Overview
ISO 15758:2014 - Hygrothermal performance of building equipment and industrial installations - Calculation of water vapour diffusion - Cold pipe insulation systems specifies a calculation method to evaluate water vapour diffusion into cold pipe insulation. The standard defines how to calculate the density of water vapour flow into insulation and the total moisture uptake over time, assuming diffusion-only transport (no airflow) and homogeneous, isotropic insulation materials. It applies to piping with medium temperatures above 0 °C, and to pipes located both inside buildings and outdoors.
Key elements covered include:
- Fundamental diffusion relations (e.g. g = −δ · dp/dx) and time integration to obtain total moisture uptake.
- Vapor diffusion resistance concepts (μ factor, diffusion-equivalent air layer thickness s) and how to compute system resistance Z′ for single- and multi-layer insulation.
- Boundary conditions, calculation procedures and worked examples (Annex A) plus procedures for systems with drying capacity (Annex B).
Key technical topics and requirements
- Diffusion-only assumption: The method presupposes that water vapour transport is by molecular diffusion only; convective airflow contributions are excluded.
- Material assumptions: Insulation materials are treated as homogeneous and isotropic so vapour pressure is constant on cylindrical surfaces equidistant from the pipe axis.
- Temperature limitation: Applicable only when the pipe medium temperature is > 0 °C (ice formation and freezing cases are excluded).
- Calculation outputs:
- Local water vapour flow rate per metre of pipe (g′).
- Cumulative moisture uptake over a specified period (G or G′).
- Multi-layer systems: Procedures for summing layer resistances and treating thin vapor retarders (large s-values) are provided.
- Boundary conditions: The standard prescribes using monthly mean conditions for the warmest month (rather than annual averages) for certain boundary assessments.
- Drying-capable systems: Guidance and experimental method for evaporation from capillary-active fabrics (Annex B) are included.
Practical applications and users
ISO 15758:2014 is used to:
- Predict interstitial condensation and accumulated moisture in chilled-water and cold process piping.
- Select and dimension insulation materials and vapor retarders to limit long-term moisture uptake.
- Evaluate remaining service life of insulation based on maximum allowable moisture content.
- Inform anti‑corrosion strategies for metal pipes where dew or condensation risk exists.
Typical users:
- HVAC and refrigeration engineers
- Building services designers and consultants
- Insulation manufacturers and suppliers
- Facility managers, maintenance engineers and corrosion specialists
- Code writers and building envelope analysts
Related standards
- ISO 9346 - Thermal insulation - Mass transfer - Physical quantities and definitions
- ISO 12241 - Thermal insulation for building equipment and industrial installations - Calculation rules
- ISO 12572 - Determination of water vapour transmission properties
- ISO 13788 - Internal surface temperature to avoid critical surface humidity and interstitial condensation
Keywords: ISO 15758:2014, water vapour diffusion, cold pipe insulation, hygrothermal, vapour retarder, interstitial condensation, diffusion resistance, insulation design, moisture uptake.
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