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: 02-Mar-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

Overview

prEN ISO 4638 - "Polymeric materials, cellular flexible - Determination of air flow permeability (ISO/FDIS 4638:2026)" is an international standard developed by CEN and ISO. This standard specifies a method for measuring the air flow permeability of flexible cellular polymeric materials, such as foams, using controlled laboratory conditions. The method is applicable to test pieces cut from products made of cellular materials. It serves both as a tool for studying the relationship between product structure and physical properties, and as a means for quality assurance in manufacturing.

Understanding the airflow permeability of cellular flexible polymers is essential, as it can provide insight into their structure, manufacturing process, and performance characteristics. This standardized approach ensures reliable, repeatable results across laboratories and applications.

Key Topics

• Air Flow Permeability Testing: The standard describes the principle of measuring airflow through a test specimen using appropriate apparatus, including a measurement cell, flow generators, manometers, and flowmeters.
• Test Specimens and Preparation: Guidance is provided on the shape, dimensions, and preparation of test pieces, ensuring representative and accurate results.
• Testing Conditions and Procedure: The document outlines standardized testing conditions and a systematic procedure-from specimen placement to data collection.
• Calculations and Reporting: Methods for calculating permeability, specific air flow resistance, and reporting the results are explained, ensuring clarity and uniformity in documentation.
• Quality Assurance and Research Utility: The standard's methodology allows for both routine quality control in manufacturing and in-depth research into material structure and properties.

Applications

prEN ISO 4638 has practical value across multiple sectors that use flexible cellular polymeric materials:

• Quality Control in Manufacturing: The method is widely used to assess uniformity and compliance with product specifications for flexible polymer foams, ensuring consistent product quality.
• Research and Development: Materials scientists and engineers use this standard for investigating how manufacturing processes or ingredients affect air flow characteristics and, by extension, material performance.
• Product Assurance in Automotive, Bedding, and Furniture: Industries that rely on comfort and ventilation properties, such as automotive interiors, bedding, and upholstered furniture, benefit from testing air permeability to optimize product design and comply with customer requirements.
• Acoustic and Thermal Insulation: Airflow permeability is a critical parameter for materials used in sound and thermal insulation, affecting their effectiveness and selection for construction or transportation applications.
• Regulatory Compliance: Reference to prEN ISO 4638 in contracts and regulatory documents provides confidence in the comparability and reliability of test results worldwide.

Related Standards

Several related standards support and complement prEN ISO 4638, enabling comprehensive material characterization:

• ISO 845: Specifies the determination of apparent density for cellular plastics and rubbers, often required alongside permeability testing.
• ISO 23529: General procedures for preparing and conditioning test pieces for physical test methods, ensuring samples are representative and tests are repeatable.
• ISO 7231: Provides a simpler quality control method for airflow through cellular materials, suitable for routine checks where detailed material characterization is not required.

By adhering to prEN ISO 4638 and its related standards, manufacturers, laboratories, and researchers ensure accurate, reproducible measurement of air flow permeability in flexible cellular materials, supporting product innovation, quality, and regulatory compliance.

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,
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
...