prEN ISO 4638

• Abstract:

  Applicable to test pieces cut from products of cellular materials. Useful for two purposes: a) studying the structure of cellular products in connection with their physical properties and their method of manufacture and b) ensuring product quality (quality assurance). Describes principle, apparatus, test piece and conditions, procedure and calculation and expression of the results. Three figures deliver the details for testing.

• Status: Not Published
• Publication Date: 30-Jan-2028
• ICS: 83.100 - Cellular materials
• Technical Committee: CEN/TC 249 - Plastics
• Current Stage: 4020 - Submission to enquiry - Enquiry
• Start Date: 06-Aug-2026
• Completion Date: 06-Aug-2026
• Ref Project: oSIST prEN ISO 4638:2026 - Polymeric materials, cellular flexible - Determination of air flow permeability (ISO/FDIS 4638:2026)

• Consolidated By: ISO/FDIS 4638 - Polymeric materials, cellular flexible — Determination of air flow permeability
• Effective Date: 12-Feb-2026
• Revises: EN ISO 4638:1995 - Polymeric materials, cellular flexible - Determination of air flow permeability (ISO 4638:1984)
• Effective Date: 14-Jan-2026

prEN ISO 4638 is a draft standard for flexible cellular polymeric materials that specifies a laboratory method for determining air flow permeability. It is prepared through ISO/CEN parallel processing for use by manufacturers, test laboratories, quality teams and buyers who need comparable results on cellular products.

What does prEN ISO 4638 specify?

prEN ISO 4638 specifies a method for measuring the permeability of flexible cellular polymeric materials to air flow. It applies to test pieces cut from products of cellular material, so it is used on samples taken from real products rather than on a special material formulation only.

The document is organized into 10 clauses:

• Clause 1 - Scope
• Clause 2 - Normative references
• Clause 3 - Terms and definitions
• Clause 4 - Principle
• Clause 5 - Apparatus
• Clause 6 - Test pieces
• Clause 7 - Testing conditions
• Clause 8 - Procedure
• Clause 9 - Calculation and expression of results
• Clause 10 - Test report

Three figures show the basic Darcy-law principle, a schematic apparatus, and a measurement cell.

What are the key requirements of prEN ISO 4638?

prEN ISO 4638 is built around passing air through a prepared test piece under controlled conditions, measuring the pressure drop, and calculating the flow characteristic from those measurements. In practice, the standard is about getting repeatable flow data from foam-like materials while avoiding edge leaks and uncontrolled compression.

Principle and calculation

Clause 4 states that the test uses controlled air flow through a cylindrical or parallelepiped specimen and measures the pressure drop between the two free faces. For homogeneous and isotropic materials, permeability is derived from Darcy’s law; for anisotropic materials, the flow direction has to be defined. This matters because the same foam can give different results depending on orientation and structure.

Clause 4 also distinguishes air flow permeability, specific air flow resistance and air flow resistance. Specific air flow resistance is used particularly in acoustics, while air flow resistance is the more practical result for non-homogeneous materials, including skinned or coated products.

Apparatus and measuring setup

Clause 5 requires a measurement cell, a steady air-flow source, flow measurement, pressure-drop measurement and thickness measurement. The source has to provide stable low flow rates, the flow measurement has to be accurate enough for the selected range, and pressure differences as small as 1 Pa must be measurable. For the user, this means the apparatus has to be sensitive enough for low-resistance foams and stable enough for traceable comparisons.

Test pieces and preparation

Clause 6 requires test pieces to match the cell shape and to be slightly oversized laterally so that side leakage is avoided. When permeability or specific resistance is being determined, surface skin or unevenness is removed so the test piece is planar and more representative of the internal structure. The faces through which air passes must be parallel, and the document says not to cut them with a hot wire.

Testing conditions and procedure

Clause 7 requires testing under ISO 23529 conditions and only after property stabilization. If wet-knife cutting is used, the test pieces have to be dried before testing. Clause 8 then sets out the sequence: place the specimen in the cell, seal the edges, measure thickness in position, establish the air flow, and take readings at five flow velocities under laminar conditions.

Results and reporting

Clause 9 requires calculation of permeability for homogeneous pieces and air flow resistance for non-homogeneous pieces. Results are reported with the specimen dimensions in test position and the apparent density. Clause 10 requires the report to identify the product, cell shape and size, specimen preparation, number and dimensions of test pieces, specimen orientation if relevant, the presence of skin, thickness and density as tested, and any deviations that could affect the result.

What terms does prEN ISO 4638 define?

Key terms used in prEN ISO 4638 include:

• Air flow permeability - the measure of how readily air passes through the cellular material under the test conditions.
• Specific air flow resistance - a parameter derived from permeability and used especially in acoustics.
• Air flow resistance - the resistance of the full specimen thickness to air flow, especially useful for non-homogeneous materials.
• Volumetric air flow rate (q) - the amount of air flowing through the specimen per unit time.
• Pressure drop (Δp) - the pressure difference between the two faces of the test piece.
• Apparent density - the density value measured for the product by ISO 845.

Who uses prEN ISO 4638?

prEN ISO 4638 is used by producers of flexible cellular polymeric materials, test laboratories, quality assurance teams and product developers. It is also relevant to buyers and specifiers who need a standard way to compare foam products, and to acoustics users who work with specific air flow resistance.

The method is useful when checking product quality, comparing materials by structure, or studying how foaming agents and manufacturing conditions change the cellular structure.

What changed in prEN ISO 4638 from the previous edition?

prEN ISO 4638 is a second edition that cancels and replaces ISO 4638:1984. The foreword states that this is a minor revision, and the listed change is the replacement of the withdrawn ISO 471 reference with ISO 23529.

Which standards are used with prEN ISO 4638?

• ISO 845 - provides the method for measuring apparent density, which is reported with the test result.
• ISO 23529 - provides the general procedures for preparing and conditioning test pieces before physical testing.
• ISO 7231 - is cited as a related quality control method based on air flow through cellular materials, useful when the aim is quality control rather than comparison of intrinsic material properties.

What does the prEN ISO 4638 document contain?

prEN ISO 4638 contains the full test method, including the apparatus layout, specimen preparation guidance, testing conditions, calculation formulas and reporting rules. It also includes three figures showing the test principle, the apparatus arrangement and the measurement cell.

Clause 9 provides the summary table format and the graphing approach for permeability versus flow velocity. The document also gives the measurement choices that support both characterization work and routine quality assurance, including how to handle homogeneous and non-homogeneous specimens.

SLOVENSKI STANDARD
01-oktober-2026
Polimerni materiali, mehke pene - Določanje prepustnosti toka zraka (ISO/FDIS
4638:2026)
Polymeric materials, cellular flexible - Determination of air flow permeability (ISO/FDIS
4638:2026)
Weichelastische Schaumstoffe - Bestimmung der Luftstromdurchlässigkeit (ISO/FDIS
4638:2026)
Matériaux polymères alvéolaires souples - Détermination de la perméabilité à l'air
(ISO/FDIS 4638:2026)
Ta slovenski standard je istoveten z: prEN ISO 4638
ICS:
83.100 Penjeni polimeri Cellular materials
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

FINAL DRAFT
International
Standard
ISO/FDIS 4638
ISO/TC 45/SC 4
Polymeric materials, cellular
Secretariat: DSM
flexible — Determination of air flow
Voting begins on:
permeability
2026-08-07
Matériaux polymères alvéolaires souples — Détermination de la
Voting terminates on:
perméabilité à l'air
2026-10-30
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/CEN PARALLEL PROCESSING LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
ISO/FDIS 4638:2026(en) © ISO 2026

FINAL DRAFT
ISO/FDIS 4638:2026(en)
International
Standard
ISO/FDIS 4638
ISO/TC 45/SC 4
Polymeric materials, cellular
Secretariat: DSM
flexible — Determination of air flow
Voting begins on:
permeability
Matériaux polymères alvéolaires souples — Détermination de la
Voting terminates on:
perméabilité à l'air
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2026
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/CEN PARALLEL PROCESSING
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland Reference number
ISO/FDIS 4638:2026(en) © ISO 2026

ii
ISO/FDIS 4638:2026(en)
Contents  Page
Foreword .iv
Introduction .v
1 Scope . 1
2  Normative references . 1
3  Terms and definitions . 1
4  Principle . 1
4.1 General .1
4.2 Air flow permeability .2
4.3 Specific air flow resistance.2
4.4 Air flow resistance .2
5 Apparatus .3
5.1 General .3
5.2 Measurement cell .3
5.3 Means of providing air flow .3
5.3.1 Source .3
5.3.2 Characteristics of the flow .3
5.4 Means of measuring volumetric flow rate .4
5.5 Means of measuring pressure drop .4
5.6 Means of measuring test piece thickness .4
6  Test pieces .4
6.1 Shape .4
6.2 Dimensions .4
6.2.1 Lateral dimensions .4
6.2.2 Thickness .5
6.3 Preparation .5
6.4 Number of test pieces .5
7  Testing conditions . 5
8  Procedure . 5
9  Calculation and expression of results .6
9.1 For homogeneous test pieces .6
9.2 For non-homogeneous test pieces .7
9.3 Precision . . .7
10  Test report .8
Bibliography .11

iii
ISO/FDIS 4638:2026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
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with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
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related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 45, Rubber and rubber products, Subcommittee
SC 4, Products (other than hoses), in collaboration with the European Committee for Standardization (CEN)
Technical Committee CEN/TC 249, Plastics, in accordance with the Agreement on technical cooperation
between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 4638:1984), of which it constitutes a minor
revision. The change is as follows:
— reference to the withdrawn document ISO 471 replaced with ISO 23529.
A list of all parts in the ISO 4638 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
ISO/FDIS 4638:2026(en)
Introduction
The air flow permeability of cellular materials indicates, in an indirect manner, some of their structural
properties. It can be used to establish correlations between the structure of these materials and some of
their physical properties. It also enables identification of the modifications to cellular structures produced
by chemical agents used in foaming, for example catalysts or surfactants.
This document is, therefore, useful for two purposes:
a) in studying the structure of cellular products in connection with their physical properties and their
method of manufacture;
b) in ensuring product quality (quality assurance).
NOTE Details of publications relating to flow behaviour in both laminar and turbulent conditions are given in
[1] [2] [3] [4] [5]
References   .
v
FINAL DRAFT International Standard ISO/FDIS 4638:2026(en)
Polymeric materials, cellular flexible — Determination of air
flow permeability
1 Scope
This document specifies a method for the determination of the permeability of flexible cellular polymeric
materials to air flow.
It is applicable to test pieces cut from products of cellular material.
[6]
NOTE ISO 7231 specifies a simple quality control method which is also based on the flow of air through cellular
materials. This can be used when it is not the intention to calculate the intrinsic properties of various materials in
order to compare them, but merely to control the quality of a given cellular material.
2  Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 845, Cellular plastics and rubbers — Determination of apparent density
ISO 23529, Rubber — General procedures for preparing and conditioning test pieces for physical test methods
3  Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4  Principle
4.1  General
Passing air under controlled conditions through a test piece in the form of a cylinder or parallelepiped.
Measurement of the pressure drop between the two free faces of the test piece.

ISO/FDIS 4638:2026(en)
4.2  Air flow permeability
K, is given by Darcy's law (see Figure 1), which describes the air flow in a homogeneous and isotropic (see
4.2, Note 1) porous medium under laminar flow conditions (see 4.2, Note 2) by Formula (1):
(1)
where
u is the linear air flow velocity, in metres per second;
q is the volumetric air flow rate crossing the test piece, in cubic metres per second;
V
A is the right cross-sectional area of the test piece, in square metres;
K is the flow permeability of the porous medium, in square metres;
Δp is the pressure drop across the test piece, in pascals;
η is the dynamic viscosity of ai
...