Air permeability and water vapour transmission are different properties. An airflow result describes air passing through a specimen under defined test conditions. A water-vapour result describes vapour transport under its own temperature and humidity conditions. One value cannot be converted into the other, and neither alone establishes the moisture performance of a finished insulation-board assembly.
The buyer’s question is: when a facer is described as “breathable”, which property has actually been measured, and which one does the project require?
Start by replacing that broad adjective with the required quantity, specimen and method. This avoids selecting a material against evidence that answers a different question.
Distinguish the test question
ISO 9073-15:2007 describes measurement of airflow perpendicular to a defined area of nonwoven material. [1] ISO 12572:2016, with its 2024 amendment, addresses cup-method determination of water-vapour transport properties for building materials and products. Its scope includes products with facings. [2]
The existence of these different methods matters more than the shared word “permeability”. Air transport and water-vapour transport do not become the same property because both results are low, high or described as “open”. The US Department of Energy also distinguishes the functions of air barriers and vapour-control layers. [4]
| Review question | Evidence to request | What it does not establish on its own |
|---|---|---|
| How much air passes through this facer? | Air-permeability result, test pressure, area and specimen identification | Water-vapour permeance or drying behaviour |
| How does water vapour pass through the tested product? | Vapour test result with temperature, humidity conditions and construction | Air leakage through the assembled board system |
| Does the facer resist liquid-water exposure? | A test relevant to the defined liquid-water exposure | Either of the two properties above |
| Will the finished construction manage moisture as required? | Evidence and assessment for the actual layered assembly and use | A conclusion from the facer name alone |
The third and fourth rows identify separate review questions; this article does not select a universal test or acceptance threshold for them.
Keep the reported quantity and unit together
For airflow, record the reported volume flow per area, its unit and the pressure difference. Those details belong with the number.
For vapour transport, distinguish a transmission rate from a permeance or a permeability. A rate describes the reported amount passing per area and time under the stated conditions. Permeance also relates transport to the vapour-pressure difference. Material permeability carries additional thickness-related meaning and should not be assigned casually to a multilayer product. [3]
Do not place all of these entries in a spreadsheet column called “permeability”. Use separate fields for property, value, unit, method, conditions and specimen. If a report gives a resistance-type quantity instead, retain that name and unit rather than reversing its interpretation informally.
For comparisons within airflow testing, use the separate guide to methods, pressure and units. That guide addresses comparability within one type of measurement; it is not a bridge from airflow to vapour transport.
Identify exactly what was tested
The loose facer, a coated facer, a facer bonded to a core, and the complete board are different specimens. Record which one appears in the report. Also record relevant additional layers, specimen orientation and the treatment of edges as described by the testing party.
A result for the supplied facer can be useful for its intended material review. It should not be silently promoted into a result for joints, fixings, cut edges or the whole installed assembly.
This distinction is especially important when comparing foil and foil-composite facer directions. The broad material label does not replace identification of the tested construction. Do not rank different facer families with a vapour value for one product and an airflow value for another.
Recognise an incomplete comparison
Illustrative document review—not product test data: a supplier provides an air-permeability result with a stated pressure. Another supplies a cup-test vapour result. Both documents may be valid for their stated specimens, but they do not answer the same selection question.
The next action is not to convert the units. First decide whether the project needs an airflow characteristic, a vapour-transport characteristic, or both. Then request the missing corresponding evidence for each identified material.
A second common gap is a report that names the property but omits the tested construction or conditions. Treat that as missing context, not as zero transmission and not as a favourable result. Ask for the relevant report section before assigning the value to the project’s comparison table.
Use a short evidence-request brief
A practical request should identify the board application, the proposed facer construction and the specific transport property being reviewed. Ask for the method, reported quantity and unit, specimen construction, conditions and the scope of any stated conclusion.
For PIR/PUR board applications, make clear whether the question concerns processing of the facer, the board as manufactured, or moisture behaviour in the intended assembly. These may require different evidence and different reviewers.
GRECHO’s role is to support application-matched material review and coordinate available technical documentation. The appropriate technical party must evaluate any proposed assembly-level conclusion using evidence for that assembly.
Frequently asked questions
Does a low air-permeability value prove a vapour barrier?
No. It is an airflow result. A claim about vapour transport needs the appropriate vapour-related evidence and a clearly identified specimen.
Can “breathable” be used as the only requirement?
It is too ambiguous for a useful technical comparison. Replace it with the property, conditions and specimen that matter to the application.
Does a water-vapour result prove resistance to liquid water?
No. Vapour transport and liquid-water exposure are different questions. Request evidence suited to the exposure rather than extending a vapour-test conclusion.
Request the right property, not a broader promise
Share the board application and the exact moisture or airflow question you need to resolve. GRECHO can help coordinate the relevant available material documentation without treating one test as proof of every transport property.
Discuss the required evidence with GRECHO.
References
[1] ISO 9073-15:2007 — Air-permeability test scope.
[2] ISO 12572:2016 — Cup-method scope and listed 2024 amendment.
[3] ISO 12572:2016 — Public publisher preview, definitions of transport quantities.
[4] US Department of Energy — Air barriers and vapour barriers.
These references establish the distinction between test purposes. The article does not reproduce the standards or provide a hygrothermal design assessment.
