Closed panel timber frame system: engineered for precision, performance and compliance
The Sigma® II closed panel timber frame system represents the highest level of technical performance within the Donaldson Timber Systems product portfolio.
Developed through years of research, testing, and collaboration with leading industry bodies, Sigma® II provides a fully engineered, factory-finished building fabric designed to deliver high thermal efficiency, structural integrity, and regulatory compliance under the Future Homes Standard.
System composition
Each Sigma® II wall panel is a prefabricated closed-panel timber frame, manufactured offsite for quality assurance and dimensional accuracy.
Typical specification:
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Stud options: Solid-Stud or I-Stud in 140mm, 184mm, 195mm, 235mm widths
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Sheathing: Structural board fixed to both internal and external faces
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Insulation: Fully filled with proprietary blown-fibre insulation under controlled factory conditions
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External finish: Reflective foil breather membrane applied to external face – ready for cavity brickwork or rainscreen cladding
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Internal finish: Factory-installed battens form a 25mm service zone, ready for foil-back plasterboard
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Air-tightness detailing: Integrated seals at vertical joints, mid-floors, and service penetrations
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Optional features: Pre-installed windows, enhanced fire-rated boards, integrated vapour control layers
This closed panel configuration minimises on-site labour, improves build accuracy, and provides a predictable, high-performance thermal envelope.
Thermal and air tightness performance
U-Value range
0.24 W/m²K to 0.12 W/m²K
Wall type and
cladding dependent
Air-tightness
3 m³/hr/m² @ 50pa
Psi-Values
Independently modelled
for common junctions
Thermal bridging
Reduced via
continuous insulation
and interlocking details
Condensation risk
Independently modelled to suit worst case conditions at:
-2.0°C external air temperature @ 90.5%
relative humidity
+21.0°C internal air temperature @ 60%
relative humidity
Sigma® II is designed to meet or exceed Future Homes Standard 2025 targets and the latest Part L Building Regulations.
Factory-fitted insulation eliminates voids and gaps, ensuring consistent performance and verified energy outcomes across projects.
Structural performance
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Load-bearing capacity: Designed and tested in accordance with Eurocode 5 (EN 1995-1-1)
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Racking resistance: Verified through laboratory testing to BS EN 594 standards
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Interlocking head and base details: Reduce vertical settlement and cross-grain shrinkage
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Panel span capability: Configurable for typical residential floor-to-ceiling heights and multi-storey applications
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Roof and floor integration: Compatible with both trussed-rafter and cassette systems for total build flexibility
Structural performance is confirmed via full engineering design and third-party assessment for each project.
Fire and acoustic performance
Sigma® II is available with 30-minute and 60-minute fire-rated wall build-ups.
Acoustic performance is verified in accordance with Approved Document E, providing robust separation and compliance for party-wall configurations.
Fire-rated solutions are achieved through enhanced sheathing materials and detailing around junctions, openings, and service zones.
Sustainability and low-carbon design
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Embodied carbon: Significantly lower than traditional masonry or steel systems
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Timber sourcing: PEFC™ certified structural timber
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Waste reduction: Offsite automation minimises site waste and packaging
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PMV (Pre-Manufactured Value): 55.7 – 58.4 % (verified by CAST Consultancy)
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Design life: ≥ 60 years under normal maintenance (BOPAS+ certified)
Compliance and accreditation
Sigma® II is recognised and approved by leading UK certification bodies. All Sigma® II products are subject to continuous internal and third-party audit processes to maintain certification validity. All Sigma® II products are subject to continuous internal and third-party audit processes to maintain certification validity.
The Buildoffsite Property Assurance Scheme (BOPAS) provides long-term confidence in durability and integrity. It validates our processes for design, manufacture, and on-site installation, guaranteeing a 60-year design life for our closed panel timber frame systems.
For developers, lenders, and housing providers, BOPAS+ accreditation removes risk and confirms that our systems are fit for modern housebuilding.
The British Board of Agrément (BBA) verifies that our systems meet the highest standards of technical compliance, durability, and fire safety. A BBA Agrément Certificate is a mark of excellence — independent proof that every product performs exactly as designed.
We’re also proud holders of the Structural Timber Association’s STA Assure Gold standard — the highest level of industry quality assurance. Together, these certifications demonstrate total reliability from factory to foundation.
All of our timber frame systems are NHBC Accepts approved, meaning they’re fully compatible with NHBC Buildmark warranties.
Design integration and technical support
Our Technical Hub provides full digital integration tools and technical guidance for Sigma® II projects:
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BIM objects and model templates for design coordination
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Construction details for walls, floors, and junctions
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U-value and condensation analysis for project-specific build-ups
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Thermal bridging calculations for custom interfaces
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Project engineering support from concept through to installation
Our in-house technical specialists work closely with architects, structural engineers, and contractors to ensure every Sigma® II project meets specification, regulation, and performance goals.
Installation and quality assurance
Sigma® II panels are installed by our qualified installation teams under dedicated project management supervision, ensuring safety, accuracy, and programme reliability.
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Factory-finished panels reduce on-site trades
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Quality inspections at each stage
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Handover documentation includes certification, as-built data, and maintenance requirements
Every frame is manufactured and erected under our audited Quality Management System, guaranteeing compliance and consistency.
