Durability of light wood frame constructive system: study of interstitial condensation

Durabilidade de sistemas construtivos light wood frame: estudo de condensação intersticial

Abstract

The Light Wood Frame (LWF) construction system has been increasingly researched and applied in Brazilian buildings, driven by the recent publication (mid-2023) of the Brazilian standard "Light Wood Frame Buildings" and by its potential for industrialization and reduced environmental impact. This reduction is mainly due to the system's ability to capture biogenic carbon through the timber used in its frames. However, a critical consideration for implementing this system in Brazilian buildings is ensuring compliance with durability requirements, particularly in addressing the risk of high humidity levels within walls or between floors, which can result from interstitial condensation. This article aims to analyze the occurrence of interstitial condensation in the walls of buildings constructed with the LWF system in the Brazilian Bioclimatic Zone 1M. To achieve this, a bibliographic review was conducted, the characteristics of the wall construction components were gathered, and computer simulations were performed using EnergyPlus and WUFI Pro 6.5 software. For the configuration studied, no interstitial condensation was detected in the internal bracing layer of the wall, suggesting a good natural drainage capacity of the construction system in the evaluated bioclimatic zone.

Keywords
Constructive system; Light Wood Frame; Durability; Humidity; Computer Simulation

Resumo

O sistema construtivo Light Wood Frame (LWF) vem sendo pesquisado e aplicado em edificações brasileiras com mais frequência em razão da recente publicação (meados de 2023) da norma brasileira intitulada “Edificações em Light Wood Frame”, aliada ao seu potencial de industrialização e redução de impacto ambiental, pela possibilidade da captura de carbono biogênico da madeira serrada usada na estrutura principal. Entretanto, um dos pontos de atenção para a implantação desse sistema nas edificações brasileiras é o atendimento ao requisito de durabilidade, especialmente quando se trata da possibilidade da ocorrência de elevados teores de umidade internos às paredes, ou entrepisos, ocasionada, por exemplo, pela condensação intersticial. Assim, este artigo objetiva apresentar um estudo sobre a análise da ocorrência de condensação intersticial em paredes de edificações construídas com o sistema LWF, na zona bioclimática 1M. Para tanto, foram feitas revisões bibliográficas, coleta das características dos componentes construtivos das paredes e simulações computacionais usando o software Energy Plus e o WUFI Pro 6.5. Para a configuração estudada não foram observadas condensações intersticiais na camada interna de contraventamento das paredes, o que sugere boa capacidade de drenagem natural do sistema construtivo na zona bioclimática avaliada.

Palavras-chave
Sistema construtivo; Light Wood Frame; Durabilidade; Umidade; Simulação computacional

Introduction

Brazil needs to build extensively and with quality to address the housing deficit, estimated at approximately 6 million homes (FJP, 2021; AGÊNCIA FAPESP, 2020). In addition to the overall housing deficit, recent climate incidents, such as landslide in São Sebastião, São Paulo, in 2023, and floods in Porto Alegre, Rio Grande do Sul, in 2024, have left thousands homeless, further compounding the housing shortage (BBC News, 2024; NBC News, 2024).

In this context, the construction market requires economically viable building systems that ensure adequate performance levels and production capacity to address the housing shortage. Over the past two decades, various innovative constructive typologies have been introduced into the Brazilian market, with particular emphasis on lightweight construction systems, such as internal partitions based on plasterboards (drywall) and lightweight structural systems like Light Steel Frame (LSF) and Light Wood Frame (LWF).

In the case of the Light Wood Frame (LWF) system, which uses solid sawn timber components, Brazil has a significant advantage in terms of natural resources. The country has extensive reforestation areas, covering approximately 4.8 million hectares of eucalyptus and pine plantations. Among these, pine is the most commonly used species for constructing solid sawn timber buildings. In this context, the southern region of Brazil hosts the largest area of planted pine forests in the country, equivalent to 1.28 million hectares, representing 82% of the national reserve of this species (SBS, 2005; ACR, 2014). The abundance of this resource, combined with the logistical advantage of serving Brazil's southern and southeastern regions, since the distance from the forest must also be considered due to road transportation, makes timber construction an attractive and sustainable alternative. Reforested timber can be planted and managed to create future stocks, ensuring the continued use of this material in the construction industry, mainly in regions quite close to the forest, as the south and southeast regions.

The use of industrialized wood panels has grown significantly in recent years in Brazil, mainly in residential projects, driven by the adoption of the wood frame construction system. According to Resende et al. (2021), the Light Wood Frame (LWF) system has been gaining traction in the national construction market due to its fast execution and sustainable appeal, mainly relating to carbon capture. One of the most recently notable projects was built in São Sebastião, where approximately five hundred (500) LWF-structured apartments were built to accommodate those displaced by the landslides caused by heavy rains in mid-February 2023 (Reportagem do Globo, 2024).

With the publication of the "DiretrizSiNAT 005: Constructive systems structured with solid timber components with panelized cladding (Light Wood Frame)" by the Ministry of Cities in 2011, a new technical framework was established. This marked the beginning of a path for certifying companies seeking to finance projects using this constructive system in Brazil. In the following years, the first projects designed and financed according to the criteria set by the Diretriz SiNAT 005 were launched. Subsequently, driven by the structured integration of light wood frame projects, the Commission of Wood Frame Study (CE-002:126.011) was established in 2016 under the Brazilian Committee for Civil Construction (ABNT/CB-002), and the NBR 16936 (ABNT, 2023) standard was published in 2023.

However, one of the challenges in implementing the Light Wood Frame (LWF) system in Brazilian buildings, despite the existence of the mentioned standard, is meeting safety and durability requirements, particularly regarding the potential for high levels of humidity within walls or between floors, which can occur due to interstitial condensation.

The structural framework of the wood frame system consists mainly of pressure-treated solid timber and wood sheathing boards that serve as bracing. According to Garay, Poblete and Karsulovic (2009), the bracing boards, made of OSB (Oriented Strand Board) or structural plywood, undergo changes in their physical and mechanical properties when exposed to critical conditions of relative humidity and temperature. These changes could include moisture absorption, swelling, bending resistance, and modulus of rupture. Since these components are part of the structural frame of the wood frame system, controlling the climatic conditions to which the panels are exposed throughout the building's lifespan becomes a crucial factor for the system's durability.

Regarding the consequences of unwanted moisture in buildings, Pires, González and Tutikian (2021) highlight the risk to the durability of constructive systems, which may suffer material and structural degradation, compromising the safety of the building. Additionally, the growth of mold and mildew can occur, which negatively impacts indoor air quality, consequently affecting the health of the occupants.

According to Vanpachtenbeke et al. (2020), depending on construction and usage factors, wooden buildings can be exposed to high levels of relative humidity due to condensation. This poses a risk of fungal growth and structural degradation, especially when untreated wood is used. Condensation is a common phenomenon in the interior of buildings and is associated with moisture. It occurs due to the excessive water vapor generation by occupants and climatic conditions. Condensation may appear on the surface of building materials (surface condensation) or within the material (interstitial condensation).

Interstitial condensation refers to moisture that forms within the layers of construction systems or at the interfaces between adjacent layers (Bellia; Minichiello, 2003). Concept adopted in this manuscript. According to Karagiozis and Salonvaara (2001), the primary causes of interstitial condensation within building envelope components are mainly attributed to four factors:

1. the initial moisture content of materials or components during the construction phase;

2. vapor diffusion between layers;

3. saturated water diffusion through contact between layers; and

4. humid air infiltration into the system through either the exterior or interior surfaces.

In addition to assessing the climatic conditions to which structural components are exposed concerning the risk of condensation, the British standard BS 5250 (BSI, 2021) – Code of Practice for the Control of Condensation in Buildings – outlines three key considerations at the design stage:

1. the climatic conditions of the coldest month of the year;

2. the restriction of conditions where relative humidity exceeds 80%; and

3. in the event of interstitial condensation, it should be evaporated during the warmer months, while the risk of degradation must be evaluated based on the maximum level of condensation that could occur within the construction layers.

These factors are especially important when evaluating a wood frame system, as specific structural components, such as bracing panels, are particularly sensitive to hygrothermal conditions.

The study of hygrothermal behavior in LWF systems has been conducted for decades in countries with cold climates, where this constructive system is well-established, such as in European countries and North America (Pasztory et al., 2012; Mundt-Petersen, 2013). However, in Brazil, there is a lack of hygrothermal studies that address the specific characteristics of the national climates and serve as a reference for the proper specification of LWF system components.

Thus, this article aims to present a study on the occurrence of interstitial condensation in the walls of buildings constructed with the LWF system in Brazilian Bioclimatic Zone 1M as classified by NBR 15220-3 (ABNT 2024), evaluating internal moisture flow, moisture content, relative humidity, and the risk of interstitial condensation in the internal bracing layer, specifically the OSB panel.

To this end, bibliographic reviews were conducted, information was gathered on the characteristics of the wall construction components, and numerical simulations were performed using Energy Plus and WUFI Pro 6.5 software.

Literature review

Characteristics used for hygrothermal simulation

One of the key factors for understanding the moisture dynamics of the construction system (and for conducting hygrothermal simulations) is the characterization of its constituent components in terms of vapor permeability. The leading international reference standard for determining the vapor permeance and permeability of a material and component is E96/96M - Standard Test Methods for Water Vapor Transmission of Materials (ASTM, 2018). This standard presents the testing method, units, and conversion factors.

Table 1
Vapor permeance for typical construction components according to E96/96M (ASTM, 2018)

Material	Permeance (perms)	Classification
Aluminum Foil 0.03 mm	0.00	Impermeable to vapor
Polyethylene 0.15 mm	0.06	Impermeable to vapor
Kraft paper with asphalt film 0.05mm	0.30	Semi-permeable to vapor
Structural plywood 6.35 mm	0.70	Semi-permeable to vapor
Ceramic brick 100 mm	0.80	Semi-permeable to vapor
Polystyrene 25.4 mm	1.20	Semi-permeable to vapor
Reinforced plasterboard with mesh 19 mm	15.00	Permeable to vapor
Gypsum board for walls 12.5 mm	50.00	Permeable to vapor

It also outlines the definitions provided by Künzel (1995):

1. vapor permeance (W): the rate of water vapor transmission per unit area, induced by a unit difference in water vapor pressure, under specific temperature and humidity conditions. It is expressed in the unit "perm," which is equivalent to 5.72 x 10-¹¹ kg/(s.m².Pa). Table 1 presents examples of vapor permeance for typical construction components;

2. permeability (δ): it is the permeance of the material multiplied by its thickness (kg/(s.m.Pa));

3. vapor resistance factor (μ): the ratio between the vapor permeability of air and the vapor permeability of the material being analyzed. This dimensionless parameter indicates how often times the resistance to vapor diffusion of a material is greater than the resistance to vapor diffusion of a still air layer; and

4. equivalent air layer (Sd): the vapor resistance factor (μ) multiplied by the thickness of the material.

In addition to permeance, another key parameter for characterizing components for hygrothermal evaluation is the equilibrium moisture content and the relative humidity. Figure 1 is an isothermal curve of hygroscopic materials, representing the state in which the material maintains a balance between its moisture content and the surrounding environment for a given relative humidity.

Figure 1
Equilibrium moisture content curve for porous materials used in foundations, at a temperature of 20 °C

Other parameters essential for characterizing components in hygrothermal simulations and related to moisture dynamics include the typical construction moisture value [kg/m³]: the characteristic initial moisture content of a construction component when it is first built, or the starting moisture content value used at the beginning of the simulations.

In addition to the above parameters, Table 2 shows other physical and thermal properties of the components that are also needed to conduct hygrothermal simulations.

Table 2
Properties of construction components for conducting hygrothermal simulations

Properties	Magnitude	Unity
Layer thickness	e	m
Bulk density	ρ	kg/m³
Porosity	-	m³/m³
Specific heat	c	J/kg.K
Thermal conductivity	λ	W/mK

Light wood frame construction in Brazil

The Light Wood Frame (LWF) system is a construction method consisting of a primary structure made from sawn wood (timber) profiles, covered with industrialized sheathing boards that are fixed to the main structure, either with or without structural functionality. Additional components, such as vapor barriers, insulation materials, and topcoats, are also part of the system.

The structural frame of the LWF system is composed of solid sawn wood pieces joined together with metal fasteners. The vertical profiles are referred to as studs, while the horizontal components that close the top and bottom of the frame are called crosspieces or top/bottom plates. Industrialized wood panels, typically treated plywood or OSB boards, are attached to the studs and top/bottom plates to brace the structural frame. Figure 2 illustrates the key components that make up the structural framework of a Light Wood Frame wall.

Figure 2
Core Components of light wood frame construction

When necessary, thermal insulation materials are used inside the structural frame, which is common in cold climates or specific bioclimatic zones. On the inner face of the structural frame, drywall plasterboards are fixed, which may be either standard or special (moisture-resistant or fire-resistant), depending on the environment in which they are applied. Before fixing the cladding panels, waterproof and vapor-permeable barriers are applied to the external surfaces. These barriers prevent the external entry of liquid water while allowing the transmission of vapor, thus maintaining the hygrothermal flow of the system. Over these barriers, cementitious boards are typically used as exterior cladding, often with cementitious mortar coatings with polymer additives (base coat) or uncoated, with only paint, and joints being sealed with sealants or metal profiles. Figure 3 illustrates the typical configuration of a Light Wood Frame wall section.

Figure 3
A typical configuration of light wood frame wall braced on both sides

Projects using the Light Wood Frame (LWF) system must specify the key characteristics of solid wood components, bracing panels, and enclosure components, addressing mechanical, physical, and chemical aspects to ensure adequate performance and structural safety throughout the building's lifespan. An important consideration regarding wood components is how the material interacts with moisture. According to Glass and Zelinka (2010), wood is a hygroscopic material, meaning it can absorb and release water from the environment. This dynamic is directly influenced by the relative humidity and temperature of the air and the moisture content present in the element. This hygroscopic nature requires that specific criteria related to the moisture dynamics of wood, as a structural material, both in solid sawn members and bracing panels, be considered during the design phase and component selection.

The main international standards for the design and selection criteria of structural wood components are:

1. EN 1995-1-1 Eurocode 5 (ECS, 2004);

2. Australian Standard - AS 1684.2 (AS, 2010); and

3. National Design Specification (NDS) for Wood Construction developed by the American Wood Council's (AWC, 2018).

These standards provide calculation methods for structural safety and criteria for the durability of wood components, considering, in addition to mechanical aspects, the influence of moisture content on the components and the minimum chemical treatments required to ensure protection against biological attacks.

As part of the factors for structural evaluation outlined by EN 1995-1-1 Eurocode 5 (ECS, 2004), two specific criteria related to the duration of the applied load and the moisture content in wood are measured, considering the hygroscopic nature of the material. These criteria are:

1. load-duration and moisture influences on strength; and

2. load-duration and moisture influences on deformations.

The inclusion of moisture content in calculating these coefficients, which govern structural design, highlights the dependence of wood's structural performance on the hygroscopic equilibrium of the designed components.

Bracing wood panels: classification influenced by exposure to moisture

There are two types of bracing panels used in Light Wood Frame (LWF) construction: OSB (Oriented Strand Board) panels and plywood panels, the main international standards for which are EN 300 (ECS, 2006) and ISO 12465 (ISO, 2007). Both standards classify these components based on their application and exposure to moisture, specifying the minimum performance criteria for each case.

The EN 300 standard categorizes OSB panels into four usage classes, as follows:

1. OSB/1: Panels for general, non-structural use in dry interior environments (including furniture applications);

2. OSB/2: Panels for structural use in dry environments;

3. OSB/3: Panels for structural use in environments exposed to moisture; and

4. OSB/4: Panels for structural use under high loads and exposure to moisture.

Plywood panels are also classified into four categories, each suited for specific applications. According to the American APA – The Engineered Wood Association, plywood panels are categorized as follows:

1. A: Plywood for decorative applications, without the need for surface treatments;

2. B: Plywood suitable for simple surface treatments, such as painting;

3. C: Plywood intended for faces that will receive a solid covering layer; and

4. D: Plywood designed for structural applications.

As wood-based materials, OSB and plywood panels are hygroscopic, meaning their properties are influenced by changes in relative humidity and the moisture content of their environment. These variations can significantly affect their performance. The British Technical Report CEN/TR 12872 (CEN, 2014) - Wood-based panels - guidance on the use of load-bearing boards in floors, walls and roofs - provides data on the typical dimensional changes observed in OSB panels with structural applications for 1% variation in moisture content. Table 3 illustrates this variation's impact on length, width, and thickness.

Table 3
Typical dimensional variations in length, width, and thickness for a 1% change in moisture content in structural OSB panels

Panel type	Dimensional change at 1% change in panel moisture content
	Length (%)	Width (%)	Thickness (%)
OSB/2	0.03	0.04	0.7
OSB/3	0.02	0.03	0.5
OSB/4	0.02	0.03	0.5

In this manuscript, the moisture analysis will focus on OSB panels, which are more commonly used in Light Wood Frame (LWF) construction in Brazil nowadays than plywood.

Waterproof and vapor-permeable barriers and their hygroscopic properties

A vapor barrier is a term that means a material used to prevent the passage of moisture in its liquid state and regulate, to varying degrees, the flow of vapor. Typically, these barriers are solid membranes or thin films made of plastic materials, either synthetic or non-synthetic fabrics. According to Quiroutte (1985), the primary function of a vapor barrier is to prevent or slow the movement of moisture through diffusion across the layers of a wall’s components. The rate at which water vapor migrates through these components depends mainly on two factors:

1. the vapor pressure difference between the interior and exterior environments of the wall; and

2. the resistance each layer has to the diffusion of moisture.

As noted by Lstiburek (2011), the proper placement of vapor barriers within the layers of a vertical enclosure system, along with the required permeability for effective moisture control, depends on external climatic conditions, the characteristics of the system components, and the temperature and relative humidity conditions within the building.

According to Geving and Holme (2013), in cold-climate countries, particularly in the Northern Hemisphere, vapor barriers are typically recommended on both sides of the wall, with the inner layer responsible for preventing interstitial condensation caused by vapor diffusion and inadvertent air infiltration from inside the building. This is because there is a relationship between the total moisture content of the wall and the interior vapor barrier, especially during the drying phase of construction moisture. In other words, the thicker the air layer equivalent, the lower the permeability and the lower the moisture content. Figure 4 shows the total moisture content in a wood frame wall during the drying phase of construction, varying resistance to vapor passage in the interior barrier (Sd) between 0.5 and 10 meters.

Figure 4
A Resistance to vapor passage in the interior barrier (Sd) between 0.5 and 10 meters

In Brazil, according to Silva and Oliveira (2019), vapor barriers have been integrated into the market as part of innovative light construction systems, designed to protect specific building components from moisture-related degradation. However, the authors note that envelope compositions are typically recommended with barriers with a certain level of vapor permeability in hot and humid climates. This allows for vapor diffusion and wall drying. Therefore, according to these authors, in Brazil, for all bioclimatic zones, it is better to adopt barriers that are vapor-permeable and impermeable to liquid water, because this barrier aims to avoid the entrance of liquid water but allow the vapor diffusion and the drying of materials.

The Canadian Standards Board (CSGB) proposes a classification system for vapor barriers based on their permeability to classify different materials. The permeability parameters are tested using E96/96M (ASTM, 2018). Table 4 presents a classification aligned with the permeability ranges associated with each material.

Table 4
Classification of vapor barriers by Canadian General Standards Board (CGSB 51.33: M89)

Classification	Permeance
Vapor barriers Class I	0.1 US perm or less
Vapor barriers Class II	1.0 US perm or less, and upper to 0.1 perm
Vapor barriers Class III	10 US perm or less, and upper to 1.0 perm

• Source: adapted from Canadian General Standards Board (1989).
• Note: US perm is a perm unit in USA.

Similar, to the CSGB, the American Organization International Building Code – IBC 2021 (ICC, 2021) also classifies vapor barriers based on their permeability parameters. However, unlike the Canadian organization, which specifies materials within classes I to III, the IBC (ICC, 2021) adopts four different categories, as outlined in Table 5.

Table 5
Classification of vapor barriers by American Organization International Building Code

Classification	Permeance
Vapor impermeable	0.1 US perm or less
Semi-impermeable to vapor	1.0 US perm or less, and upper to 0.1 perm
Semi- permeable to vapor	10 US perm or less, and upper to 1.0 perm
Permeable to vapor	Upper to 10 US perm

• Source: adapted from IBC 2021 (ICC, 2021).
• Note: US perm is a perm unit in USA.

In this manuscript, the vapor barrier is placed on the outer face of the wall analyzed.

Research method

The research that led to the results presented in this article was based on literature reviews, collection of information regarding the hygrothermal characteristics of wall components, and numerical simulations using the Energy Plus and WUFI Pro 6.5 software. The numerical simulations were used to analyze moisture flow values, relative humidity, and moisture content in the LWF wall system. Subsequently, the temperature difference in the monitored layer (internal bracing layer) throughout the year was evaluated, along with the corresponding dew point temperature to assess the risk of interstitial condensation.

Object of study and composition of the constructive system

The object of the simulations was a typical bathroom exterior wall (wet area), based on the standard geometry of a social housing unit, set on the SINAT Thermal Protocol (Brazil, 2021), with its construction components as specified by the SINAT 005 – Rev03 (Brazil, 2020) guidelines. Since state sanitary codes for social housing require only part of the bathroom's interior wall to be covered with ceramic tiles, the monitored region analyzed in this study considered the most critical in terms of moisture management is the internal face of the bathroom's exterior wall without ceramic internal cladding. Figure 5 illustrates this scenario and highlights the critical region, which is the focus of the thermodynamic and hygrothermal simulation and monitoring points.

Figure 5
Example of bathroom with partial ceramic of a Social Housing Building

Figure 6 illustrates the construction composition of the wall profile analyzed, with a 137,50mm thickness, indicating the typical building components of a bathroom exterior wall, as specified by the SINAT 005 – Rev03 (Brazil, 2020). Important to highline that the vapor barrier was considered only on the outer face. Table 6 presents the thickness of each material that compounds the wall profile analyzed.

Figure 6
Construction composition of the wall configuration analyzed, indicating the layers and the monitoring points

Table 6
Characteristics of the materials and components used in the wall subject to simulation

Material	Thickness	Apparent Density	Porosity	Specific Hot	Thermal Conductibility
	d [m]	ρ [kg/m³]	-	c [J/kg.K]	λ [W/mK]
Cement base Texture	0.003	1482	0.440	850	0.954
Basecoat	0.005	1395	0.460	850	0.540
Cement board	0.008	1380	0.479	840	0.245
OSB	0.0095	630	0.6	1400	0.13
Air layer	0.090	1.3	0.999	1000	0.523
Gypsum board	0.0125	625	0.730	850	0.200
Acrylic Paint	0.0001	1150	0.410	1600	0.220
Waterproof and vapor-permeable barrier	0.001	65	0.001	1500	2.3

Materials and components

For the input parameters of the materials and components analyzed in this study, the material database provided by WUFI Pro 6.5 was used, which is supported by extensive experimental validation carried out by the Fraunhofer IBP (Institute for Building Physics, Fraunhofer University). The only material with an alternative data source to the WUFI Pro 6.5 database was the 'economic acrylic latex paint,' which was used in the numerical simulations as the internal coating over the drywall board. This material was characterized based on experimental measurements conducted by Santos (2019). Table 6 presents the hygrothermal properties of the materials, components, and layers considered in the computational simulations.

The vapor barrier used in the simulation has a permeability of 56 US perm and is classified as "Vapor-permeable" according to the IBC 2021(ICC, 2021).

Bioclimatic zone and solar orientation

As the risk of interstitial condensation in buildings increases in cold climates—due to the greater drop in dew point temperature during colder periods the system analyzed in this study was simulated for the Brazilian bioclimatic zone 1M, as classified by NBR 15220-3 (ABNT, 2024). The climate file used was the Typical Meteorological Year (TMY) BRA_PR_Curitiba.869330_TMYx.2007-2021, representing the city of Curitiba, sourced from the Climate OneBuilding repository (https://climate.onebuilding.org/default.html).

In addition to defining the external climate, the most critical solar orientation was determined, following the recommendation of the ASHRAE 160 (ANSI, 2021) standard. This standard suggests using the solar orientation that experiences the highest annual amount of directed rainfall and the lowest solar radiation incidence. For the bioclimatic zone 1M assessed, the southeast orientation was defined as the critical one.

Software used

The computational programs used in the development of this study were WUFIPro 6.5 for hygrothermal simulations, EnergyPlus 9.4 for obtaining internal environmental conditions through thermodynamic simulations, and SketchUp Make with the OpenStudio 2.7.1 plugin for modeling and inputting the initial geometry and usage conditions of the standard SHU building design, which was used as a reference to determine the internal climate conditions.

The WUFI Pro is a software based on the calculation model proposed by Künzel (1995). It operates using dynamic modeling of simultaneous heat and moisture transport and is based on the finite volume method. This allows the analysis of systems composed of different layers by considering moisture storage functions specific to each material. It also allows the inclusion of experimental data as input. During the stage of defining the constructive system to be evaluated, WUFI provides a graphical interface for visualizing the composition of the layers. It also enables the insertion of monitoring points throughout the layers, allowing output data to be collected from any region or interface defined by the user. These monitoring points enable capturing various hygrothermal parameters, such as temperature, relative humidity, and vapor pressure in the analyzed layer (Bastos Ozelame, 2023).

Internal climate data and thermodynamic simulations

To determine the internal conditions for conducting the hygrothermal simulations (surface temperature and relative humidity of the analyzed thermal zone), thermodynamic simulations were first carried out using EnergyPlus 9.4. The windows of the bathroom were considered to be in the South direction. This process generated internal surface temperature profiles at the monitored point and the relative humidity for the analyzed thermal zone, which were then used as input data for the hygrothermal simulations. These data were entered into a standard internal climate data input spreadsheet provided by WUFI Pro 6.5, enabling them to be used in the hygrothermal simulation software to characterize the internal conditions of the evaluated system.

Then, to determine the internal data using EnergyPlus, the heat and moisture sources and the daily occupancy schedule were defined according to the SINAT Thermal Protocol (Brazil, 2021).

Regarding the ventilation schedule, it was considered to occur between 1:01 PM and 9:00 PM. The infiltration rate was converted to 0.022 m³/s•m². Window openings were modeled using the AirflowNetwork parameter, based on the recommended values from the computational simulation procedure in the performance standard, NBR 15575-1(ABNT, 2021). These values included the airflow coefficient for cracks (0.0024 kg/(s•m) for doors and 0.00063 kg/(s•m) for windows), the airflow exponent (0.59 for doors and 0.63 for windows), and the discharge coefficient (CD) of the openings (0.60 for both doors and windows). Terrain and wind pressure conditions were considered as the indicated standard.

Figure 7 illustrates the model adopted according to the SINAT Thermal Protocol (Brazil, 2021) along with the reference point used for the simulations and the North direction. The heat and moisture sources and the daily occupancy schedule were based on the guidelines set by this protocol.

Figure 7
The design modeled in SketchUp Make: (a) a view of the modeled building, (b) a view of the interior spaces, (c) adopted nomenclatures for the internal bathroom walls, (d) a reference monitoring point for extracting results from the thermodynamic simulations

Hygrothermal simulations

For conducting the hygrothermal simulations, WUFI Pro 6.5 requires, in addition to basic input data, the climate files for the evaluated location, the definition of the materials in each layer of the system, solar orientation, thermal coefficients of the simulated element, and the simulation period. Table 7 illustrates the initial conditions adopted for the simulations.

Table 7
Initial conditions for hygrothermal simulations

Variable	Adopted Condition
Facade Orientation	Southeast
Surface Inclination	90° (wall)
Building Height	< 10 meters
External Thermal Resistance (RSE)	0.04 (m²·K)/W
Absorptance (shortwave radiation)	0.5
Emissivity (longwave radiation)	0.9
Ground Reflectivity (shortwave)	0.2
Rain Reduction Factor	0.7
Calculated Directed Rain	ASHRAE 160 (ANSI, 2021) standard model for buildings less than 10 m high
Internal Thermal Resistance (RSI)	0.13 (m². K)/W
Initial Relative Humidity	80%
Initial Internal Temperature	20 °C
External Climate File	BRA_PR_Curitiba.869330_TMYx.2007-2021
Environmental Conditions Inside the Building	Obtained through thermodynamic simulations using EnergyPlus software
Simulation Duration	3 years (26.280 h)
Example building	Annex A, Diretriz SINAT 001 (Brazil, 2022)

Finally, as internal usage conditions, as described in the previous section, the hourly results of temperature and relative humidity from the thermodynamic simulations were input, thereby covering all the parameters required for conducting the hygrothermal simulations.

Results and discussion

In this section, the results of the hygrothermal simulations are presented, including the moisture flow on the interior surface of the wall, the relative humidity, the moisture content, and the risk of interstitial condensation in the monitored internal bracing layer.

Figure 8 illustrates the annual average temperature profile at the monitored point (section wall in that region – point 9 – internal OSB bracing board – Figure 6) and the indoor relative humidity levels generated by the thermal-energy simulations. These are compared to the external temperature and relative humidity values provided by the climatic data file. The results indicate that indoor temperatures consistently remain higher than outdoor temperatures throughout the year, with an average difference of 3.3 °C. Similarly, indoor relative humidity levels are consistently lower than outdoor levels, averaging 11.9% below the external relative humidity values.

Figure 8
Annual Average Values of Relative Humidity (Internal and External), External Temperature, and Internal Surface Temperature of the Evaluated Wall

Moisture flow

The internal moisture flow was evaluated and monitored on the internal surface layer of the bathroom environment. The operation of the bathroom was considered in simulation, based on the SINAT Thermal Protocol (Brazil, 2021) as indicated on item "5". Analyzing the moisture flow results aims to understand the predominant direction of moisture transfer and its intensity throughout the year. The sum of capillary moisture flow and diffusive moisture flow characterizes the total flow. Positive values of this parameter indicate vapor release into the environment (in this case, the internal environment). In contrast, negative values indicate vapor absorption by the system (towards the external side of the facade).

Figure 9 presents the profile of the total moisture flow on the system's internal surface. A predominantly positive flow can be observed, indicating a higher release of vapor into the interior of the environment throughout the year. This predominance is due to the diffusive moisture flow being the governing transport mechanism in this scenario, as the average external relative humidity is higher than the internal humidity throughout the year.

Figure 9
Internal surface moisture flow

Periods with negative flow are also observed, indicating instances where the internal moisture load exceeds the external load, resulting in vapor flow directed outward through the building envelope. This condition arises from internal moisture generation, such as evaporation caused by daily showers and other internal sources, such as cooking.

Moisture content and relative humidity

Figures 10 and 11 show the profiles of relative humidity and moisture content monitored in the internal bracing layer throughout the simulated period.

Figure 10
Annual Variation of Relative Humidity in the Internal Bracing Layer

Figure 11
Annual variation of moisture content in the internal bracing layer

As shown in Figure 10, the maximum relative humidity value recorded throughout the year reached around 80% in March, while the minimum was 65% in August. Despite these high values, it seems that there is no significant risk of interstitial condensation at the internal bracing layer because the temperature consistently remains above the dew point temperature throughout the year, as shown in Figure 12.

Figure 12
Temperature in the bracing layer and dew point temperature

Figure 11 further illustrates that the variation in moisture content within the bracing layer aligns with the relative humidity profile, reaching maximum and minimum values of 94 kg/m³ and 77 kg/m³, respectively, in March and August. This range of variation remains within the acceptable moisture content limits for the component. The database references indicate an initial moisture content for this material (OSB boards) ranging between 90 and 95 kg/m³.

Risk of interstitial condensation

To evaluate the risk of interstitial condensation, temperature values of the material and the corresponding dew (condensation) point temperature in the monitored internal bracing layer were extracted from the hygrothermal simulation results. Using this data, a spreadsheet was used to calculate the simple difference between the temperature of the bracing board monitored and the dew point temperature at the same point, based on the 8,760 hourly occurrences representing the entire last simulated year. This calculation was used to assess the potential for condensation (i.e., events where the temperature in the internal bracing layer fell below the dew point temperature).

Figure 12 illustrates the annual variation of the material temperature in the internal bracing layer alongside the corresponding dew point temperature. It is evident that the material temperature consistently remains above the dew point temperature throughout the year, indicating no significant risk of condensation.

Then, after processing the data for the 8,760 hourly occurrences representing the last simulated year, the temperatures in the monitored layer remained consistently above the dew point temperature throughout the observation period. This indicates that no risk of condensation occurred at the monitored point.

The results are consistent with Tronchin, Fabbri and Tommasino (1979), who show that interstitial condensation mainly occurs in cold climate zones and in specific construction typologies, particularly those using insulated materials. Poblete et al. (2023) also demonstrate that specific construction solutions, as timber, lead to interstitial condensation, but their simulations were conducted in Valdivia, which has a colder climate than any region in Brazil, including Bioclimatic Zone 1M. Additionally, they emphasize that indoor relative humidity is a key factor, especially in tightly sealed homes typical of cold climates.

Conclusions

For the configuration studied, no interstitial condensation was observed at the internal bracing layer, suggesting that the construction system has a good natural drainage capacity in the evaluated bioclimatic zone 1M, even though it is Brazil's coldest zone. However, the results presented in this paper only apply to the specific context and wall configuration analyzed. Therefore, to definitively conclude that interstitial condensation is not a concern for residential buildings in Brazil constructed with Light Wood Frame, further studies are required, mainly because only one configuration wall and only one bioclimatic zone were considered, and the hygrothermal behavior in different Brazilian regions is quite heterogeneous. It is also important to emphasize that, for the simulation, the walls had to be considered airtight due to the limitations of the WUFI software. Although this does not fully reflect the reality in Brazil, it can be seen as an advantage in some respects. However, this aspect must also be further studied. Such analyses would validate the simulation results against practical observations and verify the software's accuracy. It is also important to note that, even in the absence of condensation problems within the walls, the wooden components of the walls and floors must be treated against wood-destroying organisms, particularly termites, to ensure their durability over the building's Design Service Life (DSL). It would be interesting if future studies evaluated the risk of mold growth using the WUFI software.

Declaração de Disponibilidade de Dados

Os dados de pesquisa só estão disponíveis mediante solicitação.

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Edited by

• Editor:
  Enedir Ghisi

• Editora convidada:
  Luciani Somensi Lorenzi

Publication Dates

• Publication in this collection
  18 Aug 2025
• Date of issue
  2025

History

• Received
  27 Jan 2025
• Accepted
  18 May 2025