Overview
ISO 21129:2007 specifies the box method for determining the water‑vapour transmission properties of building materials and products. Intended primarily for materials with low water‑vapour resistance (where surface‑humidity transmission resistance matters), the standard describes apparatus, specimen preparation, measurement procedures and calculations to obtain water‑vapour permeance, resistance and permeability. The method measures vapour flow by recording mass changes of saturated salt solutions in a sealed test box attached to the specimen.
Key topics and requirements
- Test principle: A sealed water‑vapour permeability test box (standard size 600 × 600 × 600 mm) is attached to a 300 × 300 mm specimen. A saturated salt solution in the box establishes a controlled humidity difference between the two sides; vapour flow through the specimen is determined from mass change of the solution dish.
- Specimen: Standard area 300 × 300 mm; thickness is the product thickness (homogeneous materials >100 mm may be sliced). Preconditioning at (23 ± 5) °C and (50 ± 10) % RH until mass stabilizes (≤5 % change over three days).
- Apparatus & control: Room temperature controllable within 10–30 °C (±0.5 °C) and RH between (30 ± 3) % and (90 ± 3) %. Mixing fan to produce ~0.5 m/s air velocity at 50 mm from surface. Balance precision ±10 mg; hygrometers ±2 %.
- Humidity generation: Use saturated aqueous salt solutions (examples at 23 °C: KNO3 ≈94 % RH, NaCl ≈75 %, Mg(NO3)2 ≈53 %, MgCl2 ≈33 %, LiCl ≈11 %) to obtain a 20–40 % RH difference across the specimen.
- Measurement & calculation: Mass change measured at intervals until steady slope is obtained (at least five points with ±5 % consistency). Correct for box leakage via calibration. Compute water‑vapour permeance (W), resistance (Z) and permeability (δ). Surface‑humidity transmission resistance (1/β) is determined per Annex B and used to separate surface and material resistance.
- Accuracy considerations: Seal integrity, specimen area/thickness measurement, humidity control and box leakage calibration are highlighted for measurement quality.
Applications and users
ISO 21129:2007 is used for:
- Hygrothermal testing of building materials and components with low vapour resistance
- Product development and quality control for insulation, vapor‑open membranes, claddings and porous finishes
- Condensation risk analysis and selection of vapor control strategies in building physics
- Laboratory testing by material test houses, research institutes, certification bodies, building physicists and manufacturers
Keywords: hygrothermal performance, water‑vapour transmission, box method, water‑vapour permeance, ISO 21129:2007, building materials testing.
Related standards
- ISO 9346 - Hygrothermal performance: mass transfer quantities and definitions
- ISO 12571 / ISO 12572 - Guidance on humidity control and saturated salt solutions used for RH generation
Open preview (PDF)Hygrothermal performance of building materials and products — Determination of water-vapour transmission properties — Box method