Hygrothermal behavior modeling of the hygroscopic envelopes of buildings: A dynamic co-simulation approach

High levels of humidity in buildings lead to building pathologies. Moisture also has an impact on the indoor air quality and the hygrothermal comfort of the building’s occupants. To better assess these pathologies, it is necessary to take into account the heat and moisture transfer between the building envelope and its indoor ambience. In this work, a new methodology was developed to predict the overall behavior of buildings, which combines two simulation tools: COMSOL Multiphysics© and TRNSYS. The first software is used for the modeling of heat, air and moisture transfer in multilayer porous walls (HAM model: Heat, Air and Moisture transfer), and the second is used to simulate the hygrothermal behavior of the building (BES model: Building Energy Simulation). The combined software applications dynamically solve the mass and energy conservation equations of the two physical models. The HAM-BES coupling efficiency was verified. In this paper, the use of a coupled (HAM-BES) co-simulation for the prediction of the hygrothermal behavior of building envelopes is discussed. Furthermore, the effect of the 2D HAM modeling on relative humidity variations within the building ambience is shown. The results confirm the importance of the HAM modeling in the envelope on the hygrothermal behavior and energy demand of buildings.     

Sensitivity analysis of CFD coupled non-isothermal heat and moisture modelling

CFD (Computational Fluid Dynamics) is a useful tool to study air flow patterns in a room. Current CFD models are able to simulate air flow combined with temperature distributions and species distributions. In this paper a coupled CFD–HAM model is discussed. This model combines CFD with a HAM model (Heat, Air and Moisture) for hygroscopic materials. This coupled model is able to simulate air flow around a porous material and combines this with heat and moisture transport in the porous material. Validation with a small scale experiment in which gypsum board was used as a hygroscopic material showed good results. In this paper a further validation of the model is discussed based on a sensitivity analysis of some model parameters. Especially hygrothermal parameters like sorption isotherm and water vapour permeability proved to have a non negligible influence on the modelling outcome. Adding a hysteresis model showed improvement of the model during desorption. The model was also used to compare two modelling strategies. In one strategy the gypsum board was modelled as a uniform material, in a second approach the material was modelled as being layered. The difference between the two approaches showed to be negligible.     

长江中下游地区梅雨季节的室内热湿环境

经过调研得到长江中下游地区(以南京为例)梅雨季节住宅建筑室内热湿状况,并分析3种不同建筑能耗计算模型(整体建筑热湿空气流动耦合模型HAM,传递函数模型CTF,有效湿渗透深度模型EMPD)的准确性。数值模型基于Matlab-Simulink编写,使用调研数据进行验证,进而使用梅雨季节典型气象参数模拟分析。调研结果显示在2013年梅雨季节,多数时间内建筑室内温度高于28℃,相对湿度高于70%。数值模拟结果显示3种能耗模型对室内温度模拟的差异较小,而对室内湿度的模拟存在较大差异,特别是CTF模型误差最大。结果显示在长江中下游地区梅雨季节,当房间换气次数小于2ACH时,围护结构对于室内环境湿缓冲的作用明显,选择合适的吸放湿材料可有效降低建筑能耗30%以上。     

Moisture buffering capacity of hygroscopic building materials: Experimental facilities and energy impact

Research into dynamic moisture storage in hygroscopic building materials has renewed interest in the moisture buffering capacity of building materials and shown the potential for these materials to improve indoor humidity, thermal comfort and indoor air quality in buildings. This paper complements previous research by estimating the effect of hygroscopic materials on energy consumptions in buildings. The results show that it may be possible to reduce heating and cooling energy consumption by up to 5% and 30%, respectively, when applying hygroscopic materials with well-controlled HVAC systems. The paper also describes two different experimental facilities that can be used to measure accurately the moisture buffering capacity of hygroscopic building materials. These facilities provide different convective transfer coefficients between the hygroscopic material and ambient air, ranging from natural convection in small, sealed jars to fully developed laminar and turbulent forced convection. The paper presents a numerical model and property data for spruce plywood which will be used in a companion paper [O.F. Osanyintola, P. Talukdar, C.J. Simonson, Effect of initial conditions, boundary conditions and thickness on the moisture buffering capacity of spruce plywood, Energy and Buildings (2006), doi:10.1016/j.enbuild.2006.03.024.] to provide additional insight into the design of an experiment to measure the moisture buffering capacity of hygroscopic materials.     

Simulation of coupled heat and moisture transfer in air-conditioned buildings

The simultaneous heat and moisture transfer in the building envelope has an important influence on the indoor environment and the overall performance of buildings. In this paper, a model for predicting whole building heat and moisture transfer was presented. Both heat and moisture transfer in the building envelope and indoor air were simultaneously considered; their interactions were modeled. The coupled model takes into account most of the main hygrothermal effects in buildings. The coupled system model was implemented in MATLAB-Simulink, and validated by using a series of published testing tools. The new program was applied to investigate the moisture transfer effect on indoor air humidity and building energy consumption under different climates. The results show that the use of more detailed simulation routines can result in improvements to the building's design for energy optimisation through the choice of proper hygroscopic materials, which would not be indicated by simpler calculation techniques.     

The effect of structures on indoor humidity--possibility to improve comfort and perceived air quality

The research presented in this paper shows that moisture transfer between indoor air and hygroscopic building structures can generally improve indoor humidity conditions. This is important because the literature shows that indoor humidity has a significant effect on occupant comfort, perceived air quality (PAQ), occupant health, building durability, material emissions, and energy consumption. Therefore, it appears possible to improve the quality of life of occupants when appropriately applying hygroscopic wood-based materials. The paper concentrates on the numerical investigation of a bedroom in a wooden building located in four European countries (Finland, Belgium, Germany, and Italy). The results show that moisture transfer between indoor air and the hygroscopic structure significantly reduces the peak indoor humidity. Based on correlations from the literature, which quantify the effect of temperature and humidity on comfort and PAQ for sedentary adults, hygroscopic structures can improve indoor comfort and air quality. In all the investigated climates, it is possible to improve the indoor conditions such that, as many as 10 more people of 100 are satisfied with the thermal comfort conditions (warm respiratory comfort) at the end of occupation. Similarly, the percent dissatisfied with PAQ can be 25% lower in the morning when permeable and hygroscopic structures are applied.     

多孔调湿材料湿缓冲特性的实验研究

利用NORDTEST实验方法对中国长江中下游地区(以南京为例)多孔调湿材料的湿缓冲值(Moisture Buffer Value)进行测定,同时,研究湿缓冲值对建筑能耗及室内湿度的影响。实验结果显示,高湿度区间内材料的湿缓冲测定值较大,且不同材料的湿缓冲值存在较大差异。分析表明在长江中下游地区,使用具有吸放湿特性的建筑内表面材料可有效降低建筑能耗10%以上,同时,室内环境湿度也会得到一定程度的调节。     

The building volume with hygroscopic materials—an analytical study of a classical building physics problem

A global analytical solution covering all cases of a building volume with hygroscopic materials is given. The mathematical and physical simplifications and assumptions are quite modest. Isothermality is not assumed. Examples are rooms, attics, subfloor spaces and building cavities. All share the same physics describing the vapour pressure in the building volume and the moisture content in the hygroscopic materials as a function of building volume temperature and moisture emission rates, external vapour pressure and building volume ventilation levels, heat and mass transfer between the building volume and the hygroscopic materials, and heat and mass storage and transfer within the hygroscopic materials.     

Potential effects of permeable and hygroscopic lightweight structures on thermal comfort and perceived IAQ in a cold climate

In this study, we simulated and measured the effect of permeable and hygroscopic lightweight structures on indoor air quality (IAQ) and thermal comfort in a cold climate. The potential effect of hygroscopic mass was assessed with the simulation of extreme cases, where permeable and hygroscopic lightweight structures with unfinished surfaces were compared with impermeable and non-hygroscopic ones. Measurements were conducted in 78 rooms of 46 newly built detached timber-framed houses and analyzed according to hygroscopic surface materials and envelope permeability. From the simulations, it was shown that permeable and hygroscopic structures considerably improved perceived air quality in summer, when a ventilation rate of 6 l/s pers. in the non-hygroscopic case corresponded roughly to 4 l/s pers. in the hygroscopic case. However, window airing and furnishing will reduce this difference in practice. Both simulated and measured results showed that permeable and hygroscopic structures significantly reduced peak indoor relative humidity levels and daily changes in relative humidity, but had no long-term effects. Measured results also indicated that completely non-hygroscopic houses did not exist in reality. PRACTICAL IMPLICATIONS: Limited knowledge is available about building envelope and ventilation system interactions with consequent effects on indoor climate. To take such effects adequately into account in design and construction of buildings, solid scientific data explaining the significance of the phenomena studied are needed. We have demonstrated that moisture exchange has evidently enough importance to be taken into account in future building simulation tools.     

Transfer function model and frequency domain validation of moisture sorption in air-conditioned buildings

Moisture transfer in building components and furnishings has significant effect on indoor air humidity and latent cooling load. Many mathematical models and calculation methods have been proposed to evaluate this effect. Simple but accurate models are what the users expect. Investigation shows that the one-dimensional linear moisture transfer model, which uses the vapor pressure as the unique driving potential, is both very simple and well match the real moisture transfer characteristics of most air-conditioned buildings. Frequency analysis shows completely consistent characteristics between the transfer function model of moisture sorption by interior surface materials and the one-dimensional linear moisture transfer model through the entire wall within the frequency range that should be concerned. It also shows that the moisture penetration across the wall is neglectable. The transfer function model of moisture sorption by interior surface materials is not only very simple but also has satisfactory accuracy to evaluate the moisture transfer effect of buildings on indoor air humidity and latent cooling load.     

Moisture performance of building materials: From material characterization to building simulation using the Moisture Buffer Value concept

Predicting the indoor air relative humidity evolution is of great importance to evaluate people thermal comfort, perceived air quality and energy consumption. In building environments, porous materials of the envelope and furniture act on the indoor air humidity by reducing its variations. Solving the physical processes involved inside the porous materials requires the knowledge of the material hygrothermal properties that needs multiple and, for some of them, time-consuming experimental procedures. Recently, both the NORDTEST Project and Japanese Industrial Standard described a new Moisture Buffer Capacity index that accounts for surrounding air vapor concentration variation. The Moisture Buffer Value (MBV) indicates the amount of water vapor that is transported in or out of a material, during a certain period of time, when the vapor concentration of the surrounding air varies. The MBV evaluation requires only one experimental procedure and its value permits a direct comparison of the building materials moisture performance. However, two limitations can be distinguished: first, no relation between the MBV and the usual material hygrothermal properties has been clearly identified and second, no model has been proposed to actually use the MBV in building simulation. The present study aims to solve these two problems. First, the MBV fundamentals are introduced and discussed; followed by its relation with the usual material properties. Then, a lumped model for building simulation, whose parameters can be determined from the MBV experimental procedure, is described. To finish, examples of the use of this MBV-based lumped model for moisture prediction in buildings are presented.     

Transfer function method to calculate moisture absorption and desorption in buildings

A transfer function approach has been described to calculate dynamic moisture absorption and desorption by building materials in the hygroscopic range, and the dynamic moisture absorption and desorption processes have been theoretically modeled. Their surface vapor pressure and moisture absorption and desorption flux can be evaluated by this approach. It has turned out mathematically that the material surface attains instantaneous moisture equilibrium with the surroundings at high Biot number (Bi → ∞), and that the material moisture behavior can be described through a lumped-parameter modeling at low Biot number (Bi → 0). For building materials at intermediate Biot number, comparisons with experimental results and numerical solutions have shown satisfactory agreement with the proposed approach. Using this approach, the dynamic response of moisture absorption and desorption in interior building materials to the variation of indoor air humidity and the dynamic effect of moisture absorption and desorption by building interior materials on indoor air humidity and space latent cooling load can be simultaneously calculated.     

Moisture response of building facades to wind-driven rain: Field measurements compared with numerical simulations

Wind-driven rain (WDR) is one of the most important boundary conditions governing the hygrothermal behaviour of building facades, which is usually numerically analysed with the so-called Heat-Air-Moisture (HAM) transfer models. In the traditional approach of HAM transfer models, WDR is implemented in a simplified manner: the total mass of all raindrops impinging on a certain surface area of a building facade during the time interval of the meteorological input data (typically 1 h) is spatially and temporally averaged and is supplied to the facade as an averaged moisture flux. However, real WDR is the sum of individual raindrops that impinge on the facade in a spatially and temporally discrete modus, and that do not only spread at impact, but may also splash or bounce off the facade. Therefore the reliability of this simplification can be questioned. To investigate its validity, a new experimental set-up was developed at a full-scale test building. It allows simultaneous and continuous measurements of the reference wind speed and direction, WDR intensity, outdoor air temperature and humidity, as well as the response of facade material samples to these environmental conditions. For this purpose, a measuring device was developed that monitors the weight change of the sample with a resolution of 5 mg. Temperatures at the interior and exterior material surfaces are also monitored. The whole measurement data set is used to check the validity of the traditional numerical approach. Large differences are found between the measurement and simulation results, which cannot solely be attributed to the uncertainty in the convective moisture transfer coefficient, but may be due to two additional reasons: the occurrence of splashing and bouncing at raindrop impact on the facade, which is not included in the model, and/or errors in surface moisture evaporation and absorption due to modelling the actual random and discrete raindrop impingement as a simplified averaged moisture flux.     

The effect of combining a relative-humidity-sensitive ventilation system with the moisture-buffering capacity of materials on indoor climate and energy efficiency of buildings

Indoor moisture management, which means keeping the indoor relative humidity (RH) at correct levels, is very important for whole building performance in terms of indoor air quality (IAQ), energy performance and durability of the building. In this study, the effect of combining a relative-humidity-sensitive (RHS) ventilation system with indoor moisture buffering materials was investigated. Four comprehensive heat–air–moisture (HAM) simulation tools were used to analyse the performance of different moisture management strategies in terms of IAQ and of energy efficiency. Despite some differences in results, a good agreement was found and similar trends were detected from the results, using the four different simulation tools. The results from simulations demonstrate that RHS ventilation reduces the spread between the minimum and maximum values of the RH in the indoor air and generates energy savings. Energy savings are achieved while keeping the RH at target level, not allowing for possible risk of condensations. The disadvantage of this type of demand controlled-ventilation is that other pollutants (such as CO₂) may exceed target values. This study also confirmed that the use of moisture-buffering materials is a very efficient way to reduce the amplitude of daily moisture variations. It was possible, by the combined effect of ventilation and wood as buffering material, to keep the indoor RH at a very stable level.     

Cross validation of hygrothermal properties of historical Chinese blue bricks with isothermal sorption experiments

The historical,cultural,and social value of heritage buildings mandates special protection of these structures.Blue brick is one kind of the main construction materials of her-itage buildings,which is porous material and easily subject to deterioration due to environ-mental factors such as humidity.Therefore,determining the dynamic moisture content rule of materials under fluctuant ambient humidity is necessary for preventative conservation.This study measured the equilibrium moisture content of eight types of Chinese blue bricks under isothermal conditions with varying humidity levels.The results show that when ambient hu-midity increased from 10％to 90％,the moisture sorption of historical Chinese blue bricks tripled,which is 3 times greater than that of modern Chinese blue bricks and 10(at low humid-ity)to 20 times(at high humidity)greater than that of clay bricks in the international hand-book.The results contribute to the improvement of the international building material database and research related to hygrothermal performance of heritage buildings in East Asia.     

Cross validation of hygrothermal properties of historical Chinese blue bricks with isothermal sorption experiments

The historical, cultural, and social value of heritage buildings mandates special protection of these structures. Blue brick is one kind of the main construction materials of heritage buildings, which is porous material and easily subject to deterioration due to environmental factors such as humidity. Therefore, determining the dynamic moisture content rule of materials under fluctuant ambient humidity is necessary for preventative conservation. This study measured the equilibrium moisture content of eight types of Chinese blue bricks under isothermal conditions with varying humidity levels. The results show that when ambient humidity increased from 10% to 90%, the moisture sorption of historical Chinese blue bricks tripled, which is 3 times greater than that of modern Chinese blue bricks and 10 (at low humidity) to 20 times (at high humidity) greater than that of clay bricks in the international handbook. The results contribute to the improvement of the international building material database and research related to hygrothermal performance of heritage buildings in East Asia.     

深圳地区建筑节能工程常见问题分析

本文总结相关建筑节能设计标准和施工验收规范的建筑节能检测要求,对建筑节能材料、空调系统、配电与照明系统节能检测中的常见问题进行分析,并针对这些问题提出了相应的解决方法,以促进建筑节能工程实施质量的提高。     

Study of moisture transfer in a double-layered wall with imperfect thermal and hydraulic contact resistances

The objective of this work is to study the effect of taking into account interface contact resistance on the prediction of moisture distribution through multilayered building envelope. Therefore, two mathematical models to describe coupled heat and mass transfer in double-layered porous materials have been investigated: one that considers imperfect contact between layers and another that ignores this phenomenon. Both models are one-dimensional and were implemented using finite difference technique with an implicit scheme. Numerical results are presented in terms of moisture distribution for a double-layered wall and compared with the experimental data available in the current literature. The comparison has shown that the model that disregards interface contact resistance between layers cannot predict correctly one-dimensional heat and moisture transfer within double-layered porous materials. The sensitivity analysis of the simulation parameters and the impact of contact resistances at the whole building level are presented in detail and their effect on the whole building level was analysed. Our results suggest that the thermal contact resistance is the most influent parameter on the moisture flux across the hydraulic contact interface. On the whole building level, simulations indicate that taking into account contact resistances had a slight effect on the indoor relative humidity but a noticeable effect on heating input energy. A decrease of 10% in energy consumption is obtained when contact resistances are considered.