Inside Insulation

Inside insulation is usually employed where the façade is to be preserved for aesthetical and preservationist reasons. Apart from these aspects an inside insulation can also make sense if only certain parts of a building are to be insulated (e.g. individual flats). For constructional reasons the inside insulation of a wall quite often causes problems (dew point problem). That is why detailed and professional planning by e.g. a civil engineer is absolutely essential for the installation of an inside insulation.

How the Transport of Moisture in the Walls of Timbered Houses Works During the Heating Season.
The transport of moisture through building components works by means of vapour transport (diffusion and convection) or by liquid movement (capillarity).
Water vapour diffusion is the process of pressure compensation, where air with a higher vapour pressure (warm, humid air) tries to move in direction of air with a lower vapour pressure (cold, dry air). In a building, due to the vapour pressure gradient, the warm and humid internal air diffuses through building components (e.g. the exterior walls or the roof).

The diffusion process takes place at all times. When diffusing, the air cools down inside of the different building components. In the worst case this can lead to condensation inside of the building components.

An inside insulation causes the dew point (the point where water vapour condenses) of a wall to move further to the inside which leads to an increased risk of condensation inside of the wall.
For historic buildings there is no possibility of preventing the condensation by installing vapour barriers and retarders, as it is almost impossible to make them sufficiently air-tight. There is the danger that leaks at the vapour barrier can lead to liquid convection and thus to an increase in condensate.
Convection is a problem because compared to the process of diffusion the amount of condensation water penetrating the building components can be many times higher. That is why an inside insulation with a vapour barrier is not advisable for wall constructions that allow capillary action.
The capillarity determines how fast a building component absorbs a certain amount of water when it comes in direct contact with moisture (driving rain, splash water, soil moisture, condensate) and it determines the rate of drying of building components inside of which water vapour transported by diffusion has condensed into water.

The amount of condensate calculated according to DIN 4108 does sometimes exceed the limit allowed. That is why often an inside insulation is not advisable.
For the installation of an inside insulation we recommend a wall build-up open to diffusion as well as the use of hygroscopic materials (clay). These materials have the ability to absorb emerging condensate and to release it back to the inside air due to their capillarity and without the risk of structural damages (mould).

Over hundreds of years clay and reed have proven themselves to be an efficient combination. Due to its high concentration of silicate the reed cane does barely absorb moisture; its insulating properties are preserved permanently. Its high hygroscopicity and its good capillary conductivity help the clay to absorb and remove potential moisture. In this way the clay has a preserving effect on the inside insulation made of reed panels and protects them from damages. At the same time the clay affects the ambient air in a positive way, by providing a stable humidity and by absorbing harmful substances as for example nicotine or formaldehyde. The reed insulation has yet another advantage: It serves as a plaster base for the clay.
Below we want to show some examples for walls with state-of-the-art inside or outside insulation.

Wall build-up 1: Wall of a timbered house with inside insulation: A 60 mm reed insulation panel in clay

Existence:
30 mm lime external rendering, 140 mm straw clay on wickerwork.
U-value without reed insulation panel: 1.74 W/(m²K).

General requirements:
EnEV: U = 0.35 W/(m²K) (for the additional installation of an insulation)
DIN 4108: R ≥ 1.20 (m²K)/W.

Requirements without a mathematical condensate verification:
DIN 4108 : Ri ≤ 1.0 m² K/W ; 0.5 m ≤ sd,i
WTA: Ri ≤ 0.8 m² K/W ; 0.5 m ≤ sd,i ≤ 2.0 m.

Inside insulation:
App. 30 mm loam rendering, 60 mm Hiss-Reet reed panel, 15mm loam rendering plaster, 5 mm final loam rendering
U-value with reed insulation panel: 0.56 W/(m²K)*.

* The criteria for an exemption from the condensate calculation (WTA and DIN 4108) cannot be fulfilled. This analysis however, although it does provide a value for potential condensate does not reflect the hygroscopic qualities of the clay (loam). The wall build-up described above has proven itself.

Details on the Use and Application of Reed Insulation Panels as Inside Insulation for Timbered Houses.

  1. Check the existing wattle and daub: A wall which is to be insulated should be dry and free from mould. The possibility of moisture or salt transport inside the wall should be excluded. Furthermore, you will have to check if moisture, due to driving rain, can penetrate the wall through the gaps between the timber framework and the different compartments. The WTA recommends for timbered buildings of the exposure categories two and three a constructional rain protection of the facade.
  2. An inside insulation consisting of reed panels in a timbered house should always be open to diffusion. This means that all vapour retarding layers as for example cement plaster or bituminous paint should be removed from the exterior walls.
  3. Where reed panels come in contact with the floor (or a floor slab or the wall base) they have to protected against rising moisture (e.g. by means of bitumen board).
  4. The entire exterior wall will be covered with loam render until it forms an even surface. For better footing strips of woven reed mats of about 1 cm thickness can be fixed with a staple gun on the timbers of the building. The thickness of the loam layer should not exceed 3 cm. The clay layer is not only the even surface for the reed insulation it also helps to transport the moisture. That is why it should be applied with special care.
    If layers of more than 3 cm are necessary in order to level an especially uneven surface the first layer has to be dry before a second layer of loam and the reed insulation panel can be applied. The reed insulation panel is pressed into the loam plaster and is thus “glued” to the existing exterior wall. Alternatively, the loam rendering can be applied to the back of the insulation panel using a notch trowel. Then the panel is glued to the wall. Now the reed panel has to be pressed onto the wall and fixed. The panel is fixed to the timbers using long wood screws and insulation discs and it is attached to the partitions with insulation support. Five fixings per m² are enough.
  5. Recommendation: The reed insulation should be attached in such a way, that the individual stalks have a horizontal orientation. This makes rendering the clay easier. The production of cross joints should be avoided. Special care has to be taken to avoid hollow spaces between the clay and the insulation panel. Otherwise, there is the risk of warm inside air finding its way between exterior wall and the insulation where it would cool down and produce condensate. The inside insulation made of reed can be coated with clay at once. In order to prevent the future formation of cracks jute cloth has to be integrated into the plaster.
  6. Window reveals: In order to comply with the requirements of the EnEV for air-tightness and the reduction of thermal bridges special care has to be taken with the finishing of the window reveals. The insulation panels next to the window should, if possible, directly connect to the window frame (cp. Wall build-up 4).
  7. Wall heating: If an additional wall heating is fitted the heating pipes or coils can be attached directly to the wire of the reed panels by means of tie wraps. A horizontal chase, containing inlet and outlet pipe, can be installed at the base point of the wall.
  8. Electrical Installation: Electric cables can be laid directly into the clay plaster between the insulation layer and the exterior wall. As in the case of the wall heating the cables can be conducted in a duct at the base point of the wall. It is also possible to conduct the cables on top of the panels and below the wires. However, in this case a thicker layer of plaster may be necessary. Flush sockets can be fitted with the help of a hole saw and gypsum plaster.
  9. Fixing: Light objects as for example pictures can be fixed driving long wood screws through the insulation panels into the timber frame. For heavier objects however, as for example wall cabinets, some prior arrangements are necessary. A board of the same thickness as the insulation panel and the clay plaster is attached to the timber frame before the application of the plaster base. The corresponding insulation panels are cut out. The wall cabinets are then attached to the board.

Alternative wall build-up 1: Inside insulation wattle and daub with 60 mm reed insulation panel and light clay on an installation lathing

Existence:
30 mm lime external rendering, 140 mm straw clay on wickerwork.
U-value without reed insulation panel: 1.74 W/(m²K).

General requirements:
EnEV: U = 0.35 W/(m²K) (for the additional installation of an insulation)
DIN 4108: R ≥ 1.20 (m²K)/W.

Requirements without a mathematical condensate verification:
DIN 4108 : Ri ≤ 1,0 m² K/W ; 0.5 m ≤ sd,i
WTA: Ri ≤ 0.8 m² K/W ; 0.5 m ≤ sd,i ≤ 2,0 m.

Inside insulation:
60 mm light clay mixture (e.g. wood or hemp light clay, ρ= 500 kg/m³ ), 60 mm reed insulation panel , 15 mm loam rendering plaster, 3 mm loam final rendering
U-value: 0.48 W/(m²K)*.

* The criteria for an exemption from the condensate calculation (WTA and DIN 4108) cannot be fulfilled. This analysis however, although it does provide a value for potential condensate does not reflect the hygroscopic qualities of the clay (loam). The wall build-up described above has proven itself.

Details on the Use and Application of Reed Insulation Panels as Permanent Formwork and Insulation:

  1. See above
  2. See above
  3. See above
  4. Screwing a vertical batten (40 x 60mm) to the timber frame
  5. Fixing an installation board on the lathing (24 x 160 mm)
  6. The insulation panels are attached to the installation board with wood screws and insulation discs. If later on cupboards or similar should be attached to the walls instead of the reed insulation panel a board of the thickness of the reed insulation can be used.
  7. Filling in the reed insulation layers with a light clay mixture.
  8. Plastering the reed insulation panel with clay rendering plaster. All over the clay a jute cloth has to be applied.
  9. Wall heating, window reveals, electrical installations, fixing see wall build-up 1 with inside insulation pressed in clay.

Wall build-up 2: Masonry with 60 mm reed insulation panel in clay as inside insulation

Existence:
20 mm lime external rendering, brick wall 24 cm (ρ= ca. 1600 kg/m³)
U-value without reed insulation panel: 1.83 W/(m²K).

General requirements:
EnEV: U = 0.35 W/(m²K) (for the additional installation of an insulation)
DIN 4108: R ≥ 1.20 (m²K)/W.

Requirements without a mathematical condensate verification:
DIN 4108 : Ri ≤ 1.0 m² K/W ; 0.5 m ≤ sd,i

Inside insulation:
App. 30 mm loam rendering, 60 mm Hiss-Reet reed insulation panel, 15 mm loam rendering plaster, 3 mm final loam rendering
U-value with reed insulation panel: 0.64 W/(m²K).

Details on the Use and Application of Reed Insulation Panels as Inside Insulation for Masonry Wall Constructions:

For the handling of reed panels as inside insulation for masonry wall constructions we recommend the same procedures as for the installation of an inside insulation for timbered houses. To fix the panels however, we recommend using exclusively insulation support.

Alternative wall build-up 2: Inside insulation of masonry with 60 mm reed insulation panel and light clay on a lathing

Existence:
20 mm lime external rendering, brick wall 24 cm (ρ= ca. 1600 kg/m³)
U-value without reed insulation panel: 1.83 W/(m²K).

General requirements:
EnEV: U = 0.35 W/(m²K) (for the additional installation of an insulation)
DIN 4108: R ≥ 1.20 (m²K)/W.

Requirements without a mathematical condensate verification:
DIN 4108 : Ri ≤ 1.0 m² K/W ; 0.5 m ≤ sd,i

Inside insulation:
60 mm light clay mixture (e.g. wood light clay, ρ= 500 kg/m³ or hemp light clay ρ= 500 kg/m³), 60 mm reed insulation panel, 15 mm loam rendering plaster, 3 mm final loam rendering
U-value: 0.50 W/(m²K)

Details on the Use and Application of Reed Insulation Panels as Permanent Formwork and Insulation:

  1. See wall build-up 1.
  2. See wall build-up 1.
  3. See wall build-up 1.
  4. Attaching a vertical batten (40 x 60 mm) to the timber frame with the help of screws and wall plugs, at a distance of 1.00 m.
  5. Fixing the installation board on the lathing (24 x 160 mm).
  6. The insulation panels are attached to the installation board with wood screws and insulation discs (As a substitute also crown caps or similar can be used).
  7. Filling in the reed insulation layers with a light clay mixture.
  8. Plastering the reed insulation panel with clay rendering plaster. All over the clay a jute cloth has to be applied.
  9. Wall heating, window reveals, electrical installations, fixing see wall build-up 1.

Disclaimer of Liability

The information given above is based on the calculations and information of HISS REET Schilfrohrhandel GmbH and the product specification of the manufacturer. The content is only a selection of especially important information. The author reserves the right not to be responsible for the topicality, correctness, completeness or quality of the information provided. Liability claims regarding material or ideal damage caused by the use of any information provided, including any kind of information which is incomplete or incorrect, will therefore be rejected, unless there is evidence of intentional or gross negligence on part of the author.

The information provided cannot substitute individual specialist advice by a planner.

Authors:

Dipl. Ing. Stefan Neumann
Dipl. Kfm. Philip Kullmann