Thermal insulation in timber construction

Pure nature! You could say that this is the motto of timber construction, in which the raw material wood is increasingly being emphasised due to both aesthetic factors and its sustainable and energy-efficient quality. Despite the good insulating properties of the raw material itself, this is often not enough to fulfil today's requirements in terms of energy efficiency, technical suitability and living comfort. It is therefore not really a question of whether thermal insulation should be included in timber construction, but rather which materials are most effective in helping to fulfil these requirements.

Why thermal insulation makes sense in timber construction

 Natural roof insulation  Natural roof insulation

Climate change, energy-efficient measures and increasing technical requirements are familiar to anyone who plans, realises or implements projects in the construction industry. This dynamic brings with it a constantly growing complexity that needs to be mastered. Solutions must be found that not only fulfil the current requirements, but are also sustainable and last in the long term.

The search for effective thermal insulation in timber construction also addresses these issues and not only contributes to a pleasant living environment, but also makes a significant contribution to reducing energy consumption and thus to the sustainability of a building. Whether in the roof, on the wall, in the floor or on the façade - insulation should not be missing in any of these areas in timber construction either. High-quality, energy-efficient thermal insulation is therefore not only an investment in the future, but also a possible response to current ecological and economic challenges.

Energy-efficient insulation protection for cold and heat

Building physics aspects such as the permeability of heat, sound, moisture and air have a significant influence on the energy consumption and living comfort of a building. Accordingly, the choice of the right insulation material is a central component in the design of energy-efficient buildings. Cold and heat must not be ignored - in winter the heat should be kept in the building and in summer a pleasant coolness should be felt in the room.

Avoid thermal bridges and heat loss

Whether you are a skilled worker, builder or architect looking for the right insulation for a timber construction project for your customers, heat loss is certainly always on your mind and needs to be counteracted in the best possible way.

Interruptions in the thermal insulation at corners, connections, component transitions or penetrations often lead to so-called thermal bridges. These thermal bridges increase heat conduction and lead to unwanted heat loss, which can result in increased heating costs, uneven room temperatures, condensation and even mould growth. The seamless connection option of a suitable insulation material is therefore another important feature to guarantee your customers high-quality insulation in the long term and maximise energy efficiency.

U or lambda value? Which figure is worth more?

U-value (heat transfer coefficient) 

U-value, several building materials U-value, several building materials

The U-value is used to assess the thermal insulation properties of building components. It is a measure of thermal insulation and indicates how much heat energy flows through per area and temperature difference of 1 Kelvin between the two sides of a component. The correct unit is given in watts per square metre and Kelvin (W/m²K).

A low U-value means that the component has a low thermal transmittance and is therefore better insulated. In practice, therefore, little heat is lost to the environment on cold winter days, for example. As a rule, low U-values are therefore aimed for when planning buildings in order to minimise heat loss through the building envelope and reduce energy consumption for heating and cooling.

Lambda value (thermal conductivity)

Lambda value, single building material Lambda value, single building material

The lambda value refers to the thermal conductivity of building materials. It is stated in watts per metre and Kelvin (W/mK) and indicates how much thermal energy is conducted through a building material per metre and per temperature difference of 1 Kelvin.

Building materials with a lambda value of less than 0.1 W/(mK) are generally regarded as insulating materials, whereby a value between 0.025 and 0.075 W/(mK) is usually aimed for.

A low lambda value therefore means that the building material has a low thermal conductivity and therefore the heat is not lost elsewhere - i.e. it acts as a good insulator.

Both values provide important and different information about the thermal insulation properties of a component or material. While the U-value takes into account the thermal permeability of an entire component (thermal conductivity of the materials used - i.e. the corresponding lambda values - as well as the thickness and arrangement of the components), the lambda value takes into account the specific thermal conductivity of an individual building material. Accordingly, the lambda value is often used to compare individual materials and their properties with each other.

Sound insulation and fire protection

Technical data on sound insulation and fire protection

European standards define so-called sound insulation and fire protection classes. Sound insulation classes are defined in accordance with DIN EN ISO 16283 and enable the sound insulation of building elements and structures such as walls, ceilings and windows to be assessed. They are expressed in terms of sound insulation values (Rw values from 25 to over 75), with higher values indicating more effective sound insulation.

Fire protection classes apply throughout Europe in accordance with specific standards that classify materials according to their reaction to fire and heat. The DIN EN 13501-1 classification ranges from non-combustible (class A1) to inadequate fire protection (class F). DIN EN 13501-2 defines its classifications in a combination of letters and numbers and includes so-called fire resistance classes. These also specify the duration for which components can withstand fire and heat without losing their integrity. The abbreviations of the factors to be analysed are derived from the French as follows:

REI fire protection classes for timber construction REI fire protection classes for timber construction
  • R = Resistance (load-bearing capacity): The ability of a building component to withstand the effects of fire from one or more sides over a certain period of time without loss of stability.
  • E = Étanchéité (tightness): The component can withstand fire exposure on one side and prevents the fire from spreading to the opposite side by preventing the passage of flames or hot gases that could ignite the neighbouring material. 
  • I = Insulation (thermal insulation): The building element resists a one-sided fire load without the fire spreading to the opposite side. It prevents the transfer of heat from the side of the fire to the opposite side in order to prevent the material on that side from igniting. It also provides an effective heat barrier for a certain period of time to protect people in the vicinity of the component.

  • W = Radiation: The component with a space-enclosing function resists one-sided fire exposure, which means that the measured heat radiation on the opposite side remains below a specified value for a certain period of time.

Accordingly, REI90, for example, means that the load-bearing capacity of a building component under fire load and the thermal insulation without fire transmission are guaranteed for a period of at least 90 minutes.

Types of insulation in timber construction

Insulation in timber construction is diverse. Between natural, mineral and synthetic materials, you, like many other fabricators, builders and architects, are probably often faced with the decision of which of the numerous options is the best for your current project. After all, a number of criteria and legal requirements regarding energy efficiency, sustainability and future-orientated solutions need to be met. To give you an overview, we have listed a variety of insulation options here:

A. Synthetic insulating materials

Synthetic insulating materials are rigid foams with additives, which are often a low-cost insulating material option. As they do not rot, they are long-lasting and mainly provide insulation in the required areas. Chemical production makes them less environmentally friendly than organic materials, and moisture can encourage mould growth. Disposal is often problematic and some future disposal aspects have not yet been fully analysed.


Synthetic insulating materials Synthetic insulating materials
  • Polyurethan-Hartschaum (PUR/PIR): Polyurethan wird aus einem Kunststoff hergestellt, der durch die Reaktion von Polyol und Isocyanat entsteht. Durch die Zugabe von Isocyanurat-Gruppen entsteht die PIR Variante, die dadurch verbesserte Brandschutzeigenschaften aufweist und dementsprechend bei höheren Auflagen zum Einsatz kommt. In der Regel sind PUR/PIR-Dämmstoffe leicht entflammbar und entwickeln im Brandfall hochtoxische Gase.
  • Rigid polyurethane foam (PUR/PIR): Polyurethane is produced from a plastic that is created by the reaction of polyol and isocyanate. The addition of isocyanurate groups results in the PIR variant, which has improved fire protection properties and is therefore used for higher requirements. As a rule, PUR/PIR insulating materials are highly flammable and develop highly toxic gases in the event of fire.

B. Mineral insulation materials

The category of mineral insulation materials includes inorganic materials such as sand, stone, lime and glass. When processing mineral insulation materials, fibres can be released that can enter the lungs through inhalation and damage the surrounding cell tissue. Protective clothing and breathing masks are therefore required.


Mineral wool insulation Mineral wool insulation
  • Mineral wool
    Mineral wool is divided into rock wool and glass wool. Rock wool is usually produced by melting basalt rock, diabase rock or dolomite, while glass wool is obtained from waste glass, sand and lime. The materials are melted down and then treated with binding and impregnating agents in order to achieve the desired insulating effect. Mineral insulation materials are generally non-combustible and are usually processed into boards. Their production requires a lot of energy and at the end of their product life they are not biodegradable, which means that special procedures must be used for disposal.
Foam glass insulation Foam glass insulation
  • Cellular glass (CG)
    Cellular glass (CG) is made from recycled waste glass and is produced by melting glass granulate with carbon. The insulating material is processed in the form of panels and is generally classified as more environmentally friendly due to its production from recycled glass and its chemically inert (inactive) and emission-free properties. Protective clothing and breathing masks should be worn during installation to prevent the inhalation of dust particles.
Perlite filling Perlite filling
  • Perlite
    Perlite consists of a glassy, volcanic rock that is processed both as granulate and in slab form. The addition of organic and inorganic binders enables the stone to be produced into slabs. As perlite is considered non-combustible, it slows the spread of fire and is often regarded as fireproof. The material is also resistant to mould, rot and vermin. It is typically installed by blowing, pouring or placing panels. Although perlite is considered safe and non-toxic, it is still advisable to wear protective clothing to minimise skin irritation or dust inhalation.

C. Natural insulating materials

  • Hemp: To produce hemp insulation mats, the fibres are separated from the bast of the hemp stalk and mixed with plastic fibres. Heat is then applied to melt the plastic fibres and bind the plant fibres together. Hemp is well suited as an insulating material due to its good sound insulation, but is not always compostable due to the multiple addition of additional fibres.
  • Cork: To produce cork insulation, the bark of the cork oak is extracted and ground. Water vapour is used to enlarge the cork particles (expansion), which are processed into blocks and panels using the material's own resin. Despite its low flammability, cork is not classified as a fire protection material and therefore requires additional fire protection measures in some cases. As a renewable raw material, cork is biodegradable and can be recycled or composted at the end of its life.
  • Cellulose: Cellulose is mainly obtained from recycled newsprint fibres and is defibrated, dedusted and dried during production. The insulating material is mostly used as blown-in insulation, but is also processed in the form of cellulose mats and pellets. As the material consists mainly of recycled paper fibres, it is considered environmentally friendly and biodegradable, but may contain undesirable substances such as printing inks and fire retardants.
  • Wood fibre: As the name suggests, the insulation boards are made from the wood fibres of spruce, pine or fir trees. Depending on the manufacturing process, the fibres are softened with water, pressed together and heated (wet process) or dried, pressed and shaped (dry process). In the wet process, the naturally occurring lignin in the wood is used as a binding agent, which means that no other substances need to be added. In contrast, the dry method uses synthetic resins, which act as an adhesive at high temperatures and provide stability. Due to the addition, care must be taken to ensure appropriate disposal.
  • Sheep's wool: The fibre of sheep's wool consists of protein (keratin) and its naturally renewable function makes it a highly suitable, ecological insulating material. Sheep are usually sheared once or twice a year and thus provide the regular basis for the sheep's wool insulation material. After shearing, the wool is washed with soda and curd soap and then mechanically processed to form mats and felts. We have explained the characteristics of sheep's wool insulation in detail below

D. A fair comparison of insulation materials. 

fair comparison of insulation materials fair comparison of insulation materials
**Sources: Synthetic insulation materials acc. to Baumit-Website; mineral insulation materials acc. to Baumit-Website; renewable insulation materials using the example of wood fiber as well as ISOLENA installation according to time measurements in practice by Scandinavian Blockhaus

Sheep's wool insulation in timber construction

Wood and sheep's wool form a flawless symbiosis as two natural raw materials in house construction.

The properties of sheep's wool as an insulating material in timber construction are diverse and can be described as follows:

Benefit 1 Benefit 1

Moisture-regulating effect

Sheep's wool has the special property of absorbing moisture and transporting it by capillary action to the place where it can dry out again. Even with increased moisture (in vapour form) in the fibre itself, the thermal insulation effect of the wool hardly changes. In practice, ISOLENA sheep's wool insulation has a demonstrably low deviation value of 5% in the thermal insulation effect compared to other insulation materials (15% - 20%). Accordingly, building damage and increased investment in thermal energy can be avoided and increased safety can also be guaranteed in timber construction.

Moisture-regulating effect Moisture-regulating effect
fire protection wood constrction fire protection wood constrction
Benefit 2 Benefit 2

Fire protection

Fire protection is a particularly important issue when working with wood. Combining wood with sheep's wool makes perfect sense, as sheep's wool ignites at temperatures of around 560° to 600° C and then begins to melt instead of bursting into flames. In the event of a fire, no toxic gas is released and as soon as the source of the flame is removed, the wool extinguishes itself. The natural moisture in sheep's wool has been shown to help fires as it evaporates during the initial stages of the fire. This evaporation creates a cooling effect throughout the entire wall system, which helps to slow down the spread of the fire to other areas. Compared to insulation with mineral fibre, the inherent moisture of sheep's wool can therefore help to keep temperatures lower for a certain period of time in the event of a fire.

Benefit 3 Benefit 3

Sound insulation

Due to its diverse fibre fineness and elasticity, sheep's wool has very good sound absorption properties, which is why it is often used in the field of acoustics. The sound insulation values of ISOLENA sheep's wool are comparable to those of mineral fibre and even exceed those of soft wood fibre in some areas.

Sound insulation with sheep's wool in timber construction Sound insulation with sheep's wool in timber construction
Benefit 4 Benefit 4

Wool protection

ISOLENA has developed the globally unique, biocide-free and long-term tested plasma ion process IONIC PROTECT® to effectively protect wool from moths and insects. You can find out how the process works here in our article on wool protection.

Benefit 5 Benefit 5

Mould resistance

The protein fibre of sheep's wool remains resistant to mould and rot despite moisture and therefore provides a good basis for insulation, particularly in timber construction.

Benefit 6 Benefit 6

Indoor climate

By breaking down odours and pollutants, sheep's wool naturally ensures a pleasant indoor climate.

The molecules in the sheep's wool fibre have the ability to break down and neutralise pollutants such as formaldehyde, which is contained in wood-based materials, for example. 


Indoor climate wood construction Indoor climate wood construction

"When you enter the building, you can feel it immediately: the ecological building materials ensure an incredibly great indoor climate. Especially in our small construction projects, it is important to us that the moisture management works well and the room breathes. We have had very good experiences with ISOLENA's sheep's wool insulation in over 170 projects."

Theresa Mai, Wohnwagon Theresa Mai, Wohnwagon

Theresa Mai
Founder and Managing Director Wohnwagon

Benefit 7 Benefit 7


Disposal is extremely straightforward when using 100% sheep's wool, as it can be completely reused. No offcuts, no additional waste and no disposal costs, as leftover wool can easily be reused elsewhere due to its flexible form. It can also be recycled and reused in the production process. If sheep's wool is composted, it serves as a fertiliser in the humus created there.

Dispoal sheep wool insulation Dispoal sheep wool insulation
Benefit 8 Benefit 8


The processing of sheep's wool is very simple and efficient. Compared to other insulation materials, installation with sheep's wool can save up to 50% of the time (even without prior knowledge). The entire insulation roll can be fixed to the wall in just a few simple steps. In addition, sheep's wool is harmless to health and contains no respirable fibres, meaning that no special protective clothing such as masks, gloves or closed clothing is required during installation.

"Our team in the workshop loves the material and the pleasant, simple processing! This is a work step that is particularly popular. When the shell is in place and the sheep's wool insulation is fitted, the building feels like "home" for the first time, even if there is still a building site all around."

Theresa Mai, Wohnwagon Theresa Mai, Wohnwagon

Theresa Mai
Founder and Managing Director Wohnwagon

"Since we started working with ISOLENA sheep's wool insulation, the quality of our employees' work has improved significantly. Working hours on the construction site have been reduced and there is no fine dust pollution or allergic reactions due to the material used."

Testimonal Robert Reitinger, Scandinavian Blockhaus Testimonal Robert Reitinger, Scandinavian Blockhaus

Robert Reitinger
Site management Scandinavian Blockhaus

Project examples

Discover projects in timber construction, log house construction and tiny house construction. Here, wood and sheep's wool create a natural and healthy living space. Click for inspiring references:

Timber construction projects with sheep's wool insulation Timber construction projects with sheep's wool insulation


The question of the ideal insulation material is complex and individual. As a skilled worker, builder or architect, you always strive to offer your customers optimum results. Many factors, requirements and tests must be taken into account when selecting the ideal insulation material for timber construction. The question is: if the guiding principle in timber construction is "pure nature", why not use natural materials for insulation?

The aim should be to utilise the natural properties of the materials and work with what is naturally available. The quality of the products should be noticeable for both the customer and the installer in order to achieve consistent and satisfactory results in the long term.

Alexander Lehner von ISOLENA explains: "We offer solutions for current pressing requirements. We manufacture products that not only don't cause problems, but also represent solutions to existing problems - and we do this holistically. ISOLENA is made from 100% sheep's wool, contains no other additives and offers long-term safety."

"The combination of wood and sheep's wool brings together the best of two worlds. The stability, the simple construction method, the storage capacity in the building mass - solid wood is great!

The sheep's wool then perfectly complements this with its properties for cold and heat protection and moisture management. 

A dream team!"

Theresa Mai, Wohnwagon Theresa Mai, Wohnwagon

Theresa Mai
Founder and Managing Director Wohnwagon

Our most popular timber construction products


Heat and sound insulation
Thickness: 30-140 mm
Thermal conductivity λtr: 0,038 W/mK
Fire behaviour: D-s2, d0; CH: RF3


Heat and sound insulation
Thickness: 120-300 mm
Thermal conductivity λtr: 0,035 W/mK
Fire behaviour: D-s2, d0; CH: RF3

ISOLENA Optimal Plus

Heat and sound insulation
Thickness: 30-80 mm
Thermal conductivity λtr: 0,035 W/mK
Fire behaviour: D-s2, d0; CH: RF3