Water vapour transmission rate (WVTR) and the Sd value—the water vapour diffusion-equivalent air layer thickness—are fundamental parameters for assessing how building materials manage moisture. These values determine whether a material allows moisture to pass through (breathable) or acts as a barrier, directly influencing condensation risk, mould growth and long-term durability of the building envelope. BS EN ISO 12572 provides the standardised cup method for determining these properties, enabling architects and engineers to specify materials that balance thermal insulation with moisture control.
What is the Sd Value and Why Does it Matter?
The Sd value expresses the thickness of a static air layer that would provide the same resistance to water vapour diffusion as the tested material. Measured in metres, it offers a straightforward way to compare barrier performance across different products. A low Sd value (below 1 m) indicates a highly breathable material suitable for applications where moisture needs to escape, such as external insulation layers or insulation materials in timber frame construction. High Sd values (above 50 m) characterise vapour barriers designed to prevent moisture migration into sensitive layers.
Understanding the Sd value is essential when designing multi-layer assemblies. Incorrect sequencing—placing a vapour-tight layer on the warm side and a breathable layer on the cold side—can trap moisture inside the construction, leading to interstitial condensation, reduced thermal performance and structural damage over time. Compliance with DIN 4108 moisture control requirements often hinges on accurate WVTR and Sd data.
Testing Methods: Gravimetric and Electrolytic Approaches
BS EN ISO 12572 specifies the gravimetric cup method, which measures water vapour transmission by placing a material sample over a cup containing a desiccant or saturated salt solution. The assembly is exposed to defined temperature and humidity conditions, and weight changes over time reveal the WVTR. For diffusion-open and diffusion-inhibiting materials with Sd values of approximately 0.03 to 10 m—equivalent to a WVTR of roughly 4 to 800 g/(m²·d) depending on material and test conditions—the standard recommends a quintuple determination to ensure statistical reliability.
For materials with higher vapour resistance—Sd values from approximately 10 to over 1500 m, corresponding to WVTR values of roughly 0.005 to 125 g/(m²·d)—the electrolytic method offers a faster and more cost-effective alternative. This approach uses an electrolytic detection sensor to measure moisture transmission and is typically performed as a duplicate determination. While deviating from the strict BS EN ISO 12572 gravimetric protocol, the electrolytic method is widely accepted for vapour barriers and vapour-retarding layers where precision at very low transmission rates is required.
Related Standards and Accreditation
BS EN ISO 12572 sits within a family of testing standards for hygrothermal performance. DIN EN 1931 addresses water vapour transmission properties of flexible waterproofing sheets for roofing, including bitumen, plastic and rubber membranes. DIN 53122-1 covers plastic and elastomer films, paper and board. DIN EN ISO 15106-3 specifies the electrolytic detection sensor method for plastics film and sheeting. Accredited laboratories—such as those holding ISO 17025 accreditation for DIN EN ISO 12572 and DIN 53122-1—provide traceable results that meet regulatory and certification requirements.
Application in Breathable Construction
Breathable construction systems aim to allow moisture vapour generated inside a building to diffuse outwards through the envelope, preventing accumulation. This approach is particularly relevant in hybrid timber construction and energy-efficient retrofits where airtight insulation layers must not compromise moisture safety. Materials with Sd values in the range of 1 to 100 m, such as wood-fibre boards, certain renders and intelligent membranes, strike a balance between thermal performance and vapour permeability.
For vapour barriers—used in cold roof constructions, below-slab insulation and other applications requiring tight moisture control—Sd values typically exceed 50 m. Electrolytic measurement according to ISO 15106-3 suits these products, offering results within a shorter timeframe than gravimetric testing. Typical sample requirements for both methods include five DIN A4 sheets without folds, creases or damage, ensuring representative material behaviour.
Interpreting WVTR and Sd Values in Practice
Design teams must integrate WVTR and Sd data into hygrothermal simulations to verify that assemblies meet moisture safety criteria. Software tools compare the vapour resistance profile across all layers, checking for condensation risk under design winter and summer conditions. Materials with very low Sd values (below 0.1 m) behave almost like open air; those with Sd values above 1500 m function as near-impermeable barriers.
When specifying materials for energy-efficient envelopes—such as those targeting Passivhaus or KfW-Effizienzhaus standards—designers should consult manufacturer datasheets for tested WVTR and Sd values. These figures must derive from accredited testing to BS EN ISO 12572 or equivalent national standards. Relying on generic or untested values introduces risk, particularly in climates with significant seasonal humidity swings.
For related moisture-control topics, see our analysis of water vapour diffusion resistance, testing procedures and condensation verification. Understanding the interplay between thermal conductivity (λ-value), U-value and vapour permeability ensures that thermal upgrades do not inadvertently create moisture traps within the building fabric.
