Energy & Buildings 331 (2025) 115351

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Energy & Buildings

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Numerical analysis of heavy cob walls’ hygrothermal behavior☆

Aguerata Kabore a , Mathieu Bendouma b , Claudiane Ouellet-Plamondon a,*
a
Department of Construction Engineering, École de technologie supérieure, Université du Québec, 1100 Notre Dame Street West, Montreal, H3C 1K3, Canada
b
Département des sols et de génie agroalimentaire, Université Laval, 2325 Rue de l’université, Quebec city, QC G1V 0A6, Canada

A R T I C L E I N F O A B S T R A C T

Keywords: The development of building envelope systems with low carbon footprint materials and improved hygrothermal
Hygrothermal simulations properties is still in progress. For geosourced materials, one of the main objectives is to achieve optimal
Cob walls hygrothermal efficiency. Cob, a material made of clay, plant fibres, and water, stands out for its low carbon
Geosourced materials
footprint and ease of application on timber-framed building structures. The main goal of this study is to assess the
Hygrothermal performance
Risk of mold
hygrothermal performance of eight heavy cob wall systems in eight cities in African, European, and American
climates. An extensive laboratory characterization is carried out to measure the hygrothermal properties of each
material. The thermal conductivity obtained after the measurements is 0.75 W/m.K and 0.87 W/m.K for red and
beige clay samples, respectively, 0.52 W/m.K for the cob with 3 % fibres, and 0.2 W/m.K for the cob with 6 %
fibres samples, and the porosity rates are 21, 20, 37, and 45 for the clay and cob samples, respectively. The
hygrothermal simulation showed that the interior temperature of the walls made of cob with 6 % fibres and a
thickness of 25 cm remained stable, regardless of external climate variations. Applying beige clay plasters to the
exterior and interior surfaces of cob walls or timber structures improved thermal performance in terms of heating
or cooling energy demand but also increased the walls’ moisture absorption. This increased moisture enhances
the risk of mold growth within the wall structure. When used as infill materials in timber structures, the
simulated composite systems generally exhibit good hygrothermal performance. However, in cold climate zones
with high precipitation and heavily clouded skies, the walls may be exposed to risks of mold development. To
prevent mold in these walls, install a rain screen or an air/vapor barrier membrane between the plaster and cob.
This helps manage moisture and ensures proper wall drying.

1. Introduction mechanization, and digitization to enhance the reliability, performance,

and efficiency of construction processes while maintaining reasonable
Regarding climate emergency, a global effort is aimed at reducing costs [15–20].
carbon emissions, especially in the energy and building sectors, for a Studies conducted on a hemp concrete building envelope at various
greener economy by 2050 [1,2]. The study of local building materials thicknesses have revealed a good level of insulation with excellent
and their use in modern construction is crucial for making buildings thermal inertia [21–25], absorbing up to 90 % of daily variations in
more energy-efficient and environmentally friendly. This approach external temperature and relative humidity [22]. In the Moujalled and
promotes a sustainable architecture that considers energy efficiency and al. [22] study, using a wooden structure with hemp concrete led to air
environmental protection [3–6]. A growing awareness translates into an leakage. They recommend applying a continuous coating on the internal
increased commitment to design buildings with a low carbon footprint, walls to solve this issue. The results of these studies have shown effective
stimulating numerous in-depth research on the passive cooling and regulation of humidity, keeping it above 30 % during the heating period,
heating features specific to earthen architecture [7,8]. Recent studies and preventing the interior temperature from exceeding 27 ◦ C in sum­
highlight the benefits of building with raw earth and bio-based mate­ mer, even when the exterior temperature exceeds 35 ◦ C [21,22]. Other
rials, due to their abundant availability, affordable cost, non-toxicity, studies have also emphasized the importance of the coating layer in
simple production processes, and recyclability [9–14]. These innova­ humidity regulation and discussed the optimal thickness of this layer
tive approaches explore possibilities of industrialization, prefabrication, [26–31]. Hemp concrete, when combined with phase change materials,

☆
This article is part of a special issue entitled: ‘Decarbonising Built Env’ published in Energy & Buildings.
* Corresponding author.
E-mail address: Claudiane.Ouellet-Plamondon@etsmtl.ca (C. Ouellet-Plamondon).

https://doi.org/10.1016/j.enbuild.2025.115351
Received 16 August 2024; Received in revised form 8 December 2024; Accepted 18 January 2025
Available online 21 January 2025
0378-7788/© 2025 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
A. Kabore et al. Energy & Buildings 331 (2025) 115351

can effectively reduce fluctuations in interior temperature, provided it is affect not only the thermal and hydric characteristics of these walls but
properly positioned in the wall [32]. also energy consumption, interior air quality, and the building’s lifespan
Earth materials have been studied, just like hemp concrete materials, [53,54]. Temperature has little influence on the thermal conductivity of
and these studies have shown the benefits of using earth in eco-friendly materials reinforced with vegetable fibres [55]. Unlike temperature,
and low-carbon footprint construction [33–38]. Research also explored relative humidity significantly impacts the thermal properties of cob
the use of double walls made of hollow bricks enriched with bio-based material [56].
earth, analyzed using EnergyPlus software, based on real building The numerical modeling examines various factors such as moisture
data. The results showed that this approach improved summer thermal absorption and drying, temperature, and humidity fluctuations, poten­
comfort, reducing thermal dissatisfaction by 24.6 % compared to con­ tial moisture-related damages over time, energy losses or gains for
ventional constructions in hot climates [39]. Further simulations interior climate regulation, and moisture flows through the building
investigated the optimal thickness of these bio-based materials in envelope. Several simulation tools have been used to analyze thermal
various climates, revealing a notable reduction in heat loss and and moisture transfer in walls, including the Conduction Transfer
improved regulation of interior humidity and temperature [40,41]. Function (CTF) model [57,58], the effective moisture penetration depth
These studies emphasize the importance of sustainable local materials, (EMPD) model [59,60], the combined heat and moisture transfer
such as fibre-reinforced adobes, rammed earth, cob, and compressed (HAMT) model [55,58], COMSOL Multiphysics [61,62], and WUFI®
earth blocks (CEB), for enhancing thermal comfort while reducing the [21,63,64]. The authors Yu et al. [57] used models based on the Con­
environmental footprint of the construction sector. Earth materials duction Transfer Function (CTF) and the Coupled Heat, Air, and Mass
mixed with plant fibres, in addition to having a low carbon footprint and (HAM) algorithms to analyze the impact of coupled heat and moisture
being fully recyclable, offer an effective solution for reducing CO2 transfer on the indoor environment and energy consumption of build­
emissions in construction by contributing to the elimination of plant ings in several cities, including Harbin, Shenyang, Beijing, Shanghai,
fibres [42]. Earthen walls stabilize interior temperature and humidity and Guangzhou. Their results show that this transfer has a significant
through moisture absorption and desorption capacity. However, the use influence on the annual energy consumption for heating and cooling, as
of waterproof coatings can limit their ability to regulate humidity, well as on the thermal and humidity conditions inside buildings.
thereby increasing the risk of mold [33]. Despite these advantages, few Furthermore, Rahma et al. [58] validated the efficiency of the integrated
studies have focused on cob and cob walls with wooden structures in HAMT model in EnergyPlus to predict the hygrothermal behavior of
hot, humid, and rainy areas. A thorough analysis is necessary to assess date palm concrete, a material recommended for sustainable construc­
the influence of different external climates on building components, and tion in humid and semi-arid regions. In the work of Huibo et al. [59], the
to understand the heat and moisture transfer in these types of walls. The EMPD model was used to assess the hygroscopic performance of build­
transfer of heat and moisture through building walls is a major cause of ing materials, which influence indoor humidity and affect durability as
total building energy consumption and the appearance of mold on well as air quality. They proposed indices for evaluating hygroscopic
certain parts of these elements [34]. This phenomenon is influenced by performance, validated through theoretical and experimental analyses.
the heat input from solar radiation, exterior air humidity, driving rain, These results underline the importance of integrating environmental
and cloud cover (shading in the humid season). Studying the impact of factors, such as air velocity, to obtain a more accurate assessment of
humidity, cloud cover, and temperature on cob walls through numerical building materials. Mariana et al. and Goffart et al. [55,60] also used
simulation is essential for understanding the long-term hygrothermal these three models to check for the presence of minimum comfort pa­
behaviour of these structures, whether in wattle and daub alone or with rameters to better select the building envelope materials. They adopted
a wooden structure. This would help assess how temperature variations a statistical approach to analyze the uncertainty and sensitivity of input
and humidity levels influence the performance of these walls. data on the evaluation of the hygrothermal performance of building
Several studies on intrinsic energy, also known as embodied energy, materials. Regarding COMSOL, Gerlich et al. [61] presented in their
which corresponds to the energy consumed for the extraction of raw work the validation of the model using measured data from a segment of
materials, production, and construction of the building [43], have a real building. On the other hand, Chabani et al. [62], in their research,
shown that the embodied energy of stabilized earth materials increases examine the quality of the experimental results and the numerical
linearly with the cement content [44]. The total embodied energy of a simulation, based on the mean squared errors between the two results.
stabilized earth wall containing 8 % cement is approximately 500 MJ/ These mean squared errors serve as a measure of the accuracy of a model
m3, which represents between 15 % and 25 % of that of fired clay brick or an estimate compared to real values.
walls. The embodied energy of compressed earth bricks and unstabilized For the simulation with WUFI Pro, several authors have used this
rammed earth varies from 0 to 50 kWh/m3, with 3.94 MJ per brick, and software to evaluate the hygrothermal response of single-layer and
overall embodied carbon of 0.39 kg CO2eq per brick, or 47.5 kg CO2eq/ multi-layer walls based on meteorological data. Developed by the
m3 [45]. Generally, the embodied carbon of earth and wood materials Fraunhofer Institute for Building Physics in Germany, WUFI Pro is spe­
varies from 0.01 to 0.025 kg CO2eq/kg of material [46,47]. The cifically designed to analyze heat and moisture transfer in building en­
embodied carbon of reinforced concrete varies from 0.19 to 0.24 kg velopes. Talaiji et al. [63] used the wall model of Doouzane et al. [65] to
CO2eq/kg of concrete, and for ordinary concrete, it varies from 0.10 to validate the WUFI Pro model to assess the hygrothermal performance of
0.16 kg CO2eq/kg of concrete [47]. Regarding grey energy, some studies multi-layer straw walls, while Mesa [66] validated their numerical
conducted on buildings made of cement blocks and clay block masonry WUFI Pro model using experimental results from tests on a straw bale
have shown that buildings constructed with cement blocks emitted, wall. In this article, the model was validated in the study by Kabore et al.
during their construction, 452 kg CO2eq/m2, with a grey energy of 3198 [21] on the hygrothermal performance of hemp concrete walls, using
MJ/m2. In contrast, residential buildings constructed with clay block the models of Dhakal et al. [67] and Lamalle [68], which also integrated
masonry emitted 235 kg CO2eq/m2 and had grey energy of only 1942 experimental data to validate their model. These comparisons yielded
MJ/m2 [48]. The grey energy of materials in an earth wall is about 20 consistent results, confirming the effectiveness of WUFI Pro as a reliable
times lower than that of a hollow and solid concrete block wall [49]. tool for evaluating the moisture dynamics within walls.
Cob walls, made from a mix of clay, water, and plant fibre, are The evaluation of the hygrothermal performance of cob materials,
recognized for their thermal mass capacity [50]. As with most building through different wall system configurations, is a crucial step to ensure
materials, cob is a porous material with various porosity levels. Moisture their effective and sustainable integration into modern construction as
migration in these types of walls can lead to damage due to condensation infill materials. This evaluation maximizes the benefits of cob while
[51]. Their susceptibility to moisture results in cracks and mold for­ minimizing the risks associated with its use and promotes sustainable
mation [52,53]. Moisture migration within the walls of a building can and responsible construction practices. Studies on biosourced and

2
A. Kabore et al. Energy & Buildings 331 (2025) 115351

geosourced materials, such as hemp concrete and adobe bricks, clearly and multi-layer one-dimensional walls. Both factors can impact interior
show that these materials can play a significant role in the transition thermal comfort and wall material durability either negatively or posi­
towards more eco-friendly and high-performance buildings. By inte­ tively. The software considers parameters such as cloudiness rate,
grating these materials into construction practices, it is possible to driving rain, wall orientation for the worst possible case, and capillary
enhance not only the sustainability of buildings but also to contribute to rise in the wall’s structure. It becomes necessary to evaluate the effects
the fight against climate change. To use cob in modern construction, of these parameters on the evolution of the interior temperature, hu­
which is also a geosourced material, it is essential to understand how midity, and the risk of mold growth in cob walls. On one hand, the model
this material reacts to climatic conditions. This study aims to anticipate designed with WUFI Pro 6.7 software uses the coupled heat and mass
future impacts on the performance of cob, allowing for proactive ad­ transfer equation developed by Künzel [72,73]. On the other hand, the
justments in construction techniques. Given that cob has not yet been model designed with COMSOL 6.1 aims to evaluate the impact of the
widely studied in terms of hygrothermal performance, the results of this absence of cloudiness rates and driving rain on the evolution of tem­
research will help better understand its behavior in response to external perature and humidity through cob walls. Simulations with COMSOL 6.1
climatic conditions. This will also aid in identifying potential risks of were carried out on single-layer walls, as the simulation program is
material degradation, such as mold formation and deterioration due to specifically designed for the analysis of single-layer walls. The numerical
excessive moisture. simulation does not take into account the protection of the walls by the
The main objective of this study is to evaluate the long-term roof, as the studied walls are susceptible to driving rain. The modeling is
hygrothermal performance of two types of cob produced using tradi­ a hygrothermal modeling of the building’s walls and not of the entire
tional methods, for their application in the construction of modern building.
contemporary buildings with a wooden frame structure. In this struc­
tural case, the wood is treated with Disodium Octaborate Tetrahydrate 2.2. Wall systems for simulation
(DOT) to protect it against termites [69]. For this purpose, the hygro­
thermal performances of twelve wall systems were numerically studied The performance of eleven types of cob walls and one clay wall are
using WUFI Pro 6.7 and COMSOL 6.1, two well-known simulation en­ studied through hygrothermal simulations using COMSOL 6.1 for the
gines used to assess the hygrothermal performance of building enve­ first three single-layer wall systems and with WUFI Pro 6.7 for all walls.
lopes. Two mathematical models were used in this study, one for WUFI An illustration of the different wall systems is presented in Tables 1 and
Pro 6.7 and another for COMSOL 6.1. These software programs were 2. The hygrothermal properties of the wall materials are shown in
employed to analyze the heat and moisture transfer through eleven cob Table 3, and the values of the effective thermal resistance (R) and the
walls, including four with 3 % fibres and seven with 6 % fibres. The thermal transmission coefficients (U) of the walls obtained by WUFI Pro
evaluation of the evolution of temperature, humidity, energy, and Software are presented in Tables 4 and 5.
moisture flow of the different walls was conducted considering factors
such as orientation, cloudiness, and driving rain. As a result, the twelve 2.3. Mathematical models
wall systems, consisting of three single-layer walls and nine multi-layer
walls, were assessed through WUFI Pro 6.7 simulations taking into ac­ 2.3.1. Energy and mass balance for WUFI pro and COMSOL multiphysics
count worst-case scenarios, wall orientation, driving rain, and cloudi­ simulation
ness. For the simulation using COMSOL 6.1, only the three single-layer The building envelope made of cob, wood/cob, whether it is rein­
wall systems were analyzed. The aim of using these two models is to forced with few or many fibres, consists of porous materials. The en­
evaluate the impact of considering and not considering driving rain, velope, that separates two environments (exterior and interior), is the
cloudiness, capillary moisture, and the orientation of the walls on site of heat and moisture transfers. The modes of transfers in these types
interior thermal comfort. Additionally, to assess the impact of consid­ of envelopes are thus the transfer of heat, liquid-phase water, and/or
ering the four elements on the risk of mold development, energy gains vapour-phase water [72]. Therefore, to realistically predict the hygro­
and losses, and moisture of the walls to regulate the interior environ­ thermal behaviour of porous envelopes, it is necessary to consider the
ment. The particular challenge in modeling cob walls lies in their het­ coupling of energy and mass transfers, as well as external climate events.
erogeneity, with significant variations in density, porosity, and thermal The impact of energy and mass transfers in the envelopes is explained by
conductivity, requiring detailed information on material properties. As a the imbalance of heat flow, which results in a temperature variation
result, the production and data measurements for clay materials, clay across the envelope between the moment t and the moment t + dt, where
plaster materials, and cob materials were carried out in the laboratory dt is the time variation. Similarly, an imbalance of moisture flows
[50,70]. To achieve this, tests to determine the hygrothermal properties (vapour in the liquid state) results in a variation (storage or release) of
of the two types of cob used were conducted out in two different labo­ moisture across the envelope. An imbalance of vapour flows can lead to
ratories with samples of varying sizes. This was done to validate that the adsorption or desorption of water (vapour condensation or water
there were no significant differences between the results obtained in the evaporation), involving latent heat (Lv), which is added to the thermal
two laboratories and to ensure that sample size would not affect the balance of the heat equation (Eqs. (2) and (8)). To analyze the hygro­
thermal properties of the cob. Subsequently, the mechanical strength thermal performance of the cob formulations designed for the study
and volumetric shrinkage of the two types of cob were evaluated to project, a coupled modeling of heat and moisture transfer was imple­
ensure that these materials could be used in the construction of two- mented, and the influence of extreme temperature and humidity on the
story or taller timber-framed buildings. The results are presented in walls was evaluated. Fig. 1 shows the phenomena that occur in a wall
our article [71]. Additionally, a thorough assessment of the influence of and on the surfaces of a wall.
moisture on the hygrothermal properties of cob was also conducted The numerical simulation with COMSOL 6.1 was carried out in a
prior to the numerical simulation, and these results are presented in the transient regime. The coupled mass and energy transfer equation is
article by Kabore, A. and Ouellet-Plamondon, C. [56]. defined by Eqs. (1) and (2) [74].
∂w ∂HR ( ( ) )
2. Materials and methods ρs v + Dl
= − ∇ − DHR HR
∇HR − DTv ∇T (1)
∂HR ∂t
2.1. Simulation methodology ( )
∂T d(Wm) d(HRx ) d(Tx )
Cmat = − ∇( − λ∇T) − ρs .cpl .T. + DHR + DTv .Lv
∂t dt v
dx dx
In this study, WUFI Pro 6.7 software was used to analyze the heat and (2)
moisture flows necessary for interior climate regulation in a single layer

3
A. Kabore et al. Energy & Buildings 331 (2025) 115351

Table 1
Wall systems configuration, a) single-layer walls, b and c) Multilayer walls.
Type I Type II Type III

a)

Wall material Red clay Cob with 3 % fibre Cob with 6 % fibre
​ Type IV Type V ​
b) ​

Exterior side 2.5 cm thick beige clay plaster 2.5 cm thick beige clay plaster ​
​ 20 cm thick cob with 3 % fibre 20 cm thick cob with 6 % fibre ​
Interior side 2.5 cm thick beige clay plaster 2.5 cm thick beige clay plaster ​
​ Type VI Type VII ​
c) ​

Exterior side 2.5 cm thick spruce wood 2.5 cm thick spruce wood ​
​ 20 cm thick cob with 3 % fibre 20 cm thick cob with 6 % fibre ​
Interior side 2.5 cm thick spruce wood 2.5 cm thick spruce wood ​

With ρs is the dry mass density of the material in kg/m3, Cmat is the Anderson-de Boer) is widely used to describe water adsorption in porous
thermal capacity of the material in J/kg.K, λ is the thermal conductivity materials, thanks to its ability to account for the interactions between
in W/m.K, Lv is the constant latent heat of vaporization which is water molecules and porous surfaces [77,78]. The model accurately
simplified to 2500 kJ/kg, Cpl is the thermal capacity of liquid water represents adsorption isotherms, even in complex structures, thereby
equal to 4186 J/kg K. The diffusion coefficients of water vapour DvHR, allowing the validation of experimental adsorption data [77]. Table 6
liquid water migration DHR T
l , water vapour thermodiffusion Dv , and presents the GAB model coefficients for each material, and Fig. 2 shows
saturation vapour pressure Pvsat, are expressed by Eqs. (3)–(6) [75]. the evolution of measured and evaluated, through GAB model, water
content of the different materials.
Mv .Pvsat (T)
DHR
v = Dv (3)
R.T (WGAB .CGAB .KGAB .HR)
wm = (7)
( ) ((1 − KGAB .HR)(1 − KGAB .HR + CGAB .KGAB .HR))
dWm p2
DHR = .exp(p1 − ) (4) Where wm is moisture content absorbed by the material, in kg of
l
dHR Wm
water per kg of dry matter, WGAB is the maximum amount of water the
Dv .Mv d(Pvsat (T)) Pvsat (T) material can absorb, CGAB is the the affinity of the material surface for
DTv = HR( − ) (5) water, KGAB is the the isotherm shape correction coefficient, which ad­
R.T dT T
justs the curvature of the isotherm and HR is the relative humidity.
13.7 − 5120 These conservation equations for mass and heat used for the simu­
Pvsat (T) = Patm exp( ) (6)
T lation with the WUFI PRO 6.7 are presented by Eqs. (8) and (9) are
With Dv is the water vapour diffusion coefficient, Mv is the molar developed in the articles by Künzel [72,73].
mass of water in kg/mol, R is the ideal gas constant in J/mol K, T is the ∂w ∂HR
temperature in K, Wm is the water content of the material in kg/kg, Patm . = ∇.(Dφ ∇HR + δP ∇.(HR Psat )) (8)
∂HR ∂t
is atmospheric pressure in Pa, p1 = − 13.5 and p2 = − 0.015 are adjust­
able parameters in Eq. (4) for plaster materials in the work of Bendouma ∂H ∂T
[75]. These parameters are used by default in this article. . = ∇.(λ∇T) + Lv ∇.(δP .∇(HR Psat ) ) (9)
∂T ∂t
The software COMSOL 6.1 has already been used to solve the highly
coupled hygrothermal transfer equations of porous materials 2.3.2. Boundary conditions
[54,74–76]. The sorption curve of each material was adjusted using the The energy and mass balance of the exterior surface at x = 0 is
GAB model to determine the model coefficients for numerical simula­ determined by Eqs. (10) and (11) using meteorological data such as the
tions on COMSOL 6.1 using Eq. (7). The GAB model (Guggenheim- exterior temperature (Tout), the solar radiation (Es) determined by Eq.

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A. Kabore et al. Energy & Buildings 331 (2025) 115351

Table 2
Wall systems configuration, d and e) Multilayer walls.
Type VIII Type IX

d) ​

Exterior side 1 cm thick beige clay plaster 1 cm thick beige clay plaster ​
​ 1.5 cm thick spruce wood 1.5 cm thick spruce wood ​
​ 20 cm thick cob with 3 % fibre 20 cm thick cob with 6 % fibre ​
​ 1.5 cm thick spruce wood 1.5 cm thick spruce wood ​
Interior side 1 cm thick beige clay plaster 1 cm thick beige clay plaster ​
​ Type X Type XI Type XII
e)

Exterior side 2.5 cm beige clay 2.5 cm spruce wood 2 cm spruce wood
​ 0.1 cm air/vapor barrier 3MTM 0.1 cm air/vapor barrier 3MTM 1 cm air gap
​ 20 cm cob with 6 % fibre 20 cm cob with 6 % fibre 0.1 cm air/vapor barrier 3MTM
Interior side 2.5 cm beige clay 2.5 cm spruce wood 20 cm cob with 6 % fibre
​ ​ 2 cm spruce wood

(15), the exterior humidity (HRout), and the exterior atmospheric

Table 3 pressure. The energy and mass balance of the interior surface at x = L is
Hygrothermal properties of wall materials [50,70].
derived from the interior temperature, interior humidity, and interior
Red Beige Cob Cob Spruce atmospheric pressure and expressed by Eqs. (12) and (13).
clay clay with 3 % with 6 % wood
( )
fibres fibres 4
φT(x=0) = hcvout .(Tout − Tx=0 ) + ε.σ .F. Tsky 4
− Tx=0 + α.Es + Lv φM(x=0)
Density (kg/ 2016 1956 1654 1412 400
m3)
(10)
Thermal (W/ 0.75 0.87 0.52 0.20 0.087 ( )
conductivity m.K) MV HRout Pvsa (Tout ) HRx=0 Pvsa (T)
φM(x=0) = hMout − (11)
Specific heat (J/ 901 930 917 944 1880 R Tout T
kg.
K) ( )
φT(x=L) = hcvin .(Tin − Tx=L ) + ε.σ .F. Tin4 − Tx=L
4
+ Lv φM(x=L) (12)
Water vapour (− ) 14.52 20 15.11 14.71 552
resistance
(dry)
Water vapour (− ) 3.57 4.95 4.64 5.32 −
resistance Table 5
(wet)
Thermal resistance (R) and thermal transmittance (U) value for each wall with
Porosity (− ) 20.63 20.12 37.14 45.43 0.9
air/vapor barrier.
Water content (%) 3.22 2.32 3.57 3.63 55.8
at 80 % RH Walls Type X Type XI Type XII
Free water (%) 159.91 141.73 132.37 119.32 845
saturation R-Value (m2.K) /W 1.04 1.55 1.58
U-Value W/ (m2.K) 0.82 0.58 0.57

Table 4
Thermal resistance (R) and thermal transmittance (U) value for each wall.
Walls Type I Type Type Type Type V Type Type Type Type
II III IV VI VII VIII IX

R-Value (m2.K) /W 0.26 0.38 0.98 0.44 1.04 0.96 1.56 0.75 1.35
U-Value W/ (m2.K) 2.23 1.76 0.86 1.60 0.82 0.87 0.57 1.07 0.65

5
A. Kabore et al. Energy & Buildings 331 (2025) 115351

Fig. 1. Interactions between the external and internal environment and a hygroscopic wall, a) heat and moisture storage in the wall, b) heat or moisture exchange
between external and internal surfaces.

1
Table 6 hcvin = 1, 5.(Tx=L − Tin )3 (17)
GAB coefficients for the COMSOL 6.1 simulation.
Type of material Wgab Cgab Kgab Tsky = Tout − 9 (18)
Clay 0.011 50 0.83 Where Emeas is the sum of horizontal direct irradiation (EdirH) and
Cob3%f 0.012 35 0.82
horizontal diffuse irradiation (EdiffH) (meteorological data), Σ = π/2 for
Cob6% f 0.013 16 0.081
vertical walls, θ is the angle of incidence (meteorological data), β = 90◦
for vertical walls, and ω is the ground reflectivity equal to 0.2 in this
( )
MV HRx=L Pvsat (Tx=L ) HRins Pvsat (Tins ) study [80], and Vw is the wind speed (meteorological data).
φM(x=L) = hMins − (13)
R Tx=L Tins
2.3.3. Simulation assumptions
Where hcvout and hcvin are the coefficients of external and internal This study focuses on the numerical modeling of the hygrothermal
convection respectively determined by Eqs. (16) and (17), relationships behaviour of nine wall systems. The COMSOL 6.1 model, developed for
suggested by ASHRAE, and the fictive sky temperature (Tsky) determined the simulation of single-layer walls, was used for the numerical simu­
by Eq. (18) [79]. The mass exchange coefficient hM is derived from the lation of walls of Type I, Type II, and Type III. The assumptions made to
Lewis relation given by Eq. (14) [75]. σ is the Stefan-Boltzmann con­ carry out the simulation are presented as follows:
stant, F is the shape factor, α the short-wave absorptivity of the material,
ε is the long-wave emissivity, and Pvsat is the saturation vapour pressure. 1. The gaseous phase consisting of water vapor and air obeys the ideal
hcv gas law. The different solid–liquid-vapor phases are in thermody­
hM = (14) namic equilibrium, and the fluids absorbed in the porous medium are
ρCpLe2/3
incompressible and continuous.
1 + cos(Σ) cos(θ) Emeas (1 − cos(Σ) ) 2. Solar flows, ambient temperature, and humidity are time-dependent.
Es = .EdiffH + .EdirH + ω. (15) Climate data from January 01, 2022, to January 01, 2023 were used.
2 sin(β) 2
3. The interior temperature was set to 24 ± 2 ◦ C for cities with hot and
hcvout = 1, 53.Vw + 1, 43 (14) arid climates, and to 21.9 ± 2 ◦ C for cities with cold and humid

Fig. 2. Water adsorption isotherms for clay and cob materials.

6
A. Kabore et al. Energy & Buildings 331 (2025) 115351

climates, with interior humidity at 50 ± 10 %, in accordance with presented in section 2.2. For a simulation with multilayer walls,
ANSI/ASHRAE Standard 55 [81]. although the results presented in the authors’ works [75,85] pertain to
4. The initial relative temperature and humidity of the walls are uni­ multilayer walls, validation of the model with experimental data from
form and set at 20 ◦ C and 50 %. cob walls would be necessary.
5. The short-wave absorptivity (α) of the materials is set at 0.7 for cob
and clay walls, in accordance with the literature [82,83]. 2.3.4. Climate data
6. For long-wave radiative exchanges, the shape factors are expressed The climatic data of the cities of Djibouti, Johannesburg, Cairo,
by F = (1+cos(β)) withβ = 2π for vertical walls [84] (our case study), and Abidjan, Montreal, Paris, Rennes and Reno were used to evaluate the
2
the interior radiation temperature equal the interior temperature. hygrothermal performance of different cob walls. In total, the climatic
7. The long-wave emissivity (ε) of the materials is set at 0.9, a value data of 8 cities consisting of cities with hot and arid climates, cold and
obtained in WUFI Pro 6.7. humid climates, and temperate climates were used. The purpose of using
variable climatic data is to analyze the capacity of cob walls to regulate
For the WUFI Pro 6.7 simulations, the nine wall systems were interior temperature in climates with high temperatures and the damage
simulated with the following assumptions: that can be caused by exterior climates with very high humidity to cob
walls. The external climatic parameters were obtained from the Weather
1. The assumptions 1, 2, 3, 4, 5, 6 and 7 applied for the COMSOL 6.1 API for the years 2012 to 2023 [86]. Figures S1–S8 presented in the
simulation were applied. supplementary document illustrate the meteorological data parameters
2. The short-wave absorptivity (α) takes into account the material of the from January 1, 2022, to January 1, 2023, of the cities. Table 7 sum­
outer surface of multilayer walls. marizes the maximum and minimum values of some climatic parameters
3. The orientation of the walls is chosen for the worst case of driving of each city for the year 2022–2023. To facilitate the analysis of the
rain. The amount of rain hitting the wall is calculated according to results, the cities were grouped into two zones. The hot and temperate
ASHRAE Standard 160 based on wind speed and direction, with the climate zone represents the cities of Djibouti, Johannesburg, Cairo, and
wall’s exposure factor to rain (FE) equal to 1.4 and the rain deposi­ Abidjan, while the cold climate zone represents the cities of Montreal,
tion factor on the wall (FD) equal to 0.5. Paris, Rennes, and Reno (Table 7).
4. For the simulation period presented in point 1, climate data from 3
years were used for the WUFI Pro 6.7 simulations, from January 1, 3. Results
2020, to January 1, 2023, and the results of January 1, 2022, to
January 1, 2023, were analyzed. To identify the best construction approach for each of the eight
5. The average annual cloudiness index applied to the exterior envi­ chosen locations, we evaluated eleven different cob wall configurations
ronment for each city was taken into account, and this data is pre­ as outlined in Table 1. These walls were oriented to face the worst-case
sented in the section 2.3.4 in Table 7. scenario for each locality, where precipitation is most intense. The re­
sults obtained from WUFI Pro 6.7 and COMSOL 6.1 data include tem­
In this study, the validation of the WUFI Pro model was conducted peratures, relative humidity, as well as heat and moisture flows,
using data from Dhakal et al [67] and Lamalle [68]. The details and depending on the time and thickness of the material studied. The anal­
results are presented in the referenced works [21]. Dhakal et al. [67] ysis of the walls’ hygrothermal behaviour is based on hygrothermal
analyzed the impact of mix proportions on the properties of hemp criteria, with a critical humidity threshold set at 80 % at every point in
concrete and the hygrothermal performance of two hemp concrete the wall. The goal of these simulations is to identify risks of condensa­
walls, each 33.5 cm thick, designed for construction in Ontario, Canada. tion, mold growth, and thermal comfort issues for each climatic zone,
Meanwhile, Lamalle [68] examined five wall configurations to evaluate and to offer recommendations for a better-adapted design.
the hygrothermal performance of wood concrete for the city of Liège.
Among these configurations, a 26 cm thick wood concrete wall was used
3.1. Evaluation of temperature and humidity of the wall Type I to Type III
to validate our model. The results of the numerical simulations per­
formed with our model align perfectly with the findings of both authors,
The evolution of temperature, independent of cloudiness and pre­
thus confirming the reliability and accuracy of our approach.
cipitation rates, was analyzed using COMSOL 6.1, while the impact of
For the COMSOL model, two studies using the same model for the
these two factors was studied with WUFI Pro 6.7. The average annual
numerical simulation of insulated walls, validated by experimental data,
cloudiness rate for each city was calculated by WUFI Pro 6.7 using
allowed the model to be considered reliable. The results of the simula­
hourly climate data from each city, and this coefficient was associated
tions were compared to the experimental results, which aligned with
with precipitation. Examining Fig. 3, the effect of cloudiness and heavy
these experimental findings [75,85]. The COMSOL model was used in
rain on the evolution of the interior temperature of clay and cob walls
this study to evaluate the thermal performance of the rammed earth
with a thickness of 25 cm is visible. Moreover, the temperature of the
material in wall form, without considering cloud cover and heavy rain,
interior surface of the clay wall (Wall type I), not exposed to rain and
and was used for the simulation only with the three single-layer walls
cloudiness, reacts more to external climate variations, followed by the

Table 7
Summary of the hourly meteorological data for the year 2022–2023: Weather API [86].
Climate zone City Temperature Relative Humidity Global radiation Nebulosity Amount of rainwater
⁰C % W/m2 − l/(m2⋅h)
min max min max max Average min max

Hot-temperate-climate zone Djibouti 23 40 30 90 1050 0.31 0 2

Johannesburg 0 42 10 80 1100 0.11 0 0.1
Cairo 5 43 5 95 1050 0.18 0 2.5
Abidjan 24 30 50 90 950 0.71 0 25
Cold- temperate-climate zone Montreal − 26 36 20 100 950 0.64 0 3
Paris − 6 40 20 100 950 0.4 0 8
Rennes − 7 38 17 100 900 0.39 0 6
Reno − 20 36 7 100 950 0.45 0 12.5

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A. Kabore et al. Energy & Buildings 331 (2025) 115351

Fig. 3. Influence of driving rain on the daily interior surface temperature of uncoated clay and cob walls for the eight cities studied.

cob wall with 3 % fibres (Wall type II) and the one with 6 % fibres (Wall months, and the temperature of the interior surface of the cob wall with
type III), respectively. The exposure of the three walls to heavy rain and 6 % fibres is close to the comfort temperature set at 24 ◦ C in summer
cloudiness influences the evolution of the interior temperature for walls throughout the year.
simulated with climate data from each city. The maximum temperature for the wall type I, not exposed to rain
For walls simulated with climate data from the hot zone, the and cloudiness, varied between 28 ◦ C and 30 ◦ C, while in the presence of
maximum temperatures for Wall type I, not exposed to rain and cloud­ these elements, it varied between 24 ◦ C and 25 ◦ C, for cities in the cold
iness, varied between 28.5 ◦ C and 31 ◦ C, and between 26.5 ◦ C and zone (from Fig. 3e to Fig. 3h). The minimum temperature was 17 ◦ C for
28.8 ◦ C in case of exposure (Fig. 3a to Fig. 3d). The minimum temper­ the four cities, and in case of exposure to rain and cloudiness, it was
ature varied between 22 ◦ C and 27 ◦ C, and between 19 ◦ C and 25 ◦ C in between 13 ◦ C and 15 ◦ C. There is no heating in the modeling, which
the presence of rain and cloudiness. The cob wall with 6 % fibres (Type mean that additional heating would be required. Temperatures above
III) showed stable interior temperatures for all cities in the hot zone, 25 ◦ C were observed from the beginning of May to the end of September
except for Abidjan, where the temperature decreased from April to for clay walls. The cob wall with 3 % fibres (Type II) also recorded
September. Regarding interior thermal comfort, clay walls do not pro­ interior temperatures above 25 ◦ C between May and September, except
vide the best thermal comfort in summer. When cloudiness and pre­ for the city of Reno. The cob wall with 6 % fibres (Type II) had stable
cipitation are considered, the temperatures of the interior surface always interior temperatures with a maximum of 25 ◦ C for all cities in the cold
remain above 25 ◦ C for the eight cities. However, the cob wall with 3 % zone.
fibres shows temperatures slightly above 27 ◦ C for most of the summer Considering the rate of cloudiness and precipitation leads to lower

8
A. Kabore et al. Energy & Buildings 331 (2025) 115351

temperatures of the surface of the interior walls compared to a simula­ the wall was observed, and it always remains above 80 % throughout the
tion where these parameters are not considered. This demonstrates the year. For walls simulated with the climate data of Djibouti, Johannes­
importance of the inclusion of these parameters to accurately predict the burg, and Cairo, the humidity at 12.5 cm depth remains below 80 %
thermal behaviour of construction materials. Nonetheless, whether the throughout the year (Fig. 4a, 4b, and 4c), except for the city of Reno
cob wall contains 6 % fibres or not, and whether it is exposed to heavy where the humidity fluctuates but remains below 80 % for most days of
rain or not, it remains an acceptable option for the construction of the year (Fig. 4 h). According to Künzel et al. [87], the risk of mold
bioclimatic buildings. formation significantly increases when relative humidity exceeds 80 %
The humidity profile at a depth of 12.5 cm in the walls (marked by for more than two weeks. Excluding driving rain and cloudiness from the
the blue dot), when subjected to the climates of each city, with or simulations, the humidity profile at 12.5 cm depth is below 70 %. Fig. 5
without exposure to driving rain and cloudiness, is presented in Fig. 4. illustrates the evolution of humidity through the walls, simulated with
As the exterior climate varies over time in both simulation cases (WUFI the climate data of Montreal and Rennes. Between 5 cm and 20 cm in
Pro 6.7 and COMSOL 6.1), humidity fluctuations are more pronounced depth, the humidity varies between 60 % and 50 % and remains constant
for walls that are either exposed or not exposed to hourly cloudiness and between 10 cm and 15 cm for the clay wall. For cob walls, between 5 cm
precipitation, at a depth of 12.5 cm from the walls. Relative humidity and 20 cm in depth, the humidity maintains between 70 % and 50 % and
greater than 80 % is observed at this depth in clay and cob walls with 3 remains constant between 10 cm and 15 cm in depth in both cities. On
% fibres, exposed to driving rain throughout the year for the cities of the exterior surface of the walls, at 0 cm, a humidity of around 90 % is
Abidjan, Montreal, Paris, and Rennes (Fig. 4d, 4e, 4f, and 4 g). For the observed, thus a risk of water vapour condensation.
cob wall with 6 % fibres, a fluctuation of relative humidity throughout

Fig. 4. Influence of driving rain on the humidity at 12.5 cm of uncoated clay and cob walls for the eight cities studied.

9
A. Kabore et al. Energy & Buildings 331 (2025) 115351

Fig. 5. Evolution of humidity through the walls: a to c) Montreal and d to f) Rennes cities.

3.2. Evaluation of humidity of the wall Type IV to Type IX consecutive months, from February 1st to April 1st (two-month
average), was evaluated in order to identify the risk of mold in these
After observing the influence of driving rain and cloudiness rates on walls (Fig. 6). Six multilayer walls were simulated with the climatic data
the moisture profile at a depth of 12.5 cm of clay and cob walls under of each city, and the configuration of these wall systems is presented in
different climatic conditions (Figs. 4 and 5), the evaluation subsequently Tables 1 and 2, section 2.2. The simulation results indicated that the
focused on multilayer cob walls. The average moisture for two multilayer configurations type IV and V showed daily and two-month

Fig. 6. Average humidity over two months (February, March) at each blue point indicated on the wall (for all walls, humidity is assessed at the same positions), for
the eight cities studied. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

10
A. Kabore et al. Energy & Buildings 331 (2025) 115351

average humidity levels above 80 % for the Paris, Rennes, and Reno wall with a timber-frame structure allows for a reduction in heat losses,
cities. Moreover, in addition to configurations IV and V, the multilayer leading to a decrease in energy consumption for heating. The use of cob
configurations VIII and IX also showed humidity rates above 80 % materials for the structure of timber-frame buildings significantly
throughout the thickness of the cob walls for Montreal City. All walls reduced the heat losses from the interior surface of the walls. Moreover,
displayed humidity rates above 80 % throughout the thickness of the cob heat flow is stable with the amount of fibres present in the cob.
walls when simulated with the climatic data of Abidjan city. This issue of In terms of seasonal average, the wall type IV, made of a plaster/cob
lack of drying of the wall components for the Abidjan city can be mainly containing 3 % fibres/plaster, showed the most significant thermal
attributed to the high cloudiness rate of 0.71 with a maximum rain losses, with values of about 40 W/m2 in Montreal, 23.8 W/m2 in Paris,
amount of about 25 L/m2/h and a very high precipitation duration 25 W/m2 in Rennes, and 16 W/m2 in Reno (Fig. 8e to 8 g). Type VII wall
compared to the other cities (Figure S8 in the supplementary document). showed low thermal losses for all cities in the cold zone. The addition of
In contrast, the humidity variation in all configurations remains below a clay plaster led to an increase in heat losses. For cities in the hot zone,
70 % for the Djibouti, Johannesburg, and Cairo cities. Johannesburg and Cairo recorded thermal losses of about 10 W/m2 and
6 W/m2 in winter, while in Abidjan, the loss was about 13 W/m2 for the
wall of type V (plaster/cob with 6 % fibres/plaster). Conversely, in
3.3. Interior heat and humidity flow Djibouti, a positive thermal flow was observed throughout the year for
all simulated walls.
Fig. 7 illustrates the heat flow profile on the inner surface of cob As for heat flow, a positive moisture flow indicates a transfer of
walls, while Fig. 8 shows the seasonal average of this flow. A positive moisture from the the wall surface to the interior of the building, thereby
flow indicates a heat gain, which shows that heat moves from the wall increasing the humidity of the interior environment. Conversely, a
surface to the interior environment, while a negative flow indicates a negative moisture flow corresponds to a loss of moisture, with moisture
heat loss, with heat moving from the interior to the wall surface. In hot moving from the interior to the wall surface. A significant increase in
zones, Djibouti displayed a positive thermal flow throughout the year moisture flow in the walls can harm their durability and the building’s
(Fig. 7a and 8a). The interior surfaces of walls, simulated with climate thermal comfort. This can damage the wall materials, promote mold
data of the Johannesburg and Cairo cities, showed a loss of heat flow in growth, reduce energy efficiency, and compromise the occupants’ well-
winter and autumn, and a gain in summer (Fig. 7b to 7c and 8b to 8c). In being. Fig. 9 illustrates the moisture flow profile on the interior surface
Abidjan, a decrease of thermal flow is observed in summer for type IV of cob walls, while Fig. 10 shows the seasonal average of this flow.
and V walls, and a gain in winter and autumn (Fig. 7d and 8d). This For walls of type IV and V simulated with data from cold zone cities
decrease can be attributed to driving rain, temperature drops, and the and the city of Abidjan, moisture flow showed considerable variability.
lack of clear sky to allow the walls to dry normally after a rainstorm. This For the rest of the cities in the hot zone, the maximum moisture flow for
leads to a loss of heat from the walls to allow them to dry or balance the walls of types VI and VII was relatively low. The different local climatic
interior environment’s temperature. conditions influence the water behaviour of the walls and the manage­
The results reveal predominantly negative thermal flow throughout ment of moisture. On a seasonal average, the moisture flow of type IV
the year, except for a few days in summer, in cities of cold zones (Fig. 7e and V walls varied significantly for all simulations conducted with data
to 7h and 8e to 8 h). Type IV and V walls recorded a heat loss varying from each city. The moisture flows are positive for all types of walls
between 0 and 70 W/m2. Heat losses increase when the temperature examined, with a tendency of moisture flow approaching zero for walls
drops and humidity rises. However, examining the simulation results, a

Fig. 7. Heat flow profile of the interior wall surfaces for the eight cities studied.

11
A. Kabore et al. Energy & Buildings 331 (2025) 115351

Fig. 8. Seasonal average heat flow of the interior wall surfaces for the eight cities studied.

Fig. 9. Moisture flow profile of the interior wall surfaces for the eight cities studied.

of type VI to IX. It is crucial to note that incorporating fibres into the cob 4. Discussion
significantly increases the moisture absorption capacity, thus playing a
vital role in regulating interior humidity and, by extension, in the The humidity values, as well as the heat and moisture flow in the
comfort and health of the occupants. walls, are critical parameters for assessing hygrothermal behaviour.
These are influenced by varying climatic conditions and the properties

12
A. Kabore et al. Energy & Buildings 331 (2025) 115351

Fig. 10. Seasonal average moisture flow of the interior wall surfaces for the eight cities studied.

of the materials used, such as clay, cob, and other wall components. To temperature, humidity variations, and heat and moisture flow over time,
assess the hygrothermal performance of these materials against different based on the climatic data specific to each city used for the simulation.
external climates, hygrothermal simulations were conducted using The climatic data, encompassing both cold and warm climate countries,
COMSOL 6.1 and WUFI Pro 6.7 for nine walls, including eight cob walls offer an idea of the hygrothermal performance of cob materials in
and one clay wall. Of the eight cob walls, six are multilayered and two different climatic zones. For instance, various authors have found good
are single-layered. The single-layer clay wall serves as a reference for the consistency between the results of numerical simulations and on-site
two single-layer cob walls. These simulations provided insights into measured data for walls constructed with earth materials [88,89],

Fig. 11. Daily interior wall surface temperature in hot period.

13
A. Kabore et al. Energy & Buildings 331 (2025) 115351

geosourced materials [90], and biosourced materials [22,23,91–93]. temperature of 14 ◦ C.

The design of cob walls proves advantageous in hot climates, as it
helps reduce the need for air conditioning. However, in cold climates,
4.1. Thermal performance of the walls
using cob requires special attention, particularly because of the serious
problems that wall drying during construction can pose. These problems
In terms of thermal performance, earth construction materials are
are related to humidity and very low temperatures, which lead to delays
recognized for their ability to regulate temperature and humidity
in the drying of the walls and cause an accumulation of moisture in the
[94–96]. According to all simulations conducted with COMSOL 6.1 and
walls. For a construction made of earth or cob, it is necessary to prior­
WUFI Pro 6.7, compiling climatic data, the temperature of the interior
itize multi-layer walls by adding additional layers to the wall and, be­
surface of clay walls varied between 12 ◦ C (in the coldest city) and
forehand, to use prefabricated and dry earth or cob materials to reduce
29.6 ◦ C (in the hottest city), this is for exterior temperatures from − 15 ◦ C
drying time on the construction site. Construction should also be plan­
to 43 ◦ C (see Figs. 3, 11, and 12).
ned for warmer and drier seasons to avoid risks related to humidity and
Multi-layer walls, simulated with the climatic data of the hot zone
freezing.
and with little cloud cover (see Table 3 and Fig. 11a, 11b, and 11c),
showed temperatures of the interior surface of the walls varying be­
tween 24 ◦ C and 26 ◦ C for Djibouti and between 19.5 ◦ C and 21 ◦ C for 4.2. Evaluation of mold growth in the walls
Johannesburg and Cairo in winter. During summer and autumn, these
temperatures varied from 24.3 ◦ C to 26.6 ◦ C for Djibouti, and from 21 ◦ C To assess the risk of mold development in different types of simulated
to 26 ◦ C for Johannesburg and Cairo. walls, the WUFI Pro 6.7 software uses the LIM (Lowest Isopleth for
Regarding cob walls, although types II to V, VIII, and IX are not Mould) model [97]. A moisture isopleth for building materials is a line
suitable for very cold climates or those with significant precipitation and drawn on a diagram connecting points of constant moisture throughout
dense cloud cover, their ability to regulate interior temperature remains a material. This graphical representation allows visualization of how
satisfactory (see Fig. 11d and 12). The use of a wooden structure or a moisture distributes within the material under different environmental
vapour/air barrier for the design of cob buildings would improve ther­ conditions. Analyzing moisture isopleths is essential for understanding
mal performance, with an increase in the minimum temperature of the the hygroscopic behavior of materials, assessing risks of mold, or
interior surface of single-layer walls from 14 ◦ C to 17 ◦ C, an increase of degradation, and optimizing the thermal and sustainable performance of
3 ◦ C. Moreover, multi-layer walls made of cob with 6 % fibres (types V, constructions. In practice, this helps engineers and architects design
VII, IX to XII) offer better interior comfort, with interior temperatures more resilient structures and prevent moisture-related issues. This
varying from 17 ◦ C to 20 ◦ C in winter, 20 ◦ C to 25 ◦ C in summer, and model defines two categories of substrates, taking into account relative
21 ◦ C to 15 ◦ C in autumn, facing average daily exterior temperatures humidity, temperature, and the effect of construction materials. The LIM
oscillating between − 15 ◦ C and 35 ◦ C. Walls types VI VII, XI, and XII, I category pertains to biodegradable materials used in construction, such
combining wood and cob, showed average daily interior temperatures as wallpaper and materials for permanent elasticity joints. The LIM II
varying from 18 ◦ C to 25 ◦ C. It is recommended to keep the temperature category, on the other hand, applies to porous construction materials.
of the interior surface of the walls in cold areas above 12.6 ◦ C [40], and The analysis of the materials of the simulated walls in this study is based
this criterion has been met by the cob walls with a minimum on the LIM II curve. Events below the LIM II curve indicate no risk of

Fig. 12. Daily interior wall surface temperature in cold period.

14
A. Kabore et al. Energy & Buildings 331 (2025) 115351

mold in the walls, while events above this curve signal a risk of mold maximum driving rain of 25 l/(m2.h), increases the moisture absorption
development. Data from an experimental study obtained for rock wool in the walls. For the walls to be adapted to such a climate, the founda­
walls of buildings were compared with the results provided by WUFI Pro tions must be designed with a low water absorption coefficient to limit
to evaluate the risk of mold in the insulation [97]. The simulation results capillary rise and be covered by a steeply pitched roof to reduce rain­
and those obtained in situ were in agreement. Thus, the mold risk an­ water runoff on the wall surfaces. To thoroughly evaluate the risk of
alyses presented in this section can be used realistically. mold growth in cob walls, key parameters (isopleths) considering rela­
When analyzing the results of the simulated walls with various tive humidity and temperature, generated by WUFI Pro, as well as cli­
climate data, a significant impact of the finishing layers on the hygro­ mates presenting a high risk to cob walls, such as those of Abidjan and
thermal performance of the walls was also observed, as reported by Montreal (evolution of humidity in the walls presented by Fig. 6d and
other authors [98,99]. When the layer of clay plaster is applied to both 6e), were used.
sides of the walls, the greater the wall’s moisture absorption. Given the The isopleths for walls of types IV, V, VIII, and IX showed that the
severity of the climate, using materials such as clay or cob for con­ hygrothermal conditions in the city of Montreal were conducive to mold
struction especially in cold climate zones might not be suitable if the growth, with numerous events located above the LIM II curve (Fig. 13a,
walls do not have adequate protection against driving rain, rainwater 13b, 13e, and 13f). A low concentration of temperature/relative hu­
runoff, and capillary rise. The evolution of average humidity over two midity events above the LIM II curve is observed for the exterior surface
months through the walls, illustrated by Fig. 6, shows that the wood/ of the cob in contact with wood for walls of types VI and VII (Fig. 13c
cob/wood wall systems (Type VI and VII) is adapted to the climates of and 13d), indicating a low risk of mold development for these two types
Montreal, Paris, Rennes, and Reno (Table S1 and S2 in the supplemen­ of walls. For wood and cob walls, when a clay plaster is applied to both
tary document). However, applying clay plaster to the exterior and exterior and interior sides, a high risk of mold is observed on the exterior
interior sides of these walls (Type VIII and IX) presented a risk of water surface and in the middle of the two walls, while the risk is low for the
vapour condensation within the cob material, potentially leading to the interior surface of the wall of type IX. A high concentration of temper­
rotting of fibres due to a humidity level exceeding 80 % throughout the ature/relative humidity events above the LIM II curve was noted for all
material’s thickness for the city of Montreal. Therefore, wall systems walls simulated with the climate data of Abidjan (Fig. 14). Tables S1 and
configured with cob material with 6 % fibres and air/vapor barriers for S2 presented in the supplementary document summarize the hygro­
northern regions (Type X to Type XII walls), have been evaluated for thermal conditions for 3 years of all walls simulated with different
high-risk cities observed in this study, Montreal and Abidjan cities. climate data. It is important to note that the results used for the mold
All wall systems were suited to the climates of Djibouti, Johannes­ growth risk analysis considering scenarios of heavy driving rain, rep­
burg, and Cairo, with humidity levels below 80 % throughout the resenting the worst case of wall orientation.
thickness of the cob throughout the year, except for the results obtained According to Ghadie et al. [63], the use of a material that delays the
with the climate data of Abidjan (Fig. 4d and 6d). The humid tropical transmission of water vapour placed just on the exterior side of the cob
climate of Abidjan, characterized by a high cloudiness rate and a material could be an effective strategy to avoid the risk of mold

Fig. 13. Walls isopleths, exterior, middle, and interior sides of cob simulated with Montreal climate data.

15
A. Kabore et al. Energy & Buildings 331 (2025) 115351

exterior surface, the middle, and the interior surface of the cob indicate a
significant reduction in the risk of mold development, even in the worst-
case scenario of wall orientation (Fig. 15). There is almost no concen­
tration of temperature/relative humidity events above the LIM II curve
for these walls simulated with the climate data of Abidjan and Montreal.
This is visible when comparing Fig. 13a and Fig. 14a versus Fig. 15a to
15f. This means that there is no risk of mold development in the cob
material, indicating that this type of design is suitable for cold climates
and heavy rainfall.

5. Conclusion

Eco-friendly building design strategies, including traditional ap­

proaches, can significantly reduce the environmental impact of build­
ings while maintaining thermal comfort. This study explored the use of
materials such as clay and cob for constructing single and multilayer
walls to analyze their hygrothermal performance, heat and moisture
losses or gains, and the risk of mold development. The conclusions
drawn from the simulation results are as follows:

• By analyzing the evaluation of humidity variation across the walls,

based on simulation data covering the period from February 1 to
March 31, and the assessments of the risk of mold appearance for
wall configurations IV to XII over three years of simulation, we find
Fig. 14. Walls isopleths, exterior, middle, and interior sides of cob simulated that the higher the cloudiness index, the more the walls tend to
with Abidjan climate data. accumulate moisture, which increases the risk of mold development.
• All wall configurations demonstrated good hydric performance in
development. They also observed that adding a rain screen on the the cities of Djibouti, Johannesburg, and Cairo. For the cities of
exterior of the wall could reduce the infiltration of rainwater and hu­ Montreal, Paris, Rennes, and Reno, the configurations using exclu­
midity, thereby protecting the wall materials against the accumulation sively wood as a structure show superior hydric performance
of water vapour [63]. Consequently, wall systems with air/vapour compared to other configurations. In contrast, for the city of Abidjan,
barrier were modeled using the climate data of Abidjan and Montreal, the configurations that meet the humidity performance standards
two cities with high cloudiness rates. The air/vapour barrier was placed correspond to wall types X to XII.
on the exterior side of the cob material. The isopleths generated for the

Fig. 15. Isopleth for the Type X to XII walls with air/vapour barrier on the cob: a, b, c) Abidjan and d, e, f) Montreal.

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A. Kabore et al. Energy & Buildings 331 (2025) 115351

• Single-layer cob walls, especially those containing 6 % fibres, pro­ Declaration of competing interest
vide more favourable interior temperatures for both cold and hot
climate zones. For exterior temperatures varying from − 15 ◦ C to The authors declare that they have no known competing financial
43 ◦ C, the variation in interior surface temperatures was 12 ◦ C to interests or personal relationships that could have appeared to influence
29.6 ◦ C for clay walls, 15 ◦ C to 27 ◦ C for cob walls with 3 % fibres, the work reported in this paper.
and 19 ◦ C to 25.5 ◦ C for cob walls with 6 % fibres. Considering cloud
cover and driving rain results in a significant reduction in the Acknowledgments
maximum interior temperature of the simulated walls. Moreover,
applying a clay finish and using a wooden structure for cob walls The author wants to thank the Pole de Recherche et d’Innovation en
enhance interior thermal comfort, thus reducing the heating demand Materiaux avancés du Quebec (PRIMA Quebec (PRIMAR20-13-008)),
to maintain interior temperatures above 21.9 ◦ C in winter, and the the Natural Sciences and Engineering Research Council of Canada
cooling demand for interior temperatures below 26 ◦ C in summer. (NSERC) Alliance program (CRSNG ALLRP 560404-2020), Mitacs
The temperatures of cob walls with 6 % fibres are more stable, (IT35572), Quebec Wood Export Bureau (QWEB), AmeriCan Structures,
regardless of the boundary conditions used for the simulation. Technologies Boralife inc., ENERGIES 2050, and the Canada Research
• Regarding moisture management in walls, clay plaster significantly Chair on Sustainable Multifunctional Construction Materials (CRC-
impacts the hygrothermal behaviour of simulated cob walls. 2019-00074) to support this study.
Applying finishes, whether on a cob or wooden structure, causes a
significant increase in wall moisture. This increase can lead to a Appendix A. Supplementary data
heightened risk of mold formation, especially in walls exposed to
cold climate conditions or heavy rainfall. Walls simulated with the Supplementary data to this article can be found online at https://doi.
climatic data of cities in the cold and damp zones were particularly org/10.1016/j.enbuild.2025.115351.
vulnerable, except for types VI, VII, and Types X to XII, which did not
present a risk of mold. Data availability
• The use of an air/vapour barrier or rain screen on the exterior side of
the cob material eliminates the risk of mold development. Data will be made available on request.
• For hot and low precipitation climatic zones, the use of clay finishes
proves to be beneficial for good temperature and humidity References
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