Hygrothermal Simulation Research Articles

Stochastic modelling of hygrothermal performance of highly insulated wood framed walls

As energy consumption has become an important issue in building design, most building codes require a higher insulation level for building envelopes to improve the building's energy efficiency. However, the highly insulated walls may lead to a higher risk of moisture problems. Although hygrothermal simulation has been widely used to investigate the moisture performance of wood framed walls, the uncertainties of input parameters such as material properties, boundary conditions and moisture loads, may lead to discrepancies between simulation results and actual performance of the envelope. This paper investigates the hygrothermal performance of highly insulated wood framed walls using a stochastic approach, which combines the Latin Hypercube Sampling method and Factorial Design to take into account the uncertainties of material properties, boundary conditions and moisture loads (air leakage and rain leakage). The investigated walls include an I-joist deep cavity wall, two exterior insulated walls, and a conventional 2 × 6 stud wall as the baseline. It is found that under the moisture loads introduced (i.e. air leakage and rain leakage), the exterior insulated walls have a lower mold growth risk than the deep cavity wall and the 2 × 6 stud baseline wall. The uncertainties of material properties do not result in significant variations in simulation results such as moisture content and mold growth index as uncertainties of moisture loads do. The hygrothermal performance of these highly insulated walls is more sensitive to moisture loads and the significance of the moisture loads (air leakage and rain leakage) depends on climatic conditions.

Evaluation of the hygrothermal properties of thermal rendering systems

The search for building envelopes with enhanced thermal properties is pursued to comply with European directives for lowering reference U-values. Thermal renders present low thermal conductivity, compared to traditional renders, and combine good thermal properties with easy application by mechanical spraying. The main goal of this work is to evaluate and compare the hygrothermal performance of three thermal render systems for different European climates. An extensive laboratory characterisation, measuring physical and hygrothermal material properties, was performed. It was verified that thermal conductivity linearly increases with water content, so thermal performance can be directly compromised if hygric behaviour is unfavourable. Porosity and microstructure were found to have a significant impact on other properties because the distribution of open and enclosed pores lead to different results. The high proportion of mesopores contributes to relevant moisture content during the lifetime of the building. The hygrothermal simulation demonstrated that the finishing coatings have a significant impact on the hygrothermal behaviour of the whole system. The application of thermally improved facades implies an increase in the temperature difference across different layers, especially in the thermal render, which could promote thermal stresses. As exterior insulation, the analysed systems showed that the simulated External Thermal Insulation Composite System (ETICS) exhibits good performance in general. However, the condensation potential is higher for ETICS, in particular, compared to thermal render systems. Consequently, an optimum compromise among thermal, hygric, and physical properties should be made.

Hygrothermal and environmental performance of a perlite-based insulating plaster for the energy retrofit of buildings

In the last few years, thermal insulating plasters have started to be an attractive solution for the insulation of already existing wall structures, especially old masonry ones, where refurbishment interventions can involve the replacement of damaged plasters.Intensive research efforts are being made to reduce the thermal conductivity and the environmental impact of these materials by optimizing their mixtures (combination of lightweight aggregates, binders and additives).In the present study, the hygrothermal performance and environmental impact of the different perlite-based plasters that are currently being developed have been investigated.A series of analyses has been carried out, at a material scale, by means of heat flow meter apparatus, to determine the relationship between the perlite content and the thermal properties. Moreover, the effect of the moisture content on λ has been analyzed, and the embodied energy and embodied carbon of the four mixtures have been assessed using both the cradle-to-gate and the cradle-to-site approaches.Furthermore, in-situ measurements have been conducted at a demonstration site, at a component scale, and a series of heat and moisture transfer simulations has been carried out to evaluate the actual thermal behaviour of the plaster under real operating conditions.The thermal conductivity values of the four plaster mixtures ranged from between 0.118 W/mK and 0.059 W/mK, thus demonstrating that the perlite concentration had a significant impact on the reduction of thermal conductivity and that the embodied energy of the applied material (5 cm thickness) decreased as the perlite content increased. Moreover, the results of the measurements on the demonstration building and the hygrothermal simulations have revealed that the thermal insulating plaster is able to reduce the U-value of the wall. However, an increase of 26–30% of the actual thermal conductivity should be considered when the material is exposed to real operating conditions.

Influence of microclimate control scenarios on energy consumption in the Gallery of the 19th-Century Polish Art in the Sukiennice (the former Cloth Hall) of The National Museum in Krakow

The main aim of preventive conservation, ensuring protection of historic collections, is stabilization of relative humidity, and for some materials also temperature, in the interiors of historic buildings or museums. The improper microclimatic parameters may cause irreversible damages. On the other hand, strict control of indoor climate is expensive. Definition of optimal microclimate control in terms of energy use, taking into account most important, sometimes even contradictory factors, requires interdisciplinary cooperation, among others in the field of conservation, collections’ care, architecture, building physics and HVAC systems. Due to the type of historic objects and specific microclimate requirements, hygrothermal simulations of whole historical buildings are essential for the most part of a case study. The paper presents an analysis of energy consumption according to various microclimate control scenarios for The National Museum in Kraków: The Gallery of 19th-Century Polish Art - located in Renaissance Cloth Hall in Kraków (UNESCO World Heritage Site). The analysis is based on the results of simulations performed with WUFI®PLUS software and in situ measurements. The microclimate control variants were developed, among others, on the basis of ASHRAE and ICOM-IIC guidelines. The research was supported by The Polish National Centre for Research and Development within “HERIVERDE” project.

Influence of driving rain and vapour diffusion on the hygrothermal performance of a hygroscopic and permeable building envelope

Most natural building materials are hygroscopic and permeable to water vapour. These two characteristics have the potential to improve the longevity and indoor air quality of buildings. However, the potential of winter condensation due to vapour diffusion and the risk of mold growth should be assessed for safeguarding the longevity of building assemblies.This study investigates the relative importance of driving rain, plaster capillarity and the presence of a vapour barrier on the moisture content of building materials and the risk of mold growth for a hygroscopic and permeable building envelope (HPBE). Hygrothermal simulations of a single-family house in Denmark mainly made of wood and clay are performed with WUFI. Results indicate that the presence of an overhang is essential to ensure the durability of a HPBE rendered with a capillary active lime-based plaster while the presence of an overhang has a negligible impact for a mineral cement-based plaster. Including a vapour barrier did not introduce significant changes on the moisture content of this wall assembly. Simulation results indicate that the type of plaster and the wind-driven rain exposure are the most critical variables affecting the hygrothermal performance of this wall assembly.

Moisture Transport in Cellular Concrete Walls with the Connector for Thermal Insulation

The paper presents the results of in-situ research studies and hygrothermal simulations for a fragment of prefabricated cellular concrete wall, insulated with Styrofoam, allowing for two types of external plasters. The disruption of heat and mass transfer is effected by the connector for thermal insulation mounted in the layer of cellular concrete. The carried out research studies and simulations have demonstrated that the impact of the insulation connector with a steel core is observed only locally, but its destructive impact should not be ignored.

Calibrated hygrothermal simulation models for historical buildings

Historic buildings are a living representation of our past and it is our duty to ensure that future generations have access to their heritage. In order to accomplish this, it is necessary to determine the conditions that the buildings are in and, if needs be, to make the necessary changes. In this latter stage, it is helpful to have a validated whole-building hygrothermal model since it takes into consideration most of the processes that affect their hygrothermal performance, thus allowing us to choose the most appropriate interventions.This paper aims to establish a validation process for historic buildings based on annual indoor conditions using simulation software. Hence, the indoor conditions is a 13th century church in Lisbon were monitored over a year. The model accuracy was assessed by comparing the simulated and measured temperature and water-vapour pressure, and quantified using the coefficient of determination, coefficient of variation of the root mean square error, normalized mean bias error and goodness of fit. The hygrothermal model was then validated by comparison against other models in literature and existing standards/guidelines.Four outdoor weather files were simulated to show the importance of monitoring the outdoor climate as closely as possible to the case-study. The soil/slab interface temperature was obtained using six different models. A sensitivity analysis was developed to optimize some of the inputs. Ultimately, the church hygrothermal model was validated. However, in hygrothermal models the simulation time can be long, therefore, a simplified model was developed using two of the four tested simplifications.

Evaluating the damage degree of cracking in facades using infrared thermography

The study of facade cracking is of great importance in the investigation of pathologies and degradation, as well as in the rehabilitation processes of buildings. Cracking is one of the major defects in facades; so assessing damage severity is necessary to define the best maintenance and rehabilitation strategies. The infrared thermography has been increasingly used as an inspection technique to identify and map facade defects and their severity. The use of hygrothermal simulation to understand the dynamics of the heat flux allows to observe important references that can help in the interpretation of the thermogram under different heating or cooling conditions. This study investigates the use of the quantitative passive thermography to measure the damage degree of cracks in a building facade. The crack depth was evaluated by the Delta-T values investigated in the three cracked regions. For the width, the variation of the studied temperature in a line transversal to the crack was used. The analysis was performed at a distance of 33 m from the facade. The conclusions corroborated the importance of the hygrothermal simulation as a pre-thermography technique, which made it possible to limit the period of investigation focusing on the phenomena of interest. It is also observed that it is possible to obtain quantitative values of the degree of damage by thermographic investigation. The measurement of the damage degree, in turn, brings important contribution to the application of the inspection technique.

Hygrothermal performance of Brazilian gypsum walls

The aim of this article is to evaluate the hygrothermal behaviour of gypsum walls in the climatic diversity of Brazil, based on the action and interaction between external climatic variables and the facade element. First, a laboratory characterization of material properties for the gypsum was carried out to determine their hygrothermal behaviour, particularly water vapour permeability, capillary absorption, retention curve and thermal conductivity. Second, the wall system performance was also evaluated in terms of its watertightness. It was concluded that the addition of a silicone water-repellent, typically used by the local industry, could lead to a classification of preventive against water. It was also concluded that a content of water-repellent above 0.5% would not result in additional benefits regarding watertightness. The second part of this work was focused on the evaluation of the hygrothermal behaviour of the standard and the water-resistant gypsum walls by means of hygrothermal numerical simulation. This analysis was intended to assess the influence of various factors such as the wall components (namely, wall coatings), the climatic conditions by geographical location and also the interior conditions, the orientation of the facades and the driving rain. The results demonstrated that the Brazilian climatic diversity can lead to very different exposure conditions. The locations in Interior Northeast Brazil, which are especially dry, corresponded to the ones with lower moisture content results. The results showed that gypsum masonry application on facades, with unfavourable climatic contexts, high precipitation and low solar exposure, would result in low durability. Only future works, including in situ studies may support the justification for applying the studied solutions in facades in favourable climatic conditions.

Radon, fungal spores and MVOCs reduction in crawl space house: A case study and crawl space development by hygrothermal modelling

In this case study was to investigate how ventilation of the crawl space will influence on concentrations of radon, fungal spores and MVOCs in the crawl space and indoors of detached house. The crawl space pressurisation by exhaust air from indoors was successful to prevent the convective flow of radon from the soil, but it increased microbial growth in the crawl space. After installation of the supply and exhaust ventilation in the crawl-space and in the living space, the concentrations of fungal spores in the crawl space and also entry of radon and MVOCs into a house decreased.A microbiologically safe crawl space was determined with hygrothermal simulation utilizing the Finnish Mould Growth Model and a two year examination period. The optional structures of the crawl space being depressurised with exhaust ventilation included an open base uncovered ground and various air-sealed closed structures. When mould growth of building materials was at medium resistant sensitivity class, mould was not observed during different air change rates in any of the examined structures. Open base uncovered gravel ground is a functional solution of a crawl space, only when there are no organic materials. The air-sealed ground structure is recommended build with concrete + insulation and when air exchange rate (ach) varied from 0.2 to 1 h−1. A concrete ground in the crawl space having ach from 0.2 to 0.6 h−1 is also very effective. XPS insulation and plastic sheet covered ground are not recommendable due to their high mould index.

Development of predictive models for the probabilistic moisture risk assessment of internal wall insulation

Solid wall buildings account for a quarter of the UK building stock and need to be thermally upgraded to meet national greenhouse gas emission targets. Internal wall insulation (IWI) is often the only option for the retrofit of solid walls, especially when they are of architectural or historical interest. However, IWI can lead to moisture accumulation within the existing wall, affecting the structural integrity of the building and the health of occupants. To avoid these issues, a thorough risk assessment is necessary.This paper presents a method for developing predictive meta-models that can be used for a fast probabilistic moisture risk assessment of IWI, considering both the uncertainty and variability of input variables. First, in a Monte Carlo analysis, the uncertainty and variability of inputs were propagated through hygrothermal simulations. Then, generalised additive models for location, scale and shape (GAMLSS) were used to describe the relationship between inputs and response variables of the Monte Carlo analysis. The key input variables were identified by a global sensitivity analysis - using the elementary effects method - and in model building. Two types of response variable were considered for the models: variables based on percentage values (e.g. maximum relative humidity) and dose-response relationships (e.g. mould index). The paper shows that both risk assessment models had a good predictive power, highlighting the suitability of the developed method for the moisture risk assessment of the internal insulation of solid walls.

Hygromorphic characterization of softwood under high resolution X-ray tomography for hygrothermal simulation

Because of their complex hygromorphic shape, microstructural study of wooden materials behavior has recently been the point of interest of researchers. The purpose of this study, in a first part, consists in characterizing by high resolution X-ray tomography the microstructural properties of spruce wood. In a second part, the subresulting geometrical parameters will be incorporated when evaluating the wooden hygrothermal transfers behavior. To do so, volume reconstructions of 3 Dimensional images (3D), obtained with a voxel size of 0.5 μm were achieved. The post-treatment of the corresponding volumes has given access to averages and standard deviations of lumens’ diameters and cell walls’ thicknesses. These results were performed for both early wood and latewood. Further, a segmentation approach for individualizing wood lumens was developed, which presents an important challenge in understanding localized physical properties. In this context, 3D heat and mass transfers within the real reconstructed geometries took place in order to highlight the effect of wood directions on the equivalent conductivity and moisture diffusion coefficients. Results confirm that the softwood cellular structure has a critical impact on the reliability of the studied physical parameters.

Study of Hygrothermal Processes in External Walls with Internal Insulation

Abstract Being an important contributor to the final energy consumption, historic buildings built before 1945 have high specific heating energy consumption compared to current energy standards and norms. However, they often cannot be insulated from the outside due to their heritage and culture value. Internal insulation is an alternative. However internal insulation faces challenges related to hygrothermal behaviour leading to mold growth, freezing, deterioration and other risks. The goal of this research is to link hygrothermal simulation results with experimental results for internally insulated historic brick masonry to assess correlation between simulated and measured data as well as the most influential parameters. The study is carried out by both a mathematical simulation tool and laboratory tests of historic masonry with internal insulation with four insulation materials (mineral wool, EPS, wood fiber and granulated aerogel) in a cold climate (average 4000 heating degree days). We found disparity between measured and simulated hygrothermal performance of studied constructions due to differences in material parameters and initial conditions of materials. The latter plays a more important role than material parameters. Under a steady state of conditions, the condensate tolerating system varies between 72.7 % and 80.5 % relative humidity, but in condensate limiting systems relative humidity variates between 73.3 % and 82.3 %. The temperature between the masonry wall and all insulation materials has stabilized on average at +10 °C. Mold corresponding to Mold index 3 was discovered on wood fiber mat.

Energy efficient retrofit of a prefabricated concrete panel building (Plattenbau) in Berlin by applying an aerogel based rendering to its façades

The first application of a commercially available aerogel based rendering to the external façades of a 30-m-high building made of prefabricated concrete panels (Plattenbau) for energy efficient retrofit has been reported. Hygro-thermal simulations of the total water content for different retrofit variations and wall orientations have been carried out to find the appropriate rendering finish in order to avoid moisture accumulations. In order to get independence from inaccurate assumptions on the primary status of the building the thermal performance before and after the retrofit measures has been determined. This has been carried out by means of infrared thermography and two methods for in-situ measurement of the U-value. For comparison a conventional perlite-based insulating rendering was also applied to a similar neighbouring building and investigated. The results have been discussed with respect to preliminary assumptions and theoretical calculations. Special technical measures for the application of this novel rendering to high rise buildings have also been addressed.

A rule-based expert system applied to moisture durability of building envelopes

The moisture durability of an envelope component such as a wall or roof is difficult to predict. Moisture durability depends on all the construction materials used, as well as the climate, orientation, air tightness, and indoor conditions. Modern building codes require more insulation and tighter construction but provide little guidance about how to ensure these energy-efficient assemblies remain moisture durable. Furthermore, as new products and materials are introduced, builders are increasingly uncertain about the long-term durability of their building envelope designs. Oak Ridge National Laboratory and the US Department of Energy’s Building America Program are applying a rule-based expert system methodology in a web tool to help designers determine whether a given wall design is likely to be moisture durable and provide expert guidance on moisture risk management specific to a wall design and climate. The expert system is populated with knowledge from both expert judgment and probabilistic hygrothermal simulation results.

Analysis of the Frost‐Induced Damage of Building Enclosures on the Territory of the Czech Republic

A comprehensive analysis of environmental loads across the Czech Republic in terms of frost‐induced damage is presented. Computational simulation of hygrothermal performance of eleven characteristic types of building envelopes, composed of both contemporary and historical materials, is performed at first. The exterior boundary conditions of the computational model are defined by a set of weather data characterizing the environmental conditions in the Czech Republic, which are acquired from 64 weather stations. The results of hygrothermal simulations are assessed using several specific damage functions. In this way, the basic datasets for the frost damage analysis are obtained. Their application as input parameters of a specially developed correction procedure based on elevation makes them possible to obtain a continuous coverage of the geographic area of the Czech Republic. Finally, isopleths of the supposed frost damage are drawn, depending on the envelope type, and damage maps are produced which may help the engineers to enhance the building envelope design process. The presented results indicate the necessity of paying attention to local environmental loads in the building enclosure design process and reveal both critical and favorable locations from the point of view of frost‐induced damage to buildings.

Monte Carlo Simulation to Evaluate Mould Growth in Walls: The Effect of Insulation, Orientation, and Finishing Coating

Mould growth can have severe consequences both on the health of occupants and on constructions’ durability. Mould growth is a very complex process that depends on many factors such as temperature and relative humidity, presence of nutrients, and exposure time. Several mould prediction models, which allow estimating mould growth in building components and performing risk analysis, are available in the literature, such as the updated VTT model or the Biohygrothermal model. A Portuguese typical wall configuration was used for a sensitivity analysis. The importance of insulation (with and without insulation), orientation (north and south), and finishing coating (gypsum‐based rendering, medium density fibreboard (mdf), and untreated wood) for the mould growth phenomenon was tested using both the updated VTT model and the Biohygrothermal model. A total of 12 case studies were investigated. The influence of indoor climate was evaluated by simulating 200 scenarios previously generated using the Monte Carlo method. Each of the scenarios has been applied to the 12 case studies, and 2400 hygrothermal simulations were carried out. Initially, the case studies were simulated using WUFI 1D since both mould growth models require the superficial temperature and relative humidity as input. Simulations were carried out for a one‐year period. The updated VTT model produced results (mould index—M) ranging between 0.4 (gypsum‐based rendering, insulated, and south oriented wall) and 5.9 (untreated wood, noninsulated, and north oriented wall) and the Biohygrothermal model (mould growth) between 10.1 and 406.4 mm for the same case studies. Despite that the effect of the orientation of the wall could be identified, the importance of insulation and nature of substrate was more evident. Although the two models produced overall comparable results, some differences could be found, creating the opportunity to discuss their strengths and weaknesses as well as their sensitivity to the input parameters.

Hygrothermal simulations and In-situ measurements of ultra-lightweight concrete panels

The aim of the paper is to investigate the hygrothermal properties of a newly developed building panel, made of ultra-lightweight concrete, encased cold-formed steel elements. It describes the hygrothermal simulations of the wall and roof panels, and based on results, the heat transfer coefficients and linear thermal transmittances are determined. The hygrothermal behavior of main structural joints (wall corner, wall-roof and wall-ground connections) is also simulated using real indoor and outdoor conditions. For validating the results, a model building was investigated.

An improved explicit scheme for whole-building hygrothermal simulation

Implicit schemes require important sub-iterations when dealing with highly nonlinear problems such as the combined heat and moisture transfer through porous building elements. The computational cost rises significantly when the whole-building is simulated, especially when there is important coupling among the building elements themselves with neighbouring zones and with HVAC (Heating Ventilation and Air Conditioning) systems. On the other hand, the classical Euler explicit scheme is generally not used because its stability condition imposes very fine time discretisation. Hence, this paper explores the use of an improved explicit approach - the Dufort-Frankel scheme - to overcome the disadvantage of the classical explicit one and to bring benefits that cannot be obtained by implicit methods. The Dufort-Frankel approach is first compared to the classical Euler implicit and explicit schemes to compute the solution of nonlinear heat and moisture transfer through porous materials. Then, the analysis of the Dufort-Frankel unconditionally stable explicit scheme is extended to the coupled heat and moisture balances on the scale of a one- and a two-zone building models. The Dufort-Frankel scheme has the benefits of being unconditionally stable, second-order accurate in time O(dt^2) and to compute explicitly the solution at each time step, avoiding costly sub-iterations. This approach may reduce the computational cost by twenty, as well as it may enable perfect synchronism for whole-building simulation and co-simulation.

PHIUS 基準に基づく超高断熱・高気密住宅の熱・湿気性能の数値予測に関する研究

PHIUS 基準に基づく超高断熱・高気密住宅の熱・湿気性能の数値予測に関する研究