ICS 91.080.30; 91.120.10  
 
Reference number
DRS 524: 2023
© RSB 2023
  
 RWANDA 
STANDARD  DRS 
524 
 
First edition  
2023-mm-dd  
 
Masonry units test methods — 
Determination of thermal properties  
   
DRS 524: 2023 
©RSB 2023- All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition 
 
 
 
 
 
 
 
 
© RSB2023 
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 
prior written permission from RSB. 
Requests for permission to reproduce this document should be addressed to: 
Rwanda Standards Board 
P.O Box 7099 Kigali-Rwanda 
KK 15 Rd, 49 
Tel. +250 788303492 
Toll Free: 3250 
E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw  
ePortal: www.portal.rsb.gov.rw  DRS 524: 2023 
iii                ©RSB 2023- All rights reserved 
 Contents Page 
Foreword ............................................................................................................................................................. v  
1 Scope ...................................................................................................................................................... 1  
2 Normative references ............................................................................................................................ 1  
3 Terms and definitions ........................................................................................................................... 1  
4 Symbols (and abbreviated terms) ........................................................................................................ 3  
5  Procedures to determine λ10,dry, unit-values for solid masonry units and λ10,dry,mor-values for 
mortars ................................................................................................................................................... 5  
5.1 General ................................................................................................................................................... 5  
5.2 λ10,dry, mat-values for solid masonry units and mortars ....................................................................... 5  
5.2.1 Model S1. Determination of λ10,dry,unit-values from tabulated λ10,dry,mat/net dry density 
relation .................................................................................................................................................... 5  
5.2.2 Model S2. Determination of λ10,dry,unit-values based on λ10,dry,mat /net dry density 
curve ....................................................................................................................................................... 6  
5.2.3 Model S3. Procedures to determine λ10,dry,unit-values from determining the thermal 
transmittance ( Umas) of masonry built from solid masonry units and mortar ................................. 9  
5.3 Test methods and numbers of samples to be taken for the different models ................................ 9  
6 Procedures to determine equivalent λ10,dry,unit-values for masonry units with formed voids 
and composite masonry units ............................................................................................................. 9  
6.1 General ................................................................................................................................................... 9  
6.2 Calculation methods ........................................................................................................................... 10  
6.3 λ10,dry,unit-values of masonry units ...................................................................................................... 10  
6.3.1 Determination of λ10,dry,unit-values from tabulated λunit /λmat relation ................................................ 10  
6.3.2 Determination of λ10,dry,unit-values based on calculation .................................................................. 11  
6.3.3 Model P5. Determination of λ10,dry,unit-values from determining the thermal transmittance 
(Umas) of masonry built from masonry units with formed voids or composite masonry 
units and mortar .................................................................................................................................. 12  
6.4 Test methods and numbers of samples to be taken for the different models .............................. 15  
7 Moisture conversion ........................................................................................................................... 16  
8 Procedures to determine design thermal values ( Rdesign,mas or λdesign, mas ) for masonry built 
from masonry units and mortar ......................................................................................................... 17  
8.1 General ................................................................................................................................................. 17  
8.2 Rdesign,mas - or λdesign,mas -values based on calculation .......................................................... 17  
8.2.1 Rdesign,mas- or λdesign,mas-values based on λdesign-values for the masonry units and the mortar ..... 17  
8.2.2 Rdesign,mas- or λdesign,mas-values using a numerical calculation method based on the 
design thermal conductivity of the materials used ......................................................................... 18  
8.3 Rdesign,mas- or λdesign,mas-values of masonry built from masonry units with formed 
voids or composite masonry units and mortar based on tabulated values .................................. 18  
8.3.1 Tabulated values ................................................................................................................................. 18  
8.3.2 Application of Annex B ....................................................................................................................... 18  
8.3.3 Alternative application of Annex B .................................................................................................... 19  
9 Determination of the thermal transmittance of masonry ................................................................ 20  
10 Specific heat capacity ......................................................................................................................... 20  DRS 524: 2023 
©RSB 2023- All rights reserved                  iv 
 11 Rounding rules for λ-values for masonry ........................................................................................ 20  
Annex A (normative)  Tabulated λ 10,dry,mat-values of materials used for masonry products .................... 21  
Annex B (normative)  Determination of thermal resistance by hot box method using heat flow 
meter — Masonry ................................................................................................................................ 32  
B.1 Test procedure .................................................................................................................................... 32  
B.1.1 Applicability of the method ................................................................................................................ 32  
B.1.2 Conditioning of the specimen ........................................................................................................... 32  
B.1.3 Specimen selection and mounting ................................................................................................... 33  
B.2 Measurement ....................................................................................................................................... 34  
B.2.1 Test conditions ................................................................................................................................... 34  
B.2.2 Measurement period ........................................................................................................................... 34  
B.2.3 Calculations ......................................................................................................................................... 34  
Annex C (normative)  Test specimen ............................................................................................................. 36  
C.1 General ................................................................................................................................................. 36  
C.2 Selection and size ............................................................................................................................... 36  
C.3 Specimen preparation and instrumentation .................................................................................... 36  
C.3.1 Conformity with product standards .................................................................................................. 36  
C.3.2 All specimens except loose-fills ....................................................................................................... 36  
C.3.3 Loose-fill materials ............................................................................................................................. 39  
Annex D (informative)  R dry,mas- or λ10,dry,mas-values of masonry built from a range of masonry 
units containing formed voids .......................................................................................................... 40  
Annex E (informative)  Example of how to use the tables in Annex B ....................................................... 65  
Annex F (informative)  Requirements for appropriate calculation procedures ........................................ 67  
F.1 Capabilities of the program ............................................................................................................... 67  
F.2 Input data and results ........................................................................................................................ 67  
F.3 Testing of the program accuracy ...................................................................................................... 69  
F.4 Reference cases.................................................................................................................................. 69  
Annex G (informative)  Evaluation of conformity ......................................................................................... 78  
Annex H (informative)  Alternative procedure for the moisture correction of units with formed 
voids ..................................................................................................................................................... 80  
. DRS 524: 2023 
v                ©RSB 2023- All rights reserved 
 Foreword 
Rwanda Standardsarepreparedby Technical Committees and approved by Rwanda Standards Board (RSB) 
Board of Directors in accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and application of standards. 
The main task of technical committees is to prepare national standards. Final Draft Rwanda Standards 
adopted by Technical committees are ratified by members of RSB Board of Directors for publication and 
gazettment as Rwanda Standards. 
DRS524 was prepared by Technical Committee RSB/TC 9, Civil engineering and Building materials.  
In the preparation of this standard, reference was made to the following standard: 
BS EN 1745:2012 : Masonry and masonry products — Methods for determining thermal properties    :  
The assistance derived from the above source is hereby acknowledged with thanks. 
Committee membership  
The following organizations were represented on the Technical Committee on Civil engineering and Building 
materials (RSB/TC 9) in the preparation of this standard. 
A+Construction Group Ltd 
Africeramics Ltd 
Consultants Engineers Group (CEG) Ltd 
D&D Resources Ltd 
Dutureheza Ltd 
Enabel Rwanda 
Greenpack Africa Ltd 
Integrated Polytechnic Regional Centre (IPRC) - Musanze  
Mass Design Group 
NP Construction Company (NPCC) Ltd DRS 524: 2023 
©RSB 2023- All rights reserved                  vi 
 Road Transport Development Agency (RTDA) 
Rwanda Housing Authority (RHA) 
Rwanda Inspectorate, Competition and Consumer Protection Authority (RICA) 
Rwanda Quarries Association (RQA) 
SKAT Consult  
St Joseph Engineering Company (SJEC) Ltd 
Standards for Sustainability (SfS) 
Stonecraft Industries Ltd 
University of Rwanda - College of Science and Technology (UR - CST) 
Rwanda Standards Board (RSB) – Secretariat 
 DRS 524: 2023 
vii                ©RSB 2023- All rights reserved 
 Introduction 
This Draft Rwanda Standard provides rules for the determination of dry and design thermal conductivity and 
thermal resistance values of masonry products and masonry.  
It describes how dry thermal values are determined. It also describes the correction methods to derive design 
values from a dry value. The dry value is a characteristic of a masonry material, masonry unit or of masonry. 
On the basis of dry thermal conductivity values determination methods of design thermal values are given.  
Three procedures (model S1 – S3) for the determination of dry thermal conductivity ( λ10,dry,unit) of solid 
masonry units are described and five procedures (model P1 – P5) for the determination of equivalent dry 
thermal conductivity ( λ10,dry,unit) of masonry units with formed voids and composite masonry units are 
described, see Figure 1.  
For mortars according to EN 998-1 and EN 998-2, the models S1 – S2 can be used.  
Additionally, three procedures for the determination of thermal resistance are described. These procedures 
are:  
 the use of tabulated R-values;  
 the measurement of R-value;  
 the numerical calculation of R-value.  
The following major types of masonry units are covered by this European Standard:  
 solid masonry units;  
 masonry units with formed voids;  
 composite masonry units.  
In Figure 1, the different models and procedures are illustrated.  
The design value of a product characteristic is the value determined for a specific application and for use in 
calculations.  
Design thermal values are determined, according to the procedure given in this European Standard according 
to the intended application, environmental and climatic conditions, bearing in mind the purpose of this 
determination, such as:  
 energy consumption;  
 design of heating and cooling equipment;  DRS 524: 2023 
©RSB 2023- All rights reserved                  viii 
  surface temperature determination;  
 compliance with national building regulations;  
 consideration of non-steady state thermal conditions in buildings. DRS 524: 2023 
ix                ©RSB 2023- All rights reserved 
  
DRS 524: 2023 
1                ©RSB 2023- All rights reserved 
 Masonry and masonry products — Methods for determining thermal 
properties 
1 Scope 
This Draft Rwanda standards specifies procedures for the determination of thermal properties of masonry and 
masonry products. 
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. 
DRS 526, Natural stone test methods : Determination of real and bulk density and of total and open porosity 
DRS 526, Natural stone test methods — Determination of real and apparent density and total porosity 
ISO 6946, Building components and building elements — Thermal resistance and thermal transmittance — 
Calculation method 
ISO 7345, Thermal insulation — Physical quantities and definitions 
ISO 10211,Thermal bridges in building construction — Heat flows and surface temperatures — Detailed 
calculations 
ISO 10456, Building materials and products — Hydrothermal properties — Tabulated design values and 
procedures for determining declared and design thermal values 
 
3 Terms and definitions 
For the purposes of this standard, the following terms and definitions given in ISO 7345 and the following 
apply. 
3.1 
masonry 
assemblage of masonry units laid in a specified pattern and joined together with masonry mortar Comment [SN1]: Replaced by Annex B  
©RSB 2023- All rights reserved                  2 
 3.2  
masonry product  
masonry units, masonry mortars, rendering and plastering mortars  
3.3  
solid masonry unit  
masonry unit containing no perforations except external indentations such as grip holes, grooves, etc.  
3.4  
masonry unit with formed voids  
masonry unit with a system of intentionally formed voids  
3.5  
composite masonry unit  
masonry unit incorporating one or more layers of additional material to enhance performance  
3.6  
thermal value  
common term for either the thermal conductivity [W/(m·K)] or the thermal resistance [m2·K/W]  
3.7  
dry state  
state after drying under conditions stated in the relevant standards  
3.8  
dry thermal value  
value of a thermal property of a building material or product in a dry state determined according to this 
Rwanda standards as a basis for the calculation of design thermal values  
Note 1 to entry: The dry thermal value can be expressed as thermal conductivity or thermal resistance.  
3.9  
design thermal value  
value of a thermal property of a building material or product under specific external and internal conditions 
which can be considered as typical of the performance of that material or product when incorporated in a 
building component or building  DRS 524: 2023 
3                ©RSB 2023- All rights reserved 
 3.10  
masonry thermal conductivity  
value which is derived by dividing the thickness of a given masonry element by its thermal resistance 
excluding surface resistance  
3.11  
reference conditions  
set of conditions identifying a state of equilibrium selected as the base to which the thermal values of building 
materials and products are referred  
3.12  
equivalent thermal conductivity  
value derived by dividing the width of a masonry unit with formed voids or a composite masonry unit or 
masonry by its thermal resistance excluding surface resistance 
4 Symbols (and abbreviated terms) 
The order of the indices for thermal values is temperature, condition and subject  
 
Symbol  Quantity  Unit 
λ10,dry,mat thermal conductivity at an average temperature of 10 °C in dry state for 
the material  W/(mK)  
λ10,dry,mas  thermal conductivity at an average temperature of 10 °C in dry state for 
the masonry  W/(mK)  
λ10,dry,mor  thermal conductivity at an average temperature of 10 °C in dry state for 
the mortar  W/(mK)  
λ10,dry,unit  thermal conductivity at an average temperature of 10 °C in dry state for 
the unit. For solid units the λ10,dry, unit  is the same as λ10,dry, mat  and for units 
with formed voids and composite units the λ10,dry, unit  is the equivalent 
thermal conductivity.  W/(mK)  
λdesign,mas  design thermal conductivity for the masonry  W/(mK)  
λdesign,mor  design thermal conductivity for the mortar  W/(mͼK)  
λdesign,unit  design thermal conductivity for the unit  W/(mK)   
λi  individual measured or calculated thermal conductivity  W/(mK)  
Ri individual measured thermal resistance  m2ͼK/W   
©RSB 2023- All rights reserved                  4 
 Rdry,mas  thermal resistance of masonry  m2K/W  
Rdesign,mas  design thermal resistance of masonry  m2K/W  
Rsi, Rse internal and external surface resistance  m2K/W  
Rt,mas  the true thermal resistance of the masonry  m2K/W  
amor  percentage area of mortar joint in the measured masonry  %  
aunit percentage area of units in the measured masonry  %  
d  thickness of the masonry  m  
T  temperature  K  
µ water vapour diffusion coefficient   
cp specific heat capacity  J/(kg·K)  
l length of a masonry unit  mm  
w width of a masonry unit  mm  
hunit height of a masonry unit  mm  
hmor thickness of a mortar joint  mm  
Fm moisture conversion factor   
fu moisture conversion coefficient by mass  kg/kg  
fψ moisture conversion coefficient by volume  m3/m3 
u moisture content mass by mass  kg/kg  
ψ moisture content volume by volume  m3/m3 
U10,dry,mas  thermal transmittance of the masonry in dry state   W/(m2.K) 
Umas thermal transmittance of the masonry  W/(m2.K) 
Umor  thermal transmittance of the mortar  W/(m2.K) 
Uunit thermal transmittance of the units  W/(m2.K) 
P fractile of population  %  
ρg,dry  gross dry density  kg/m3 
ρn,dry  net dry density  kg/m3 
v  
 percentage of voids  %  DRS 524: 2023 
5                ©RSB 2023- All rights reserved 
 3.3 Subscripts   
10  average test temperature of 10 °C  
dry  state after drying under conventional conditions as stated in the relevant standards  
mas masonry  
mat   material  
mor   mortar 
unit   masonry unit  
 
5  Procedures to determine λ10,dry, unit-values for solid masonry units and λ10,dry,mor-
values for mortars 
5.1 General 
λ10,dry,unit-values for solid masonry units and λ10,dry,mor-values for mortars are identical to the λ10,dry,matvalues. 
The λ10,dry,mat-values of solid masonry units and of mortars can be determined from tests carried out on 
samples of the material or from tables or graphs which relate λ10,dry,mat to density or from determining the 
thermal transmittance ( Umas) of masonry built from masonry units and mortar. In all cases the λ10,dry,mat-value 
is to be representative of the material.  
5.2 λ10,dry, mat-values for solid masonry units and mortars 
5.2.1 Model S1. Determination of λ10,dry,unit-values from tabulated λ10,dry,mat/net dry density relation 
Tabulated λ10,dry,mat-values for different materials used for masonry products are given in Annex A, 
differentiated by material and dry density. This annex also contains values for the water vapour diffusion 
coefficient, the specific heat capacity and the moisture conversion coefficient.  
These tabulated values are valid for materials where there is factory production control of the net dry density 
but no directly measured λ-values. λ10,dry,mat - values are given as 50 % and 90 % fractiles ( P).  
  
©RSB 2023- All rights reserved                  6 
 5.2.2 Model S2. Determination of λ10,dry,unit-values based on λ10,dry,mat /net dry density curve 
5.2.2.1 General 
To determine a λ10,dry,mat-value from a λ10,dry,mat /net dry density relationship the following procedure shall be 
used:  
5.2.2.2 Test specimens 
Test specimens shall be in accordance with the requirements of Annex C . Care should be taken that the test 
specimens are representative of the masonry product itself.  
NOTE  An appropriate way to ensure this is to cut specimens from masonry units.  
5.2.2.3 Conditioning of specimens  
Normally masonry materials are tested in a dry condition. It is also possible to carry out tests in a moist 
condition (e.g. conditioned to constant mass in an environment of (23 ± 2) °C and 50 % ± 5 % relative 
humidity), in which case the measured value has to be converted to the dry state following one of the 
procedures given in Clause 6.  
5.2.2.4 Test measurement  
The reference test method is given in EN 12664. The test shall be carried out at a mean temperature of 10 °C.  
Alternative test methods, which may require different test specimens and different conditioning methods, may 
be used, if the correlation between the reference test method and the alternative method can be given.  
4.2.2.5 Establishing a product related λ10,dry,mat /net dry density-curve  
Three items of information are necessary for this determination procedure:  
1. the tabulated λ10,dry,mat /net dry density-correlation for the given material (see Annex A);  
2. the product net dry density range, which can be derived either from the production history or from the net 
dry density tolerances which are given in the relevant product standards;  
3. at least three individual test measurements of the net dry density and λi , on material which is 
representative for the current material produced. The measurements of net dry density and λ shall be 
carried out on the same specimens. The three tests have to be carried out on specimens from different 
production batches to represent the manufactured product net dry density range. These three 
measurements are used to determine the distance of the individual  
λ10,dry,mat /net dry density-curve, for a defined production, from the tabulated  DRS 524: 2023 
7                ©RSB 2023- All rights reserved 
 λ10,dry,mat/net dry density curve.  
Determine the measured λi-value as prescribed in 5.2.2.1 to 5.2.2.3 and calculate the arithmetic mean value 
of the 3 λi -results. 
Measure the net dry density of each of the three samples following the procedure prescribed in DRS 526 and 
calculate the arithmetic mean value of the 3 results.  
Then use the following procedure.  
Through the point A representing mean thermal conductivity and mean net dry density draw a λ/net dry 
density-curve parallel to the general λ10,dry,mat/net dry density-curve obtained from plotting the tabulated λ- and 
net dry density-values for the product (material) given in Annex A.  
Derive the mean λ-value of the product from the average net dry density. Derive the upper and lower limit 
values as the values that represent 90 % and 10 % of the manufactured product under consideration density 
range with a confidence level of 90 % according to ISO 10456.  
Use the product related λ10,dry, mat/net dry density-curve to determine the λ10,dry,mat-value related to the mean 
net dry density the manufacturer is confident to achieve.  
Express the λ10,dry,unit-values for solid masonry units or the λ10,dry mor-values for mortars as the mean λ10,dry mat-
value together with the difference between the limit and the mean value.  
Figure 2 shows this process in the form of a graph.  Comment [SN2]: I think, since they propose 
more options, we can remain other one option 
and omit others.   
©RSB 2023- All rights reserved                  8 
  
 
NOTE For factory production control purposes thermal conductivity may be controlled from the net dry density of the 
material, see Annex E.  
DRS 524: 2023 
9                ©RSB 2023- All rights reserved 
 5.2.3 Model S3. Procedures to determine λ10,dry,unit-values from determining the thermal 
transmittance ( Umas) of masonry built from solid masonry units and mortar 
To determine a λ10,dry unit-values from test measurements of the thermal transmittance of masonry 
built from masonry units and mortars, the procedure in 6.3.3 shall be used.  
5.3 Test methods and numbers of samples to be taken for the different models 
In the following table test methods and numbers of samples to be taken for the different models is given. 
Table 1 — Test methods and minimum numbers of specimens within the test 
Test methods  Minimum numbers of 
specimens  
Model S1:  
Material density, DRS 526 (natural stone units) Model S2:  
Material density, DRS 526 (natural stone units)  
Thermal conductivity, Annex B  
Gross dry density, DRS 526  (natural stone units)  
Thermal transmittance, Annex B  
6  
 
3  
3  
 
3 × 6  
3  
 
6 Procedures to determine equivalent λ10,dry,unit-values for masonry units with 
formed voids and composite masonry units 
6.1 General 
The thermal properties of masonry units with formed voids cannot fully be determined by the  λ10,dry,mat-value 
of the material, there is also a high influence from the shape and the geometry of the voids in the unit. The 
thermal conductivity of the materials can be derived from tables or measurements.  
The λ10,dry,unit-values of masonry units with formed voids can be determined:  
 from tables;  
 from calculations;  
 from test measurements carried out on masonry samples.  
The λ10,dry,unit-values of composite masonry units can be determined:  
 from calculations;   
©RSB 2023- All rights reserved                  10 
  from measurements carried out on masonry samples.  
6.2 Calculation methods 
There are several different numerical methods in use (e.g. Finite Difference, Finite Element) for the calculation 
of the thermal properties of masonry units with formed voids or composite masonry units. The thermal 
conductivities of the materials and the configuration of the units are necessary input parameters for such 
calculations.  
The requirements for appropriate calculation programs (accuracy, boundary conditions, etc.) are given in 
Annex D. The method described in EN ISO 6946 may also be used.  
6.3 λ10,dry,unit-values of masonry units 
6.3.1 Determination of λ10,dry,unit-values from tabulated λunit /λmat relation 
6.3.1.1  General  
λ10,dry,unit-values used for masonry units with different void patterns are given in Annex B. Annex C provides an 
example of how to use Annex B.  
No tabulated values for composite masonry units are given in Annex B.  
NOTE The types of units shown and the pattern of voids are intended as examples of units typically found on the market. 
They are not intended to cover every size and type of unit or void pattern produced.  
6.3.1.2 Application of Annex B  
Examples for λ10,dry,unit-values of masonry units with formed voids given in Annex B, are 
differentiated by:  
 material;  
 geometry of the units and geometry of formed voids;  
 λ-value of the material of the masonry units;  
Linear interpolation may be used for material conductivities between the values given in the tables in Annex B.  
6.3.1.3 Model P1. The determination of the λ10,dry,unit-value using Annex B using measured thermal 
conductivity of the masonry unit material  
To determine a λ10,dry,unit-value from using Annex B using measured thermal conductivity of the masonry unit 
material, the following procedure shall be used:  DRS 524: 2023 
11                ©RSB 2023- All rights reserved 
 Select the table relevant for the actual units. Express the λ10,dry,unit-value as the value given in the relevant 
table for the λ10,dry,mat-value the manufacturer is confident to achieve. The λ10,dry,mat-value is a measured 
thermal conductivity of the masonry unit material as specified in 4.2.2. 
6.3.1.4 Model P2. The determination of the λ10,dry,unit-value using Annex B using tabulated value 
from Annex A  
To determine a λ10,dry,unit-value from using Annex B using tabulated value from Annex A, the following 
procedure shall be used:  
Select the table relevant for the actual units. Express the λ10,dry, unit-value as the value given in the relevant 
table for the λ10,dry,mat-value the manufacturer is confident to achieve. The λ10,dry,mat-value is a tabulated value 
from Annex A.  
6.3.2 Determination of λ10,dry,unit-values based on calculation 
6.3.2.1  General  
To determine a λ10,dry,unit-value for a masonry unit by calculation methods following 5.2, the following 
procedure shall be used:  
Based on:  
 the geometry of the units;  
 the geometry of formed voids;  
 the λ10,dry,mat-value;  
 the orientation of the unit in use a numerical model of the unit can be established and the thermal 
transmittance can be approximated.  
This method is also suitable for composite masonry units, where the calculation is dealt with 
separately for each layer.  
6.3.2.2 Model P 3. Determination of λ10,dry,unit–values using measured thermal conductivity of the 
masonry unit material  
Express the λ10,dry,unit-value as the result of the calculation using the λ10,dry,mat-value the manufacturer is 
confident to achieve. The λ10,dry,mat-value is a measured thermal conductivity of the masonry unit material as 
specified in 4.2.2.  
©RSB 2023- All rights reserved                  12 
 6.3.2.3 Model P 4. Determination of λ10,dry,unit-values using tabulated value from Annex A  
Express the λ10,dry,unit-value as the result of the calculation using the λ10,dry,mat-value the manufacturer is 
confident to achieve. The λ10,dry,mat-value is tabulated values from Annex A.  
6.3.3 Model P5. Determination of λ10,dry,unit-values from determining the thermal transmittance ( Umas) 
of masonry built from masonry units with formed voids or composite masonry units and mortar 
6.3.3.1 General  
To determine λ10,dry,unit -values from test measurements of the thermal transmittance of masonry built from 
masonry units and mortars, the following procedure shall be used.  
6.3.3.2 Testing procedure  
 Select test samples from 3 different production batches for the product under consideration. Determine 
their mean gross dry density.  
 From each of these batches erect one wall.  
 Measure the thermal transmittance on each of those walls following Annex B . If the measured wall is not 
in a dry state, the measured value has to be converted to the dry state following the procedure given in 
Clause 6. 
6.3.3.3 The determination of the λ10,dry,unit–value  
Calculate the λ 10, dry, mas - value using the equation: λ 10,dry,mas = 𝜆ଵ଴,ௗ௥௬,௠௔௦=ௗ
భ
ೆభబ,೏ೝ೤,೘ೌೞିோೞ೔ିோೞ೐ 
where  
U10,dry,mas  is the thermal transmittance of the masonry in dry state, in W/m2K;  
Rsi, Rse are the internal and external surface resistance in m2ͼK/W according to EN ISO 6946;  
d  is the thickness of the masonry in m; λ10,dry,mas is the thermal conductivity of the masonry in dry 
state in W/(m·K). 
𝜆ଵ଴,ௗ௥௬,௨௡௜௧=100𝑥𝜆ଵ଴,ௗ௥௬,௠௔௦−𝑎௠௢௥𝑥𝜆ଵ଴,ௗ௥௬,௠௢௥
𝑎௨௡௜௧ 
amor  is the percentage area of mortar joint in the measured masonry;  
aunit  is the percentage area of units in the measured masonry;  
λ10,dry,moris the thermal conductivity of the actual mortar joint;  DRS 524: 2023 
13                ©RSB 2023- All rights reserved 
 λ10,dry,unit is the thermal conductivity of the units. 
The thermal conductivity of the mortar joints shall take into account mortar pockets and strip bedding and the 
use of insulating material between the strips.  
If the units are intended to be used with unfilled vertical mortar joints the masonry tested shall also be with 
unfilled joints and the λ10,dry,unit-values for the units will take into account the effect of the unfilled joints 
calculated according to EN 6946.  
Take the 3 individual calculated λ10,dry,unit-values and calculate the arithmetic mean value.  
Measure the gross dry density of each of the three samples taken from each batch of masonry units and 
mortar following the procedure prescribed in DRS 526  and calculate the arithmetic mean value of the 3 
results.  
To the given λ10,dry,mat–values in the relevant table in Annex B find the corresponding net dry densities values 
in Annex A. From the corresponding net dry density values calculate the related gross dry density values 
using the following equation:  
ρ௚,ௗ௥௬=100−𝑉
100ρ௡,ௗ௥௬ 
ρg,dry =100100−vρn,dry  
where    
ρg,dry is the gross dry density in kg/m3; 
ρn,dry is the net dry density in kg/m3;  
v   is the percentage of voids taken from the relevant table in AnnexB.  
Through the point A representing mean thermal conductivity and mean density draw a λ/gross dry density-
curve parallel to the general λ10,dry,unit/gross dry density-curve obtained from plotting the tabulated λ10,dry,unit–
values in Annex B and the corresponding calculated gross dry density-values for the product.  
Use the product related λ10,dry,unit/gross dry density-curve to determine the λ10,dry,unit-value related to the mean 
gross dry density the manufacturer is confident to achieve.  
Figure 3 shows this process in the form of a graph.  
 
 
  
©RSB 2023- All rights reserved                  14 
  
 
 
DRS 524: 2023 
15                ©RSB 2023- All rights reserved 
 NOTE For factory production control purposes thermal conductivity may be controlled from the gross dry density of the 
product, see Annex E.  
6.4 Test methods and numbers of samples to be taken for the different models 
In the following table test methods and numbers of samples to be taken for the different models is 
given.  
6.4.1.1 Table 2 — Test methods and minimum numbers of specimens within the test  
Test methods  Minimum numbers 
of specimens  
Model P1:  
Material density, DRS 526 13 
Thermal conductivity, Annex B  
Material density, DRS 526 :  
Material density, DRS 526 13 
Thermal conductivity, Annex B  
Material density, DRS 526  
Gross dry density, DRS 526  
Thermal transmittance,  ISO 1934   
6  
3  
 
6  
 
3  
3  
 
6  
 
3×6  
3  
 
  
©RSB 2023- All rights reserved                  16 
 7 Moisture conversion 
Design thermal conductivity values/resistance values for masonry units or mortars may be determined using 
one of the following 3 procedures:  
From the λ10,dry-value calculate the corresponding λdesign-value using the moisture conversion coefficient given 
in Annex A for each material and the design moisture content from the tables in  EN ISO 10456 or the 
nationally given design moisture content for a specific material and application.  
Alternatively, moisture conversion coefficients and moisture conversion factors can be derived from tests, 
carried out at several practical moisture contents.  
Procedure 1 (for materials, mortar and solid masonry units):  
𝜆ௗ௘௦௜௚௡=𝜆ଵ଴,ௗ௥௬𝑥𝐹௠              Or alternatively           𝑅ௗ௘௦௜௚௡=ோభబ,೏ೝ೤
ி೘ 
𝐹௠=𝑒௙ೠ ୶ ௨೏೐ೞ೔೒೙                      Or alternatively           𝐹௠=𝑒𝑓𝜓 x 𝜓𝑑𝑒𝑠𝑖𝑔𝑛 
Fm  is the moisture conversion factor      [1];  
fu   is the moisture coefficient by mass      kg/kg;  
udesign  is the design moisture content mass by mass   kg/kg;  
fψ   is the moisture coefficient by volume     m³/m³ ;  
ψdesign         is the design moisture content volume by volume   m³/m³. 
Procedure 2 (for masonry units with formed voids and composite masonry units):  
Moisture conversion has to be carried out for the thermal conductivity of each constituent material according 
to procedure 1, followed by a calculation of the thermal conductivity of the unit according to 5.2.  
For composite masonry units and partially filled units with formed voids the moisture conversion factors of 
each material have to be taken into account.  
Procedure 3 (for masonry units with formed voids):  
As an alternative to procedure 2 an approximate method taking into account the percentage of voids can be 
used. Details of this procedure can be found in informative Annex F.  
 
 DRS 524: 2023 
17                ©RSB 2023- All rights reserved 
 8 Procedures to determine design thermal values ( Rdesign,mas or λdesign, mas ) for 
masonry built from masonry units and mortar 
8.1 General 
Design thermal resistance or design thermal conductivity for masonry may be determined using one 
of following procedures.  
The Rdesign,mas-values or λdesign,mas-values of masonry built from masonry units can be determined 
from calculations, from tables or from tests.  
8.2 Rdesign,mas - or λdesign,mas -values based on calculation 
8.2.1 Rdesign,mas- or λdesign,mas -values based on λdesign-values for the masonry units and the mortar 
Determine the Rdesign,mas- or λdesign,mas–values according to the following procedure:  
Calculate the λdesign, mas -values using the equation 
λ
design,mas =a morλ
design,mor +aunitλ
design,unit  
were 
amor  is the percentage area of mortar joint;  
aunit  is the percentage area of units;  
λdesign,mor is the design equivalent thermal conductivity of the mortar joint;  
λdesign,unit is the design thermal conductivity of the units. 
Calculate the design thermal resistance Rdesign,mas using the equation:  
𝑅𝑑𝑒𝑠𝑖𝑔𝑛,𝑚𝑎𝑠 =d
𝜆ௗ௘௦௜௚௡,௠௔௦ 
where d  is the thickness of the masonry in m. 
 
 
  
©RSB 2023- All rights reserved                  18 
  
8.2.2 Rdesign,mas- or λdesign,mas-values using a numerical calculation method based on the 
design thermal conductivity of the materials used 
There are several numerical methods in use (e.g. Finite Difference, Finite Element) for the calculation of the 
thermal properties of masonry units. The thermal conductivities of the materials as necessary input 
parameters for such calculations shall be the design-value for the masonry product used.  
The requirements for appropriate calculation programs (accuracy, boundary conditions, etc.) are given in 
Annex D.  
The method described in EN ISO 6946 may also be used.  
8.3 Rdesign,mas- or λdesign,mas-values of masonry built from masonry units with formed 
voids or composite masonry units and mortar based on tabulated values 
8.3.1 Tabulated values 
Equivalent λ 10,dry,mas-values for masonry built with units having different void patterns are given in Annex B.  
No tabulated values for composite masonry units are given in Annex B.  
NOTE The types of units shown and the pattern of voids are intended as examples of units typically found on the market.  
They are not intended to cover every size and type of unit or void pattern produced.  
8.3.2 Application of Annex B 
Examples for material λ 10,dry,mat-values for the determination of R dry,mas- or λ 10,dry,mas-values of masonry built 
from masonry units with formed voids are given in Annex B, differentiated by:  
 material;  
 geometry of the units and geometry of formed voids;  
 λ10,dry,mat-value of the material of the masonry units;  
 λ10,dry,mor-value of the mortar.  
The tabulated Rdry,mas- or λ10,dry,mas-values should be taken as the basis for the calculation of any 
national design values, which are dependent on the climatic conditions and corrections for the application 
using Clause 6.  
Linear interpolation may be used for material conductivities between the values given in the tables in Annex B.  
 DRS 524: 2023 
19                ©RSB 2023- All rights reserved 
 8.3.3 Alternative application of Annex B 
The tabulated values have been calculated assuming a specific height and length of the masonry units, a 
specific thickness of the horizontal mortar joints and no mortar in the vertical joints (the "basic dimensions" are 
given for each geometry class). For masonry built from units with a different height, a correction for the mortar 
joints may be taken into account as follows. The same procedure may be used to determine values for 
masonry with vertical mortar joints in those cases where no separate values are given. These methods are 
suitable for all available masonry units.  
Calculate the Umas-value of the masonry from the λmas-value of the masonry in the table using the equation: 
𝑈𝑚𝑎𝑠 =1
𝑅௦௜+ୢ
ఒ೘ೌೞ+𝑅௦௘ 
Where: 
Umas is the thermal transmittance of the masonry, in W/(m2·K);  
Rsi, Rse are the internal and external surface resistance in m2ͼK/W according to EN ISO 6946;  
d is the thickness of the masonry in m;  
λmas is the tabulated value of the thermal conductivity of the masonry in W/(m·K).  
Calculate the thermal transmittance of the masonry units without mortar as follows: 
𝑈𝑢𝑛𝑖𝑡 =𝑈௠௔௦xℎ−𝑈௠௢௥xℎ௠௢௥
ℎ௨௡௜௧ 
Where:  
hunit   is the unit height which was the basis for the calculation of the tabulated value in mm;  
hmor    is the height of the mortar joint which was basis for the calculation of the tabulated value in mm;  
h = hunit + hmor in mm;  
Uunit   is the thermal transmittance of the units without mortar influence in W/(m2 K);  
Umor   is the thermal transmittance of the mortar joint in W/(m2K) given by: 
 
  
©RSB 2023- All rights reserved                  20 
 𝑈𝑚𝑜𝑟 =1
𝑅௦௜+ୢ
ఒ೘೚ೝ+𝑅௦௘ 
λmor is the thermal conductivity of the mortar in W/(m·K).  
Calculate the thermal transmittance of masonry built from units with another height, as follows 
𝑈𝑚𝑎𝑠,𝑎𝑐𝑡 =𝑈௨௡௜௧ x ℎ௨௡௜௧,௔௖௧+𝑈௠௢௥xℎ௠௢௥,௔௖௧
ℎ௔௖௧ 
where  
Umas,act   is the thermal transmittance of masonry made from units with the height hunit in W/m2·K;  
hunit,act     is the actual height of the unit in mm;  
hmor,actis the actual height of the mortar joint in mm;  
hact = hunit, act + hmor,act in mm.  
A similar calculation may be applied for different length of masonry units and thickness of vertical mortar 
joints.  
If there are no vertical mortar joints the differing lengths of the units may be ignored.  
NOTE 1 The thermal transmittance of masonry made from masonry units with a length > 250 mm which have a tongue 
and groove system instead of vertical mortar joints will be lower than the tabulated value, which means that the tabulated 
value is on the safe side. For masonry units with shapes as shown in Figures B.23 to B.28, where the voids run 
continuously over the vertical joint, the length of the unit has no influence on the thermal transmittance.  
NOTE 2 The heat flow direction is indicated in the drawings in Annex B by means of an arrow.  
9 Determination of the thermal transmittance of masonry 
Calculation of the thermal transmittance U shall be made according to EN ISO 6946.  
10 Specific heat capacity 
The thermal mass of the construction has a significant influence on the heating and cooling  
requirements of buildings. Values for the specific heat capacity cp are therefore given in Annex A.  
11 Rounding rules for λ-values for masonry 
The value should be rounded according to EN ISO 10456. DRS 524: 2023 
21                ©RSB 2023- All rights reserved 
 Annex A 
(normative)  
 
Tabulated λ 10,dry,mat-values of materials used for masonry products 
The water vapour diffusion coefficient µ is defined as the factor, which describes how many times higher the 
diffusion resistance of a material layer is, than the resistance of an air layer with the same thickness under the 
same conditions. To compare the diffusion resistance of two building elements, it is necessary to multiply the 
µ-factor by the thickness of the respective layer, which leads to a figure with the dimension m. The diffusion 
behaviour is different, whether it is diffusion into a building component (lower values) or out of the building 
component (drying period, higher value). 
Table A.1 — Clay units (fired clay)  
Density of the material 
(net dry density)  λ10,dry,mat  [W/(m·K)] Water vapour  
diffusion coefficient  Specific heat capacity 
cp 
[kg/m3]  P = 50 % a P = 90 %  µ [J/(kg·K)]  
1 000  0,20  0,27  5/10  1 000  
1 100  0,23  0,30  5/10  1 000  
1 200  0,26  0,33  5/10  1 000  
1 300  0,30  0,36  5/10  1 000  
1 400  0,34  0,40  5/10  1 000  
1 500  0,37  0,43  5/10  1 000  
1 600  0,41  0,47  5/10  1 000  
1 700  0,45  0,51  5/10  1 000  
1 800  0,49  0,55  5/10 b 1 000  
1 900  0,53  0,60  5/10 b 1 000  
2 000  0,58  0,64  5/10 b 1 000  
2 100  0,62  0,69  5/10 b 1 000  
2 200  0,67  0,74  5/10 b 1 000  
2 300  0,72  0,79  5/10 b 1 000  
2 400  0,77  0,84  5/10 b 1 000   
©RSB 2023- All rights reserved                  22 
 fψ = 10 (m3/m3)  
Calculations in support of the Energy Performance of Buildings Directive are related to buildings and not to individual products. U-
values used in such calculations are based on the mean thermal resistance of masonry elements. Therefore the recommended 
material λ value is the 50 % fractile.  
For clay materials with a density between 1 800 kg/m3 and 2 400 kg/m3 used as facing materials the µ-value is 50/100 instead of 
5/10. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 DRS 524: 2023 
23                ©RSB 2023- All rights reserved 
 Table A.2 — Calcium silicate units  
Density of the material  
(net dry density)  λ10,dry,mat  [W/(m·K)] Water vapour  
diffusion coefficient  Specific heat capacity 
cp 
[kg/m3]  P = 50 % a P = 90 %  µ [J/(kg·K)]  
900  0,22  0,29  5/10  1 000  
1 000  0,24  0,30  5/10  1 000  
1 100  0,26  0,32  5/10  1 000  
1 200  0,30  0,36  5/10  1 000  
1 300  0,34  0,41  5/10  1 000  
1 400  0,40  0,46  5/10  1 000  
1 500  0,47  0,53  5/25  1 000  
1 600  0,55  0,61  5/25  1 000  
1 700  0,64  0,70  5/25  1 000  
1 800  0,75  0,81  5/25  1 000  
1 900  0,86  0,92  5/25  1 000  
2 000  0,98  1,05  5/25  1 000  
2 100  1,14  1,20  5/25  1 000  
2 200  1,31  1,37  5/25  1 000  
2 300  1,49  1,56  5/25  1 000  
2 400  1,68  1,76  5/25  1 000  
fψ = 10 (m3/m3)  
a Calculations in support of the Energy Performance of Buildings Directive are related to buildings and not to individual products. 
U-values used in such calculations are based on the mean thermal resistance of masonry elements. Therefore the recommended 
material λ value is the 50 % fractile.  
Table A.3 — Dense aggregate concrete units and manufactured stone units  
Density of the material 
(net dry density)  λ10,dry,mat  [W/(m·K)] Water vapour 
diffusion coefficient  Specific heat 
capacity cp 
[kg/m3]  P = 50 % a P = 90 %  µ [J/(kg·K)]  
1 600  0,69  0,88  5/15  1 000  
1 700  0,75  0,93  5/15  1 000  
1 800  0,82  1,01  5/15  1 000   
©RSB 2023- All rights reserved                  24 
 1 900  0,90  1,09  5/15  1 000  
2 000  1,00  1,19  5/15  1 000  
2 100  1,11  1,30  5/15  1 000  
2 200  1,24  1,42  30/100  1 000  
2 300  1,37  1,56  50/150  1 000  
2 400  1,52  1,72  50/150  1 000  
fψ = 4 (m3/m3)  
a Calculations in support of the Energy Performance of Buildings Directive are related to buildings and not to individual products. 
U-values used in such calculations are based on the mean thermal resistance of masonry elements. Therefore the recommended 
material λ value is the 50 % fractile. 
 
Table A.4 — Concrete units with no other aggregate than pumice  
Density of the material  
(net dry density)  λ10,dry,mat  
[W/(m·K)] Water vapour  
diffusion coefficient  Specific heat capacity 
cp 
[kg/m3]  P = 50 % a P = 90 %  µ [J/(kg·K)]  
 500  0,11  0,14  5/15  1 000  
 600  0,13  0,16  5/15  1 000  
 700  0,16  0,18  5/15  1 000  
 800  0,19  0,21  5/15  1 000  
 900  0,22  0,24  5/15  1 000  
1 000  0,26  0,28  5/15  1 000  
1 100  0,30  0,32  5/15  1 000  
1 200  0,34  0,36  5/15  1 000  
1 300  0,38  0,41  5/15  1 000  
fψ = 4 (m3/m3)  
a Calculations in support of the Energy Performance of Buildings Directive are related to buildings and not to individual products. 
U-values used in such calculations are based on the mean thermal resistance of masonry elements. Therefore the recommended 
material λ value is the 50 % fractile. 
 
 
 DRS 524: 2023 
25                ©RSB 2023- All rights reserved 
 Table A.5 — Concrete units with polystyrene aggregate  
Density of the material 
(net dry density)  λ10,dry,mat  [W/(m·K)] Water vapour  
diffusion coefficient  Specific heat capacity 
cp 
[kg/m3]  P = 50 % a P = 90 %  µ [J/(kg·K)]  
 500  0,13  0,16  5/15  1 000  
 600  0,14  0,19  5/15  1 000  
 700  0,17  0,22  5/15  1 000  
 800  0,18  0,25  5/15  1 000  
fψ = 5 (m3/m3)  
a Calculations in support of the Energy Performance of Buildings Directive are related to buildings and not to individual products. 
U-values used in such calculations are based on the mean thermal resistance of masonry elements. Therefore the recommended 
material λ value is the 50 % fractile.  
Table A.6 — Concrete units with expanded clay aggregate  
Density of the material 
(net dry density)  λ10,dry,mat  [W/(m·K)] Water vapour  
diffusion coefficient  Specific heat capacity 
cp 
[kg/m3]  P = 50 % a P = 90 %  µ [J/(kg·K)]  
 400  0,10  0,12  5/15  1 000  
 500  0,12  0,15  5/15  1 000  
 600  0,16  0,18  5/15  1 000  
 700  0,19  0,21  5/15  1 000  
 800  0,22  0,25  5/15  1 000  
 900  0,26  0,28  5/15  1 000  
1 000  0,30  0,32  5/15  1 000  
1 100  0,34  0,36  5/15  1 000  
1 200  0,39  0,41  5/15  1 000  
1 300  0,43  0,46  5/15  1 000  
1 400  0,48  0,51  5/15  1 000  
1 500  0,53  0,56  5/15  1 000  
1 600  0,60  0,63  5/15  1 000  
1 700  0,67  0,70  5/15  1 000  ed 
co
py
: 
Vo
ca
tio
na
l 
Tr
ad
in
g 
C
ou
nc
il, 
Vo
ca
tio
na
l 
Tr
ai
ni
ng 
C
ou
nc
il, 
Ve
rsi
on 
co
rre
ct 
as 
of 
31
/0
8/
20
12 
00
:2
4, 
(c) 
Th
e 
Bri
tis
h 
St
an
da
rd
s 
In
sti
tut
io
n 
20
12  
©RSB 2023- All rights reserved                  26 
 fu = 4 (kg/kg) if expanded clay is the predominant aggregate  fu = 2,6 (kg/kg) if expanded clay is the only aggregate  
a Calculations in support of the Energy Performance of Buildings Directive are related to buildings and not to 
individual products. U-values used in such calculations are based on the mean thermal resistance of masonry 
elements. Therefore the recommended material λ value is the 50 % fractile.  
Table A.7 — Concrete units with more than 70 % expanded blast-furnace slag aggregate a 
Density of the material  
(net dry density)  λ10,dry,mat  
[W/(m·K)]  Water vapour  
diffusion coefficient  Specific heat capacity 
cp 
[kg/m3]  P = 50 % b P = 90 %  µ [J/(kg·K)]  
1 100  0,19  0,21  5/15  1 000  
1 200  0,23  0,24  5/15  1 000  
1 300  0,28  0,29  5/15  1 000  
1 400  0,33  0,34  5/15  1 000  
1 500  0,39  0,40  5/15  1 000  
1 600  0,45  0,47  5/15  1 000  
1 700  0,52  0,54  5/15  1 000  
fu = 4 (kg/kg)  
A lightweight aggregate produced by the expansion of molten blast-furnace slag with water. Blast-furnace slag is a 
by-product of the extraction of iron haematite ores.  
Calculations in support of the Energy Performance of Buildings Directive are related to buildings and not to 
individual products. U-values used in such calculations are based on the mean thermal resistance of masonry 
elements. Therefore the recommended material λ value is the 50 % fractile.  
Table A.8 — Concrete units with the predominant aggregate derived from pyroprocessed colliery 
material 
Density of the material  
(net dry density)  λ10,dry,mat  [W/(m·K)] Water vapour  
diffusion coefficient  Specific heat capacity 
cp 
[kg/m3]  P = 50 % a P = 90 %  µ [J/(kg·K)]  
1 100  0,31  0,35  5/15  1 000  
1 200  0,33  0,37  5/15  1 000  
1 300  0,35  0,39  5/15  1 000  
1 400  0,37  0,41  5/15  1 000  
1 500  0,39  0,43  5/15  1 000  ed 
co
py
: 
Vo
ca
tio
na
l 
Tr
ad
in
g 
C
ou
nc
il, 
Vo
ca
tio
na
l 
Tr
ai
ni
ng 
C
ou
nc
il, 
Ve
rsi
on 
co
rre
ct 
as 
of 
31
/0
8/
20
12 
00
:2
4, 
(c) 
Th
e 
Bri
tis
h 
St
an
da
rd
s 
In
sti
tut
io
n 
20
12 DRS 524: 2023 
27                ©RSB 2023- All rights reserved 
 fu = 4 (kg/kg)  
a Calculations in support of the Energy Performance of Buildings Directive are related to buildings and not to 
individual products. U-values used in such calculations are based on the mean thermal resistance of masonry 
elements. Therefore the recommended material λ value is the 50 % fractile  
   
©RSB 2023- All rights reserved                  28 
 Table A.9 is to be used for concrete units with lightweight aggregates, where no history for λexists (e.g. for 
new products). Therefore, no 50 % and 90 % fractiles ( P) can be given, the given λ-values are to be 
understood as safe values for all different types of aggregates.  
Table A.9 — Concrete units with other lightweight aggregates 
Density of the material  
(net dry density)  λ10,dry,mat a Water vapour diffusion 
coefficient  Specific heat  
capacity cp 
[kg/m3]  [W/(m·K)]  µ [J/(kg·K)]  
500  0,24  5/15  1 000  
600  0,27  5/15  1 000  
700  0,30  5/15  1 000  
 800  0,33  5/15  1 000  
 900  0,37  5/15  1 000  
1 000  0,41  5/15  1 000  
1 100  0,46  5/15  1 000  
1 200  0,52  5/15  1 000  
1 300  0,58  5/15  1 000  
1 400  0,66  5/15  1 000  
1 500  0,74  5/15  1 000  
1 600  0,83  5/15  1 000  
1 800  1,08  5/15  1 000  
2 000  1,33  5/15  1 000  
fψ = 4 (m3/m3)  
a Calculations in support of the Energy Performance of Buildings Directive are related to buildings 
and not to individual products. U-values used in such calculations are based on the mean thermal 
resistance of masonry elements. Therefore, the recommended material λ value is the 50 % fractile.  
 
Table A.10 — Autoclaved aerated concrete units  
Density of the material  
(net dry density)  λ10,dry,mat  
[W/(m·K)]  Water vapour  
diffusion coefficient  Specific heat capacity 
cp DRS 524: 2023 
29                ©RSB 2023- All rights reserved 
 [kg/m3]  P = 50 % a P = 90 %  µ [J/(kg·K)]  
 300  0,072  0,085  5/10  1 000  
 400  0,096  0,11  5/10  1 000  
 500  0,12  0,13  5/10  1 000  
 600  0,15  0,16  5/10  1 000  
 700   0,17  0,18  5/10  1 000  
 800  0,19  0,21  5/10  1 000  
 900  0,22  0,24  5/10  1 000  
1 000  0,24  0,26  5/10  1 000  
fu = 4 (kg/kg)  
a Calculations in support of the Energy Performance of Buildings Directive are related to buildings and not to 
individual products. U-values used in such calculations are based on the mean thermal resistance of masonry 
elements. Therefore the recommended material λ value is the 50 % fractile.  
 
Table A.11 — Natural stone units a 
 
Type of stone  Density of the  
material  
(net dry 
density)  λ10,dry,mat  Water vapour diffusion 
coefficient µ  Specific 
heat  
capacity  
 c p 
 [kg/m3]  [W/(m·K)]  
 dry   wet  [J/(kg·K)]  
1. METAMORPHIC AND PLUTONIC ROCKS  
 
Gneiss, porphyry  
Marble  
Granites  
Shale, slates  2 300 to 2 900  3,5  10 000  10 000  
1 000  
2 600 to 2 800  3,5  10 000  10 000  
1 000  
2 500 to 2 700  2,8  10 000  10 000  
1 000  
2 000 to 2 800  2,2  1 000  800  
1 000  
2. VOLCANIC ROCKS    
 
Basalt  
Trachytes, andesites  
Vacuole lava  2 700 to 3 000  1,6  10 000  10 000  
1 000  
2 000 to 2 700  1,1  20  15  
1 000  
 
©RSB 2023- All rights reserved                  30 
 ≤ 1 600  0,55  20  15  
1 000  
3. LIMESTONE    
 
- very hard stone  2 200 to 2 590  2,3  250  200  
1 000  
- hard rock   2 000 to 2 190  1,7  200  150  
1 000  
- compact stone  1 800 to 1 990  1,4  50  40  
1 000  
- soft stone  1 600 to 1 790  1,1  40  25  
1 000  
- very soft stone  ≤ 1 590  0,85  30  20  
1 000  
4. SANDSTONE    
 
- Quartz sandstone  2 600 to 2 800  2,6  40  30  
1 000  
- Siliceous sandstone  2 200 to 2 590  2,3  40  30  
1 000  
- Calciferous sandstone  2 000 to 2 700  1,9  30  20  
1 000  
5. FLINT, MILLSTONE, PUMICE    
 
- Flint  2 600 to 2 800  2,6  10 000  10 000  
1 000  
- Millstone  1 900 to 2 500  1,8  50  40  
1 000  
- Millstone  1 300 to 1 900  0,9  30  20  
1 000  
- Natural pumice  ≤ 400  0,12  8  6  
1 000  
a For these materials the 50 % / 90 % approach is not applicable.   
 
Table A.12 — Mortar (masonry mortar and rendering mortar)  
Density of the material 
(net dry density)  λ10,dry,mat  
[W/(m·K)] Water vapour  
diffusion coefficient  Specific heat capacity 
cp 
[kg/m3]  P = 50 % a P = 90 %  µ [J/(kg·K)]  
200  0,074  0,081  5/20  1 000  
300  0,086  0,094  5/20  1 000  
400  0,10  0,11  5/20  1 000  DRS 524: 2023 
31                ©RSB 2023- All rights reserved 
 500  0,12  0,13  5/20  1 000  
600  0,14  0,15  5/20  1 000  
700  0,16  0,17  5/20  1 000  
800  0,18  0,20  5/20  1 000  
900  0,21  0,23  5/20  1 000  
1 000  0,25  0,27  5/20  1 000  
1 200  0,33  0,36  5/20  1 000  
1 400  0,45  0,49  5/20  1 000  
1 600  0,61  0,66  15/35  1 000  
1 800  0,82  0,89  15/35  1 000  
2 000  1,11  1,21  15/35  1 000  
fψ = 4 (m3/m3)  
a Calculations in support of the Energy Performance of Buildings Directive are related to buildings and not to individual products. 
U-values used in such calculations are based on the mean thermal resistance of masonry elements. Therefore the recommended 
material λ value is the 50 % fractile. 
 
 
 
 
 
 
 
 
 
 
 
  
©RSB 2023- All rights reserved                  32 
 Annex B 
(normative)  
 
Determination of thermal resistance by hot box method using heat flow 
meter — Masonry 
B.1 Test procedure 
B.1.1 Applicability of the method 
The applicability of the method to a particular specimen should be assessed, see also the introduction. 
Layered specimens with highly conducting facings and a core material of low thermal conductivity may induce 
a lateral heat flow larger than permitted by 6.2.1, hence measurements of the individual layer sand 
subsequent calculations on the whole structure may be necessary. 
Simplified calculations are also a suitable means to assess measured values and related accuracy. 
B.1.2 Conditioning of the specimen 
Specimens shall be stored for a sufficient period of time to achieve an even moisture distribution within them. 
Three conditioning alternatives apply. 
a) Dry specimen: as a general rule, drying shall be done at 105 8C until constant mass is reached. For 
specimens containing materials susceptible to property changes at this temperature, the drying should be at 
70 8C (e.g. polystyrene plastic foams) or at 40 8C (e.g. materials containing gypsum). The specimen shall 
then be allowed to cool before starting the test. To keep the specimen dry it may be necessary to enclose the 
specimen in a vapour-tight envelope. 
b) Conditioning under a temperature gradient: the test specimen shall be conditioned at the same boundary 
conditions (temperature and relative humidity) as those foreseen for the hot box test. The conditioning shall 
continue until moisture equilibrium is reached. 
c) Conditioning to a specified moisture content: the test specimen shall be conditioned at (23 ± 2) 8C and a 
certain relative humidity (not exceeding 95 %, in order to stay in the hygroscopic range), until constant mass is 
reached. After conditioning, it may be necessary to enclose the test specimen in a vapour-tight envelope. 
For measurements on masonry walls in a moist condition, according to options b) and c) of this subclause, the 
range of moisture content of the specimens should correspond to the practical moisture content or at least not 
exceed this value by more than 3 % by volume. 
To keep the influence of moisture redistribution during the test within acceptable limits, the duration of the 
measurements shall be kept as short as possible, 
i.e. testing should be finished as soon as the thermal steady-state condition is reached. DRS 524: 2023 
33                ©RSB 2023- All rights reserved 
 The mass of the specimen before and after the test shall be reported, or core samples shall be taken before 
and after the test. 
The dry bulk densities of all building materials are to be determined. The size and bulk density of masonry 
have to be examined in accordance with the relevant product standard. For masonry mortar, the dry bulk 
density shall be measured; for light masonry material both the bulk density and mortar density shall be 
measured. The bulk density of concrete and similar materials shall be examined in accordance with the 
respective product standards. Aggregates and their mixing ratio and grain size distribution shall be measured. 
For loose fill insulation materials, the bulk density in the dry state and, if possible, grain size distribution are to 
be determined in accordance with respective product standards. 
B.1.3 Specimen selection and mounting 
The test specimen shall be selected or constructed in such a way that it is representative and according to 
common practice; areas likely to have different surface temperatures should be included in the metering 
section in a representative way. For specimens of this type, the local density of heat flow rate can be 
estimated by preliminary additional measurements before the installation of the heat flow meter. For this 
purpose small size heat flow meters can be used whose thermal resistances do not significantly alter the local 
density of heat flow rate (thickness: less than 1 mm) and whose areas correspond to the influence areas of 
the thermal bridges. Also thermographic inspections can help in evaluating inhomogeneities. Specimens not 
meeting the homogeneity criteria defined in 5.3.2, see also 6.2.1 and 6.3.2, shall be tested according to 
another method or calculations shall be undertaken. 
If the specimen is modular, the metering section should be an integral multiple of the module. The edge of the 
metering section should either coincide with the module lines or be midway between module lines. Specimens 
should be tested without plaster. Air flow through the specimen shall be avoided by sealing the surfaces. 
To ensure perfect contact with the heat flow meter and guard section cover sheet, specimens with coarse-
pored or uneven surfaces have to be smoothed with plaster or other suitable material, having the minimum 
thickness necessary to achieve flatness. 
The specimen shall be mounted or sealed in such a way that neither air nor moisture will gain ingress into the 
specimen from the edges or pass from the hot side to the cold side or vice versa. 
It is necessary to control the dew-point of the air on the hot side, to ensure that in the selected testing 
condition no condensation will take place on the hot side of the specimen. 
It should be considered whether continuous cavities in the specimen require barriers at the periphery of the 
metering section, and whether high conductivity facings should be cut at the perimeter of the metering section. 
Surface temperature sensors shall be mounted in such a way that there is no air pocket between the sensor 
and the specimen surface. The sensors have to be mounted such as to avoid heat flow rates to or from the 
metering section. 
The emissivities of sensors and specimen surfaces should be equal. Surface temperature sensors between 
the heat flow meter (and guard section sheet) and the specimen shall be mounted as indicated in 6.3.2. 
The heat flow meter and the guard sheet shall be glued to the hot side surface of the specimen or shall be 
installed with a two-sided adhesive foil, avoiding the formation of any air pocket. The glue shall be a thin layer 
not adding a significant thermal resistance, shall not penetrate deeply into the specimen, shall avoid the  
©RSB 2023- All rights reserved                  34 
 presence of any air pocket between the specimen and the heat flow meter and shall allow the disassembling 
of the heat flow meter without causing severe stresses to it that could cause a change in the heat flow meter 
calibration characteristics. 
In general, test specimens are in a vertical position. If results can be influenced by convection (for specimens 
with hollow cores), tests shall be conducted in the position that corresponds to the practical end use 
application. 
B.2 Measurement 
B.2.1 Test conditions 
Test conditions shall be chosen to suit the product standard requirements or specific end use applications, 
having in mind the effect of testing conditions on accuracy. Both mean test temperature and temperature 
differences affect the test results. NOTE Mean temperatures of 10 8C to 20 8C and a difference of at least 20 
K are common in building applications. 
The moisture content of the specimen during the measuring period is determined by recording its mass before 
and after measurement and in the dry state after the test has been completed. The specimen can also be 
broken down into its constituents, and the individual building materials can be dried at their allowed maximum 
temperatures. 
B.2.2 Measurement period 
The required time to reach stability for steady-state tests depends upon such factors as thermal resistance 
and thermal capacity of the specimen, surface coefficient of heat transfer, presence of mass transfer and/or 
moisture redistribution within the specimen, type and the performance of automatic controllers associated with 
the apparatus. Due to variation of these factors, it is impossible to give a single criterion for steady-state. 
To ensure that steady-state conditions have been reached, measurements of some relevant temperatures and 
the density of heat flow rate shall be plotted versus time to detect any monotonic variation. 
B.2.3 Calculations 
This test method allows the measurement of the specimen surface-to-surface thermal resistance, Rt. 
Rt = (Tsi 2 Tse)/q (2) 
From this measured quantity and the surface resistances according to EN ISO 6946 the heat transfer 
properties are calculated according to the following equations: 
RT = Rsi + Rt + Rse (3) 
U = 1/RT (4) 
If the surface-to-surface thermal resistance Rt of a masonry test specimen is determined according to this 
standard for at least three levels of moisture content, a value Rt* of the surface-to-surface thermal resistance 
for each defined practical moisture content (moisture condition under national service conditions) and a DRS 524: 2023 
35                ©RSB 2023- All rights reserved 
 moisture factor describing the influence of moisture content on the thermal properties of the tested material, 
can be derived. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
©RSB 2023- All rights reserved                  36 
 Annex C 
(normative)  
 
Test specimen 
C.1 General 
Testing may be split into specimen handling, see below, and actual testing procedure, see clause 7. Some 
decisions about measurable heat transfer properties, specimen handling and testing conditions shall be taken 
when starting testing, see A.5. Directions on these decisions shall only be sought in this standard and/or in the 
relevant product standard applicable to the specimen to be tested. 
C.2 Selection and size 
One or two specimens shall be selected (from each sample) according to the type of apparatus (see 5.2.2 or 
5.2.3 for guarded hot plate apparatus and 5.3.1 for heat flow meter apparatus). The specimen or specimens 
shall meet the general requirements outlined in A.3 and A.4. When two specimens are required they shall be 
as identical as possible with thicknesses differing by less than 2 %. The specimen or specimens shall be of 
such size as to cover the heating unit surfaces completely (including the guard section), without exceeding the 
overall linear dimension of the heating unit or heat flow meter by more than 3 %. They shall have a thickness 
according to the relevant product standard and additionally the relationship between the thickness of the test 
specimen used and the dimensions of the heating unit shall be restricted so as to limit the sum of the 
imbalance error (guarded hot plate apparatus only) and edge heat loss errors to 0,5 %, see thickness limits of 
Table A.1 in A.3. For the minimum specimen thickness see A.3.4 and Tables A.1 and A.2. 
C.3 Specimen preparation and instrumentation 
C.3.1 Conformity with product standards 
The preparation of the specimens shall be in accordance with the appropriate product standard; unless 
otherwise specified in product standards, the general criteria in 6.3.2 and 6.3.3 should be fulfilled. 
C.3.2 All specimens except loose-fills 
C.3.2.1 Preparation 
As building materials and in particular masonry materials exist in many forms, e.g. bricks, blocks and cast 
slabs, use the method appropriate to the particular form available, e.g. edge to edge gluing of smaller pieces 
of similar density, (but never with joint surfaces parallel to the faces of the specimen) machining directly from 
larger pieces or casting directly from a mix. In all cases, ensure that there is a high degree of flatness of the 
test specimen and that other dimensions are within the tolerances specified. 
When edge-to-edge joining or joint gluing is necessary, make the joints between machined surfaces 
perpendicular to the main specimen surfaces and preferably symmetrical with respect to the line passing 
through the centre of the specimen and perpendicular to its main surfaces. Keep the number of pieces used to 
the minimum dictated by the module size of the product, especially in the metering DRS 524: 2023 
37                ©RSB 2023- All rights reserved 
 area. Ensure that, within the metering area, the total joint cross-section does not exceed 0,1 % of the metering 
area. Use a cold setting adhesive, such as an epoxy or polyester resin for bonding; adhesives approaching 
the specimen conductivity are preferred. Apply the adhesive to the mating surfaces avoiding impregnation as 
far as possible. After bonding, either grind flat the faces of eachspecimen using a surface grinder or milling 
machine, or hand finish the faces by rubbing with abrasive paper fixed to an engineer’s surface plate. 
Cure cement and masonry materials from which specimens are to be prepared for 28 days prior to testing and 
record the date of manufacture. Specimens may initially contain a large amount of water as a consequence of 
manufacture and preparation. Great care shall be taken in selecting testing conditions with reference to 
moisture content in order to eliminate mass transfer during the test, or establish conditions for which mass 
transfer effects are repeatable and well understood, see 7.2.2 and 7.2.3. 
The surface of the test specimens shall be made plane by appropriate means (sandpapering, face-cutting in a 
lathe, and grinding are often used), so that close contact between the specimens and apparatus or interposed 
sheets can be effected. 
For rigid materials, the faces of the specimens shall be made as flat as the apparatus surfaces (see A.3.6.2) 
and shall be parallel over the total surface area within 2 % of the specimen thickness. 
The planeness of the surfaces can be checked with, for example, a good quality engineer's straightedge 
(straight to 0,01 mm) held against the surface and viewing at grazing incidence with a light behind the 
straightedge. Departures as small as 25  m are readily visible. Large departures can be measured using feeler 
gauges and the straightedge as follows. The straightedge should be supported on a gauge block of known 
thickness, say 1 mm, at each end of the surface to be checked. 
Positive and negative deviations can be measured using feeler gauges along a straight line. Eight straight 
lines should be investigated as follows: the four edges of the surface, the two diagonals and a central cross 
(two lines parallel to the edges of the surface). When this procedure, applicable to both apparatus and 
specimen surfaces, is applied to specimen checking, it should be repeated for each face of the specimen. 
Scratches, chips or similar defects over and above the naturally occurring surface irregularities in the finished 
surfaces of cellular or aggregate materials are accepted provided that the total of their surface areas is an 
acceptable fraction of the metering area and that their maximum depth is an acceptable fraction of the 
specimen thickness, so as to keep the added thermal resistance due to the corresponding air pockets low. For 
the purpose of this standard: 
 if (Ad/Am)(Ra/R)< 0,000 5 the effect may be ignored; 
 if 0,000 5 ≤ (Ad/Am)(Ra/R)≤  0,005 the test may be undertaken, but the presence of the defectshall be 
mentioned in the test report; 
where: 
Adis the overall cross-sectional area of the defects; 
Amis the area of the metering section; 
Rais the thermal resistance of an air layer of thickness equal to the maximum depth of anydefect;  
©RSB 2023- All rights reserved                  38 
 Ris the thermal resistance of the specimen. 
C.3.2.2 Selection and installation of contact sheets 
Imperfect apparatus and/or specimen flatness produces contact thermal resistances, see in A.3.6.2 the 
maximum allowed limit when testing rigid specimens without contact sheets. 
When the specimens’ thermal resistance is less than 0,3 m2·K/W, or if their flatness does not meet the 
requirements of A.3.6.2, thin sheets of an adequately compressible material shall be inserted between the 
specimen surfaces and the plates of the apparatus to establish good thermal contact between them. The thin 
sheets shall also insulate electrically the thermocouples which are then to be placed on the specimen surfaces 
to determine the temperature difference across the specimen (see 6.3.2.3). 
If all other requirements are met (homogeneity, compressibility, etc.), sheets of the highest thermal 
conductivity material available should be chosen. When using contact sheets, the thermal resistance of the 
sheets should be the smallest compatible with the elimination of air pockets. 
NOTE 1 Foamed silicone rubber of density about 600 kg/m3 and thickness about0,5 % to 1 % of the overall apparatus 
size (typically 2 to 3 mm for medium size apparatus)have been found to meet the requirements satisfactorily. 
Sufficient clamping pressure, in excess of 10 kPa, is required to produce uniform thermal contact between 
apparatus surfaces, thermocouples, thermal contact sheets and specimens. 
NOTE 2 These high pressures can damage the heat flow transducer of the heat flow meter apparatus, and thus the 
guarded hot plate apparatus is preferred for tests on specimens having a thermal resistance less than 0,3 m2·K/W. 
The use of contact sheets introduces the errors described in A.3.6.3. When the contact resistance is deemed 
to be too high, an alternative solution to the use of contact sheets is to improve the surface finish of the 
specimen and/or that of the plates of the apparatus. 
NOTE 3 The lowest measurable thermal resistance according to B.5, is 0,02 m2·K/W (e.g. 0,04 m of structural concrete), 
but the overall accuracy of 2 % around room temperature may be achieved only when the specimen thermal resistance is 
equal to or greater than 0,1 m2·K/W. 
C.3.2.3 Thermocouples mounted on the specimen 
When contact sheets are used, thermocouples mounted on the specimen surfaces, or cemented in shallow 
grooves accurately machined to a known depth in the specimen surfaces, shall be used to measure the 
temperature difference through the specimens. The number of uniformly distributed thermocouples on each 
side of the specimen in the area corresponding to the metering section of the apparatus should be not less 
than N A , or 2, whichever is greater, where N = 10 m-1 and A is the area in square metres of one side of the 
metering section. For the error in the temperature difference when using contact sheets and thermocouples 
mounted on the specimen, see A.3.6.3. 
It is recommended that at least two more thermocouples are added on each side of the specimen in the area 
corresponding to the metering section of the apparatus. 
Thermocouples mounted directly into the surfaces shall either be: 
a) flattened fine wire, or DRS 524: 2023 
39                ©RSB 2023- All rights reserved 
 b) thin foil types which may be bought ready-made, or 
c) prepared by rolling or pressing the junctions together with about 20 mm of the adjoining wire of 
conventional thermocouples. 
The thermocouples shall be fabricated from a stock of calibrated thermocouple wire, or from wire that has 
been certified by the manufacturer to comply with Table B.1 of ISO 8302:1991, or they shall be individually 
calibrated. 
NOTE 1 It is important to establish good contact between the junctions and the specimen surfaces. In some 
circumstances it will be necessary to improve the mounting area locally to achieve this. This can be done effectively and 
with minimal loss of accuracy by smoothing the surface locally using a machinable, quick-drying filling compound using a 
thickness of less than 0,5 mm. 
The thermocouples shall be taped in position on these prepared surface areas (approximately 15 mm to 20 
mm in diameter) using narrow strips of adhesive tape about 2 mm to 4 mm wide placed some 5 mm from the 
junction itself. 
NOTE 2 Wider strips of tape can be used in the guard area to insulate and protect the wire around the edges of the 
apparatus. 
NOTE 3 The clamping pressure exerted on the thermal contact sheets ensures that the thermocouples are firmly in 
contact with the specimen surfaces during the test. However, when practicable (i.e. if the prepared surface is non-
absorbent), a smear of thermoconductive compound loaded with zinc oxide can be introduced between the thermocouple 
junctions and the surfaces to further improve thermal contact. 
C.3.3 Loose-fill materials 
When testing loose-fill materials, the thickness of the specimen shall be at least 10 times the mean dimension 
of the beads, grains, flakes, etc. of the loose-fill material. To prepare the specimen(s) it is recommended that a 
representative portion, slightly greater than the amount needed for the test, be taken from the sample and 
weighed before and after it has been conditioned as in 7.2, where applicable. From these masses the 
percentage mass loss is calculated. An amount of the conditioned material is weighed out such that it will 
produce one (two) specimen(s) of the desired density using the procedure described in the relevant product 
standard. As the ultimate volume of the specimen is known, the required mass can be determined. The 
specimens are then quickly mounted in the apparatus or left to reach equilibrium with the standard laboratory 
atmosphere (23 °C, 50 % relative humidity), in accordance with the guidelines given in the relevant product 
standard or in 7.2.  
©RSB 2023- All rights reserved                  40 
 Annex D 
(informative)  
 
Rdry,mas- or λ10,dry,mas-values of masonry built from a range of masonry 
units containing formed voids 
NOTE The range of size and types of unit and void pattern is intended to be representative of units typically found on the 
market. It is not intended to be an exhaustive list covering all combinations of material, unit size, void configuration and 
size. The procedure according to 7.3.3 will need to be followed for configurations of units not covered by these tables.  
The geometries are defined numerically by two figures:  
 the number of rows of voids; and  
 the number of voids in a row.  
For example, 3,7/1,6 means that this type of unit has 3,7 rows of voids per 100 mm thickness and 1,6 voids in 
a row per 100 mm length, which means 11 rows of voids in the case of a masonry thickness of 300 mm and 4 
voids in a row in the case of a unit length of 250 mm. The transverse web portion is defined as the sum of the 
thicknesses of the transverse webs divided by the unit length expressed as a percentage and is given for each 
geometry as additional information.  
Further information is given for each geometry about the dimensions, which were the basis for the numerical 
calculation.  
The following tabulated values should be used as a basis for the determination of unit equivalent λ 10,dry,unit-
values or R dry,mas- or λ10,dry,mas-values of the masonry if neither an individual test measurement nor a 
calculation are available for a specific product.  
The values in this annex were calculated using a three-dimensional Finite-Difference-Program.  
The equivalent thermal conductivity of the air in the voids was determined according to EN ISO 6946:2007, 
B.2. The program used was checked through the examples shown in Annex D and fulfils all the requirements 
for appropriate calculation procedures.  
The theoretical background for the selection of geometries was knowledge about the principal geometrical 
influences on the thermal resistance:  
 number of rows of voids;  
 thickness of the material webs between the voids (transverse web portion);  
 voids staggered or "in line";  
 shape of the voids. (Experience shows that the last two factors can be neglected-for the purpose of 
tabulated values).  DRS 524: 2023 
41                ©RSB 2023- All rights reserved 
 The tabulated values in the following tables are generally given for masonry with only horizontal mortar joints.  
In some cases, the tabulated values are split into two, one of which is valid without vertical mortar joints and 
the second is valid with vertical mortar joints. For those geometry classes, where no separate values are given 
use the calculation procedure in 7.3.3. 
The thermal resistance of the mortar joints on which the calculation results are based can be derived in 
different ways. Full bed mortar joints may be provided using an insulating mortar or it is possible to reach the 
same resistance/equivalent conductivity by making twin strip mortar joints from a general-purpose mortar, 
possible with a strip of insulating material in between.  
The values are grouped according to the material of the masonry units, nevertheless, the calculation results 
are also valid for other materials, if the geometry and the thermal conductivity of the material is the same.  
The resistance-values are tabulated as resistance per 100 mm, which means for example that for a masonry 
of 300 mm thickness the values have to be multiplied by 3. As additional information, the calculation results 
are also given as λ 10,dry,mas -values of the masonry, which are calculated according to the following equation:  
λଵ଴,ௗ௥௬,௠௔௦=0.1
λௗ௥௬,௠௔௦ 
The values for the percentage of voids given in the tables are related to the cross section of the units. 
 
 
©RSB 2023- All rights reserved                  42 
  
(transverse web portion: 26,4 %; percentage of voids : 38,4 %)  
basic dimensions: l = 250 mm, w = 300 mm, hunit= 238 mm, hmor = 12 mm 
 
DRS 524: 2023 
43                ©RSB 2023- All rights reserved 
  
 
 
 
©RSB 2023- All rights reserved                  44 
  
(transverse web portion: 48,0 %; percentage of voids: 38,4 %) 
basic dimensions: l = 300 mm, w = 250 mm, hunit = 238 mm, hmor = 12 mm 
NOTE There are no values given for a combination of such a masonry unit with a mortar with a conductivity of 0,16 
W/mͼK, because such a combination would not be sensible. 
DRS 524: 2023 
45                ©RSB 2023- All rights reserved 
  
 
 
©RSB 2023- All rights reserved                  46 
  
 
DRS 524: 2023 
47                ©RSB 2023- All rights reserved 
  
 
 
©RSB 2023- All rights reserved                  48 
  
 
 
DRS 524: 2023 
49                ©RSB 2023- All rights reserved 
  
 
 
©RSB 2023- All rights reserved                  50 
  
 
 
 
 
DRS 524: 2023 
51                ©RSB 2023- All rights reserved 
  
 
 
 
 
©RSB 2023- All rights reserved                  52 
  
 
 
 
 
DRS 524: 2023 
53                ©RSB 2023- All rights reserved 
  
 
NOTE Tables B.23 to B.28 inclusive were calculated without vertical mortar joints. Tables B.29 to B.33 inclusive were 
calculated with mortar pockets. Table B.34 is based on a continuous vertical mortar joint.  
The values in Tables B.29 to B.33 inclusive are valid for more than one shape, the drawings shown are only examples for 
the geometries covered. 
 
©RSB 2023- All rights reserved                  54 
  
 
DRS 524: 2023 
55                ©RSB 2023- All rights reserved 
  
 
 
 
 
 
 
 
©RSB 2023- All rights reserved                  56 
  
 
 
 
 
 
 
 
DRS 524: 2023 
57                ©RSB 2023- All rights reserved 
  
 
 
(transverse web portion: 7 % - 14 %; percentage of voids: 42,7 %)  
 
©RSB 2023- All rights reserved                  58 
 basic dimensions: l = 380 mm, w = 300 mm, hunit = 221 mm, hmor = 12 mm 
 
DRS 524: 2023 
59                ©RSB 2023- All rights reserved 
  
 
 
 
 
 
 
 
 
©RSB 2023- All rights reserved                  60 
  
 
 
 
 
 
 
 
DRS 524: 2023 
61                ©RSB 2023- All rights reserved 
  
 
 
 
 
 
 
 
©RSB 2023- All rights reserved                  62 
  
 
 
 
 
 
 
DRS 524: 2023 
63                ©RSB 2023- All rights reserved 
  
 
 
 
 
 
 
 
 
 
 
©RSB 2023- All rights reserved                  64 
  
 
 
 
 
 
 
 
DRS 524: 2023 
65                ©RSB 2023- All rights reserved 
 Annex E 
(informative)  
 
Example of how to use the tables in Annex B 
 
The unit has 19 rows of holes, which means 5 rows of holes per 100 mm thickness, and either 3 or 4 holes per 
row, which means 1,2 or 1,6 holes per 100 mm length. There is no geometry class 5/1,2, therefore the 
relevant table is Table B.3 - Geometry 5/1,6. The values from this table are on the safe side, because of the 
number of voids per row and also because of the thickness of the webs. From Annex A the λ10,dry,mat-value for 
a clay unit material with a density of 1 500 kg/m3 can be taken as 0,43 W/m ͼK (if an individual test 
measurement for λ is available the measured value can be taken). From the first column in Table B.3 
(λ10,dry,mor = 0,16 W/mK) a resistance per 100 mm thickness of 0,58 m2K/W and a λ10,dry,masof 0,17 W/m ͼK is 
obtained. As the unit has a thickness of 38 cm, the R-value for the dry masonry is 0,58 × 3,8 = 2,204 m2K/W. 
The unit has a tongue and groove system in the vertical joint, therefore no mortar correction is necessary 
(even if there was a vertical mortar joint, it could be neglected, because a thermal insulating mortar is used).  
 
©RSB 2023- All rights reserved                  66 
 No correction is necessary for deviating dimensions, because the length and height of the unit are identical 
with the "basic dimensions" of geometry in Table B.3).  
To produce a design thermal value, the dry resistance has to be corrected according to moisture. The 
moisture correction coefficient is taken as 6 % per volume percent change of moisture as no individual 
measurement is available. Therefore, for a practical moisture content of 1 % by volume the dry resistance has 
to be multiplied by 0,94, which leads to a design resistance of 2,204 × 0,94 = 2,072 m2K/W; a practical 
moisture content of 1,5 % by volume leads to a design resistance of 2,204 × 0,91 = 2,006 m2K/W. The design 
U-value would then be in the first case 0,45 W/(m2ͼK) in the second case 0,46 W/(m2K). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 DRS 524: 2023 
67                ©RSB 2023- All rights reserved 
 Annex F 
(informative)  
 
Requirements for appropriate calculation procedures 
F.1 Capabilities of the program 
The user shall be supplied with the necessary information about the capability of the program to simulate the 
relevant characteristic properties of the physical component under considerations. Therefore, the following 
aspects of the heat flow model shall be defined:  
 2 or 3 dimensional;  
 rectangular or non-rectangular shape;  
 isotropic or non-isotropic conductivity. In this case:  
 general anisotropy;  
 partial anisotropy (with respect to the eigenvalues or eigenvectors of conductance);  
 voids;  
 equivalent conductivity or resistance (convective and radiative part);  
 radiation exchange and equivalent conductivity (convective part);  
 radiation exchange and internal air flow model;  
 the thermal resistance of the voids has to be calculated according to EN ISO 6946;  
  mass transfer (air-, moisture-transport from environment to environment);  
 surface resistances to be taken from EN ISO 6946.  
There is no specific preference related to the numerical methods incorporated; on the other hand, the user 
shall be informed about the advantages and restrictions of each method.  
F.2 Input data and results 
Input data shall be presented, to make it possible for a third party to do the same calculation.   
©RSB 2023- All rights reserved                  68 
 The following calculation results shall at least be provided:  
 minimum surface temperature of the test component on all sides;  
 maximum surface temperature of the test component on all sides;  
 2D or 3D thermal coupling coefficient (W/(mͼK) or W/K respectively);  
 number and type of elements.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 DRS 524: 2023 
69                ©RSB 2023- All rights reserved 
 F.3 Testing of the program accuracy 
The program shall be tested by calculating reference cases according to EN 10211.  
F.4 Reference cases 
 
 
©RSB 2023- All rights reserved                  70 
  
 
DRS 524: 2023 
71                ©RSB 2023- All rights reserved 
  
 
Where:   Rt,mas is the true thermal resistance of the masonry.  
                NOTE  The term            ௗ೔
஛௜          relates to the twplaster layers 
 
©RSB 2023- All rights reserved                  72 
                Symbols and definitions given in EN ISO 10211. 
 
 
 
 
 
DRS 524: 2023 
73                ©RSB 2023- All rights reserved 
  
D.4.3 CASE 3:  Calculation of thermal resistance Rt of masonry consisting of masonry units, horizontal mortar 
layers, vertical mortar pockets and additional external insulation layer.  
©RSB 2023- All rights reserved                  74 
  
DRS 524: 2023 
75                ©RSB 2023- All rights reserved 
  
 
 
 
 
 
 
 
 
©RSB 2023- All rights reserved                  76 
  
DRS 524: 2023 
77                ©RSB 2023- All rights reserved 
  
 
©RSB 2023- All rights reserved                  78 
 Annex G 
(informative)  
 
Evaluation of conformity 
Information about in what way the parameters used in the determination of the λ 10,dry,unit or equivalent λ 10,dry,unit–
values will be part of the evaluation of conformity system is given in the following table: 
Table E.1 — Parameters for evaluation of conformity 
 
 Model  Initial type testing (ITT) Factory production control (FPC) 
S1  Net dry density  Net dry density  
S2  Net dry density λ10,dry, mat /net dry density 
relationship a  Net dry density  
S3  Net dry density  
Thermal transmittance of masonry  Net dry density  
P1  Net dry density  
Configuration  
Thermal conductivity of the masonry unit 
material  Net dry density  
Configuration  
P2  Net dry density  
Configuration  Net dry density  
Configuration  
P3  Net dry density  
Configuration  
Thermal conductivity of the masonry unit 
material  Net dry density  
Configuration  
P4  Net dry density  
Configuration  Net dry density  
Configuration  
P5  Gross dry density Configuration λ10,dry,unit /gross 
dry density relationship b  Gross dry density  
Configuration  
If the net dry density of the unit under consideration deviate less than 2 times the declared tolerance from the net dry density forming the 
basis for the established relationship, then the λ10,dry,mat /net dry density relationship does not need to be repeated.  
If the pattern is the same and the gross dry density of the unit under consideration deviates by less than twice the declared tolerance from 
the gross dry density forming the basis for the established relationship, then the λ10,dry,unit/ gross dry density relationship does not need to 
be repeated.  DRS 524: 2023 
79                ©RSB 2023- All rights reserved 
 As part of a FPC system the λ 10,dry, mat–value for a batch of masonry unit may be determined based on direct 
testing of thermal conductivity. If so the following procedure should be used:  
Establish a correlation between  results and the alternative test method. The λ 10,dry,unit–value may be based 
on the value obtained from the alternative test method after applying the established correlation correction.  
NOTE Care should be taken that the test specimens are representative of the masonry product itself. An appropriate way 
to ensure this is to cut specimens from masonry units. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
©RSB 2023- All rights reserved                  80 
 Annex H 
(informative)  
 
Alternative procedure for the moisture correction of units with formed 
voids 
The principle of this method is to correct the design moisture content according to the percentage of voids. It 
is a safe approximation and may be used as an alternative to procedure 2 in Clause 6. 
From the moisture correction coefficient and the design moisture content, the following formulae can be used: 
 
 
DRS 524: 2023 
 
Price based on80pages  
©RSB 2023- All rights reserved