Passive House (Passivhaus) represents the most rigorous energy performance standard in construction — requiring buildings to achieve annual heating demand below 15 kWh/m²a, a reduction of 75–90% compared to standard construction. Timber is the ideal structural material for Passive House because its low thermal conductivity minimises structural thermal bridging, and its dimensional precision enables the airtightness levels the standard demands.
This guide provides architects with the technical detail needed to design and specify Passive House timber buildings — from wall build-ups and U-value calculations to airtightness strategies and the certification pathway.
Passive House Performance Requirements
Core Criteria (Passive House Classic)
| Criterion | Requirement | Verification Method |
|---|---|---|
| Specific heating demand | ≤ 15 kWh/m²a | PHPP energy model |
| Specific cooling demand | ≤ 15 kWh/m²a | PHPP energy model |
| Primary energy demand (PER) | ≤ 60 kWh/m²a | PHPP energy model |
| Airtightness (n50) | ≤ 0.6 ACH | Blower door test (EN 13829) |
| Overheating frequency | ≤ 10% of hours > 25°C | PHPP dynamic simulation |
Passive House Plus and Premium
| Class | Renewable energy generation | Primary energy (PER) |
|---|---|---|
| Classic | No requirement | ≤ 60 kWh/m²a |
| Plus | ≥ 60 kWh/m²a generated | ≤ 45 kWh/m²a |
| Premium | ≥ 120 kWh/m²a generated | ≤ 30 kWh/m²a |
Why Timber Excels for Passive House
Thermal Conductivity Comparison
| Material | Thermal Conductivity (λ, W/mK) | Relative to Timber |
|---|---|---|
| Timber (softwood) | 0.13 | 1× (baseline) |
| Concrete | 1.5–2.0 | 12–15× worse |
| Steel | 50 | 385× worse |
| Aluminium | 160 | 1,230× worse |
| Mineral wool insulation | 0.035 | 3.7× better |
| Wood fibre insulation | 0.038–0.042 | 3.1–3.4× better |
Timber's thermal conductivity is low enough that structural members create only minor thermal bridges — easily addressed with a continuous external insulation layer. Steel and concrete structures require complex thermal break details that are difficult to execute consistently.
Timber Frame Advantages for Passive House
- Integrated insulation: Insulation fills the structural depth — no wasted space
- Low thermal bridging: Studs at 0.13 W/mK vs insulation at 0.035 W/mK — ratio of only 3.7:1
- Dimensional precision: Factory-fabricated panels achieve ±1 mm tolerance — essential for airtightness
- Airtight layer integration: OSB sheathing serves as both structural bracing and airtight layer
- Lightweight: Reduced foundation requirements compared to masonry
- Speed: Weathertight in days, protecting insulation and airtight layer during construction
- Carbon benefit: Net-negative embodied carbon offsets operational carbon
Wall Build-Ups for Passive House
Build-Up 1: Standard Timber Frame (U = 0.12 W/m²K)
| Layer (outside to inside) | Thickness | λ (W/mK) | R (m²K/W) |
|---|---|---|---|
| Timber cladding (ventilated) | 22 mm | — | — (ventilated cavity) |
| Ventilated cavity + battens | 38 mm | — | — |
| Breather membrane | — | — | — |
| Wood fibre insulation (external) | 80 mm | 0.038 | 2.11 |
| OSB sheathing (structural + airtight) | 15 mm | 0.13 | 0.12 |
| Timber frame + mineral wool | 235 mm | 0.035* | 6.71* |
| Vapour control layer | — | — | — |
| Service void + insulation | 50 mm | 0.035 | 1.43 |
| Plasterboard | 12.5 mm | 0.25 | 0.05 |
| Total | ~453 mm | — | ~10.42 |
Corrected for timber fraction (15% studs at 0.13 W/mK, 85% insulation at 0.035 W/mK)
Calculated U-value: 0.096 W/m²K (including surface resistances)
This build-up comfortably exceeds Passive House requirements and provides margin for junctions and thermal bridges.
Build-Up 2: CLT + External Insulation (U = 0.13 W/m²K)
| Layer (outside to inside) | Thickness | λ (W/mK) | R (m²K/W) |
|---|---|---|---|
| Timber cladding (ventilated) | 22 mm | — | — |
| Ventilated cavity + battens | 38 mm | — | — |
| Breather membrane | — | — | — |
| Wood fibre insulation | 200 mm | 0.038 | 5.26 |
| CLT panel (5-layer) | 100 mm | 0.13 | 0.77 |
| Service void (optional) | 50 mm | 0.035 | 1.43 |
| Plasterboard (or exposed CLT) | 12.5 mm | 0.25 | 0.05 |
| Total | ~423 mm | — | ~7.51 |
Calculated U-value: 0.127 W/m²K (including surface resistances)
Build-Up 3: Double-Stud Wall (U = 0.09 W/m²K)
For ultra-low energy or extreme climates:
| Layer (outside to inside) | Thickness | λ (W/mK) | R (m²K/W) |
|---|---|---|---|
| Timber cladding (ventilated) | 22 mm | — | — |
| Ventilated cavity | 38 mm | — | — |
| Breather membrane | — | — | — |
| Outer stud frame + cellulose | 140 mm | 0.038* | 3.68* |
| Cellulose (continuous, no studs) | 100 mm | 0.038 | 2.63 |
| Inner stud frame + cellulose | 140 mm | 0.038* | 3.68* |
| OSB (airtight layer) | 15 mm | 0.13 | 0.12 |
| Service void | 50 mm | 0.038 | 1.32 |
| Plasterboard | 12.5 mm | 0.25 | 0.05 |
| Total | ~518 mm | — | ~11.48 |
Corrected for timber fraction
Calculated U-value: 0.085 W/m²K
Airtightness Strategy
The Airtight Layer
In timber construction, the airtight layer is typically one of:
OSB/plywood sheathing (most common): 15 mm OSB on the warm side of the main insulation, with all joints taped using airtightness tape (e.g. pro clima Tescon Vana, Siga Wigluv)
Dedicated membrane: Intelligent vapour control layer (e.g. pro clima Intello Plus) stapled to the inner face of the frame, with all laps and penetrations sealed
CLT panels: The CLT panel itself provides airtightness when all panel-to-panel joints are sealed with appropriate tape or sealant
Critical Airtightness Junctions
| Junction | Strategy | Detail |
|---|---|---|
| Wall-to-floor (ground) | OSB folded onto sole plate, sealed to DPM | Continuous tape connection |
| Wall-to-floor (intermediate) | OSB continuous behind floor zone | Service void allows continuity |
| Wall-to-roof | OSB connected to roof airtight layer | Tape or membrane overlap |
| Window perimeter | Airtight tape from OSB to window frame | Compriband + internal tape |
| Service penetrations | Grommets or airtight collars | Pro clima Kaflex or equivalent |
| Electrical boxes | Airtight back-boxes | Sealed membrane boxes |
Achieving n50 ≤ 0.6 ACH
The Passive House airtightness requirement (0.6 air changes per hour at 50 Pa) is demanding but achievable with timber construction:
Design phase:
- Define the airtight layer on every drawing (red line exercise)
- Detail every junction, penetration, and transition
- Minimise penetrations through the airtight layer
- Locate services in the service void (inside the airtight layer)
Construction phase:
- Brief all trades on airtightness importance
- Protect the airtight layer during construction
- Tape all OSB joints immediately after installation
- Use airtight grommets for all service penetrations
- Conduct interim blower door tests (at weathertight stage)
- Final blower door test before internal finishes conceal the airtight layer
Typical results:
- Standard timber frame (no special attention): n50 = 3–7 ACH
- Good practice timber frame: n50 = 1–3 ACH
- Passive House timber frame: n50 = 0.2–0.6 ACH
- Best practice (factory-built panels): n50 = 0.1–0.3 ACH
Thermal Bridge-Free Design
What is a Thermal Bridge?
A thermal bridge is a localised area of the building envelope with significantly higher heat transfer than the surrounding construction. In timber buildings, thermal bridges occur at:
- Studs: Timber (λ = 0.13) vs insulation (λ = 0.035) — 3.7× difference
- Junctions: Wall-floor, wall-roof, wall-wall corners
- Openings: Window and door perimeters
- Penetrations: Structural connections through insulation
- Balconies: Cantilevered elements penetrating the thermal envelope
Passive House Thermal Bridge Criterion
The Passive House standard requires all linear thermal bridges to have a Ψ-value (psi-value) ≤ 0.01 W/mK. This is effectively "thermal bridge-free" — the thermal bridge is so small it can be ignored in the energy calculation.
Strategies for Thermal Bridge-Free Timber Construction
Stud thermal bridging:
- External insulation layer (60–100 mm) overlaps all studs
- Reduces stud thermal bridge from Ψ = 0.03–0.05 to Ψ < 0.01 W/mK
- This is the single most important detail for Passive House timber walls
Junction details:
- Insulation continuity at all junctions (no gaps, no compression)
- Overlapping insulation layers at corners
- Insulated sole plates (thermal break between foundation and frame)
- Warm roof construction (insulation above rafters or at ceiling level with continuity)
Window installation:
- Windows positioned in the insulation plane (not at inner face of wall)
- Insulated window reveals (minimum 30 mm insulation around frame)
- Thermally broken window sills
- Certified Passive House windows (Uw ≤ 0.80 W/m²K)
Mechanical Ventilation with Heat Recovery (MVHR)
Why MVHR is Essential
In a Passive House with n50 ≤ 0.6 ACH, natural ventilation through infiltration is negligible. MVHR provides:
- Continuous fresh air supply (30 m³/h per person minimum)
- Heat recovery from exhaust air (75–95% efficiency)
- Filtered air (pollen, dust, pollution removal)
- Controlled humidity management
- No draughts or cold spots
MVHR Specification for Timber Passive House
| Parameter | Requirement | Notes |
|---|---|---|
| Heat recovery efficiency | ≥ 75% (certified: ≥ 85%) | Passive House certified units achieve 85–95% |
| Electrical efficiency | ≤ 0.45 Wh/m³ | Low energy fans essential |
| Supply air temperature | ≥ 16.5°C (after heat recovery) | Comfort requirement |
| Noise level | ≤ 25 dB(A) in living spaces | Acoustic attenuation on ducts |
| Filter class | Minimum F7 supply, G4 extract | Replaceable filters |
| Frost protection | Required below –5°C design temp | Pre-heater or earth tube |
Ductwork in Timber Construction
MVHR ductwork integrates well with timber construction:
- Service void: 50 mm void inside the airtight layer accommodates distribution ducts
- Floor voids: Timber floor construction provides space for main duct runs
- Manifold systems: Radial duct distribution from central manifold minimises duct sizes
- Penetrations: Minimise penetrations through the airtight layer — route ducts within the thermal envelope
Windows for Passive House Timber Buildings
Performance Requirements
| Parameter | Passive House Requirement | Typical Specification |
|---|---|---|
| Installed U-value (Uw,installed) | ≤ 0.85 W/m²K | 0.70–0.80 W/m²K |
| Glazing U-value (Ug) | ≤ 0.70 W/m²K | 0.50–0.60 W/m²K (triple) |
| Frame U-value (Uf) | ≤ 0.80 W/m²K | 0.70–0.80 W/m²K |
| Solar heat gain (g-value) | ≥ 0.50 (south-facing) | 0.50–0.62 |
| Spacer bar | Warm edge (Ψ ≤ 0.04 W/mK) | Stainless steel or composite |
Window Installation in Timber Walls
For thermal bridge-free installation:
- Position window in the insulation plane (typically 1/3 from outside face)
- Support on insulated sill (compressive insulation or bracket)
- Insulate reveals with minimum 30 mm insulation overlapping frame
- Seal airtight layer to window frame with appropriate tape
- Seal weather layer (external) with compriband or membrane
Certification Pathway
Steps to Passive House Certification
| Step | Action | Timing |
|---|---|---|
| 1 | Appoint Certified Passive House Designer (CPHD) | Design stage |
| 2 | Develop PHPP energy model | Detailed design |
| 3 | Optimise design to meet criteria | Iterative design |
| 4 | Submit pre-construction documentation to certifier | Before construction |
| 5 | Construct with quality assurance | Construction |
| 6 | Conduct blower door test | Before finishes |
| 7 | Commission MVHR and heating systems | Completion |
| 8 | Submit as-built documentation and test results | Post-completion |
| 9 | Certifier issues Passive House certificate | 4–8 weeks after submission |
Certification Costs
| Item | Cost Range | Notes |
|---|---|---|
| PHPP energy modelling | €2,000–€5,000 | Included in designer's fee |
| Passive House certifier fee | €2,000–€4,000 | Depends on project size |
| Blower door test | €500–€1,500 | May need interim + final test |
| Thermal bridge calculations | €1,000–€3,000 | 2D/3D modelling of junctions |
| MVHR commissioning report | €500–€1,000 | Flow rates, balance, noise |
| Total certification costs | €6,000–€14,500 | 1–3% of build cost |
Cost Premium for Passive House Timber
Additional Costs vs Standard Building Regulations
| Element | Standard Spec | Passive House Spec | Additional Cost |
|---|---|---|---|
| Wall insulation | 150 mm mineral wool | 235 mm + 80 mm external | +€3,000–€6,000 |
| Roof insulation | 200 mm mineral wool | 300–400 mm | +€2,000–€4,000 |
| Floor insulation | 100 mm PIR | 200 mm PIR or equivalent | +€1,500–€3,000 |
| Windows | Double-glazed PVC | Triple-glazed timber/alu (PH certified) | +€8,000–€18,000 |
| Airtightness | Standard (n50 ~5) | PH standard (n50 ≤ 0.6) | +€2,000–€5,000 |
| MVHR system | None (or basic MEV) | PH-certified MVHR | +€5,000–€10,000 |
| Thermal bridge details | Standard | Thermal bridge-free | +€1,000–€3,000 |
| Certification | None | PH certification | +€6,000–€14,500 |
| Total premium | — | — | +€28,500–€63,500 |
| Premium per m² (150 m² home) | — | — | +€190–€423/m² |
| Percentage premium | — | — | +10–20% |
Payback Through Energy Savings
A Passive House uses 75–90% less heating energy than a standard home:
- Standard home heating cost: €1,200–€2,400/year
- Passive House heating cost: €150–€400/year
- Annual saving: €800–€2,000/year
- Simple payback: 14–32 years (without energy price inflation)
- With 5% annual energy inflation: 10–18 years payback
Related Resources
- Structural Timber Specification Guide — comprehensive pillar guide
- Timber Frame vs CLT vs Glulam — structural system comparison for Passive House
- Carbon Footprint of Timber — embodied carbon benefits of timber Passive House
- Cost of Building a Timber Home — detailed cost breakdown including Passive House premium
- Timber Moisture Content — critical for airtightness and long-term performance
- C24 Structural Construction Timber — structural framing for Passive House walls
- BSH Glued Laminated Beams — engineered timber for Passive House structures