FAQs

Our wall panel system offers strong acoustic insulation capabilities, contributing significantly to occupant comfort, privacy, and noise reduction. The inherent density and thickness of straw bale walls contribute to their sound insulation properties, aligning with the principle that mass is a primary factor in blocking sound.

Reported Rw values for plastered straw bale external walls typically range from 43 to 59.4 dB. Comparable systems like EcoCocon panels achieved an Rw rating of 54 dB, which is considered excellent for typical wall assemblies and suitable for various building types requiring significant noise reduction.

The high density of compressed straw within the panel core limits oxygen availability, causing tightly packed straw to smoulder rather than rapidly ignite. Lime plaster, a mineral-based material, is naturally fire-resistant, does not catch fire or spread flames, can withstand temperatures over 500 degrees Celsius, and releases carbon dioxide when disintegrating, which helps extinguish fires.

Importantly, it does not produce toxic fumes or smoke when burned. The exterior plaster layer incorporates borax, which, when exposed to heat, forms a protective vitreous layer and promotes a carbonaceous char, slowing pyrolysis and suppressing flame combustion and smoke formation.

General plastered straw bale walls have achieved an FRR of 120-135 minutes (REI 120-135) for non-load bearing elements and 2 hours for structural integrity under ASTM E119.

Comparable systems like EcoCocon panels achieved an REI 120 rating and a B-s1, d0 classification according to EN 13501-1 for both internal and external surfaces, indicating very limited contribution to fire, weak or absent smoke production, and no flaming droplets.

The application of hygrothermal analysis, typically using WUFI, is indispensable for predicting the long-term durability of moisture-sensitive materials like the straw within the Hiberna Modular system.

Accredited WUFI analyses for comparable straw panel systems, such as EcoCocon, demonstrate no long-term moisture accumulation, even under stress-test conditions. These analyses show that the straw panel layers dry out quickly, maintaining moisture content consistently below critical limits for mould growth.

Hygrothermal analysis, typically using WUFI software (Wärme- und Feuchtetransport instationär) is used to analyse heat, air, and moisture (HAM) transport in building components and assemblies. It is indispensable for accurately predicting the long-term durability of moisture-sensitive materials like the straw within the Hiberna Modular system.

Accredited WUFI analyses for comparable straw panel systems, such as EcoCocon, demonstrate no long-term moisture accumulation, even under stress-test conditions. These analyses show that the straw panel layers dry out quickly, maintaining moisture content consistently below critical limits for mould growth. Hiberna Modular has used WUFI simulation to model how it performs in a number of locations around the country. The modelling is project-specific, so we might recommend modelling for your project as a way of verifying for a building consent application.

Hygrothermal modelling tools like WUFI Pro 6 and WUFI VTT are superior to simpler models because they allow for one-dimensional analysis of coupled heat and moisture diffusion, and capillary effects through building assemblies, accounting for material moisture buffering and non-linear humidity distribution.

These tools can also simulate mould growth risk based on Viitanen's mould model, with the mould index ideally not exceeding 1.0 on exposed interior surfaces and not exceeding 3.0 on any internal surface. Analysis should account for potential variations in natural material properties and be performed for critical orientations and appropriate weather data.

Our wall assembly proactively promotes a healthier indoor environment by prioritising naturally sourced, low-toxicity materials. Its primary components—straw, clay, and lime—are inherently safe and free from Volatile Organic Compounds (VOCs) and formaldehyde, which can off-gas from many conventional building materials.

This directly contributes to superior indoor air quality from the very first day of occupancy.

Accredited test results for VOC emissions from various building materials, including straw products, consistently show very low levels of Total VOC (TVOC), Total Semi-Volatile Organic Compounds (TSVOC), and specific carcinogens like formaldehyde. These tests frequently report emission levels below quantification limits, indicating minimal release of harmful substances.

Its design philosophy emphasises "breathability" and vapour permeability. Both its interior and exterior lime-stabilised earth plasters and the straw panel core are highly vapour-permeable.

This approach allows moisture to easily diffuse through the wall, distinguishing it from conventional walls that might trap moisture.

The Hiberna system incorporates a 30mm layer of lime-stabilised earth plaster on the interior and a 10mm layer of lime-stabilised earth plaster on the exterior of the compressed straw core.

Both of these plaster layers are highly vapour-permeable, facilitating the diffusion of moisture through the wall assembly and its escape to the outside via the ventilated cavity.

Our system employs a multi-layered defence. The outermost cladding provides the primary defence against bulk water. Behind this, a ventilated cavity acts as a rapid drainage plane and promotes continuous drying. A breathable building membrane is installed over the exterior plaster for temporary weather protection and as a continuous moisture barrier.

The 10mm exterior lime-stabilised earth plaster acts as a moisture buffer and a vapour-open membrane, allowing the wall to breathe outwards. Both the interior (30mm) and exterior (10mm) lime-stabilised earth plasters, along with the compressed straw core, are highly vapour-permeable, allowing incidental moisture to diffuse and escape, preventing accumulation and inhibiting mould growth.

Be cautious with internal surface coatings, and avoid vapour barriers. While a normal roller-applied interior paint can reduce moisture at the surface, additional vapour resistance on the outside of the straw (like a lower permeability WRB or cementitious render) combined with lower vapour resistance straw or internal plaster can lead to failure of mould criteria.

Preventing water intrusion through a robust building envelope design, including proper flashings and detailing for external moisture features, is paramount.

Additionally, protecting straw bale wall segments from rain during construction until they are under a roof and the Water Resistive Barrier (WRB) is weathertight is crucial.

Avoid closing up the building during construction to prevent condensation and ensure materials like timber are dried to below 12% average moisture content before insulation or interior lining is installed. Sufficient ventilation and time for interior plaster and concrete to dry are also necessary before sealing the building. 

The WRB needs to have as low permeability as possible to prevent moisture ingress from the exterior while still allowing drying.

Permeable plaster skins (typically 20 to 60mm thick) are vital to allow moisture to escape from the straw bales. Increasing lime content in stucco mixes enhances vapour permeance, with pure lime-sand stuccos being among the most permeable.

Pure cement-sand stuccos are highly vapour-resistant and should be avoided.

A proposed assembly includes a 10mm thick clay slip layer sprayed onto compressed straw, followed by a 20mm lime-stabilised earth plaster and an 8mm internal lime plaster, all chosen for appropriate vapour permeability.

The Hiberna Modular system functions as an engineered composite structure where the synergistic interaction of the straw infill, the interior and exterior lime plaster skins (which contribute as structural skins), and structural timber framing. This allows the wall to effectively resist both lateral and vertical loads. Bracing is incorporated where required with an integrated diagonal tensioned 10mm metal rods.

Plastered straw bale walls have shown effective performance against wind loads, resisting up to 2.4 kilopascals (kPa).

Studies on prefabricated straw-bale panels reveal significant improvement in stiffness and resistance with the complete rendered panel, sustaining loads far exceeding those of bare timber frames or straw-filled panels alone. Full-size plastered straw bale walls reinforced with wire mesh have achieved ultimate shear capacities of 67.1 kilonewtons per meter (kN/m).

Regarding vertical loads, plastered straw bale prisms show a maximum average compressive strength of 0.98 N/mm2 with a 20mm plaster thickness. EcoCocon panels have declared characteristic vertical load-bearing capacities of 60.0 kN/m without struts.

The system, with its compressed straw core, achieves exceptional U-values, leading to a dramatic reduction in both heat loss and heat gain, resulting in consistent indoor comfort and significantly lower energy bills. It prioritises the elimination of air gaps and thermal bridges for optimal thermal performance and contributes to overall airtightness. Straw at 105 kg/m3 is conservatively R1.9 per 100mm.

R-values for straw bale walls typically range from 5.3 to 7.0 (m2K/W). Comparable systems like EcoCocon panels achieve declared U-values as low as 0.12 W/(m2K) for a complete system with clay plaster, and even 0.106 W/(m2K) with increased insulation thickness. The Hiberna system itself claims a U-value in the range of 0.12−0.15 W/(m2K) ( ~ R 1.9/100mm)

Continuous mechanical ventilation appropriate for the occupancy is strongly recommended because modern, well-sealed buildings, especially with high internal occupancy levels.

Ventilation rates should meet or exceed building code requirements (e.g., NZBC's 0.35 ACH or 7.5 L/s per person minimum), as these rates significantly impact internal moisture, comfort levels and loads on the building enclosure.

Continuous ventilation (exceeding 0.5 ACH for the entire building) should be maintained during construction, even before mechanical ventilation systems are commissioned, using fans or open windows.

Continuous mechanical ventilation appropriate for the occupancy is strongly recommended because modern, well-sealed buildings, especially with high internal occupancy levels, are risky without assured ventilation.

Ventilation rates should meet or exceed building code requirements (e.g., NZBC's 0.35 ACH or 7.5 L/s per person minimum), as these rates significantly impact internal moisture levels and loads on the building enclosure.