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Structural Timber · For Architects
Passivhaus timber frame specification requires more than insulation — air tightness (n50 ≤ 0.6 ach), eliminated thermal bridges, and certified detailing. This template covers the complete wall, junction, and certification requirements for Passivhaus-compliant projects.
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NBS-format clause, common errors checklist, and standards reference. Editable PDF format ready for your tender documents.
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Specification Checklist
n50 ≤ 0.6 ach (Passivhaus) or n50 ≤ 1.0 ach (EnerPHit). Ψ-values ≤ 0.01 W/mK at all junctions.
Typically 0.10–0.15 W/m²K. Combination of insulation depth and quality.
Continuous, uninterrupted, with detailed connection at every junction.
PHPP-verified Ψ-values for all linear thermal bridges (eaves, jambs, sills, etc.).
Wood fibre, mineral wool, or cellulose. Avoid PIR for cavity-only fill (vapour issues).
Passivhaus Certified Designer involvement, PHI certified components where possible.
Certified Passivhaus Tradespeople (CEPHEUS) for installation.
Example Clause (NBS Format)
PASSIVHAUS TIMBER FRAME Performance target: - Annual heat demand: ≤ 15 kWh/m²/year - Air tightness: n50 ≤ 0.6 air changes per hour at 50 Pa - Verified by: Passivhaus Certified Designer, PHPP modelling Wall build-up (external to internal): 1. Cladding: Larch on ventilated cavity (separate spec) 2. Breather membrane: Class W1, Sd ≤ 0.05m, UV stable 3. Wood fibre sheathing: 60mm rigid wood fibre board, taped joints 4. Studs: C24, 38×235mm at 600mm centres 5. Insulation: 235mm wood fibre between studs, density 50 kg/m³, lambda 0.038 W/mK 6. Service void: 38×38mm battens with 38mm wood fibre 7. Air tightness layer: Intelligent membrane, Sd 0.5–10m variable, all joints taped 8. Internal lining: 12.5mm gypsum plasterboard U-value: 0.13 W/m²K verified by PHPP Air tightness layer (CRITICAL): - Material: Pro Clima Intello Plus or equivalent BBA-certified intelligent membrane - Joints: All overlaps taped with manufacturer-specified tape - Penetrations: All wires, pipes, ducts sealed with proprietary collars - Junctions: Continuity at all wall-floor, wall-ceiling, and corner junctions - Testing: Pre-completion air test required Junctions: - All thermal bridges PHPP-modelled, Ψ-values ≤ 0.01 W/mK - Eaves, jambs, sills, base plate, ridge: Each detailed and verified Workmanship: - Lead installer: Certified Passivhaus Tradesperson - Pre-test inspection: Complete envelope inspection before plasterboard Certification: - Project: Passivhaus Certification (PHI) target - Documentation: PHPP file, air test certificate, junction details, photo records
Copy and paste this clause into your tender. Adjust dimensions, grades, and project-specific requirements.
Common Errors
Polythene VCL instead of intelligent membrane
Correction: Intelligent membranes adapt to seasonal moisture flow. Polythene traps interstitial moisture and risks structure decay.
Missing junction detailing
Correction: Wall U-value alone does not deliver Passivhaus. All junctions must be PHPP-modelled and detailed.
No air test before completion
Correction: Test BEFORE plasterboard goes up. Finding leaks behind plasterboard is much more expensive.
Standard plumber doing penetrations
Correction: Air tightness layer penetrations need certified Passivhaus Tradesperson. Sloppy penetrations destroy n50 result.
Reference Standards
Frequently Asked
Typically 300–400mm total external-to-internal. 235mm insulation between studs + 60mm continuous external insulation + service void + linings is a common build-up achieving 0.13 W/m²K.
n50 ≤ 0.6 ach for Passivhaus Classic. ≤ 1.0 ach for EnerPHit (retrofit). For comparison, UK Building Regs minimum is around 8 ach.
For external rigid layer (continuous insulation), yes. For cavity-only fill, generally no — PIR risks interstitial condensation issues. Wood fibre, mineral wool, or cellulose preferred between studs.
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