Robust moisture reference year methodology for hygrothermal simulations
Introduction
Good hygrothermal performance of building envelopes is crucial as moisture damage is one of the main causes of building envelope deterioration. Excessive moisture accumulation in building envelopes may lead to structural damage, mold growth, decrease of thermal resistance of building materials and degradation of indoor air quality. An effective moisture control in the building envelope ensures building long-term service life [1]. It is essential for designers and builders to understand the moisture conditions of a climate region to assess correctly building envelope durability. Traditional steady-state methods such as Glaser [2] and dew point methods [3] cannot predict the long-term moisture response of a wall assembly, in particular when rainwater is taken up by the walls. Numerical hygrothermal models, simulating the coupled transport of heat and moisture over varying environmental conditions, have the ability to provide the data necessary to assess long-term heat and moisture performance of wall assemblies and predict the risk of moisture damage. But hygrothermal models require the selection of representative climatic data including rainfall, air temperature, relative humidity, solar radiation, longwave radiation, wind speed and velocity as input, on yearly basis [4]. Representative climatic data should reflect the climatic variability subjected to the building envelope under consideration and provide the required level of safety with regard to moisture problems [5]. A common approach is to determine a reference year from available long-term climatic data.
Compared to energy reference years that use mean values of climate data for the locations under consideration [6], [7], [8], a moisture reference year should represent a climate that allows a correct evaluation of the moisture stress on the building envelope. Moisture problems are often caused by a combination of several extreme weather conditions. A 10% level criterion is often suggested to select moisture reference years. The choice of a 10% damage risk is justifiable, since this choice refers to a return period of 10 years, which for hygrothermal problems is appropriate, allowing the moisture accumulated during a bad year to dry out in the following years preventing long term deterioration [9]. Different methods have been used to define moisture reference years. Ali Mohamed and Hens [10] suggested that the critical moisture reference year (MRY) could be related with the annual mean outdoor temperature. By comparison, Geving [11] suggested that the MRY could be related to the annual mean outdoor relative humidity. Hagentoft and Harderup [12] presented that the MRY could be related to a Π factor, which describes the drying potential of a wall by defining the difference between the absolute humidity by volume at saturation and the actual absolute humidity by volume of the outdoor air. Kalamees and Vinha [5] used a method similar to a Π factor to select the MRY for evaluating the risk of water vapor condensation. In the above methods, the selection of the MRY is either based on a wetting potential or drying potential, not on both.
Cornick et al. [13] formulated a Moisture Index (MI) method to select MRY, which comprises both wetting and drying indices. The wetting index (WI) is based on the mean annual total horizontal rainfall or annual wind-driven rain load. The drying index (DI) represents the annual evaporation potential. It is defined as the sum of the hourly difference between the saturation vapor ratio and actual vapor ratio of the ambient air. As the wetting index and drying index have different units, they are normalized. The normalization scheme is given as follows:where I represents the annual wetting index or drying index. and represent respectively the minimum and maximum annual wetting or drying index over the considered years.
The Moisture Index is calculated based on the normalized wetting and drying indices using the following equation:
The advantage of the Moisture Index method is that it reflects actual environmental conditions as subjected to a wall during wetting or drying. However, there are several drawbacks with this method. Firstly, the drying potential is actually primarily influenced by the difference between the saturation vapor ratio at the wall surface and the water vapor ratio in the air, rather than the difference between saturation vapor ratio of the surrounding air and actual vapor ratio in the surrounding air as used in the MI method [14]. Secondly, wetting and drying potentials are very different physical quantities, showing different units. Finally, meteorological factors such as wind speed, short-wave solar radiation and long-wave radiation are not taken into account in the calculation of the drying index.
Therefore, an index with a more accurate representation of the drying process at the wall envelope surface is required. The drying of moisture from building envelopes occurs through evaporation. The evaporation process is affected by temperature, air humidity, wind speed, water availability and net radiation. Coming from soil science, the concept of potential evaporation is a measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation [15]. It is defined as the evaporation rate that occurs when a sufficient water source is available. Compared to actual evaporation, which is the quantity of water that is actually removed from a surface by evaporation, potential evaporation is independent of material type and structure and only depends on climatic conditions.
Moisture reference years can be selected based on the evaluation of different climatic variables. It is noted that the relation between the criteria for selecting a moisture reference year and the criteria for evaluating moisture performance is complex. Salonvaara et al. [16] stated that none of the existing methods for selecting a critical moisture reference year for hygrothermal simulations is satisfactory in terms of providing a known level of moisture performance using the RHT Index [17] to select moisture reference years that are among the most severe for the simulated structures.
The objective of this paper is to develop a methodology for the proper selection of moisture reference years required for hygrothermal performance studies. We propose the combined use of a Climatic Index for evaluating the climatic conditions and the RHT Index which allows the evaluation of the hygrothermal behavior. The Climatic Index comprises wetting and drying components, where the wetting component is based on the annual wind-driven rain load and the drying component on the annual potential evaporation. Hygrothermal simulations for four regions in Switzerland and for three different masonry wall systems are performed to predict temperature and moisture content distributions in the wall components. The RHT Index has been used to evaluate the hygrothermal performance of the different wall systems. The correlation between the Climatic Index and the RHT Index is evaluated. Moisture reference years are then selected using a combination of Climatic Index and RHT Index.
Section snippets
The Climatic Index approach
In general, the moisture conditions of building envelopes result from their wetting and drying behavior. A reference year for hygrothermal calculations should consider the critical climatic conditions which allow to evaluate the hygrothermal performance of a building envelope providing a required level of safety in terms of moisture damage. The moisture reference year should be selected by considering both wetting and drying components. Regarding the wetting component, wind-driven rain (WDR) is
Hygrothermal simulations
Hygrothermal modeling is used to calculate the temperature and moisture distributions in building envelope assemblies. The governing equations for moisture and heat transport are the same as those in HAMFEM [25], and are solved using the finite element solver COMSOL. The numerical heat and mass transfer model was validated in a HAMSTAD benchmark [26]. In this paper, we consider only 1D heat and mass transport in multilayer envelopes. In order to consider different wall assemblies, three types
Results
Hourly meteorological data from MeteoSwiss stations from the year 1981–2010 are used to calculate the annual WDR and the potential evaporation. Fig. 3, Fig. 4 show the WDR and potential evaporation roses for the year 1994. The WDR is dependent on the horizontal rainfall intensity, wind speed and wind direction. For Zurich and Geneva, a southwest-facing wall receives the largest amount of WDR. For Lugano, the wall orientation with the largest WDR is northwest. For Davos, due to the influence of
New procedure for selecting a moisture reference year
The common procedure for selecting a moisture reference year aims at finding that year that shows a 10% failure level, or a 90% safety level. For a 30-year dataset, this 10%-level year corresponds to the third wettest year. Using the Climate Index as moisture damage indicator, these years are the years from Table 2 with the third largest Climatic Index: 1993, 1983, 1993 and 1986 for Zurich, Geneva, Lugano and Davos, respectively. In order to evaluate whether these years are indeed 10%-level
Conclusions
A new index, called the Climatic Index, is proposed to evaluate the severity to which building envelopes are exposed to a certain outside climate considering both the wetting load and evaporation potential for a typical climate region. The Climate Index evaluates the balance between wetting and drying potentials and is used for the selection of a moisture reference year. For this analysis, three types of masonry wall assemblies (masonry wall without insulation, internally insulated masonry wall
Acknowledgement
The research presented in this paper is supported in part by funds from the Swiss Competence Center for Energy Research project ‘‘Future Energy Efficient Buildings and Districts’’, CTI.1155000149.
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