillary transport of liquids the WTA recommends not using any kind of vapour-tight layers. Instead, the construction should be non-vapour retarding and the Sd-value of the inside insulation including interior lining and an air layer should lie between 1.0 m and 2.0 m. Thus condensate and rainwater that might possibly have penetrated can also dry to the interior. To ensure the effective removal of possible moisture the insulation boards should be used in combination with hygroscopic materials as for instance clay.

Protection Against Driving Rain (DIN 4108-3)
A danger which should not be underestimated is the exposure of the façade of timbered houses to driving rain. Through the gaps between the wooden framework and the compartments driving rain can penetrate into the walls and cause damages. DIN 4108-3 classifies according to location, orientation and region into different exposure groups. For the groups two and three a constructional protection of the façade is recommended (lining, plastering or similar). For the protection against driving rain according to DIN 4108-3 it seems appropriate to examine the timbered house in question and to find an individual solution.