Vapor Diffusion Resistance Factor (mu-value)

What is the vapor diffusion resistance factor?

The vapor diffusion resistance factor, written as the Greek letter mu (μ), is a dimensionless material property that quantifies how much slower a substance transmits water vapour compared to still air. It is one of the most fundamental inputs in residential building physics, used in every moisture-safety calculation for walls, roofs, and floors designed to European standards. A higher mu-value means the material resists vapour more strongly. Knowing the mu-value is the only way to predict whether hidden condensation will occur inside a building assembly.

How does the mu-value become the sd value?

The sd value (Sd in metres) is the quantity you actually use in calculations. It is derived by multiplying the mu-value by material thickness in millimetres and dividing by 1000 to convert to metres. In formulae: sd = mu x d (in mm) / 1000. If a material has a mu of 50 and is 100 mm thick, then sd = 50 x 100 / 1000 = 5 m. This sd value is entered into moisture-risk software or STN EN ISO 13788 calculations. Product datasheets for membranes, foils, and coatings always declare the sd value directly for convenience.

How does this differ from a vapor barrier or retarder?

A vapor barrier and a vapour retarder are finished products that slow or stop vapour diffusion. The mu-value is the material property underlying them. A polyethylene film has a very high mu (around 100,000), which means a thin sheet achieves a high sd value and functions as a barrier. A wood-fibre board has a lower mu (around 5 to 10), delivering much less resistance at the same thickness. The mu-value determines what a product can do; the product itself is the application of that property to a specific thickness and context.

Which materials have high mu-values and which have low?

Still air itself defines mu = 1. Insulation materials are close to air: glass-fibre is around 1.3 to 2.5, mineral wool 1.5 to 3, and open-cell foam 3 to 10. These are vapour-open materials. Timber products vary with density: plywood is 50 to 200, solid timber 20 to 50, and chipboard 10 to 50. Concrete and dense masonry sit at 70 to 100. Plastics are highly resistant: rigid polyurethane foam is 30 to 40, polyethylene film 100,000 or more, and aluminium foil essentially infinite. Always check the product datasheet rather than assuming category averages, as manufacturing methods and porosity vary widely.

How is the mu-value tested and verified?

The standard test method is EN ISO 12572 (Cup Test Method). A material sample sits as a lid over a cup containing either distilled water (wet-cup test) or silica gel (dry-cup test), creating a sealed chamber with a controlled vapour gradient. Cup mass is recorded daily over 14 to 21 days to measure vapour transmission. The rate is compared to a reference still-air layer to derive the mu-value. High-quality products come with test certificates from accredited laboratories; datasheets listing mu without a test reference may be estimated rather than measured.

Why do passive-house designers care about mu?

Passive-house and low-energy designs aim for very low air leakage and high thermal resistance, creating cold surfaces inside the envelope. This makes interstitial condensation (hidden moisture inside the wall or roof) a real risk if vapour is not controlled. The mu-value and resulting sd values verify that the design will not accumulate dangerous moisture in winter and that any small amount will evaporate in summer. Without knowing mu-values for every layer, you cannot certify a passive-house design is safe.

What does a condensation-risk calculation measure?

A moisture calculation using STN EN ISO 13788 models the temperature and relative humidity inside each layer across a full year, typically a worst-case winter month. The calculation uses mu-values (converted to sd values) to predict how much vapour diffuses into the cold part each month and how much evaporates in summer. If cumulative condensation mass falls below material tolerance (near zero for timber and insulation, a few kilograms per square metre for some products), the design passes. If not, thermal performance must improve, vapour resistance must increase, or ventilation strategy must change.

Are mu-values used in dew-point calculations?

Dew-point calculations and mu-values are related but serve different purposes. A dew-point check asks whether a single temperature-humidity combination will cause vapour to condense on a specific surface; it is a snapshot test, often used to check whether an fRsi temperature factor will lead to surface mould. The mu-value is used in the annual mass-balance calculation accounting for diffusion, condensation, and evaporation over a full heating season. Both are part of the toolkit in STN 73 0540 Part 2 (Slovak energy-performance standard), but applied at different design stages.

These are qualitative bands; product datasheets are authoritative. Manufacturers test each formulation, density, and thickness combination. The ranges illustrate why material selection drives strategy: a wall of insulation and timber permits drying both directions; the same assembly with a plastic film on the warm side becomes a trapped condensation chamber.

Table 2 shows how sd value determines design strategy. A construction with low sd values must be vapour-open in both directions or protected from rain; one with high sd values must be sealed airtight and kept dry. Choosing mu-values is a strategic design decision affecting how the building will age and whether drying is possible in an emergency.

The mu-value is a material property every building physicist in Slovakia needs to understand. It is not a product; it is the property making a product suitable for one application and unsuitable for another. When verifying a passive-house design or approving a renovation detail, you are ultimately verifying that mu-values and the resulting sd profile allow the building to manage moisture safely over a 50-year service life.