Compilation of relevent research
By Kim Brooks
August 2018
Hempcrete is a lightweight fibre-reinforced concrete material, a mixture of the inner woody core as well as the long fibres of the industrial hemp plant, hydrated lime-based binder, and water. This material is used internationally for construction, as a wall infill system which offers insulation and moisture regulator.
We have built three modular hempcrete houses to date; one in Alberta for a client, one on our property, and a third on our commercial site on Bowen Island. This third unit has been sold and will be moved (by Nichol Brothers) onto a traditional foundation on Bowen Island.
The following describes our houses and the properties of hempcrete material which has been researched extensively over the last 16 years. Sources from the reference material at end of the document.
| Properties of Hempcrete- details below | |
| Hygroscopic properties – absorption and movement of moisture | |
| Vapor permeability: | 3.4 x 10-5 PSI. |
| Moisture Buffer Value | 2 g/(m2.%RH) |
| Porosity | 80% in volume |
| Thermal Performance | |
| Conductivity and Thermal Mass Insulation | |
| R-value | 2.4 – 4.8 per inch |
| Thermal Resistance | (typical U-value at 300 mm = 0.21 W/m2 K) |
| Thermal transmittance (also measured in U value) | 300mm wall (0.35 W/m2K) |
| Thermal Conductivity | 0.06-0.12 W/mK low thermal conductivity (λ = 0.047 to 0.058W/m 2 K) low thermal conductivity (λ = 0.047 to 0.058W/m 2 K) low thermal conductivity (λ = 0.047 to 0.058W/m 2 K) (λ = 0.047 to 0.058W/m 2 K) |
| Air Permeability (air leakage rate per hour per square meters of envelope area) | <2:m3/m2 hr @ 50 pa 1.0 x 10-6 PSI |
| Strength | |
| Compressive strength ranges, depending on the casting process; | 116 to 145 pounds per square inch (PSI) 29=>1006.56 (0.2 to 6.94 MPa ) |
| Tensile | Composites made of 20% hemp fibre were found to display a tensile strength and modulus of 35 MPa and 4.4 GPa, respectively. |
| Flexural strength: | 44 to 58 PSI Flexural strength: 44 to 58 PSI |
| Weight | 15% that of traditional concrete |
| Dry density: | 270–330 kg/m3to 93.6 to 136.4 Kgs/M3 5.9 to 8.6 lbs/cf Concrete-150lbs. Hempcrete is 17=>20 times lighter than concrete |
| Fire rating: | Approximately 60-120 minutes per 4 inches of thickness Limecrete, Ltd. (UK) reports a fire-resistance rating of 1 hour per British/EU standards. Flame spread index = 0 Smoke Developed Index = 0 |
| Mean Acoustic Absorption Coefficient | 0.69 NRC. It absorbs 90% of airborne sound |
| Carbon Negativity of Hemp Concrete | Test Wall of 1m x 1m x 0.3m sequestrated 82.71 kg of carbon dioxide. This compensated for the 46.43 kg of carbon dioxide emitted during growing, manufacturing and construction processes such as the manufacture of lime, transportation of material, etc., but also enabled an additional 36.08 kg of carbon storage |
| Carbon Sequestration | 7 lbs/cf 108kg/M3 50 Kg /1 M2 |
| Co2 emissions | Significantly reduced (.545kg/kWh) |
(HempLime) primary ingredients
Lime-Based binder
The high percentage of lime is important due to the highly alkaline. It is an excellent binder with unique binding properties to hemp due to the high silica content. Lime is poisonous to pests, fire-resistant, and prevents moulds.
Hemp plant Chemical Properties
| Hemp hurds component | Content (%) |
| Toluen-ethanol extract | 3.57 |
| Holocellulose | 77.28 |
| Cellulose | 44.5 |
| Hemicellulose | 32.78 |
| Lignin | 21.03 |
| Ash | 3.04 |
As hempcrete is a natural product, performance can vary slightly. Due to this, when test results have varied, a range is included. Multiple researchers’ data is included- Please see reference sources below.
Hemp concretes have high levels of porosity enabling excellent thermal behaviour, hygric and acoustic advantages. Hemp lime provides a lightweight material with the characteristics of a heavier material that both insulates stores heat and mediates fluctuations in external temperature. Although thermal conductivity is low relative to other high capacitance materials, the thermal transmittance is higher than walls with ample widths of classic insulation materials.
The thermal inertia characteristics of hemp-lime compensate and enable comfortable thermal indoor conditions and low energy operation. Future research will investigate dynamic thermal diffusivity evaluation to enable greater confidence in claims of hemp-lime as a material with optimum thermal inertia and mass characteristics.
Further information on the ‘Properties of Hempcrete’ table, above
Hygroscopic properties -absorption and movement of moisture
Vapor permeability: 3.4 x 10-5 PSI.
Moisture Buffer Value of 2 g/(m2.%RH)
The breathability and ability to act as moisture buffer reduces the risk of damage from trapped humidity and the application technique prevents thermal bridges.
Hemplime concrete ‘works without any additional vapor retarder.’ (Hygrothermal Performance Assessment for Super SSR Modular Block Wall System. Smart Solutions Manfred Kehrer, President 171 Glassboro Dr. Oak Ridge, TN 37830 www.WUFI.us Date Prepared – November 2016)
Thermal Performance;
Thermal parameters of hemp-lime are increasingly well characterised with a proliferation of studies in recent years publishing performance results for a range of binder types, densities, moisture contents, and construction/application methods.
Conductivity and Thermal Mass Insulation
Thermal Resistance
The higher the R-value, the more thermal resistance the material has and therefore the better its insulating properties. Hempcrete has high Thermal Resistance
R-value
Hempcrete ranges from 2.4 – 4.8 per inch of thickness
In comparison to other free-form construction materials has a much higher R-value. Concrete has a typical R-value of about 0.08 per inch of thickness compared to hempcrete
Thermal Conductivity:
(typical U-value at 300 mm = 0.21 W/m2 K)
0.06-0.12 W/mK
low
thermal conductivity (λ = 0.047 to 0.058W/m
2K)low thermal conductivity (λ = 0.047 to 0.058W/m2
K)low thermal conductivity (λ = 0.047 to 0.058W/m
2K)(λ = 0.047 to 0.058W/m
2K) 0.07–0.09 W/mK
0.09 – 0.115 W/mK
Thermal transmittance (also measured in U value)
300mm wall, 0.35 W/m2K (UK building code standards (0.21 W/m2K (domestic), 0.27 W/m2K). Evidence in the UK from a BRE study at Haverhill Housing Project indicates that the walls of a hempcrete house, performed at a better (lower) actual U value than the notional calculation. The monolithic nature of the wall construction in and of itself should also lead to lower thermal bridging, which may also have been a factor in the higher performances measured at Haverhill. The construction technique promotes good airtightness and minimal thermal bridging of the building envelope. All these factors combine to produce low carbon, hygrothermally efficient buildings that are low energy both in construction and in use, and offer opportunities for recycling at the end.’Hygrothermal performance of an experimental hemp–lime building. (Andy Shea, Mike Lawrence, Pete Walker 2012/11/1JournalConstruction and Building MaterialsVolume36Pages270-275)
Thermal Mass or Inertia the capacity of a material to store energy.
Hempcrete has high thermal inertia and demonstrates all the characteristics of Thermal Mass with the added benefit of being lighter weight than concrete and other masonry building materials and is able to STORE and RELEASE ENERGY (heat) that gives hempcrete its superb ability to insulate.
Due to hemp concrete’s high porosity and Moisture Buffer Value of 2 g/(m2.%RH) moisture gets entrapped within the many pores of the material resulting in the material gaining thermal mass. This thermal mass prevents fluctuations in temperature of the indoor atmosphere much like phase change materials (PCM).
Materials like concrete cannot absorb and emit heat quickly and do not have the ability to buffer moisture. There is no other construction material on the market, which has such a high R-value to low production value ratio.
‘…hempcrete has serious ecological and energy credentials.’ Passive House
• Lower mechanical (HVAC) requirements. Our NotSo Tiny home requires 7,000 btu (relative to what, for comparison)
These properties also indicate a reduction in the requirement for air conditioning by a minimum of 10%
- Air Permeability (air leakage rate per hour per square meters of envelope area)
<2:m3/m2 hr @ 50 pa or 1.0 x 10-6 PSI
Hempcrete is inherently airtight. Better than best practices which is approximately 3:m3/m2 hr @ 50 pa. A range of 5 -9 is common (https://www.cibse.org/getmedia/. Stroma-Air-Tightness-Presentation-Slides.pdf.aspx)There are air permeability benefits by the nature of lime hemp’s monolithic construction, (spray or infill around openings etc.) and plastering, rendering finishes.
- Strength
Compressive
Compressive strength ranges, depending on the casting process;
116 to 145 pounds per square inch (PSI)
29=>1006.56 (0.2 to 6.94 MPa )
‘One of the most important parameters for a building material used as a walling material is compressive strength. Researchers around the world that have worked with hemp concrete have reported various values of hempcrete’s compressive strength. The typical compressive strength is around 1 MPa, around 1/20 that of residential grade concrete. It is a low-density material and resistant to crack under movement thus making it highly suitable for use in earthquake-prone areas.
Unlike other organic concrete additives, wet hemp breaks down very slowly, enabling the cement to cure to maximum strength without cracks due to off-gassing. Research also indicates a clear proportional relationship between density and compressive strength.
Tensile Composites made of 20% hemp fibre were found to display a tensile strength and modulus of 35 MPa and 4.4 GPa, respectively.
Flexural strength: 44 to 58 PSI Flexural strength: 44 to 58 PSI
- Weight
Density:
dry density: 270–330 kg/m3to 93.6 to 136.4 Kgs/M3
5.9 to 8.6 lbs/cf
15% that of traditional concrete
Concrete-150lbs. Hempcrete is 17=>20 times lighter than concrete
- Durability
the longevity of the structure made from the material Interestingly, hemp concrete, because of the lime content undergoes carbonation across a span of several years and turns into limestone, rendering strength to the structure and the micro bonds in the binder matrix (Cultrone et al., 2005; Lawrence, 2006). This gain in strength over time could possibly signify increasing durability with time and can lead to houses lasting for centuries; and afterward they can even be recycled.
Mean Acoustic Absorption Coefficient
0.69 NRC. It absorbs 90% of airborne sound
- Carbon
Carbon Negativity of Hempcrete
Hemp shivs are composed of 45% carbon, meaning 1 kg of hemp shivs sequester about 1.6 to 1.8 kg of carbon dioxide through photosynthesis during the plant’s growth. A study conducted by Ip and Miller, 2012 indicated that a functional wall unit of dimensions 1m x 1m x 0.3m had sequestrated 82.71 kg of carbon dioxide, which not only compensated for the 46.43 kg of carbon dioxide emitted during growing, manufacturing and construction processes such as the manufacture of lime, transportation of material, etc. but also enabled an additional 36.08 kg of carbon storage. This means that structures made of hemp concrete essentially compensate for all of the carbon emissions made from all other processes related to constructing the structures. Hence, if the goal is to achieve a carbon-neutral building, hemp concrete must be used in a considerable quantum to offset the carbon emissions.
Carbon Sequestration
7 lbs/cf
108kg/M3
50 Kg /1 M2
A unique result of combining lime and hemp is their ability to act as a carbon sink. Data from Lime Technology Ltd in the UK estimate that a typical lime hemp house equates to roughly 50 tonnes of carbon per house, which in addition to embodied energy savings is a critical one-off carbon saving.
Co2 emissions Significantly reduced (.545kg/kWh)
Human Comfort Model
Recent studies of standard and hempcrete buildings have shown that assessment of performance based on either R or U-values alone is unrepresentative of true performance. The interplay between the warmth, oxygen and moisture content plays a strong role in how comfortable humans feel, regardless of the R or U values.
A material with high thermal inertia is one in which dynamic effects are prevalent, and one for which steady-state analysis could prove inaccurate. It is a bulk material property related to thermal conductivity and volumetric heat capacity and can modulate the impact of varying environmental conditions. Constantly varying external environment conditions continuously impact the building envelope and don’t regularly allow for prolonged steady-state operation.
With respect of thermal performance characterization, studies have shown that hemp-lime exhibits high
moisture buffering capacity and a good balance between low mass and storage capacity. A recent
salient study of whole hemp-building performance presents insightful evidence of the ability of hemp to
attenuate oscillations in the external environment. Other studies have undertaken simulation-based
investigation of dynamic whole hemp-building performance.
The majority of laboratory-based studies however have focused on steady-state thermal characterization with less focusing on non-steady-state situations. Transient behavior appears after an imposed change in temperature at a boundary of the material, as occurs during changing climate conditions or indoor environmental conditions. (A. Evrard and A. De Herde, “Bioclimatic envelopes made of lime and hemp concrete,” presented at the CISBAT, Lausanne, Switzerland, 2005.)
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Latif
et al., 2015Fire Testing, example; ASTM E119-14 CAN/ULC S101-07
DATE(S) OF TESTING: June 22, 2018
TESTING REQUESTED: Testing to the mandatory requirements of the following
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Materials, and CAN/ULC S101-07,Standard Methods of Fire Endurance Tests of
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Hygrothermal performance of an experimental hemp–lime buildingAndy Shea, Mike Lawrence, Pete Walker 2012/11/1JournalConstruction and Building MaterialsVolume36Pages270-275
Hygrothermal Performance Assessment for Super SSR Modular Block Wall System justSmartSolutions Manfred Kehrer, President 171 Glassboro Dr. Oak Ridge, TN 37830 www.WUFI.us Date Prepared – November 2016
Active research Centers;
The Université de Reims Champagne Ardenne in France
• The Université Catholique de Louvaine in Belgium
• The University of Bath in UK
• The University of Bristol (Interface Analysis Centre) in UK
• Collaboration of The University of Nantes (Institut de Recherche en Génie
Civil et Mécanique) with the University of Rennes (LGCGM – EquipeMatériaux
Thermo Rhéologie) and the Universitè de Bretagne Sud (Centre de
Recherche de Saint-Maudé, LIMATB)
(PDF) Performance of Hempcrete Walls Subjected to a Standard Time-temperature Fire Curve.. Available from: https://www.researchgate.net/publication/277533751_Performance_of_Hempcrete_Walls_Subjected_to_a_Standard_Time-temperature_Fire_Curve
[accessed Sep 17 2018]
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