Building Physics - No Way around It

Transcription

1 I Building Physics - No Way around It G. J6hannesson and P. Levin Division of Building Technology, KTH - Royal Institute of Techno Sweden lm3 Abstract In recent years, growing interest in life cycle cost analysis regarding environmental and economic factors has focused on the choice of materials with regard to the chemical composition, embodied energy, emissions of gasses and other more or less quantifiable processes. In some cases, this concentration on material properties has lead to inferior solutions regarding thermal performance, moisture protection and durability in general. In this paper, an attempt is made to establish a relation between design that neglects common theories of building physics and the consequent environmental and economic cost. The study is based on a Swedish single-family house having a normal design where components are successively exchanged to materials and constructions that are today commonly known as being environmentally feasible. For each solution, the method estimates the impact on the use of energy and emission of greenhouse gasses. With additional information the need for maintenance, repair and replacement at some intervals should also be included in such a study. The analysis will always be partly built on unreliable input data and subjective reasoning. The aim here is not to give exact results but to place emphasis on the importance of a proper perspective for the technical design when considering the life cycle. For both analysed cases with the timber-framed wall and the different methods for airtightness it is evident that their role in the operation of the buildings dominates the differences in the environmental impacts of the construction itself. It can be concluded, from these examples that recommendations on environmentally feasible constructions based on unreliable information regarding the environmental impact of materials without considering the functionality of the material and the constructions as a whole over the lifetime of the building or building component, are to be considered as more or less organised desinformation to builders and buyers. Finding alternative building materials and more effective means for the building process is in no way to be considered as unimportant in our efforts for a better environment. However, the examples above show that the functionality of alternative solutions and materials must be considered when designing from the perspective of the life cycle of a construction before these can be recommended as fulfilling the requirements of both sustainability and functionality. Keywords: durability, embodied energy, emissions, energy use, performance, service. life

2 1 Introduction In recent years, growing interest in life cycle cost analysis regarding environmental and economical factors has focused on the choice of materials with regard to the chemical composition, embodied energy, emissions of gasses and other more or less quantifiable processes. In some cases, this emphasis on material properties has lead to inferior solutions regarding thermal performance, moisture protection and durability in general. In this paper, an attempt is made to establish a relation between design that neglects common theories of building physics and the consequent environmental and economic cost. The study is based on a Swedish single-family house having normal design where components are successively exchanged for materials and constructions that are today commonly known as being environmentally feasible. 2 Methodology When assessing the environmental impact of different building materials, the chemical composition and emissions have to be weighed to a comparative quantity. This is difficult but has been done using different approaches [ 11. The next difficulty is to choose between different materials based on an assessment of environmental impact. The difficulty here is that the environmental impact partly refers to the material and partly to the functionality of the material as a part of a complex building structure in the exterior envelope of a whole building. When making a choice between two materials with the same functionality the pure material properties can be considered. When choosing between materials with different functionalities the obvious risk is that the role of the material properties will dominate the functionality aspect and that this will lead to sub-optimisation that will not ultimately benefit the global environment. Also when a material with the same functionality but at a higher price is chosen, this has to be matched against the cost efficiency of other measures that could benefit the environment. 3 Case study of the vapour barrier in a timber-framed wall 3.1 Technical description of the timber-framed wall The timber-framed wall built in Sweden today normally consist of materials that have been developed to fulfil one or two functions, i.e. separate material layers are used for vapour barriers, efficient insulation, surfaces etc. This permits the wall to meet high demands on performance but also it becomes vulnerable to damages and poor workmanship. The layers of the timber-framed wall, starting from inside the interior cladding, consists of: gypsum board with paint, vapour barrier made of 0.2 mm polyethylene (PE) film, timber frame and light weight insulation where the timber part is assumed to be 15 %, wind barrier of low density tarred fibre board, a ventilated air gap and exterior cladding consisting of a wood panel.

3 The timber-framed wall does not have the same built-in natural airtightness as does, 3.2 Cost-benefit analysis for replacing the plastic film One of the major aims of the enthusiasts for a better environment, has been to remove or replace the plastic film that over the years has traditionally been used as a vapour barrier on the inside of timber-framed constructions. The function of the plastic film is partly to provide a high vapour resistance on the inside of the construction and to provide adequate airtightness. There are two different dominating strategies for replacing the plastic film. The first is to eliminate it and use insulation materials that are supposed to absorb and release moisture over an annual cycle. The second is to replace the film with some other technology, such as an interior surface cladding with surface treatment and carefully made joints to provide good airtightness. A cost-benefit analysis for plastic film and mineral wool batts replaced by cellulose insulation can serve as an example of the actual complexity of such problems: Eventual cost - Extra work and materials for interior cladding surface treatments and joints - Higher material cost for cellulose based insulation materials - Increased energy use due to reduced airtightness - Increased wall thickness due to higher thermal conductivity - Increased energy use due to higher humidity level in the insulation layer - Increased environmental cost due to reduced service life - Higher risk for emissions from the interior of the construction to the inside. Eventual benefits - Materials from renewable sources - The environmental cost of the plastic film is saved - Material and installation cost for the plastic film is saved - Has a market value since it is easy to convince ignorant buyers about the health hazard connected to the use of plastic film. Environmental data on construction materials, components and elements can vary within wide limits depending on where the products are produced and applied. We have chosen data provided by other researchers [l, 21 and have studied two alternative constructions, one with plastic film vapour barrier and mineral wool insulation batts (thermal conductivity W/K) and one without a vapour barrier and with cellulose insulation (thermal conductivity W/K). For the construction we summed the energy used and the greenhouse gasses emitted to the atmosphere in relation to the production of materials and construction. We referred to the references for the basic values. To relate these values to possible emissions owing to the energy used during the operation of the building over 70 years we used oil burned in a rather effective plant as a reference. These values represent a possible scenario but are by no means valid in all cases. The choice of oil as a reference source for heating is of course

4 disputable. Together with hydro power and nuclear fission the burning of oil is still a major source of heat in Sweden and one can assume that a great part of a marginal increase in energy consumption will be met with oil burners. Table 1 shows the total energy used and the most important greenhouse gas emissions for the two construction alternatives. Table 1. Used energy and air emissions for two wall construction alternatives including effects of energy use for heating during 70 years of operation. I Energy co so NO, Timber framed walls MJ/m2 g/m 2 g/m 2 g/m 2. Wood Vapour barrier Nails and screws Glasfibre wool batts Paint Gypsum board Wood fibre board Sum for construction Operation ,1 Constr. and operation Wood Nails and screws Cellulose insulation Paint Gypsum board Wood fibre board Sum for construction Operation Constr. and operation Total difference I I Replacing the mineral wool and plastic film with cellulose insulation reduces the embodied energy by 20 %, the CO2 and NOx emissions by 40 % and 5 % respectively, whereas the SOx emissions are increased by 4 %. When the operation of the wall with 130 degree-days per year over a period of 70 years is included, CO2 emissions increased by 140 %, SOx emissions by over 200 % and NOx emissions increased by 20 Oh. The above examples shows that choosing a material with lower thermal performance very rarely will be environmentally feasible even though the material itself may have less embedded energy and fewer greenhouse gas emissions. The difficulty with such a comparison is the choice of cases for comparison. Replacing the cellulose fibre insulation with mineral wool with higher thermal conductivity would also have given increased environmental impact. There is also an economic angle. If the two

5 construction alternatives had been given different insulation thickness to reach the same price per m2 for the construction a more expensive product like the cellulose fibre insulation alternative would have shown much higher environmental impact. The above analysis also assumes the same airtightness for both constructions. 3.3 Life cycle cost assessment of the timber-framed wall The building design process is a series of choices. In earlier times it was sufficient to make the monetary cost - benefit analysis. That is to regard the economy of the project from a present value perspective for a given specification based on technical and construction related aspects and as well on estethic performance. Now we should also consider the environmental impact of our choices. The economic and environmental impact can also be closely related. In a timber-framed construction the thickness of the timber frame for structural purposes is approximately 100 mm. To maintain a low U-value the thickness of the frame has to be increased beyond this just for insulation purposes. A wall with 200 mm mineral wool and a crossing-stud timber frame gives a U-value of approximately 0.20 W/m2K. If the wall is instead insulated with cellulose fibre insulation the U-value can be increased to 0.22 W/m2K. To compensate for the higher thermal conductivity the thickness has to be increased to 220 mm. The additional cost for the cellulose fibre insulation is about 40 SEK/m2 for the original thickness and about 25 SEK/m2 for the extra construction thickness. The additional cost for the same insulation performance is therefore 65 SEK/m2 or about SEK for a 150 m2 house. This amount could instead be invested in an air-to-air heat exchanger or an exhaust air heat pump saving 3000 kwh per year or 2.5 m3 of oil over a 70 year lifetime. The other choice would be to insulate with the same thickness resulting in an increase in energy use by 400 kwh per year or about 0.4 m3 of oil which is still large compared to the 30 1 of oil saved by not using the plastic film. The airtightness of a timber framed construction has normally been provided with a plastic film behind the interior cladding. The oil consumption for heating infiltrated air 2 3 is for a 150 m house with 0.5 air exchanges per hour approximately 50 m over a lifetime of 70 years. An inferior construction in term of airtightness can easily give an increase in infiltration of air exchanges per hour resulting in an extra consumption of 5-10 m3 of oil over the lifetime of the building which can be compared with the oil saved by not installing a plastic film being less than 100 kg. Owing to the relatively small impact of the construction compared with the operation, the impact of functionality of the construction dominates the total picture. This also means that the question of service life is suppressed looking at the construction for itself. If however the construction can be seen as the weakest link in the service life estimation for the whole building with its infrastructure and also including the possible negative effects of the decay on the construction on the indoor environment the picture becomes different.

6 4 Case study of caulking joints between walls and windows/doors This example is illustrated by a rectangular one story house of 10 by 12 meters. The window area is assumed to be 18 % of the floor area and the joint length is 4 m per window with two doors giving a total crack length of about 100 m. For simplicity we assume 25 m on each face of the building and no other air leakage paths present. The house is equipped with a mechanical exhaust and supply ventilation system that takes care of the desired airchange rate. Additional air leakage is undesired. The house is placed in Stockholm with an average heating season temperature of +2 C and 4 m/s wind speed. Indoor temperature is 20 C. Results from laboratory measurements of air leakage of different joints between windows/doors and wall frame, and the corresponding air leakage curves were found in literature [3, 41. The relations between air leakage flow and pressure difference from these measurements were expressed as: q = Ap.86 m3/hm for a joint sealed with mineral wool packing but without elastic compound and q = Apom7 m3/hm for a joint with elastic compound. For a well performed joint using polyurethane foam sealing the value was practically zero. To find a relevant seasonal average pressure difference for these air leakage paths, standard pressure coefficients for wind perpendicular to the long face of the building were assumed. Thus, the air infiltrates through the windward wall of the building and exfiltrates through the other walls. The building, including pressure coefficients, wind speed and temperatures were inserted in the IDA-MAE computer program to find the resulting pressure differences and air flows [5]. The calculated reduced air flow from using elastic compound in addition to mineral wool packing amounted to 8.6 m3/h for this house, which could be estimated to reduce heating need with about 300 kwh/year in the Stockholm climate. Using the polyurethane foam, which normally is installed without mineral wool packing, the reduction will be slightly larger. In Table 2, the environmental impact of three types of joints is estimated by total energy use and emissions to air. Also the saved emissions originating from energy saving (reduced oil burning) over a 20 year service life owing to the reduced air infiltration is also provided. In this example, a 20 year service life was chosen as reasonable for the three types of joints considering functionality and seasonal moisture and temperature movements. Environmental impact data is taken from previously mentioned sources and [6,7].

7 Table 2. Environmental impact for three types of joints for a single-storey house (100 m joint length) compared to the saved emissions over 20 years from reduced energy losses. Type of joint Mass Total co 2 so X NO, energy use kg/m joint MJ g g g 1 Polyurethane foam Butyl (PP base ext. filler) Backing rod (LDPE) Glassfibre wool packing Sum of materials Glassfibre wool packing Saved emissions over years (joint 2 compared to joint 3) As can be seen from Table 2, the impact of the saved emissions totally dominates over the impact from the materials for all of the investigated parameters. This example shows the importance to create building detail solutions that gives as low energy use as possible. A new trend that can be seen is to recommend packing with cellulose fibre insulation and omitting the caulking. Measurements are not available but assuming that the cellulose fibre insulation will give similar results as packing with only mineral wool, it is obvious that small environmental benefits looking only at the construction itself will quickly be eaten up during the first years of operation. 5 Conclusions From the above examples it can be concluded that recommendations on environmentally feasible constructions based on unreliable information concerning the environmental impact of materials, without first considering the functionality of materials and constructions as a whole over the lifetime of the building, must be considered as more or less organised desinformation to builders and buyers. Whether these are commercially or otherwise motivated, these kinds of recommendations could be harmful to the environment because they could lead to wrong decisions and also because they undermine the more serious long-time work for a better environment based on science and technology. Finding alternative building materials and more effective means for the building process is in no way to be considered as unimportant in our efforts for a better

8 environment. However, the examples above show that the functionality of alternative solutions and materials must be considered when designing from the perspective of the life cycle of a construction before these can be recommended as fulfilling the requirements of both sustainability and functionality. 6 References Nilsson, M., Gullberg, M. (1998) Externalities of Energy. Swedish Implementation of the ExternE Methodology. ISBN: Stockholm Environmental Institute. Erlandsson, M. (1995) Environmental Assessment of Building Components. TRITA-BYMA 1995: 1, KTH, Stockholm. Hoglund, I., Jansson, B. (1984) Joint Sealing between Window (Door) Frames and Walls. Conference Proceedings, Windows in Building Design and Maintenance, Gothenburg June Swedish Council for Building Research D13: 1984, Stockholm. Levin, P. (1991) Building Technology and Air Flow Control in Housing, Swedish Council for Building Research D 16: 199 1, Stockholm. Sahlin, P., Bring, A. (1995) The IDA Multizone Air Exchange Application, Version 1.Ol, May Bris Data AB and the Department of Building Sciences, Div. of Building Services Engineering, KTH, S Stockholm, Sweden. Boustead, I. (1993) Eco-profiles of the European plastics industry. Report 3 Polyethylene and polypropylene. The European Centre for Plastics in the Environment, Brussels. Boustead, I. (1997) Eco-profiles of the European plastics industry. Report 9: Polyurethane precursors (TDI, MDI, Polyols) (second edition). Association of Plastics Manufacturers in Europe, Brussels.