Insulation is the single most important factor determining the thermal performance, comfort, and energy efficiency of a timber frame home. The choice of insulation material affects not only U-values and heating bills, but also moisture safety, fire performance, acoustic comfort, summer overheating risk, and the environmental footprint of the building.
This guide provides a comprehensive comparison of all insulation materials suitable for timber frame construction — covering thermal performance, moisture behaviour, fire safety, acoustic properties, installation methods, cost, and environmental impact.
Understanding Timber Frame Thermal Performance
The Thermal Bridging Challenge
A timber frame wall is not uniformly insulated — the C24 structural studs that provide structural support also create thermal bridges:
| Component | Thermal conductivity (λ) | Compared to insulation |
|---|---|---|
| Mineral wool insulation | 0.035 W/m·K | Baseline |
| Wood fibre insulation | 0.038 W/m·K | 1.1× |
| C24 Spruce stud | 0.13 W/m·K | 3.7× |
| C24 Pine stud | 0.15 W/m·K | 4.3× |
| Steel stud (for comparison) | 50 W/m·K | 1,430× |
While timber is a far better insulator than steel, the studs still reduce overall wall performance. A wall with 15% timber fraction (47 mm studs at 400 mm centres in a 300 mm deep wall) loses approximately 15–20% of its theoretical insulation value due to thermal bridging.
U-Value Calculation Method
The combined U-value of a timber frame wall is calculated using the proportional area method (BS EN ISO 6946):
U_combined = f_timber × U_timber_path + f_insulation × U_insulation_path
Where f_timber = fraction of wall area occupied by timber (typically 12–18%)
Target U-Values (2026)
| Standard | Wall U-value | Roof U-value | Floor U-value |
|---|---|---|---|
| Building Regs Part L 2025 | 0.18 W/m²·K | 0.15 W/m²·K | 0.18 W/m²·K |
| RIBA 2030 target | 0.15 W/m²·K | 0.12 W/m²·K | 0.15 W/m²·K |
| Passive house | 0.10–0.12 W/m²·K | 0.08–0.10 W/m²·K | 0.10–0.12 W/m²·K |
| Net zero best practice | 0.12–0.15 W/m²·K | 0.10–0.12 W/m²·K | 0.12–0.15 W/m²·K |
Insulation Materials Compared
Overview Table
| Material | λ value (W/m·K) | Vapour permeable | Hygroscopic | Fire class | Embodied carbon | Cost index |
|---|---|---|---|---|---|---|
| Mineral wool (glass) | 0.032–0.040 | Yes | No | A1 | Moderate | 1.0 |
| Mineral wool (stone) | 0.034–0.038 | Yes | No | A1 | Moderate | 1.1 |
| Wood fibre (flexible) | 0.036–0.040 | Yes | Yes | B–E | Negative | 1.8 |
| Wood fibre (rigid) | 0.038–0.043 | Yes | Yes | B–E | Negative | 2.0 |
| Cellulose (blown) | 0.035–0.040 | Yes | Yes | B–C | Negative | 1.2 |
| PIR/PUR (rigid board) | 0.020–0.023 | No | No | C–D | High | 1.5 |
| EPS (expanded polystyrene) | 0.030–0.034 | Low | No | E | Moderate | 0.8 |
| XPS (extruded polystyrene) | 0.029–0.036 | No | No | E | High | 1.3 |
| Sheep wool | 0.035–0.040 | Yes | Yes | B–D | Low | 2.5 |
| Hemp fibre | 0.038–0.042 | Yes | Yes | E | Negative | 2.2 |
1. Mineral Wool (Glass Wool and Stone Wool)
The industry standard — proven, economical, non-combustible.
| Property | Glass wool | Stone wool |
|---|---|---|
| Thermal conductivity | 0.032–0.040 W/m·K | 0.034–0.038 W/m·K |
| Density | 12–80 kg/m³ | 30–200 kg/m³ |
| Fire classification | A1 (non-combustible) | A1 (non-combustible) |
| Vapour resistance (μ) | 1–2 | 1–2 |
| Moisture absorption | Hydrophobic (treated) | Hydrophobic (treated) |
| Acoustic performance | Good (low density) to excellent (high density) | Excellent |
| Thermal mass | Very low | Low |
| Embodied carbon | 1.2–2.5 kg CO₂/m² (at R=1) | 1.5–3.0 kg CO₂/m² (at R=1) |
Installation in timber frame:
- Cut to friction-fit between studs (oversized by 10–15 mm)
- No adhesive required — held by friction
- Multiple layers can be staggered to reduce air gaps
- Easily cut around services
- Requires VCL on warm side (mineral wool does not resist vapour flow)
Advantages: Non-combustible (critical for fire safety); proven track record; widely available; economical; easy to install; good acoustic performance.
Disadvantages: No hygroscopic buffering (cannot manage moisture actively); settles slightly over time in vertical applications; irritant during installation (PPE required); no thermal mass (poor summer performance).
2. Wood Fibre Insulation
The premium choice for breathable, moisture-safe timber frame construction.
| Property | Flexible batts | Rigid boards |
|---|---|---|
| Thermal conductivity | 0.036–0.040 W/m·K | 0.038–0.043 W/m·K |
| Density | 40–60 kg/m³ | 110–240 kg/m³ |
| Fire classification | E (treated: B) | B–D |
| Vapour resistance (μ) | 1–3 | 3–5 |
| Moisture absorption | Hygroscopic (up to 15% by weight) | Hygroscopic |
| Specific heat capacity | 2,100 J/kg·K | 2,100 J/kg·K |
| Thermal mass | High | Very high |
| Embodied carbon | –1.0 to –2.0 kg CO₂/m² (at R=1) | –1.5 to –3.0 kg CO₂/m² (at R=1) |
Why wood fibre excels in timber frame:
Hygroscopic buffering: Absorbs up to 15% moisture by weight without losing insulation performance or causing damage. This provides a safety margin against condensation — moisture that reaches the insulation is absorbed, stored temporarily, and released when conditions allow drying.
Thermal mass: Specific heat capacity of 2,100 J/kg·K (vs 1,030 for mineral wool) provides significant summer overheating protection. A 200 mm wood fibre wall delays heat transmission by 10–12 hours, keeping interiors cool during hot days.
Compatibility with timber: Wood fibre and structural timber have similar moisture behaviour — both absorb and release moisture in response to humidity changes. This creates a harmonious wall assembly where all components work together rather than against each other.
Carbon storage: Wood fibre insulation stores approximately 1.4 kg CO₂ per kg of material, making it carbon-negative. A fully insulated timber frame house with wood fibre stores an additional 2–4 tonnes CO₂ in its insulation alone.
Installation: Flexible batts friction-fit between studs (similar to mineral wool). Rigid boards used externally, fixed with insulation screws through to studs.
3. Cellulose Insulation (Blown)
Excellent performance at moderate cost — the recycled option.
| Property | Value |
|---|---|
| Thermal conductivity | 0.035–0.040 W/m·K |
| Density (installed) | 45–65 kg/m³ (open blow) / 55–75 kg/m³ (dense pack) |
| Fire classification | B–C (treated with borate/aluminium hydroxide) |
| Vapour resistance (μ) | 1–2 |
| Moisture absorption | Hygroscopic (similar to wood fibre) |
| Specific heat capacity | 1,900–2,100 J/kg·K |
| Raw material | 85% recycled newspaper + 15% fire retardant |
| Embodied carbon | –0.5 to –1.5 kg CO₂/m² (at R=1) |
Installation methods:
| Method | Application | Density | Advantages |
|---|---|---|---|
| Open blow | Horizontal (loft floors) | 25–35 kg/m³ | Fast, economical |
| Dense pack | Vertical (walls, slopes) | 55–75 kg/m³ | No settling, fills all voids |
| Damp spray | Open walls (before lining) | 45–55 kg/m³ | Adheres to surfaces, no netting |
| Injection | Closed cavities (retrofit) | 55–65 kg/m³ | Non-destructive retrofit |
Dense pack in timber frame walls: Cellulose is blown at high density (55–75 kg/m³) into closed wall cavities through small holes in the sheathing or lining. At this density, it will not settle and completely fills all voids around services and irregular framing. This eliminates the air gaps that reduce performance in batt insulation installations.
Advantages: Excellent void-filling (no gaps around services); hygroscopic moisture buffering; good thermal mass; lowest cost of natural insulation options; recycled content; carbon-negative.
Disadvantages: Requires specialist installation equipment; cannot be DIY installed in walls; needs containment (netting or board) in open cavities; slightly higher λ than best mineral wool.
4. PIR/PUR Rigid Foam
Maximum thermal performance in minimum thickness.
| Property | PIR | PUR |
|---|---|---|
| Thermal conductivity | 0.020–0.023 W/m·K | 0.022–0.025 W/m·K |
| Density | 30–45 kg/m³ | 30–40 kg/m³ |
| Fire classification | C–D (foil-faced: B) | C–D |
| Vapour resistance (μ) | 30–150 | 30–100 |
| Moisture absorption | Very low | Very low |
| Compressive strength | 100–200 kPa | 100–150 kPa |
| Embodied carbon | 3.5–5.0 kg CO₂/m² (at R=1) | 4.0–6.0 kg CO₂/m² (at R=1) |
Use in timber frame — CAUTION:
PIR/PUR is vapour-closed and creates moisture management challenges in timber frame walls:
| Position | Suitability | Risk level | Notes |
|---|---|---|---|
| External (outside sheathing) | Good | Low | Keeps frame warm, prevents condensation |
| Between studs (full fill) | Risky | High | Cold bridges at studs, no drying path |
| Between studs (partial fill) | Very risky | Very high | Cold cavity behind PIR traps moisture |
| Internal (warm side) | Acceptable | Low | Acts as VCL, but reduces drying inward |
Recommended use: PIR is best used as continuous external insulation board outside the timber frame sheathing. In this position, it keeps the entire frame warm (above dew point), eliminates thermal bridging, and the vapour-closed nature is beneficial (prevents external moisture reaching the frame).
5. Sheep Wool
Natural, breathable, and pleasant to install — premium niche product.
| Property | Value |
|---|---|
| Thermal conductivity | 0.035–0.040 W/m·K |
| Density | 20–35 kg/m³ |
| Fire classification | B–D (naturally self-extinguishing) |
| Vapour resistance (μ) | 1–3 |
| Moisture absorption | Hygroscopic (up to 35% by weight without performance loss) |
| Embodied carbon | Low (natural product, minimal processing) |
Advantages: Exceptional moisture buffering (absorbs 35% by weight); naturally fire-resistant (high ignition temperature, self-extinguishing); pleasant to handle (no irritation); excellent acoustic performance.
Disadvantages: Expensive (2–3× mineral wool); limited availability; requires moth treatment; lower density means less thermal mass than wood fibre; variable quality between suppliers.
Wall Build-Up Options
Option A: Standard Compliance (Part L 2025)
U-value: 0.17 W/m²·K | Total wall thickness: 310 mm
| Layer (outside to inside) | Thickness | Material |
|---|---|---|
| Timber cladding | 22 mm | Larch or treated pine |
| Ventilation cavity | 25 mm | On battens |
| Breather membrane | — | Vapour-permeable |
| Sheathing (OSB3) | 9 mm | Structural racking |
| Studs + insulation | 140 mm | C24 studs + mineral wool (λ=0.035) |
| VCL | — | Polyethylene or intelligent membrane |
| Service void + insulation | 50 mm | Battens + mineral wool |
| Plasterboard | 12.5 mm | Standard or moisture-resistant |
| Total | ~310 mm | — |
Option B: Enhanced Performance
U-value: 0.13 W/m²·K | Total wall thickness: 390 mm
| Layer (outside to inside) | Thickness | Material |
|---|---|---|
| Timber cladding | 22 mm | Larch or treated pine |
| Ventilation cavity | 25 mm | On battens |
| Breather membrane | — | Vapour-permeable |
| External insulation | 80 mm | Rigid wood fibre (λ=0.040) |
| Sheathing (OSB3) | 9 mm | Structural racking |
| Studs + insulation | 140 mm | C24 studs + wood fibre batts (λ=0.038) |
| Intelligent membrane | — | Variable-permeability VCL |
| Service void + insulation | 50 mm | Battens + wood fibre |
| Plasterboard | 12.5 mm | — |
| Total | ~390 mm | — |
Option C: Passive House Standard
U-value: 0.10 W/m²·K | Total wall thickness: 470 mm
| Layer (outside to inside) | Thickness | Material |
|---|---|---|
| Timber cladding | 22 mm | Larch |
| Ventilation cavity | 25 mm | On thermally-broken brackets |
| Breather membrane | — | Vapour-permeable |
| External insulation | 120 mm | Rigid wood fibre (λ=0.040) |
| Sheathing (OSB3) | 15 mm | Structural + airtightness layer |
| Studs + insulation | 200 mm | C24 studs + cellulose dense-pack (λ=0.038) |
| Intelligent membrane | — | Variable-permeability VCL |
| Service void + insulation | 50 mm | Battens + wood fibre |
| Plasterboard | 12.5 mm | — |
| Total | ~470 mm | — |
Moisture Management
The Moisture Balance Principle
A timber frame wall must be designed so that any moisture entering the assembly can dry out faster than it accumulates. The key principles:
- Vapour resistance decreases from inside to outside (warm side more resistant than cold side)
- Hygroscopic materials provide buffering (temporary moisture storage during peak events)
- Ventilation cavity enables drying (air movement removes moisture from outer face)
- Airtightness prevents convective moisture transport (warm moist air must not enter the wall assembly)
Vapour Resistance Ratios
| Wall assembly | Inner VCL (Sd value) | Outer membrane (Sd value) | Ratio (inner:outer) | Risk level |
|---|---|---|---|---|
| Polyethylene VCL + breather membrane | 50–100 m | 0.1–0.3 m | 150–1000:1 | Low |
| Intelligent membrane + breather membrane | 0.5–10 m (variable) | 0.1–0.3 m | 2–100:1 | Low |
| OSB as VCL + breather membrane | 3–5 m | 0.1–0.3 m | 10–50:1 | Moderate |
| No VCL + PIR external | — | 30–150 m | Reversed | High risk |
Condensation Risk Analysis
For any timber frame wall build-up, a condensation risk analysis should be performed to BS EN ISO 13788 (Glaser method) or dynamic simulation (WUFI):
- Glaser method: Simple steady-state calculation. Conservative. Required for Building Regulations.
- WUFI simulation: Dynamic hygrothermal modelling. More realistic. Accounts for moisture storage, solar drying, and rain absorption. Recommended for innovative build-ups.
For moisture content management in timber frame construction, the target is to keep structural timber below 20% MC at all times — the threshold above which fungal decay becomes possible.
Cost Comparison
Material Cost per m² of Wall (at U=0.15 W/m²·K)
| Insulation material | Thickness required | Material cost/m² | Installation cost/m² | Total/m² |
|---|---|---|---|---|
| Glass mineral wool | 270 mm | £12–18 | £8–12 | £20–30 |
| Stone mineral wool | 260 mm | £15–22 | £8–12 | £23–34 |
| Wood fibre (flexible) | 290 mm | £28–40 | £10–15 | £38–55 |
| Wood fibre (rigid external) | 120 mm (external only) | £35–50 | £15–25 | £50–75 |
| Cellulose (dense pack) | 270 mm | £14–20 | £12–18 | £26–38 |
| PIR (external board) | 100 mm (external only) | £25–35 | £12–18 | £37–53 |
| Sheep wool | 280 mm | £35–55 | £10–15 | £45–70 |
Whole-Life Cost (Including Energy Savings)
Over a 60-year building life, the energy cost savings from better insulation often outweigh the initial material cost premium:
| Insulation choice | Initial premium (vs mineral wool) | Annual energy saving | 60-year net benefit |
|---|---|---|---|
| Mineral wool (baseline) | £0 | £0 | £0 |
| Wood fibre (same U-value) | +£3,000–5,000 | +£50–100 (summer cooling) | +£0–1,000 |
| Cellulose (same U-value) | +£500–1,500 | £0 | –£500–1,500 |
| Enhanced U-value (0.12 vs 0.18) | +£4,000–8,000 | +£200–400 | +£4,000–16,000 |
Installation Best Practice
General Rules for All Insulation Types
- Fill all voids completely — gaps as small as 5 mm create convection loops that dramatically reduce performance
- Cut accurately — oversized by 10–15 mm for friction fit (batts); exact fit for rigid boards
- Stagger joints — offset joints between layers by minimum 100 mm
- Seal around services — pack insulation tightly around pipes, cables, and boxes
- Maintain VCL continuity — tape all joints, seal around penetrations, lap at junctions
- Protect from weather — install insulation only in dry conditions; protect from rain until enclosed
- Verify moisture content — do not insulate around timber above 20% MC
Common Installation Defects
| Defect | Performance impact | Prevention |
|---|---|---|
| Gaps behind batts | 25–50% U-value reduction locally | Push batts fully to back of cavity |
| Compressed insulation | Reduced thickness = higher U-value | Cut to correct size, don't force |
| Missing insulation at edges | Severe thermal bridge | Check all perimeter junctions |
| VCL not sealed at joints | Air leakage, condensation risk | Tape all laps with compatible tape |
| Insulation installed wet | Reduced performance, decay risk | Store dry, install in dry conditions |
| Services in insulation zone | Gaps around pipes/cables | Use service void; pack around services |
Environmental Comparison
Embodied Carbon per m² of Wall (at U=0.15 W/m²·K)
| Material | Embodied carbon (kg CO₂/m²) | Carbon stored (kg CO₂/m²) | Net carbon (kg CO₂/m²) |
|---|---|---|---|
| Glass mineral wool | +3.5 | 0 | +3.5 |
| Stone mineral wool | +4.5 | 0 | +4.5 |
| Wood fibre | +2.0 | –8.0 | –6.0 |
| Cellulose | +1.0 | –5.0 | –4.0 |
| PIR | +8.0 | 0 | +8.0 |
| EPS | +5.0 | 0 | +5.0 |
| Sheep wool | +1.5 | 0 | +1.5 |
Wood fibre and cellulose insulation are carbon-negative — they store more carbon than is emitted in their production. Combined with C24 structural timber, a timber frame wall insulated with wood fibre stores approximately 25–35 kg CO₂ per m² of wall area in its structure and insulation alone.
For projects targeting net zero, the combination of timber frame + wood fibre insulation + timber cladding creates a wall assembly that is a net carbon sink — actively removing CO₂ from the atmosphere.
Related Resources
- Passive House Timber — Achieving passive house with timber frame
- Timber Frame vs CLT — Structural system comparison
- Timber Moisture Content — Moisture management in walls
- Cost of Timber Home 2026 — Build cost analysis
- Modular Homes vs Traditional — Construction methods
- Timber Fire Resistance — Fire performance of insulated walls
- Structural Timber Guide — Complete timber construction overview
- C24 Structural Construction Timber — Structural timber products
- BSH Glued Laminated Beams — Engineered timber products