(Note: Opinions in the articles are of the authors only and do not necessarily reflect the opinion of the European Union)

The word 'breathable' is a poor starting point for specifying an insulation-board facing. It can mean air passes through a material, water vapour diffuses through it, or a product is simply marketed as moisture-tolerant. Those are different propositions. A useful specification replaces the adjective with a property, a test method and an identified construction.

For teams working on European building-envelope projects, the distinction matters at two levels: selecting a facer for a board manufacturing process, and deciding what the resulting board can contribute to an installed wall or roof. A result that helps the first decision may not answer the second. Material selection should therefore start with the intended function, rather than with the highest or lowest number on a supplier comparison sheet.

Air movement and vapour diffusion are separate mechanisms

Water vapour can travel with moving air through openings, or pass through materials by diffusion in response to a vapour-pressure difference. Liquid water introduces further mechanisms, including rain penetration and capillary movement. Building-science guidance treats these separately because controlling one does not establish control of the others. [1]

In a textile air-permeability test, the question concerns airflow through a specimen. ISO 9073-15:2007 addresses flow perpendicular to a defined area of nonwoven fabric, including treated or untreated laminates. A reported value needs its measurement conditions; it is not an unqualified declaration of how a wall will behave. [2]

Water-vapour testing asks a different question. ISO 12572:2016, with Amendment 1:2024, describes cup-based determination of water-vapour permeance and permeability for building products and materials under isothermal conditions. Its scope includes insulation products with facings and integral skins. It is not an airflow test. [3]

The two properties are not interchangeable, and no general unit conversion exists from an air-permeability result to a water-vapour transmission result. Treating them as separate evidence requirements does not mean they can never be studied together. It means that evidence for one is not a substitute for measuring the other.

Start by naming the function and the test object

A useful review separates the following questions before requesting data:

• Design or production question: How does air pass through this facer specimen?
  Evidence to request: Air-permeability result, method, pressure difference, exposed area and specimen identity
  What the evidence cannot establish by itself: Water-vapour transport or installed-building airtightness 
• Design or production question: How does water vapour pass through the stated material or product?
  Evidence to request: Vapour test, reported quantity, temperature, humidity conditions, and construction
  What the evidence cannot establish by itself: Rain resistance or air leakage 
• Design or production question: Does the installed envelope resist unwanted air leakage?
  Evidence to request: Evidence for the continuous air-control arrangement and an appropriate building-level assessment
  What the evidence cannot establish by itself: Performance of every individual facer grade 
• Design or production question: Can the proposed wall or roof manage moisture over time?
  Evidence to request: Assessment of the complete build-up, climate, internal conditions and relevant transport properties
  What the evidence cannot establish by itself: A universal answer based on the facer name

This is a review framework, not a replacement for a test standard or a project specification. Its value is that it prevents a supplier, a board manufacturer, and a designer from answering three different questions while believing they are discussing the same property.

Also identify the physical object behind each result. A bare veil, a coated facer, a facer bonded to insulation, and a complete installed assembly are not equivalent specimens. Record coating state, layers, and relevant interfaces before transferring a figure into a design or procurement document. A report for the incoming material should not silently become a declaration for the finished board.

A facer result is not a building airtightness result

A material can be part of an air-control strategy, but a continuous system also needs connections across joints, edges, penetrations and interfaces. Guidance on air-sealed construction emphasises continuity and sealing of those leakage paths. A favourable specimen result alone does not establish that continuity. [4]

The difference is also visible in the scope of the test methods. ISO 9972:2015 concerns fan-pressurisation measurement of buildings or parts of buildings over a range of pressure differences. It explicitly does not evaluate individual components. A textile air-permeability value and a building airtightness result therefore belong to different assessment levels. [5]

For a faced insulation board proposed as part of an envelope, the review should ask who is responsible for the air-control layer, how adjacent boards connect, and what happens at changes of material. If the facer is not intended to provide that function, the specification should not suggest otherwise. Equally, the presence of insulation should not be treated as proof that an air-control layer has been detailed.

Vapour data needs a reported quantity, not just a number

Water-vapour transmission rate, permeance and permeability are related terms, but they do not name the same reported quantity. In broad terms, a transmission rate expresses transported mass per area and time; permeance additionally relates that transport to the vapour-pressure difference; permeability is a material quantity for which thickness is relevant. A value should be copied with its full unit and definition, not relabelled to fit a spreadsheet column. [3]

A practical request should identify the method and edition, the measured construction, the temperature and humidity conditions on each side, and whether the stated result describes a facer alone or a faced product. The applicable laboratory procedure must determine how the test is carried out. This article does not prescribe a cup configuration, conditioning period, or acceptance threshold.

For a layered construction, avoid assigning a convenient single-layer interpretation without checking its validity. A coating, an adhesive, or an integral skin can be part of what was tested. Removing those details from the description makes it difficult to know whether the number belongs to the proposed product.

Production compatibility and envelope suitability need separate decisions

Public insulation-facer documentation identifies processing considerations such as adhesion to mineral wool, stability during foam lamination, and resistance to bleed-through. These illustrate the kinds of requirements a board manufacturer may need to evaluate; they do not establish a universal specification for all facers. [6]

Consider a hypothetical change from one coated facer to another on an insulation-board line. The manufacturing team may need to review feeding, bonding, and the condition of the finished surface. The design team may separately need evidence for vapour transport through the finished product and compatibility with the intended wall or roof build-up. Passing a line trial does not, by itself, answer the design questions. Nor does a suitable vapour result demonstrate that the material will process successfully on that line.

The useful output is therefore two linked decisions: whether the material is suitable for the defined manufacturing trial, and whether the resulting product has the evidence needed for the intended use. Keeping the decisions separate makes it easier to identify what a proposed substitution has changed.

Check the whole build-up before making a moisture conclusion

A single vapour value cannot describe the changing conditions experienced by a multilayer wall or roof. Fraunhofer IBP's WUFI framework, for example, evaluates coupled heat and moisture transport in building components under climatic conditions. This illustrates the difference between a material datum and a component-level assessment. It does not mean that every facer comparison requires a simulation. [7]

Where a hygrothermal assessment is appropriate, the reviewer should identify the model inputs, boundary conditions, and mechanisms actually included. A model is not a substitute for the missing material properties, and a calculation should not be described as having evaluated air leakage if that mechanism was not included.

This is particularly important when the word 'breathable' is used as a design conclusion. Whether moisture can accumulate or dry out depends on the proposed construction and exposure, not on that label alone. The specification should state the intended moisture-control function and the evidence supporting it.

A short specification handover that prevents long arguments

Before a facer is released for an application-specific trial, prepare one shared evidence record containing the material and lot identity; the layer or assembly tested; the method, edition and conditions; the reported quantity and unit; and the decision the result is intended to support. Keep unresolved questions visible rather than filling them with assumed equivalence.

The review can then end with three explicit statements: what is supported for the facer, what is supported for the faced board, and what still needs assessment for the installed construction. A supplier-authored companion resource on the distinction between insulation-facer airflow and vapour data is listed in reference [8]; the test-method authorities remain the standards cited above.

Good specification does not begin by deciding whether a facer should be 'more breathable'. It begins by deciding which transport mechanism matters, at which level of the construction, and under which conditions the evidence will be used.