Traditional and passive house energy footprint calculation methods

Transcription

1 Chapter 6: Case-studies Traditional and passive house energy footprint calculation methods m. arh. Dan Stoian Politehnica University of Timisoara, Timisoara, Romania arh. Ioana Botea Politehnica University of Timisoara, Timisoara, Romania ph.d. eng. Valeriu Stoian Politehnica University of Timisoara, Timisoara, Romania ABSTRACT: We build our houses according to current norms regarding energy consumption and economy. But how accurate are the calculations in the case of new technologies? Can they deal with super insulated structures? Do they take into account the entire energy consumption and generation that takes place inside the building? We propose a critical approach upon two methods of energy consumption calculation and two energy-wise different technologies of building the same house. First we will analyze the traditional calculation method (as per current Romanian standards), second we will analyze the passive house calculation method (as defined by the Passivhaus Institut in Darmstadt, Germany). Both methods will be applied first for a traditionally built house as per current Romanian energy standards and second for the same house built this time as a passive house as defined by the Passivhaus Institut. The comparison aims to show the greater precision of the passivhaus method and also the lack of applicability of the traditional method in current passive house design. 1 INTRODUCTION 1.1 General Passive houses vs. traditional building techniques is an ongoing debate since a few years. The issue becomes more and more obvious when we take into account the very volatile energy prices on the market today. We propose an approach that takes into account the characteristics of each method and tries to shed some light onto the passive-traditional debate. 2 TRADITIONAL PASSIVE HOUSE PRINCIPLES IN COMPARISON 2.1 General A passive house is defined as a building that has a very low energy consumption (max. 15 kwh/m 2 /year for heating and cooling and a total energy footprint of less than 120 kwh/m 2 /year). A dwelling which achieves passive house standards usually includes a few features that distinguish it from mainstream builds. 2.2 Title, author and affiliation frame First of all passive houses will distinguish themselves through compact form (good A/V ratio) and good insulation. As such all components of the outer shell should achieve a U-value of at least 0.15 W/m 2 K, all this while conventional housing takes less into account the A/V ratio and limits the U-values at about W/m 2 K. 833

2 Portugal SB10: Sustainable Building Affordable to All Second, southern orientation and shading are taken into consideration as passive use of solar energy is a significant factor in pasive house design, whereas in traditional building planning consideration is given to a certain measure to north/south orientation but the improvements resulting from passive site design are often not taken into account. Windows are a crucial part of passive house design as they usually provide the weakest part of the outer shell energy-wise. As such the windows used in passive houses should have U- values not exceeding 0.80 W/m 2 K (glazing and frames combined) with a solar heat-gain coefficient of around 50%. Typical U-values in traditional houses are in the range of W/m 2 K. Figure 1 - Proposed ground floor plan of dwelling The building envelope should be airtight in passive houses insuring an air leakage rate of maximum 0.6 volumes per hour at a 50 Pa pressure difference between the interior and the exterior. Mainstream builds require only some airtightness, the norm actually takes into account a natural air change rate of at least 0.5 volumes per hour achievable by opening the windows. Other standards require an airtightness at least 10 times poorer than passive house standards. Other key points include a ventilation system with heat recovery running at an efficiency of at least 80% so that most of the heat is contained inside the house, preheating of the intake air for the ventilation system through underground heat exchangers (air-soil) so that the fresh air will be preheated to ca. 5 C even in wintertime. These measures have little significance for a traditional house with high air permeability where ventilation is usually achieved by opening the windows/doors or by using trickle vents or extract fans. Energy-saving appliances are a must in a passive house as the requirement of a total energy footprint of less than 120 kwh/m 2 /year imposes certain restrictions. Figure 2 - First floor plan of proposed dwelling 834

3 Chapter 6: Case-studies 2.3 Insulation As previously stated the building components of a passive house should have superior insulating properties. The limiting U-value of 0.15 W/m 2 K places the thickness of the insulation at around cm (Fig. 3, 4) given the use of traditional materials like polystyrene foam or mineral wool. With such a highly insulating outer shell extraordinary care must be taken to avoid thermal bridges altogether, posing a series of technical difficulties.the continuity of the insulation in a passive house is an important matter since heat losses through even small thermal bridges have a very large impact on the global energy balance. The entire heated volume of the building should be wrapped in a continuous insulating shell, from below the foundations, running up the walls and over the roof. Windows with U-values not exceeding 0.80 W/m 2 K (glazing and frames combined) with a solar heat-gain coefficient of around 50% usually have triple glazing and provide in wintertime for most of the heat gain necessary to maintain a confortable indoor climate. Special care must be shown towards better quality frames for the windows as they constitute ca % of the window surface and only contribute to the heat loss, exhibiting no solar gain. The superior insulating materials of a passive house also provide for a generally better indoor climate through higher inner surface temperatures in wintertime and lower in summertime, providing for a lack of cold surfaces detrimental to confort and a decreased chance of condensation as well as protection from the high summer temperatures. Figure 3 - Cross-section of proposed dwelling 2.4 Mechanical Passive houses must use, in order to take advantage of the great insulation otherwise and to insure a good quality of the inside air in an otherwise airtight house, a ventilation system with heat recovery. In order to make the most out of it the heat exchange efficiency should be higher than 80%, not difficult apparently, since commercial units achieve up to 92% efficiency. 835

4 Portugal SB10: Sustainable Building Affordable to All Figure 4 - Cross-section of traditional wall structure Figure 5 - Cross-section of passive house wall structure 836

5 Chapter 6: Case-studies 2.5 Building technique Insulation The requirements for passive houses also mean that certain problems have to be solved in order to attain strict conditions. The high thickness of insulation poses certain problems as it must be continuous. Special types of insulation must be used under the foundations in order to insure stability of the building. Particular details have to be addressed in suspending the insulation on the exterior of the walls, in order not to create thermal bridges. These problems are usually dealt with less care in a traditional building because of the lesser influence they have on the overall result Thermal bridges Thermal bridges become really relevant only in a highly insulated outer shell as an uninsulated building consists mostly of thermal bridges. Thermal bridge definition is somewhat different in a passive house compared to a traditional house. Generally passive houses are designed that the ψ factor of any thermal bridge is kept at a maximum of 0,010 W/(m²K) (Fig. 6). In these conditions any protrusion through the insulating shell must be carefully planned because otherwise the effect can be catastrophic on the energy balance Airtightness Particular attention is paid to good airtightness of a passive house. If a passive house is built in a brick and mortar fashion, usually the inner render layer doubles as an airtight layer. If the building technology is in wood or steel, different measures have to be taken: usually an airtight layer of PE membrane or a similar material is applied on the interior of the building, beneath the final render layer Windows Also important are the window details: windows should be mounted in such a manner that the fixing method does not allow for thermal bridges and provides for good airtightness (Fig. 7). Usually a separate PE or similar membrane is used to provide this level of airtightness. Figure 6 - Passive house roof to wall detail Figure 7 - Passive house window mounting detail 837

6 Portugal SB10: Sustainable Building Affordable to All 3 COMPARATIVE ENERGY CONSUMPTION EVALUATION 3.1 Algorithms for estimating energy consumption The energy consumption assessment has been developed according to two methods of calculation: the traditional calculation method and the passivehouse calculation method. While the traditional method can be applied to passive houses the results do not correspond with the passive ouse calculation method results. Opposite, the passive house method can be applied to traditional houses results being closer to each other. 3.2 Differences in assessment methods First and foremost we must point out that there are fundamental differences in the approach to energy consumption assessment according to each method. The traditional energy consumption calculation method uses a global insulation coefficient for the entire building (G) an average thermal transmittance value for the entire shell of the building. G is then used to determine the necessary energy per 1 m 2. The passivehouse method involves a calculation of all the heat losses separately through all the surfaces comprising the building envelope, taking into account different U-values for different materials which eventually yields a total energy consumption for the entire house, which is then divided by the area of the building in order to obtain the necessary energy per 1 m 2. Different factors also apply for shading, window to window connection heat loss, free heating energy as a result of building use resulting in a different result between the two methods. 3.3 Solar heat gain / shading While the passivehouse calculation method provides for detailed analysis of solar heat gain according to precise (to the nearest degree) cardinal orientation, angle of the window around a horizontal axis, the traditional method only differentiates between 5 main cardinal directions and assumes all windows at an angle equal to or higher than 30 degrees as vertical. The latter method does not allow for a precise calculation of solar heat gain. 3.4 Ventilation In the passivehouse calculation method there is provision for taking into account artificial ventilation with dehumidification factors while the traditional method applies a bulk coefficient for natural ventilation and assesses heat loss according to an estimated hourly ventilation rate. 3.5 Internal heat gain Passive house method takes into account for a dwelling ca. 2.1 W/m 2 of heating energy while the traditional method allows for 7 kwh/m 3 *year. The results are normalized in the table below. Table 1- Internal heat gain comparison (kwh/m 2 *year) Traditional Passivehouse Thermal bridges Both methods allow for thermal bridge loss calculation but the methods differ. This was not pursued here as both buildings were designed without thermal bridges. 838

7 Chapter 6: Case-studies 4 CONCLUSIONS 4.1 Calculation results The results are centralized in the following table: Table 2- Internal heat gain comparison (kwh/m 2 *year) Calculation method Difference Building method Traditional Passivehouse Traditional % Passivehouse % 4.2 Interpretation The assessment presented here takes into account the energy consumption needed for heating a residential building with a net flor area of 142 m 2 in winter, calculated according to the two methods. As the results show the difference in assessing a higly insulated dwelling are rather large. It is interesting to notice that the passivehouse method calculates a higher energy consumption for the traditional house while the traditional method assesses a larger energy need for the passive house, each one respective to the other calculation method. Basically the traditional method is useless in assessing a passivehouse, mainly because it can not take correctly into account the solar heat gain and because it lacks provision for assessing the ventilation system heat loss. The passivehouse calculation method shows results closer to the traditional method when applied to a traditional dwelling. REFERENCES *** Romanian norm for design and execution of thermal building insulation (In Romanian:Normativ pentru proiectarea si executia lucrarilor de izolatii termice la cladiri), Bucuresti:Institutul National de Cercetare-Dezvoltare in Constructii si Economia Constructiilor Sommer, A Passivhauser. Planung Konstruktion Details Beispiele. Koeln: Rudolf Muller GmbH & co. KG. Feist, W. & Pfluger, R. & Kaufmann, B. & Schnieders, J. & Kah, O Passive House Planning Package Technical information PHI-2007/1. Darmstadt: Passivhaus Institut. 839

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