Study Of A Moisture Sorption Calculation Method Of Building Wall Layer Of Hygroscopic Material And Its Simplified Model

Hygroscopic material in building wall layer,which can draw in the water vapour from indoor air in a high humidity profile and breathe out the water vapor in a low humidity profile,is usually used to moderate the indoor humidity passively without adding energy costs.However,as one kind of the new-type functional materials,the studies of its moisture buffering performance and its simplified dynamic moisture transfer models are not sufficient.In this paper,a simple moisture transfer calculation method of the hygroscopic material plate in building wall is proposed to investigate its moisture buffering performance.In addition,a simplified dynamic moisture transfer model as well as its parameter identification method are proposed and developed to simulate the moisture performance of the hygroscopic material under different conditions for practical application.Firstly,a theoretical moisture transfer model is built based on the theoretical moisture transfer theory.After that,the description of some traditional calculation methods as well as their advantages and disadvantages are briefed.To avoid their limitations,a simple moisture transfer calculation method,named as the Fourier Transform-Transfer Function(FT-TF)calculation method,is proposed.More details about the calculation principle,calculation equations and the procedure are presented later.Besides,an experiment test rig was established to measure the moisture adsorption and desorption between the hygroscopic material plate and indoor air for investigating the moisture performance.Meanwhile,the proposed FT-TF calculation method is validated by using the experiment measurements.The results show the predicted moisture flux by using the FT-TF calculation method can match the measured moisture flux well.Compared with the experimental measurements,the average relative errors of the moisture sorption between the model prediction and the measurement are 3.6%for Case 1 and 8.2%for Case 2 respectively.The results further indicate the proposed calculation method has a good accuracy and high efficiency.The FT-TF calculation method can be used to calculate the moisture buffering capacities(MBCs)of hygroscopic materials with various thicknesses,and an optimal moisture buffering thickness can be determined from the curve of MBC vs thickness.The optimal moisture buffering thickness of the clay plaster is 13.2 mm while the indoor air relative humidity maintains at a high-level for 12 hours and at a low-level for 12 hours.The proposed method for determining the optimal moisture buffering thickness can be achieved with high efficiency and good accuracy for practical applications by only using the physical properties of the hygroscopic material without any experiments.On the other hand,a theoretical optimal moisture buffering thickness is firstly deduced by using mathematical transform based on the physical properties of the hygroscopic material.Two kinds of optimal thicknesses,i.e.the ideal optimal moisture buffering thickness and the practical optimal moisture buffering thickness,are presented with coincident equation system based on the types of indoor humidity profiles.This study further proposes a building wall with double-layer hygroscopic material structure,and investigates its moisture buffering performance under multiple-period moisture load profiles.In addition,the arrangement and the thickness distribution of the double-layer hygroscopic material structure are optimized.The results show the semi-steady moisture buffering capacity and the semi-steady stored moisture of the double-layer hygroscopic material structure can be greatly improved by placing a hygroscopic material with high moisture moisture capacity as the deep layer and another hygroscopic material with high vapor permeability as the surface layer.For example,placing a thin layer of lime silica brick behind the cellulose insulation may greatly improve the stable daily absorbed moisture from 40.0g/m~2 to 58.4g/m~2 and the stable stored moisture from 20.1g/m~2 to 29.3g/m~2 under a specific composited moisture load profile.A proper combination of two kinds of hygroscopic materials can significantly improve their moisture characteristics.Finally,this paper proposes a simplified dynamic moisture transfer model of building wall layer of hygroscopic material,and presents the parameter identification method of the simplified model by using genetic algorithm,which compares the frequency characteristics of the simplified model with that of the theoretic model.The optimation of the order of the simplified is also presented.This simplified dynamic model is then validated by the experiment results of the established experiment rig.Based on the simplified model,a simulation platform is developed to simulate the indoor humidity environment of the building with hygroscopic materials,and then is validated by using some common exercises in IEA 41 project.This paper further investigates the moisture control performance of various hygroscopic materials with different area and various thicknesses under different climate for practical applications.