Overview
ISO 7783:2018 - Paints and varnishes - Determination of water‑vapour transmission properties - Cup method - specifies a laboratory cup method to measure the water‑vapour transmission rate (V) of coatings (self‑supporting films and coatings on porous substrates). It supplements ISO 12572, adopts its procedures, definitions and calculations where possible, and defines both the wet‑cup (reference, high humidity) and dry‑cup (low humidity) approaches. Note: the method does not accurately quantify very high transmission rates above 680 g/(m2·d) (equivalent to diffusion‑equivalent air layer thickness s_d < 0.03 m).
Key topics and requirements
- Test principle: seal a test piece to the rim of a cup (containing either a saturated ammonium dihydrogen phosphate solution for the wet‑cup or a desiccant for the dry‑cup), place in a controlled enclosure and monitor mass change over time to determine water‑vapour flow.
- Test specimens: covers self‑supporting coatings (film alone) and non‑self‑supporting coatings applied to porous substrates. Substrate examples and requirement include homogeneous porous materials with V > 240 g/(m2·d).
- Apparatus and materials: test cup, sealing material, test enclosure (temperature/RH control), balance, desiccant or NH4H2PO4 solution, substrate for non‑self‑supporting coatings.
- Measurements and calculations: determination of mass change, water‑vapour transmission rate (V), water‑vapour diffusion‑equivalent air layer thickness (s_d) and water‑vapour resistance factor (μ) following ISO 7783 formulas and conventions. Test conditions (temperature and relative humidity) should match intended use as transmission may vary with conditions.
- Limitations: not suitable for reliably quantifying very high transmission rates (>680 g/(m2·d)). Wet‑cup is preferred for high humidity; dry‑cup for low humidity.
Applications and users
This standard is practical for:
- Paint, varnish and coating manufacturers - product development and quality control
- Independent testing laboratories - routine permeability testing and certification
- Specification writers, architects and building envelope consultants - assessing breathable coatings and moisture management
- Materials scientists and R&D teams - comparing barrier properties, evaluating film performance under humidity gradients
Practical uses include evaluating moisture permeability of decorative and protective coatings, comparative ranking of formulations, and supplying objective data for specifications and compliance.
Related standards
- ISO 12572 - Building materials - Determination of water vapour transmission properties (procedures and definitions referenced)
- ISO 4618 - Paints and varnishes - Terms and definitions
- ISO 1513, ISO 2808, ISO 3233‑1, ISO 3696 - referenced for sample preparation, film thickness, non‑volatile content and water quality
Keywords: ISO 7783:2018, water‑vapour transmission, cup method, wet‑cup, dry‑cup, paints and varnishes, s_d, water‑vapour resistance, permeability testing.