Study On Heat And Moisture Transfer In Exterior Wall Of Buildings In Severe Cold Area

Heat and moisture transfer process of the exterior wall has great influence on the performance of the buildings,especially in the severe cold area with great indoor and outdoor temperature difference.The common building damages caused by moisture transfer include shedding of the external surface and deliquescence powder of the inner surface.And the damages mentioned above are more serious in severe cold area than in other areas in China.This paper is based on the study of heat and moisture transfer of t he exterior wall in severe cold area,so as to have a better understanding of the heat and moisture transfer law of the exterior wall,then provide guidance for the architectural design in this area.The theory of heat and moisture transfer in the porous medium and the mathematical model of coupled heat and moisture transfer put forward by other researchers in the past ten years were introduced systematically in this paper.On the basis of the existing research achievements,a mathematical model had been established.Then,a series of calculations of the exterior wall which meet the building energy efficiency standards of the severe cold area were conducted using the special numerical calculation software CHAMPS-BES and influence of the heat and moisture transfer mechanism,wall structure,material properties and outdoor environment conditions were studied.Finally,the results of theoretical research and numerical simulation were combined to simplify the mathematical model of coupled heat and moisture transfer in wall.Through the study of this subject,it is found that the influence of air convection under indoor and outdoor air pressure difference on the heat and moisture transfer of wall is very small,and the influence of this factor can be ignored in mathematical modeling.When using the same kind of external thermal insulation material,the wall structure of ceramsite block can increase the volumetric moisture content of the insulation material slightly,as well as the risk of moisture damage.The wall structure with thermal insulation materials installed closer to the external surface makes the yearly decreasing of the volumetric moisture content more obviously.And it is beneficial to reduce the accumulation of moisture in the material and the ris k of condensation in the structural layer,meanwhile,the temperature fluctuation within the structure layer is reduced.Using the annual average value of environmental parameters outdoor to calculate the heat and moisture transfer of the wall can lead to great errors,and the accuracy of the calculation results can be improved by adopting the method of subsection average.