Hygrothermal performance of a vapor-open envelope for subtropical climate, field test and model validation

Highlights

• A vapor-open wooden building envelope for subtropical conditions was applied to a residential building in central Japan.

• An insulation optimization design scheme was applied considering environmental, economic and hygrothermal aspects.

• The building has been monitored using a large number of temperature and humidity sensors inside the walls and the roof.

• The hygrothermal performance of the envelope was validated by the measurements and transient heat and moisture simulations.

• It was shown that the model can simulate the hygrothermal behavior of the envelope with a high accuracy.

Abstract

The construction industry is known to be a key contributor to manmade climate change. Amongst other nations, Japan has a building energy efficiency goal which does not yet emphasize the importance of the moisture safety design of well-insulated building envelopes considering its diverse climatic conditions. The authors developed a vapor-open wooden building envelope for the subtropical conditions of Japan and optimized a design method, which considers environmental, economic and hygrothermal aspects. As a case study, a detached residential building has been constructed in Ohmihachiman (central Japan). The building has been monitored using a large number of temperature and humidity sensors inside the walls and the roof. Results have been obtained from measurements over a period of one year. Furthermore, transient hygrothermal simulations using the measured exterior and interior climates have been carried out. It was shown that 1) the construction of the wall was successful with the desired level of air-tightness and 2) the simulation model by a commercial software is applicable for predicting the hygrothermal performance of the wall with the envelope system in the actual use condition.

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.