Load-bearing straw construction: dimensions, density, lambda and μ factor

The load-bearing straw construction, combined with a raw earth coating on the interior and a lime coating on the exterior, constitutes a construction system with specific thermal and hygroscopic properties. This article presents the technical characteristics of our project built with prefabricated and prestressed straw walls.

Characteristic	Value
Dimensions of a boot	37 x 47 x 120 cm
Density	90 to 120 kg/m³
Thermal conductivity (λ)	0.045 to 0.050 W/(m K)
Value λ retained for the calculation	0.048 W/(m K)
Factor μ (resistance to vapor diffusion)	About 2 to 3
Thickness of straw in the wall	47 cm
Thermal resistance of straw (R)	Approximately 9.79 m² K/W
Thermal transmission coefficient (U) of the wall	Approximately 0.10 W/(m²·K)

Composition of the Wall

The exterior wall is made up of three distinct layers, organized from the interior to the exterior to provide structure, insulation and hygrometric regulation at the same time:

• Inner layer: 4 to 6 cm of raw earth coating. This material ensures natural regulation of relative humidity and provides surface thermal inertia.
• Heart of wall: 47 cm of straw in bales. This thickness constitutes the supporting element and the main insulating body.
• Exterior layer: 4 to 5 cm of lime plaster. This mineral coating protects the straw from bad weather while remaining permeable to water vapor.

The total thickness of the wall is approximately 56 cm.

Origin and Characteristics of Boots

The straw bales used for this project came from a local order from a farmer in the region. Their dimensions do not result from a standardized industrial format, but correspond directly to the specific setting of the baler used during harvest.

• Dimensions noted: 37 cm thick, 47 cm high and 120 cm long.
• Density: Between 90 and 120 kg/m3.
• Implementation: The walls were prefabricated in the workshop. The vertical prestressing technique was applied to ensure the stability of the structure.

This choice of local supply illustrates the ability of the straw sector to adapt to the technical constraints of a construction project while maintaining a short circuit logic.

Contribution of Structural Prestressing

A particularity of this achievement lies in the prestressing applied during the manufacture of the wall panels. Each wall, with a height of 2.40 m (corresponding to the stack of 6 bales), was subjected to a compressive load of 1,000 kg per linear meter.

This prestressing has several technical advantages:

• Limitation of settlement: Initial compression reduces the risk of subsequent differential settlement, thus limiting the appearance of cracks in the coatings.
• Structural rigidity: It increases the cohesion of the whole, allowing the wall to take the loads of the frame and ensuring the stability of the structure.
• Homogeneity: It guarantees a regular density of the straw over the entire height of the wall, avoiding less dense areas which could create thermal bridges or mechanical weaknesses.

Thermal Performance and Calculation of the U Coefficient

The thermal performance of the wall results directly from the thickness of the straw and its density.- Thermal conductivity (lambda): For straw with a density between 90 and 120 kg/m3, the thermal conductivity is generally between 0.045 and 0.050 W/(m.K). We use an average value of lambda = 0.048 W/(m.K) for the calculation.

• Thermal resistance (R): For a thickness of 47 cm of straw, the thermal resistance is calculated according to the formula R = e / lambda:

  • R_straw ~= 0.47 / 0.048 ~= 9.79 m2.K/W.
  • By adding the resistance of the coatings (earth and lime), the total resistance of the wall (R_total) reaches approximately 10.0 m2.K/W.

• Thermal transmission coefficient (U): The coefficient U is the inverse of the total resistance (U = 1 / R_total).

  • Calculation: U = 1 / 10.0 = 0.10.
  • Result: U ~= 0.10 W/(m2.K).

This value is significantly lower than the requirements of current thermal regulations, both in Switzerland and in France. It greatly exceeds the criteria of the Canton Energy Model (MuKEn) in Switzerland, which often recommends a maximum U of 0.20 W/(m2.K) for new constructions, as well as the RE2020 standards in France (maximum U of 0.28 W/(m2.K) for walls). This level of performance approaches the criteria of the passive construction standard (generally set at U <= 0.15 W/(m2.K)).

Thermal Phase Shift and Inertia

The thermal phase shift represents the time necessary for a heat wave to pass through the wall. This is an estimated data which directly depends on the thickness of the wall, the density of the materials and their volumetric thermal capacity.

• Estimated value: For our 47 cm wall of dense straw, completed by the interior raw earth coating, the phase shift is estimated at approximately 12 to 14 hours.
• Dependence on thickness: The phase shift is proportional to the thickness of the insulation. The thicker and denser the wall, the longer the heat travel time. With a standard thickness of 35 cm, the phase shift would be approximately 9 to 10 hours; the addition of an additional 12 cm in our configuration makes it possible to achieve this cycle greater than 12 hours.
• Concrete impact: This phase shift makes it possible to delay the arrival of summer heat indoors until the end of the evening or at night, when it is possible to cool the building by natural ventilation, thus ensuring summer comfort without mechanical air conditioning.

Vapor Permeability and Interior Comfort

The design of the wall respects the principle of perspirance, that is to say the ability of materials to allow water vapor to diffuse.

• Resistance factor to vapor diffusion (mu): Straw has a low mu factor (around 2 to 3), similar to that of raw earth (3 to 5) and lime (5 to 8).
• Humidity management: This homogeneity of properties allows the water vapor produced inside the building to pass through the wall and escape to the outside, without encountering a blocking synthetic vapor barrier.

This operation makes it possible to maintain a stable interior humidity, generally between 40 and 60%, which contributes to the comfort of the occupants and prevents the risk of condensation in the heart of the wall.

Durability and Resistance

The durability of a straw wall relies on its protection against liquid humidity. The chosen implementation includes:

• A base out of water to keep the straw away from rising capillaries and rain splashes.- Sufficient roof overhangs to protect the facades.
• An exterior lime coating impervious to rainwater but permeable to vapor.

Concerning fire resistance, the high density of the straw (90-120 kg/m3) and the presence of mineral coatings on both sides limit combustion. In the event of a fire, the surface layer of the coating or the outer layer of the boot burns slowly, forming a charred layer which protects the core of the boot and maintains the load-bearing capacity of the wall for a significant period of time.

Conclusion

The use of pre-stressed straw bales, from local production and offering 47 cm of insulation, makes it possible to achieve high levels of thermal performance (U ~= 0.10 W/(m2.K)) with a phase shift estimated at more than 12 hours, while ensuring the structural stability of the structure. Combined with natural coatings (earth and lime), this technique offers a coherent construction system, controlling thermal and hygrometric flows for a sustainable and energy-efficient building, meeting in advance the requirements of Swiss (MuKEn) and European energy standards.