Thermal insulation made from technical hemp
view from the perspective of building physics
Publisher
Tisková zpráva
20.10.2010 10:00
Tisková zpráva
20.10.2010 10:00
Interest in a healthy lifestyle is increasingly leading to the use of natural building materials. Due to their properties, thermal insulation made of technical hemp is being used again to a greater extent.
In construction, a large number of building materials with excellent properties are utilized. However, their production has direct or indirect impacts on the environment, just like the products themselves. For example, to create fibers for mineral insulation, it requires melts of stone at a temperature of 1600 °C; in comparison, hemp mats are thermally stabilized at a temperature of 150 to 160 °C. Another comparison can be made regarding the amount of production emissions when processing petroleum products for synthetic materials versus the fragrant work involved in drying and "breaking" plant raw materials.
Plant-based materials are no novelty in buildings; on the contrary, they were used much more frequently in the past due to their availability and affordability. It is no wonder that many of them are experiencing a large comeback today. Hemp has a thousand-year tradition in Europe. The advantage of hemp is its short growing season - in some regions, hemp can be harvested up to twice a year. For technical purposes, industrial hemp is used, from which building insulation is made.
For the use of materials in construction, their mechanical and construction-physical properties and processability are most important. Naturally, for thermal insulations, their thermal-technical properties are also crucial.
Mechanical Properties
Hemp insulation is sufficiently flexible due to the toughness of the hemp fiber; after short-term compression, it returns to its original shape. This property is particularly appreciated during assembly when it is inevitable to compress the mats while inserting them between structural elements. At the same time, they maintain their shape in the long term, preventing settling and the formation of unwanted cavities where insulation should have been.
For load-bearing insulations, their strength is important (which depends on the degree of compression under long-term loading) and dynamic stiffness, which affects acoustic damping.
In laboratory measurements of mechanical properties, for boards with a bulk density of 112 kg/m³, the stress value at 10% deformation was found to be 36.9 kPa and dynamic stiffness 20.8 MPa/m. These values declare that hemp boards with a bulk density of over 100 kg/m³ are very well usable for both light and heavy floating floor constructions.
For comparison: mineral wool insulation Orsil N has a stress at 10% deformation of 15 kPa and Orsil T-P 40 kPa. (Results of laboratory testing at the Brno University of Technology were provided by Ing. Jiří Zach).
Thermal Conductivity
The most important property of thermal insulation is its thermal conductivity, or rather the coefficient of thermal conductivity. This parameter varies among different materials. When we divide it by the thickness of the insulator, we obtain the partial thermal resistance of the insulation layer. Thus, we can choose different materials and influence the resulting thermal resistance by their thickness. The inverted value is the now commonly stated coefficient of heat transfer (U), which, according to current standards, should be for a wall in a lightweight construction, for example, 0.3 W/(m².K). The standard recommended value is 0.2 W/(m².K). If, for example, we want to meet the conditions of the new subsidy program "Zelená úsporám" and build a low-energy house, this value should be even lower, for passive houses even less than 0.15 W/(m².K). However, the overall design and assessment are much more complex and depend on many other aspects.
The coefficient of thermal conductivity (λ) for insulators ranges between 0.02 to 0.05 W/(m.K) or higher, with the rule being - the lower it is, the better. Polyurethane insulations are the best off with a value of λ = 0.02 W/(m.K), closely followed by extruded and expanded (common) polystyrenes with λ = 0.038 to 0.043 W/(m.K). For quality mineral insulation, the coefficient of thermal conductivity ranges between 0.036 to 0.043 W/(m.K). Hemp insulation has a thermal conductivity coefficient of 0.040 to 0.042 W/(m.K), placing it among the best common insulators.
Thermal Accumulation and Specific Heat
Thermal accumulation is the ability of a material to absorb, retain, and gradually release heat or vice versa. We best describe this property using thermal capacity, or rather its relative part per unit volume. The value of specific heat capacity expresses how much heat is absorbed by 1 kg of material for its temperature to increase by 1 K. The unit of specific heat capacity (c) is J/(K. kg). The amount of heat here is given by the relation
Q = c.m.DT
If hemp insulation has a value of c = 1,600 J/(K.kg), it means that one kilogram of this insulation absorbs 1,600 J with a temperature difference of 1 K.
In practice, this means that when the temperature inside the house rises, materials here absorb heat. This also applies conversely - during a temperature drop, heat is released. This creates a certain inertia or delay in the increase or decrease of temperature during external changes, which is a very beneficial property. With a specific weight of m = 35 kg/m³, 1 m³ of hemp insulation material absorbs heat Q = 56 kJ. Mineral insulation, which has a value of c = 840 J/(K.kg), absorbs heat Q = 33.6 kJ with a specific weight of m = 40 kg/m³.
When comparing these materials, we find that hemp insulation absorbs almost twice as much heat, thus affecting the thermal comfort in the interior twice as effectively, which plays a significant role especially in lightweight wooden constructions.
Diffusion of Air Moisture
One of the greatest enemies of lightweight constructions and thermal insulation in general is moisture. The outer structure faces balancing the internal stable temperature and relative humidity with significantly varying values outside throughout the year. Moisture enters the structure mainly through the diffusion of water vapor and moisture flow through the airtight layers' leaks.
The principle of proper design of the external structure layers is to limit the diffusion flow into the structure to such an extent that it prevents increased concentration of airborne moisture and the formation of condensation inside the structure. This means that an impermeable layer must be created from the interior side, which minimizes diffusion to an acceptable minimum, either by using a vapor retarder or a vapor barrier. Layers must further be composed from the least to most diffusion-open towards the exterior.
High diffusion permeability for insulation means that it is breathable and easily discharges and ventilates moisture. The diffusion resistance factor of hemp insulation is μ = 1.9, which is a parameter of a very permeable material.
For comparison: mineral insulation has similar values, while polystyrene has μ = 20 to 30.
Redistribution of Moisture
In addition to high diffusion permeability, hemp insulation is also capable of redistributing moisture. In general, this is the capacity of a material to balance and transmit moisture throughout its volume, thus a kind of moisture conductivity. This prevents local moisture concentration. Moisture is transmitted (distributed) to the entire volume of the material, creating a significantly larger surface area for easy ventilation. Hemp insulation can absorb and cope with a large amount of moisture. The bulk moisture may increase up to 20% without reducing the effectiveness of insulation properties.
Thus, the correct functioning of the assembly does not rely only on the diffusion of airborne moisture, but the ability to transfer moisture through organic fibers significantly contributes to it. It effectively prevents local flooding and particularly protects wooden structural elements from increased local humidity and subsequent degradation.
For comparison: with mineral insulation, the loss of thermal insulation capacity occurs at only 2% bulk moisture. With currently minimal capacity for redistribution, this material becomes highly risky with the entry of even a very small amount of moisture, especially with local leaks and damages to the vapor barrier, where moisture flow is concentrated into a very small area. There is a risk that mineral insulation may start to become locally wet to such an extent that the concentrated moisture cannot be ventilated through the limited surface and will begin to slide under its own weight to the lower sections. There, adjacent structures may become damp, accompanied by the emergence of mold, and in the case of wooden structures, significant damage to the wooden elements may occur due to increased moisture, rotting of the lower ends of posts or rafters, and foundational thresholds and purlins.
Diffusion-Open Assembly
Thanks to high thermal and moisture accumulation, hemp insulation positively affects the indoor climate, especially when used in a diffusion-open assembly of external construction. A diffusion-open assembly has significantly higher permeability than an assembly with a vapor barrier, allowing for a greater effect of transmitting the accumulated moisture back into the interior. This will occur conversely when the relative humidity inside is very low.
In any case, including in a diffusion-open assembly, the principle of airtightness and the correct order of individual layers of the assembly applies. The assembly is assessed during the design via calculations, where the year-round balance of moisture condensation inside the structure must not yield a positive result, thus the amount of moisture entering the structure must not be greater than the amount of vented moisture. The correctness of the assembly is influenced by numerous other factors, such as the properties of the materials used for the individual layers, their order, airtight sealing of the internal surface, and conversely, the permeability of the external surface, the effectiveness of venting ventilation cavities, and so forth, both during design and particularly during the construction process. Due to its properties, hemp insulation is suitable for use in diffusion-open assemblies.
Acoustic Properties
Natural fibers are flexible and tough. They are capable of vibrating at the same frequency as sound waves. The fibers are not interconnected; in insulating mats, they create a loose structure, and due to vibrations, there is additionally friction between the fibers. Long, flexible, and tough fibers significantly reduce the intensity of sound waves through their behavior in the structure. These facts suggest that hemp insulation can be used as quality acoustic insulation in sandwich assemblies to reduce airborne sound transmission. However, official laboratory measurements are not yet available.
On the other hand, the use of hemp boards as impact sound insulation is supported by research into their mechanical properties (see the section on Mechanical Properties). In conclusion, it can be stated that using technical hemp as acoustic insulation is at least comparable to its main competitor - mineral insulation.
Flammability
According to ČSN EN 13501-1: 2007, hemp insulation is classified in reaction to fire in class E, meaning it is a flammable material in contact with flame.
This issue can be easily addressed by enclosing it with a fire-resistant cladding with appropriate certification; for load-bearing elements of wooden constructions, it is necessary to use a certified assembly with corresponding fire resistance. These certified assemblies and systems are offered by various building material manufacturers, and it is a matter of proper design to determine the most advantageous one. There are certain restrictions on the use of hemp insulation based on fire regulations for more significant buildings, such as hospitals, schools, and so on, where the use of non-combustible materials is required for fire-resistant constructions. Hemp insulation can be used without difficulty in the vast majority of common buildings.
Evaluation
If we assess additional properties of hemp insulation, we conclude that its advantages outweigh its disadvantages. Its positives, i.e., flexibility and non-settling, excellent thermal insulation properties, high accumulation capabilities, high diffusion permeability, ability to redistribute moisture, excellent acoustic properties, natural and healthy indoor climate, easy and health-safe processing and handling, resistance to mold and pests, place hemp insulation in a significant, ecologically beneficial, and traditional material. The disadvantage of this material is its aforementioned flammability, which can be solved by appropriate modifications to the construction.
Use
Hemp thermal insulation can be used in all buildings as an equivalent to mineral thermal insulation. Its ideal use is in wooden structures and common building roofs, where diffusion-open assemblies can be advantageously utilized. The assembly with hemp insulation is significantly safer with regard to moisture redistribution. It can remove and ventilate many more amounts of moisture from the structure. A clear condition remains to monitor the occurrence of excessive moisture in the structure, use a vapor retarder, and functional external ventilation.
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| Completed assembly of the wooden structure skeleton and installation of the roof covering with a diffusion-open protective membrane in Vonoklasy near Prague |
Plant-based materials are no novelty in buildings; on the contrary, they were used much more frequently in the past due to their availability and affordability. It is no wonder that many of them are experiencing a large comeback today. Hemp has a thousand-year tradition in Europe. The advantage of hemp is its short growing season - in some regions, hemp can be harvested up to twice a year. For technical purposes, industrial hemp is used, from which building insulation is made.
For the use of materials in construction, their mechanical and construction-physical properties and processability are most important. Naturally, for thermal insulations, their thermal-technical properties are also crucial.
Mechanical Properties
Hemp insulation is sufficiently flexible due to the toughness of the hemp fiber; after short-term compression, it returns to its original shape. This property is particularly appreciated during assembly when it is inevitable to compress the mats while inserting them between structural elements. At the same time, they maintain their shape in the long term, preventing settling and the formation of unwanted cavities where insulation should have been.
For load-bearing insulations, their strength is important (which depends on the degree of compression under long-term loading) and dynamic stiffness, which affects acoustic damping.
In laboratory measurements of mechanical properties, for boards with a bulk density of 112 kg/m³, the stress value at 10% deformation was found to be 36.9 kPa and dynamic stiffness 20.8 MPa/m. These values declare that hemp boards with a bulk density of over 100 kg/m³ are very well usable for both light and heavy floating floor constructions.
For comparison: mineral wool insulation Orsil N has a stress at 10% deformation of 15 kPa and Orsil T-P 40 kPa. (Results of laboratory testing at the Brno University of Technology were provided by Ing. Jiří Zach).
Thermal Conductivity
The most important property of thermal insulation is its thermal conductivity, or rather the coefficient of thermal conductivity. This parameter varies among different materials. When we divide it by the thickness of the insulator, we obtain the partial thermal resistance of the insulation layer. Thus, we can choose different materials and influence the resulting thermal resistance by their thickness. The inverted value is the now commonly stated coefficient of heat transfer (U), which, according to current standards, should be for a wall in a lightweight construction, for example, 0.3 W/(m².K). The standard recommended value is 0.2 W/(m².K). If, for example, we want to meet the conditions of the new subsidy program "Zelená úsporám" and build a low-energy house, this value should be even lower, for passive houses even less than 0.15 W/(m².K). However, the overall design and assessment are much more complex and depend on many other aspects.
The coefficient of thermal conductivity (λ) for insulators ranges between 0.02 to 0.05 W/(m.K) or higher, with the rule being - the lower it is, the better. Polyurethane insulations are the best off with a value of λ = 0.02 W/(m.K), closely followed by extruded and expanded (common) polystyrenes with λ = 0.038 to 0.043 W/(m.K). For quality mineral insulation, the coefficient of thermal conductivity ranges between 0.036 to 0.043 W/(m.K). Hemp insulation has a thermal conductivity coefficient of 0.040 to 0.042 W/(m.K), placing it among the best common insulators.
Thermal Accumulation and Specific Heat
 |
| Detailed view of installed hemp insulation in a sloped roof construction |
Q = c.m.DT
If hemp insulation has a value of c = 1,600 J/(K.kg), it means that one kilogram of this insulation absorbs 1,600 J with a temperature difference of 1 K.
In practice, this means that when the temperature inside the house rises, materials here absorb heat. This also applies conversely - during a temperature drop, heat is released. This creates a certain inertia or delay in the increase or decrease of temperature during external changes, which is a very beneficial property. With a specific weight of m = 35 kg/m³, 1 m³ of hemp insulation material absorbs heat Q = 56 kJ. Mineral insulation, which has a value of c = 840 J/(K.kg), absorbs heat Q = 33.6 kJ with a specific weight of m = 40 kg/m³.
When comparing these materials, we find that hemp insulation absorbs almost twice as much heat, thus affecting the thermal comfort in the interior twice as effectively, which plays a significant role especially in lightweight wooden constructions.
Diffusion of Air Moisture
One of the greatest enemies of lightweight constructions and thermal insulation in general is moisture. The outer structure faces balancing the internal stable temperature and relative humidity with significantly varying values outside throughout the year. Moisture enters the structure mainly through the diffusion of water vapor and moisture flow through the airtight layers' leaks.
The principle of proper design of the external structure layers is to limit the diffusion flow into the structure to such an extent that it prevents increased concentration of airborne moisture and the formation of condensation inside the structure. This means that an impermeable layer must be created from the interior side, which minimizes diffusion to an acceptable minimum, either by using a vapor retarder or a vapor barrier. Layers must further be composed from the least to most diffusion-open towards the exterior.
High diffusion permeability for insulation means that it is breathable and easily discharges and ventilates moisture. The diffusion resistance factor of hemp insulation is μ = 1.9, which is a parameter of a very permeable material.
For comparison: mineral insulation has similar values, while polystyrene has μ = 20 to 30.
Redistribution of Moisture
In addition to high diffusion permeability, hemp insulation is also capable of redistributing moisture. In general, this is the capacity of a material to balance and transmit moisture throughout its volume, thus a kind of moisture conductivity. This prevents local moisture concentration. Moisture is transmitted (distributed) to the entire volume of the material, creating a significantly larger surface area for easy ventilation. Hemp insulation can absorb and cope with a large amount of moisture. The bulk moisture may increase up to 20% without reducing the effectiveness of insulation properties.
Thus, the correct functioning of the assembly does not rely only on the diffusion of airborne moisture, but the ability to transfer moisture through organic fibers significantly contributes to it. It effectively prevents local flooding and particularly protects wooden structural elements from increased local humidity and subsequent degradation.
For comparison: with mineral insulation, the loss of thermal insulation capacity occurs at only 2% bulk moisture. With currently minimal capacity for redistribution, this material becomes highly risky with the entry of even a very small amount of moisture, especially with local leaks and damages to the vapor barrier, where moisture flow is concentrated into a very small area. There is a risk that mineral insulation may start to become locally wet to such an extent that the concentrated moisture cannot be ventilated through the limited surface and will begin to slide under its own weight to the lower sections. There, adjacent structures may become damp, accompanied by the emergence of mold, and in the case of wooden structures, significant damage to the wooden elements may occur due to increased moisture, rotting of the lower ends of posts or rafters, and foundational thresholds and purlins.
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Diffusion-Open Assembly
Thanks to high thermal and moisture accumulation, hemp insulation positively affects the indoor climate, especially when used in a diffusion-open assembly of external construction. A diffusion-open assembly has significantly higher permeability than an assembly with a vapor barrier, allowing for a greater effect of transmitting the accumulated moisture back into the interior. This will occur conversely when the relative humidity inside is very low.
In any case, including in a diffusion-open assembly, the principle of airtightness and the correct order of individual layers of the assembly applies. The assembly is assessed during the design via calculations, where the year-round balance of moisture condensation inside the structure must not yield a positive result, thus the amount of moisture entering the structure must not be greater than the amount of vented moisture. The correctness of the assembly is influenced by numerous other factors, such as the properties of the materials used for the individual layers, their order, airtight sealing of the internal surface, and conversely, the permeability of the external surface, the effectiveness of venting ventilation cavities, and so forth, both during design and particularly during the construction process. Due to its properties, hemp insulation is suitable for use in diffusion-open assemblies.
Acoustic Properties
Natural fibers are flexible and tough. They are capable of vibrating at the same frequency as sound waves. The fibers are not interconnected; in insulating mats, they create a loose structure, and due to vibrations, there is additionally friction between the fibers. Long, flexible, and tough fibers significantly reduce the intensity of sound waves through their behavior in the structure. These facts suggest that hemp insulation can be used as quality acoustic insulation in sandwich assemblies to reduce airborne sound transmission. However, official laboratory measurements are not yet available.
On the other hand, the use of hemp boards as impact sound insulation is supported by research into their mechanical properties (see the section on Mechanical Properties). In conclusion, it can be stated that using technical hemp as acoustic insulation is at least comparable to its main competitor - mineral insulation.
Flammability
According to ČSN EN 13501-1: 2007, hemp insulation is classified in reaction to fire in class E, meaning it is a flammable material in contact with flame.
This issue can be easily addressed by enclosing it with a fire-resistant cladding with appropriate certification; for load-bearing elements of wooden constructions, it is necessary to use a certified assembly with corresponding fire resistance. These certified assemblies and systems are offered by various building material manufacturers, and it is a matter of proper design to determine the most advantageous one. There are certain restrictions on the use of hemp insulation based on fire regulations for more significant buildings, such as hospitals, schools, and so on, where the use of non-combustible materials is required for fire-resistant constructions. Hemp insulation can be used without difficulty in the vast majority of common buildings.
Evaluation
If we assess additional properties of hemp insulation, we conclude that its advantages outweigh its disadvantages. Its positives, i.e., flexibility and non-settling, excellent thermal insulation properties, high accumulation capabilities, high diffusion permeability, ability to redistribute moisture, excellent acoustic properties, natural and healthy indoor climate, easy and health-safe processing and handling, resistance to mold and pests, place hemp insulation in a significant, ecologically beneficial, and traditional material. The disadvantage of this material is its aforementioned flammability, which can be solved by appropriate modifications to the construction.
Use
Hemp thermal insulation can be used in all buildings as an equivalent to mineral thermal insulation. Its ideal use is in wooden structures and common building roofs, where diffusion-open assemblies can be advantageously utilized. The assembly with hemp insulation is significantly safer with regard to moisture redistribution. It can remove and ventilate many more amounts of moisture from the structure. A clear condition remains to monitor the occurrence of excessive moisture in the structure, use a vapor retarder, and functional external ventilation.
text: Ing. Jan Škopek (the author is a designer at OMEGA project, s.r.o.)
photo: archive of CANABEST and OMEGA project
photo: archive of CANABEST and OMEGA project
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