oSIST prEN ISO 4638:2026

• 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
• Public Enquiry End Date: 19-Oct-2026
• ICS: 83.100 - Cellular materials
• Technical Committee: IPMA - Polimer materials and products
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 14-Aug-2026
• Due Date: 01-Jan-2027
• Ref Project: prEN ISO 4638 - Polymeric materials, cellular flexible - Determination of air flow permeability (ISO/FDIS 4638:2026)

• Revises: SIST EN ISO 4638:1999 - Polymeric materials, cellular flexible - Determination of air flow permeability (ISO 4638:1984)
• Effective Date: 14-Jan-2026

Overview

oSIST prEN ISO 4638:2026 defines a standardized method for assessing the air flow permeability of flexible cellular polymeric materials. Developed by the Slovenian Institute for Standardization (SIST) and harmonized with ISO/FDIS 4638:2026, this international standard is essential for evaluating the permeability characteristics of foam and similar cellular products. The method described is applicable to test pieces extracted directly from finished products, making it highly relevant for quality control as well as for research and product development in the field of flexible polymeric foams.

Key Topics

• Standardized Permeability Measurement: Specifies precise procedures and apparatus requirements for measuring how easily air passes through cellular flexible polymeric materials.
• Material Structure Analysis: Supports the study of the internal structure of cellular materials, offering insights into how manufacturing processes and chemical additives (such as foaming agents or surfactants) affect end-product performance.
• Quality Assurance: Provides a repeatable, objective foundation for product quality assurance, enabling manufacturers and users to detect variations in material consistency.
• Apparatus and Test Conditions: Detailed guidance on the setup, including the measurement cell (cylindrical or rectangular), means for generating and measuring airflow, as well as test specimen preparation and conditioning.
• Analysis and Reporting: Instructions on calculating key parameters (such as permeability and specific air flow resistance) and preparing comprehensive test reports according to international practice.

Applications

The standardized measurement of air flow permeability in flexible polymeric foams is valuable across multiple sectors:

• Quality Control in Manufacturing: Enables consistent production of cellular materials such as polyurethane foams, ensuring compliance with industry and safety standards.
• Product Development: Assists researchers in correlating the cellular structure of polymeric materials to their physical properties, promoting the design of high-performance materials for specialized applications.
• Compliance and Certification: Forms the basis for third-party verification and certification of product claims regarding material properties, aiding manufacturers in meeting regulatory and customer requirements.
• Performance Optimization: Useful in industries where acoustic absorption, filtration, or cushioning properties are dependent on air flow characteristics (e.g., automotive, construction, and consumer goods sectors).
• Comparative Analysis: Permits direct comparison of permeability among different products or materials, using rigorously defined test conditions and calculation methods.

Related Standards

For a comprehensive approach to polymeric materials characterization and quality assurance, consider the following related standards:

• ISO 845, Cellular plastics and rubbers - Determination of apparent density: Specifies the method for measuring the density of cellular plastics, an essential parameter often correlated with permeability.
• ISO 23529, Rubber - General procedures for preparing and conditioning test pieces for physical test methods: Outlines best practices for sample preparation and conditioning, ensuring consistency across different testing protocols.
• ISO 7231, Polymeric materials, cellular, flexible - Determination of air flow value at constant pressure-drop: Provides a simpler quality control method for airflow testing, useful as a supplementary or preliminary check.

Practical Value

Adopting oSIST prEN ISO 4638:2026 improves reliability in product testing, enhances traceability, and facilitates international trade by aligning with globally recognized ISO standards for flexible polymeric materials. Implementing this standard supports improved product performance, safer end use, and ongoing innovation in the development of cellular foam-based technologies.

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
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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,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
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
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
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
...