ISO 21129:2007

INTERNATIONAL ISO
STANDARD 21129
First edition
2007-07-15
Hygrothermal performance of building
materials and products — Determination
of water-vapour transmission
properties — Box method
Performance hydrothermique des matériaux et produits pour le
bâtiment — Détermination des propriétés de transmission de la vapeur
d'eau — Méthode de box
Reference number
©
ISO 2007
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.

©  ISO 2007
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2007 – All rights reserved

Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, symbols and units . 1
4 Principle. 2
5 Apparatus . 2
6 Test specimen . 4
7 Procedure . 4
8 Calculation and expression of results. 5
9 Measurement accuracy . 5
10 Test report . 6
Annex A (normative) Conversion of water-vapour pressure. 7
Annex B (normative) Method for calculating the water vapour surface resistance. 8
Annex C (informative) Double-box method . 9
Bibliography . 11

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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 21129 was prepared by Technical Committee ISO/TC 163, Thermal performance and energy use in the
built environment, Subcommittee SC 1, Test and measurement methods.
iv © ISO 2007 – All rights reserved

Introduction
The general principles are applicable to all hygroscopic and non hygroscopic building materials and products.
It is required to search for hygrothermal performance and thermal performance of building materials in order to
solve the condensation phenomenon of building walls and to establish the condensation-preventive measures.
In the box method, a specimen is fixed to a measurement box. The mass of saturated salt solutions contained
in two pans hanging on the two side boxes of a specimen is measured. The amount of water vapour
penetrating through a specimen is calculated from change of the mass of the saturated salt solutions in the
two pans.
In a cup method, the mass of the saturated salt solution is measured together with that of the specimen;
however, in the box method, it is not necessary to measure the mass of the specimen.
The reason that the box method is more accurate than the cup method is that there are thick materials and/or
heavy materials in building materials or products.
If the principle of the box method is applied, it is possible to measure the water-vapour transmission properties
of not only materials but also some building products with good accuracy.
INTERNATIONAL STANDARD ISO 21129:2007(E)

Hygrothermal performance of building materials and
products — Determination of water-vapour transmission
properties — Box method
1 Scope
This International Standard specifies a box method for determining the water-vapour permeability of building
materials. The box method is used primarily to measure the water-vapour permeance of materials that have
low water-vapour resistance, in which the influence of the surface-humidity transmission-resistance cannot be
ignored.
9 2
NOTE Materials with low water-vapour resistance includes those with no greater than 1,8 × 10 ⋅ m ⋅ s ⋅ Pa/kg of
−8 2
resistance [water vapour permeability coefficient of 5,5 × 10 kg/(m ⋅ s ⋅ Pa or above)].
2 Normative references
The following referenced documents are indispensable for the application 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 9346, Hygrothermal performance of buildings and building materials — Mass transfer — Physical
quantities and definitions
3 Terms, definitions, symbols and units
3.1 Terms and definitions
For the purposes of this document, the definitions given in ISO 9346 and the following apply.
3.1.1
density of water vapour flow rate
mass of water vapour transferred through per unit area per unit time under specified conditions of temperature,
humidity and thickness
3.1.2
water vapour permeance
density of water vapour flow rate divided by the water vapour pressure difference between the two surfaces of
the specimen
3.1.3
water vapour conductivity
water vapour permeance per unit thickness
3.1.4
water vapour resistance
reciprocal of water vapour permeance
3.1.5
water vapour permeability
product of the water vapour permeance and the thickness of a homogeneous specimen
3.2 Symbols and units
Symbol Quantity Unit
W Water-vapour permeance of specimen kg/(m ⋅s⋅Pa)
p
Z Water-vapour resistance of specimen m ⋅s⋅Pa/kg
p
G Water-vapour flow rate through specimen kg/h
A Area of specimen m
P Water-vapour pressure in box or room Pa
ϕ Relative humidity
1/β Water-vapour surfaces resistance m ⋅s⋅Pa/kg
β Surface coefficient of water-vapour transfer kg/(m ⋅s⋅Pa)
δ Water-vapour permeability of specimen kg/(m⋅s⋅Pa)
p
d Thickness of specimen m
Subscripts:
High High water vapour pressure side.
Low Low water vapour pressure side.
See References [1] to [5] for further details.
4 Principle
Install a box (called the water-vapour-permeability test box) for measuring the amount of permeated water
vapour in a room maintained at a constant temperature and humidity. To create a difference in water vapour
pressure, i.e., a difference in humidity, between the two sides of the specimen, attach the test box to one face
(opening) and then place a high- or low-density aqueous salt solution into the box and maintain the box at a
constant humidity. During this time, water vapour flows from the box to the room or from the room to the box
by permeating the specimen. The amount of vapour flow can be determined by measuring the change in mass
of the dish containing the aqueous salt solution by taking measurements at constant intervals using a scale
(electronic balance); see Figure 1. In this way, the total water-vapour resistance, including the surface-
transmission-resistance of the material, can be calculated by measuring the vapour-pressure difference on
both sides of the specimen at a steady state and a constant water-vapour permeance. The water-vapour
resistance of the material can be calculated by subtracting the surface-humidity transmission-resistance on
both sides of the specimen, obtained according to Annex B, from the total water vapour resistance.
5 Apparatus
NOTE See Figure 1.
5.1 Room, in which the temperature can be freely set within a range of 10 °C to 30 °C to an accuracy of at
least ± 0,5 °C and the relative humidity of the room can be freely set within a range of (30 ± 3) % to (90 ± 3) %.
5.2 Water-vapour permeability test box, having the standard dimensions of 600 mm × 600 mm × 600 mm.
The measuring box should be made of material such as metal, glass or hard plastic that is impermeable to
water vapour and, under the measurement criteria, it should be resistant to corrosion (anti-moisture material).
The box joint shall be properly sealed to prevent water-vapour leakage. The box shall be constructed to
provide sufficient strength for usage. The part connected to the sample installation frame at the opening
(flange) shall be made of anti-moisture rubber packing or other material to enable air tightness.
2 © ISO 2007 – All rights reserved

5.3 Mixing fan, capable of blowing air along the surface of the specimen as uniformly as possible.
Average velocity of the air movement at a distance of 50 mm from the surface shall be adjusted to be 0,5 m/s.
5.4 Saturated-aqueous-salt solution dish, having an area at least equal to the area of the specimen.
The dish shall be resistant to corrosion from the salt solution and be of a construction to enable mass
measurement by a balance.
5.5 Balance, capable of weighing the pan with a precision of ± 10 mg.
5.6 Specimen installation frame, made of non-distorting, anti-moisture material.
It shall be constructed with an opening in the centre where the specimen can be placed, and enable secure
air-tight attachment of the frame to the flange of the water-vapour test box. The opening dimensions shall be
300 mm × 300 mm. It shall have a flange-type shape to keep in thick specimens and have gaps to enable the
sealing of small surface holes in the specimen.
5.7 Thermometers, two, capable of measuring the air temperature on both sides of the specimen to an
accuracy of ± 0,1 °C.
5.8 Hygrometers, two, capable of measuring the relative humidity of both sides of the specimen to an
accuracy of ± 2 %.
Key
1 specimen 7 mixing fan
2 measuring box 8 thermometer and hygrometer
3 frame for specimen 9 sealant
4 suspended weighing pan 10 rubber gasket
5 balance 11 rubber plug
6 room at constant temperature and humidity 12 clamp
Figure 1 — Test apparatus
6 Test specimen
6.1 Size and thickness of specimen
The standard size of the test specim
...