🧊 Thermal Conductivity Coefficient λ
Determination of thermal conductivity coefficient λ of insulation and building materials by guarded hot plate (GHP) or heat flow meter (HFM) method.
Overview
Thermal conductivity coefficient λ [W/(m·K)] is key thermal insulation parameter of building materials. Lower λ means better thermal insulation properties. Typical values: EPS foam 0.031–0.040, mineral wool 0.032–0.040, concrete 1.0–1.5, brick 0.6–0.8, wood 0.13–0.18.
Standard PN-EN 12667 describes two reference methods: - Guarded Hot Plate (GHP) — absolute method, most accurate - Heat Flow Meter (HFM) — comparative method, faster
Value λ is essential for building thermal calculations (MIiR Regulation 2015), insulation thickness selection, Declaration of Performance (DoP) of building materials, and building energy certification.
Method principle
GHP method (guarded hot plate): Material specimen of known thickness d is placed between hot plate (hot) and cooling plate (cold). Guard zone eliminates lateral heat losses. After reaching steady state, heating power Q, plate temperatures T₁ and T₂, and thickness d are measured.
λ = Q × d / (A × ΔT) [W/(m·K)]
where Q — heating power [W], d — thickness [m], A — heating area [m²], ΔT = T₁ - T₂ [K].
HFM method: analogous, but heat power measured by heat flux transducer calibrated on reference material.
Applications
- Certification of insulation products (EPS, wool, PIR, XPS)
- Declaration of Performance (DoP) — CE marking
- Building thermal calculations (U-value of partitions)
- Quality control of insulation material production
- R&D of new thermal insulation materials
- Assessment of natural material insulation (cellulose, hemp, sheep wool)
Key parameters
| Parameter | Value |
|---|---|
| Range λ (GHP) | 0.01–0.5 W/(m·K) |
| Range λ (HFM) | 0.02–0.5 W/(m·K) |
| Accuracy (GHP) | ±2% |
| Accuracy (HFM) | ±3–5% |
| Specimen dimensions | 300×300 mm or ø300 mm |
| Specimen thickness | 20–100 mm (typical) |
Standard
- Standard number
- PN-EN 12667:2002
- Title (PL)
- Właściwości cieplne materiałów i wyrobów budowlanych — Określanie oporu cieplnego metodami osłoniętej płyty grzejnej i przepływomierza cieplnego — Wyroby o dużym i średnim oporze cieplnym
- Title (EN)
- Thermal performance of building materials and products — Determination of thermal resistance by means of guarded hot plate and heat flow meter methods — Products of high and medium thermal resistance
Step-by-step procedure
1. Specimen preparation
Cut specimen to apparatus dimensions (e.g., 300×300 mm). Measure thickness (min. 5 points, average). Weigh (to calculate density). Condition at 23°C/50% RH min. 24 h.
2. Apparatus configuration
Set plate temperatures: T_hot = 35°C, T_cold = 15°C (average 25°C, ΔT = 20 K). Or per product standard requirements.
3. Specimen placement
Place specimen centrally between plates. Ensure full contact (no gaps). In GHP — check guard zone.
4. Measurement
Start measurement. Apparatus automatically reaches steady state (ΔT stability < 0.1 K for 30 min). Measurement time: 2–12 h (depends on material).
5. Result reading
Software calculates λ, R (thermal resistance), heat power. Perform min. 3 repetitions. Calculate average and standard deviation.
6. Reporting
Report λ with accuracy 0.001 W/(m·K). Specify measurement conditions (temperatures, specimen moisture, density). Determine declared value λD per PN-EN ISO 10456.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| GHP apparatus | Netzsch GHP 456, Lambda-Messtechnik, LaserComp FOX | 200,000–500,000 PLN |
| HFM apparatus | Netzsch HFM 446, LaserComp FOX 314/600, Hukseflux TRSYS | 100,000–300,000 PLN |
| Laboratory oven | For specimen conditioning (70°C) | 5,000–15,000 PLN |
| Caliper / thickness gauge | For specimen thickness measurement with accuracy 0.1 mm | 200–1,000 PLN |
| Analytical balance | For specimen density determination | 5,000–15,000 PLN |
Health and safety (OHS)
- Hot plates — temperature up to 80°C, burn risk
- Insulation materials — dust (mineral wool), FFP2 mask
- Specimen cutting — caution with cutting tools