Elsevier

Energy and Buildings

Volume 62, July 2013, Pages 486-495
Energy and Buildings

Effect of heat and mass coupled transfer combined with freezing process on building exterior envelope

https://doi.org/10.1016/j.enbuild.2013.03.012Get rights and content

Highlights

  • The relations of some parameters are presented to close the governing equations.

  • The changing of some envelope parameters with drying was analyzed.

  • Effect of freezing process on building exterior envelope parameters was analyzed.

Abstract

In cold severe area, the exterior layers of building envelope usually experience seasonal freezing/thawing in winter. However, it would lead to severe problems especially in the newly completed building. In order to analyze the drying of envelope on building initial use, the heat and moisture coupled transfer of building envelope in severe cold area Harbin, China was simulated. The modeled result was analyzed. It is concluded that the drying rate of the newly completed building envelope is significantly high in the first year, especially in the first few months. Insulation performance of the wall in the first winter is most serious due to the high initial moisture and freezing ice content in insulation layer. For the simulated envelope, the freezing of the moisture content especially that in insulation layer had notable effect on heat transfer coefficient (thermal resistance), and the maximum of modeled envelope heat transfer coefficient in the first year winter is about 7% higher than that in the tenth year, which is taken as final hygral state. Some parameters as apparent density, specific heat, average thermal diffusivity, heat storage coefficient and thermal inertia index, which were always considered as constants for a fixed building envelope are also changed with the heat and mass transfer of building envelope.

Introduction

The building envelope is typical multilayer porous media construction. The heat transfer, moisture transfer and air infiltration is a typical heat and mass coupled transfer process [1]. The moisture transferring in building materials can cause serious results as metal corrosion, structure deterioration and decreasing performance of building insulations. Also, the expansion and deformation of building envelope framework during freezing process have an important impact on building durability and energy consumption in the cold regions. So the study on heat and moisture transfer of building envelope is of great important.

The building porous material is full of damp air, liquid water or ice [2]. In the moist building materials, the heat transfers in following ways: the conduction happened between solid framework, liquid moisture and gas in pores; the convection and radiation happened in pores; the phase changing of water, vapor or ice happened in pores. Therefore, it is difficult to obtain an exact analysis of the transfer process. Most published literatures on heat and mass coupled transfer of building envelope were focus on that in quasi-steady state. However, for a newly completed building envelope with high initial moisture content, especially in its initial drying period, the heat and mass transfer is not in quasi-steady state but in a state with continually decreasing moisture content, which is a more complex process. So it is more difficult to study the drying process of a newly completed building envelope. Moreover, the published models are most focus on buildings in hot and humid region [3], [4], [5], which did not consider the freezing of liquid moisture. The temperature in building elements frequently decreases below 0 °C during winter in severe cold area, and the exterior layers of envelope would experience seasonal freezing and thawing. However, the seasonal freezing/thawing cycle can lead to many severe problems as significant effects on ecosystem and human infrastructures in cold regions, etc. Recently, there has been an increase in the need for evaluation of freeze damage and its effect on building thermalphysical parameters and energy consumption.

In published references, the literatures on other fields, as soil freezing and thawing in the cold regions [6], food preservation [7], wood drying [8], agriculture [9] and so on, related heat and mass transfer model with freezing process. However, their attentions were not accordant with that of building envelope. The literatures [10], [11], [12] especially on that of building envelope are surveyed as follows: the literature [10] studied the freezing process in building envelope by introducing an parameter of moisture chemistry potential. But they determined the liquid moisture freezing temperature of 0 °C. The literature [11] and [12] studied the heat and moisture coupled transfer model of building envelope in cold severe regions and its effect on the building heating energy performance. No references modeled the heat and moisture transfer of building envelope in severe cold area under realistic indoor and outdoor climate for years.

In this paper, the multilayer building envelope with wide variation of moisture content in severe cold area was modeled. Taking newly completed building envelope with 100 mm thickness EPS wall in the climatic condition of Harbin, China as example to model and analyze the freezing process character and thermophysical parameters changing with the drying process of building envelope. The modeling results are used to examine thermal effects of the newly completed building envelope drying process, and also can provide new insight and basic data for later study of building indoor humidity level, energy consumption, ecosystem and human infrastructures and so on in severe cold area.

Section snippets

Modeling

The moisture transfer through composite wall systems, even in a one-dimensional process, is a complex phenomenon that involves both liquid and vapor transport, and when combined with the wall thermal behavior, phase change may be present. According to Ref. [11], the elementary mass conservation equation can be rewritten as(ρiθi+ρlθl+ρvθv)t+(ρlθlklpckvθvpv)=m˙i+m˙l+m˙v

The energy conservation equation can be expressed asρcpTthm˙=(λeffT)When there is no freezing/thawing phenomenon

Envelope heat and mass coupled transfer model related parameters

In order to close the governing equations, the coupled parameters as the effective heat conductivity coefficient of porous material envelope, the relative humidity in pores, freezing temperature of liquid moisture, etc. deriving as a function of control parameters are presented in this section.

Simulation properties

As shown in Fig. 3, the analyzed 240 mm slag concrete wall with 100 mm exterior heat-insulation using EPS board was typical envelope composition for Harbin, China. The wall was covered with 10 mm layer of cement-lime mortar on the interior surface and 10 mm cement mortar on the exterior surface. The material properties used in the modeling are listed in Table 1. The wall initial temperature was 295 K. The initial liquid moisture volume percent of slag concrete was 22% [11], that of EPS board was

Result discussion

Fig. 5 shows the changing of liquid moisture volume percent distribution with time in the first year. When there was no exterior insulation layer, the slag concrete had a high drying rate at surface. However, when the insulation layer that had a high vapor diffusion resistance was installed, the drying rate decreased. And also the insulated performance of EPS board decreased due to the obvious damp accumulating at the interface around of slag concrete and insulation layer. The accumulated damp

Conclusions

In this paper, modeling method derived was used to simulate heat and moisture transfer of building envelope with freezing/thawing processes. For closing the governing equations, equilibrium relations of the effective heat conductivity coefficient of porous material envelope, the relative humidity in porosity, the freezing temperature of liquid moisture, etc. with control parameters are presented. As an example of simulation, the drying performance of a newly completed building envelope in

Acknowledgements

The project is supported by National Natural Science Foundation of China (Grant No. 51206190), the Postdoctoral Science Foundation of Central South University and Beijing Municipality Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering.

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