Hygrothermal performance of a vapor-open envelope for subtropical climate, field test and model validation
Introduction
The construction industry plays a significant role with regards to the global warming and resource depletion. 30–40% in the anthropogenic greenhouse gas emissions can be attributed to the built environment [1] [2]. In order to decrease the environmental impact of the construction sector, the governments of many countries have implemented building regulations with an emphasis on energy efficiency. Globally many countries orient themselves on the strategies of the European Union, having one of the longest traditions and most solid experiences of energy efficient buildings.
One of the European Union's goals is to reduce by 20% the primary energy consumption by 2020 [3]. In the construction sector which has the share of 40% of the European Union's total energy consumption, there is the requirement to construct all new buildings as nearly zero-energy building (nZEB) by 2020 [4].
Besides the standard driven improvements in the European construction industry, there are considerable voluntary efforts in the private sector which go beyond the minimum requirements by national regulations. Examples are German Passivehouse [5] and the Swiss MINERGIE® label [6] [7], in which the reduction of the energy consumption and the use renewable energy is strongly emphasized. These technologies and know-hows are seen as important references for the realization of the nZEB buildings in the future and also as potential export platforms for the building industry. Meanwhile, Passivehouses have been realized on all continents and under most climatic conditions.
The Japanese standard as of January 2016 is based on different climatic conditions from north to south. Each climate zone, which is shown in Fig. 1, has its own energy efficiency target value (for example 127.8 kWh/(m2·a) for Zone IV). The general plan is that 1) the current energy standard changes from a recommendation to an obligation for newly built buildings by 2020 and 2) all the newly built buildings are nZEBs on average by 2030.
When designing well insulated and air-tight buildings, the climate dependent moisture risk needs to be thoroughly investigated, in order to avoid humidity-related damage. Especially wooden buildings would require careful designing as wood is prone to biological degradation and mold growth under warm and humid conditions [8]. A number of empirical and numerical research have been carried out to investigate the thermal and hygric conditions in wooden buildings' envelope under various conditions. Kalamees et al. [9] conducted laboratory tests to validate various heat and moisture simulation software on the hygrothermal performance of various wooden envelopes. Simonson et al. [10] investigated vapor-open envelope in an actual building and proposed the diffusion resistance setup across the envelope under a cold climate. Pilot et al. [11] and Labat et al. [12] measured and simulated different wall configurations at an unoccupied building under the South-East France climate for three years.
The moisture transport occurs due to air leakage and moisture diffusion across the envelope. During the first deployments of the highly insulated houses in northern Japan, moisture accumulation has been observed in the external walls and in the foundations. This resulted in the growth of the mold Serpula lacrymance, which caused severe structural damage by deteriorating wooden elements [13] [14]. Eventually, it was revealed that the infiltration of warm room air towards the exterior through gaps between building elements was the cause of the moisture accumulation [15] [16]. These studies showed that the air infiltration was due to a lack of consideration of the airflow through the structural elements of the conventional post and beam construction, and it was causing not only the growth of mold but also resulted in substantial heating energy loss. As this type of infiltration is a common problem regarding condensation and the efficiency of heating all over Japan [17], several detail solutions were proposed by private associations and have been applied currently in conventional wooden frame buildings (for example [18]).
The moisture risk due to diffusion in building envelopes in Japan is not commonly studied. In fact, Japan has a challenging situation due to the diversity in climate from the north to the south. Especially in the central and southern parts of Japan, the subtropical conditions with hot-humid summer and cold-dry winter may require a robust solution to cope with the moisture risk which may be caused by the large vapor pressure gradient between the inside and outside of the envelope. The key problem is the change of the moisture flux direction throughout the year. The water vapor tends to flow inwards in summer and outwards in winter. In order to prevent moisture accumulation which may result in biological deterioration of building components and mold growth, it is important to take into account both the hygrothermal properties of the building envelope and the local climatic conditions. Saito et al. [19] [20] have established a practical design method which gives the requirement for the moisture resistance ratio of wooden wall assemblies to avoid problems caused by condensation. Although this method was validated by means of experiments and numerical simulations, it can be applied only to Japanese conventional post and beam constructions. Therefore, the modelling and its validation of the hygrothermal performance of different types of building envelopes under the Japanese climate is a research field which needs to be investigated extensively. Furthermore, even outside Japan few studies have investigated the hygrothermal performance of building envelopes applied to solid timber panels as a complete house system [21], however, some studies have carried out field and laboratory tests of such timber panel systems without actual occupancy behaviors [22] [23] [24].
An envelope system which has the characteristics to cope with the subtropical conditions and the diversity of the climate of Japan was developed by Goto et al. [22]. The layered structure of the envelope is shown in Fig. 2. The envelope consists of three main components; 1) clay board as the inner plating as thermal mass, 2) laminated timber panel as the load bearing element and 3) wood fiber board as the insulation. The concept's central idea is that all the main components have moderate moisture sorption capacity and water vapor permeability. Thanks to the layered structure, the thickness of each layer can be changed without influencing the other layers according to individual design conditions (local climate, targeted energy efficiency, user's preference and so on). The interior and exterior surface can be covered with any vapor open layer such as clay plaster and cladding for weather protection, respectively.
The hygrothermal performance of the wall was modeled by using a commercial transient heat and moisture transfer simulation software and the model was successfully validated by laboratory measurements [22]. Furthermore, the whole building heat and moisture balance model, which takes into account the moisture buffering by the interior materials, was developed in Ref. [25]. Finally, these models were applied to optimize the thickness of the insulation layer by considering the environmental impact and economic cost of the material used, energy consumption for heating and the durability of the building envelope based on the energy supply, economic condition and the diverse climates of Japan [26].
A residential building with the above envelope system was realized in Ohmihachiman (Japan) in June 2013 to be used as a test house in order to realize the final validation of the concept. The relative humidity and temperature at different points in the external walls and roof have been measured since October 2013. The present study aims to validate the transient heat and moisture simulation model of the envelope system using the measured data and assessing the moisture risk under real life conditions.
Section snippets
The design of the test house
The test house is a detached residential house for a family (two adults and a child) and situated at Ohmihachiman City (see Fig. 1). Fig. 3 shows the AMeDAS (Automated Meteorological Data Acquisition System [27]) temperature and relative humidity in daily average for 2014 in Hikone City, where the closest weather station to Ohmihachiman is located. The plan views of both the ground and the first floor as well as the south façade elevation are shown in Fig. 4.
The wall make-up considered two
Numerical model for transient heat and moisture transfer
In order to design moisture safe building envelopes, it is important to design them with validated numerical models. Several transient heat and moisture transfer simulation tools have been developed and became publicly available over the past decade (for example [32] [33] [34]). The application to wooden wall assemblies was investigated by various research (for example [9]) and it is widely accepted to be a valid methodology to simulate the hygrothermal performance of such walls. WUFI Pro 5.3
Measurement results
Fig. 7, Fig. 8 show the measurement results of temperature and relative humidity respectively throughout the year 2014. For the clarity of the graphic presentations, the figures show only the monitoring positions of 1 (the ventilated air gap as the exterior), 3 (the warm side of the insulation) and 5 (the interior). Due to technical problems (battery problems and the interruption of the local electricity supply), there were periods with no data acquisition.
The temperature of the monitoring
Conclusion
A vapor-open building envelope was applied in the subtropical condition taking into account the environmental impact, lifetime cost and the longevity of the wall elements. After the realization of a detached house in Ohmihachiman with the insulation thickness of 18 cm for the wall and 22 cm for the roof, the hygrothermal performance of the building envelope was measured with the actual users living in the house for a one-year period in 2014. The measured outdoor and indoor conditions were
Acknowledgement
The innovation promotion agency CTI of the Swiss Confederation (grant 9755.1 PFIW-IW) and the Building Technology Accelerator by EIT Climate-KIC is acknowledged for financial support. The authors express their special gratitude also to R. Paul (✝) of Swiss Building Components AG and Family Iida for the realization of the test house in Ohmihachiman. Further the technical support by R. Vonbank (Empa) and Decentlab GmbH is also greatly acknowledged.
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2022, Environmental Technology and InnovationCitation Excerpt :Some studies (Xu et al., 2016; Goto et al., 2016; Yang et al., 2009; Jin et al., 2021) have focused on humid subtropical climates with hot, humid summers and warm, humid winters. Wood is prone to biodegradation and mold growth under warm and humid conditions, therefore; Goto et al. (2016) developed a wooden building envelope open to water vapor for Japan. Yang et al. (2009) showed that different types of natural ventilation buildings in subtropical humid areas have insufficient indoor ventilation to meet the comfort of occupants, and 66.3% of the occupants hope for warmer winter.
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2021, Journal of Building EngineeringCitation Excerpt :Lee et al. [28] pointed out that the contradictory opinions on constructions are often based on overly simplistic evaluation models, and outlined a holistic approach to hygrothermal modelling for framed wall assemblies in both the field area of the wall and in the framing material itself. Goto et al. [29] suggested that when designing buildings with well insulation and airtightness, it is necessary to thoroughly investigate the climate-dependent moisture risk to avoid moisture-related damage. For example, the hygrothermal responses of wooden envelopes has been investigated by empirical and numerical research under different climatic conditions [30–32].
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