Using passive solutions to improve thermal summer comfort in timber framed houses in South-west France

Transcription

1 Using passive solutions to improve thermal summer comfort in timber framed houses in South-west France Sylvain Boulet 1, Stéphanie Armand-Decker 2, FCBA Technological Institute I2M-TREFLE laboratory - Bordeaux University SUMMARY The timber construction offers many advantages for sustainability: wood is a renewable material and has a very low carbon footprint. One frequently advanced advantage of timber frame construction is that it can provide a very well insulated envelope. People living in such houses of course appreciate the low energy consumption, and many of them also enjoy comfortable winter living conditions, in particular. However, due to a lack of thermal mass, timber frame houses have a very low inertia causing summer discomfort in warm climate. The main purpose of the study was to develop passive systems for two timber-frame adjoining houses aim to improve thermal comfort in summer while reducing energy loads in winter. The buildings are located in the department of Gironde, in the South-west of France, which meteorological conditions are slightly warm enough in summer and temperate in winter. KEYWORDS Building Design, Measurements, Natural Ventilation, Thermal Comfort 1 INTRODUCTION The first part of the project focuses on the energy design of buildings using thermodynamic simulation tools and regulatory framework. Many technical solutions for the bioclimatic design of residences and the selection of energy systems were then proposed. Then we concentrated on the search of intelligent assemblies of space configurations, technical principles and innovative devices, suited for timber construction to meet the objectives of energy efficiency and comfort. Including the control of heat, air flow, sunlight, solar energy The final part focus on the validation of the design, once the residences are delivered and occupied, measurements were made using a complete instrumentation system to verify the energy performance of the buildings and the respect of summer comfort conditions. This paper presents a first analysis of the summer thermal comfort of two adjoined timber framed houses of the same surface. 2 CONTEXT Use of maritime pine wood The project plans to use mainly maritime pine wood as construction material. Are considered: wood frame, panels, fibrous insulation, joineries. Localization The Aquitaine climate is an oceanic moderated climate, characterized by mild and humid winters, while summers are relatively dry and warm. In a previous work undertaken by the TREFLE (Guiavarch and al., 2007), based on the analysis of the specific South-west France climate and the behaviour of occupants in wooden building, reveals the primacy of summer constraints compared to winter constraints as for the comfort of users.

2 Wood construction In winter energy consumption levels in timber framed houses are quite low thanks to the combination of wooden structure and efficient thermal insulation. However, summer is the Achilles heel of timber framed constructions; the low inertia is the main obstacle. Thus it is necessary to use several tricks to reduce heat gain into the building during the summer. In winter, we want to maximize solar heat gain and internal gain in winter and minimize them in summer. Architectural design Thermal inertia is necessary but not sufficient for comfort in winter or summer, whatever the mode of use of buildings. For summer comfort, the thermal inertia must be associated with cooling ventilation system. Indeed, once the building is hot after a sunny day: it is still hot... The timber-framed houses are very light: they have very low thermal inertia. We can add them by placing small concrete panels or sand in floors and walls but this is limited. Thus, in a hot climate like the Mediterranean, it is not clear that a timber-framed house can ensure a good summer comfort despite the undeniable advantages of wood construction. Bioclimatism is a way to design a building using passive solutions, thanks to its location, orientation, insulation, and interior design spaces. While distinguishing between the concepts of summer comfort and winter comfort: - Winter comfort: it will be useful to focus on a compact house (limit thermal bridges), well insulated, with few openings to the north, using building materials with high thermal inertia, with dense vegetation that protects the prevailing winds. - Summer comfort: avoid overheating (external blinds for windows, advanced roofs to block sunlight), deciduous vegetation in the south (their leaves provide shade in summer, and their bare branches let in the sun in winter). 3 MANAGEMENT OF SUMMER COMFORT Many studies have been carried out by varying multiple parameters such as components of the envelope, building orientation, solar protection, location and size of the window surfaces... These various studies coupled with architectural studies, structural calculations and economic studies led to the design of two architectural choices using two different constructive principles: A partially timber-framed house with additional thermal inertia including an intermediate floor of nailed laminated maritime pine wood. This residence is equipped with a natural ventilation system, to cool the building structure and storing freshness in the mass of the building. In the following section it will be called a housing 1. A timber-framed house with a ground coupled heat exchanger (geothermal system). The freshness of the soil is used to cool fresh air blown into the house. In the following section it will be called a housing 2. The addition of thermal inertia, mainly in the housing 1, coupled with an efficient night ventilation let us store the excess heat through high thermal inertia materials (plasterboard, insulating wood fiber board and massive timber floor) during the day and then the night-time remove with natural ventilation. To maximize the natural ventilation of the housing 1, ventilation shutters have been added to each window and between each room.

3 Figure 1. Ventilation shutters Housing 1 The free flow of air has proven itself as a vector of cooling in the vernacular architecture and in various achievements of bioclimatic architecture today. Due to a specific central staircase geometry and the creation of transfer windows in the envelope, controlled by specialized shutters. Housing 2, lighter, has been equipped with a ground coupled heat exchanger coupled to the ventilation system. The air management is provided by a group of double-flow ventilation with high yield heat recovery for the two houses. 4 INSTRUMENTATION AND RESULTS SUMMER THERMAL COMFORT MONITORING Continuous measurements The residences were equipped with full instrumentation system that allows continuous monitoring of comfort parameters. Monitoring is done on a full year. Two indicators of comfort were mainly used for the analysis: The maximum temperature reached in the house and the level of discomfort of Brager zone (Brager, 2000) (adaptive comfort approach). Adaptive indoor thermal comfort was analyzed according to the outdoor temperature on each residence. The figures below result from the results corresponding to the ground floor of both houses. Dotted and continuous lines corresponding to a range of temperatures with 90% and 80% acceptability. Figure 2. Adaptive comfort analysis Housing 1

4 Figure 3. Adaptive comfort analysis Housing 2 It is observed that the residence the most comfortable is the housing 2 with less than 5% of points outside the zone of 90% satisfaction. Figure 4. Analysis of measured temperatures An initial analysis of temperature recorded in both residences shows that the average temperature is acceptable, less than 26 C with an outdoor average close to 28 C. However, looking closer there is a significant overheating in the housing 1 (massive housing with a natural optimized ventilation system) with a temperature exceeding 31 C. This overheating is not significant in the housing 2 with the ground coupled heat exchanger. A survey was then conducted to understand the modes of occupation of the residences and their influence on the comfort of the building. It was thus requested from the occupants to define the comfort or discomfort perceived in housing and to detail the actions carried out such as closing or not shutters and windows. In housing 1, occupants have defined a comfort rather well with, however, a few moments of discomfort during the heat wave day. It turned out that the occupants enjoy the space and prefer to open the windows and shutters throughout the day at the risk to undergo the thermal consequences rather than feeling locked up. Housing being occupied most of the day and with the design of optimized natural ventilation, the temperature inside the housing generally follows the outside temperature. The occupants were still satisfied by the conditions of summer comfort. In housing 2 windows are generally closed during the day, sometimes the shutters also. The occupants find that housing has a very significant summer comfort.

5 Specific measures In parallel with measurements taken over the whole year, an evaluation of the hygrothermal summer comfort is conducted on both residences. Thermal comfort for the occupants of the houses depends on several physical quantities measured in the inside environment. These quantities, defined in the standards (ISO 7730, 2006), are the following: air temperature, airspeed, wet bulb globe temperature, humidity and the temperature of the walls. We use a thermal environment monitor which integrates the various measurement systems and permits a description of the environment coupled with several temperature sensors, independent of the thermal environment monitor. Based on the calculation of heat balance, the PPD index allows the prediction of the Percentage of People Dissatisfied by the indoor environment, the PPD index is calculated using the equation of Fanger (1970). Figure 5. Comparison of predicted percentages of dissatisfied The curves above correspond to the results of measurements on the ground floor in the living rooms of both residences. Through the comparison of the predicted percentages of dissatisfied calculated we find that the housing 2 has a degree of comfort much better than housing 1. Indeed, with a PPD between 5% and 15%, hygrothermal comfort conditions in the housing 2 prove to be quite acceptable for a summer day type. Instead, the housing 1 has a PPD over 30% dissatisfied when the heat peaks are found, in the middle of the afternoon. This is confirmed by the various interviews with the residents of these houses, the occupants of the housing 2 found the conditions of thermal comfort to be quite acceptable for the period. The responses of residents of the housing 1 corroborate the results of measurements, the surveys show that thermal comfort conditions in summer are considered quite good during periods of morning and night. As for the period the most "critical" in the afternoon, residents state that they feel uncomfortable situations, judging the environment of the ground floor generally warm for this period. As mentioned above, it appears that the residents of housing 1 tend to open the windows and shutters in the afternoon. This is not the through-ventilation, but the opening of the various external joinery which will create a draft, which is why we notice a very marked discomfort during the entire period of the afternoon: the temperature interior is always very close to the outdoor temperature. While residents of the housing 2 have a solar gain management allows them to keep a very acceptable comfort in their homes.

6 It is interesting to recall that it is very important to have a large part natural to have a satisfactory visual relationship with the outside to ensure visual comfort for residents. A multi-criteria analysis taking into account the criteria of hygrothermal and visual comfort seems interesting to pursue in order to find a solution best suited for residents who would satisfy the conditions of thermal and visual comfort. 5 CONCLUSION Finally the design carried out towards buildings with very low energy consumption with heating requirements of 7 kwh/m²/year and 6 kwh/m²/year corresponding to a primary energy consumption of 18 kwh/m²/year and 15,5 kwh/m²/year, which met the objectives. A major challenge of the project has been the management of summer comfort. Indeed, the constructive system chosen is a light envelope system with low thermal inertia. Many solutions have been studied to compensate for the lack of inertia of housing and ensure optimal comfort in summer. Finally two different residences were conceived: a massive housing with a massive intermediate floor associated with an optimized natural ventilation system and a lighter housing associated with the use of a ground coupled heat exchanger allowing to benefit the inertia of the ground. The residences were then built and an instrumentation system was installed. A first study was conducted during the summer 2010, showing that housing equipped with the ground coupled heat exchanger met the objectives and performance thanks to the ground exchanger and an adequate behavior of the users. These results were not as encouraging for massive housing, where the temperature interior bordered sometimes 32 C. But after analyzing the data and a survey carried out by the residents it appears that this was due to a permanent opening of the shutters and windows. Both homes have been designed to facilitate natural ventilation, they remarkably evacuate excess heat, but when the windows are deliberately open, interior temperatures can become very high. The systems used to manage the summer comfort seem to operate in both houses. But the coherence of such systems is hard to control and depends largely on the circumstances of human behavior. Installed systems require occupants warned and informed. A sociological study becomes essential in an area where the buildings become more efficient and where the change requires a modification in human habits. 6 REFERENCES Brager G.S. and De Dear R.J., «A standard for natural ventilation», October 2000, ASHRAE Journal, Vol 42. Fanger P. O., «Thermal Comfort» Mac Graw-Hill Book Company, New York, ISO 7730, «Ergonomics of the thermal environment - Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria», Guiavarch A., Clottes F. and Lagière P., «Démarche de conception de bâtiments passifs à usage tertiaire - Application à la construction bois en région Aquitaine», XXVèmes Rencontres Universitaires de Génie Civil, 2007.