The role of the functional equivalence in lca of buildings and building products

Transcription

1 The role of the functional equivalence in lca of buildings and building products Monica Lavagna (1) (1) Building Environment Science and Technology Department, Politecnico di Milano, Italy Monica Lavagna Abstract The definition of the functional unit (or functional equivalent) is one of the most sensitive aspects of a LCA when the purpose is the comparison of alternatives. The results of comparative LCA evaluations of products and technical solutions can change significantly in relation to the different functional equivalent setted with reference to various performances in use. In this paper, it is stressed the importance that the comparisons (and the functional equivalent) should be based on the specific needs of the project and on the performances to be achieved at the level of building. The functional equivalent is a critical aspect also in the LCA comparison of buildings. The need of normalization of the results of LCA of buildings is often obtained by referring at a reference unit (as the surface or the volume of the building). The paper investigates different possible assumption of reference unit that can drastically change the results of LCA. 1. COMPARATIVE LCA STUDY AND FUNCTIONAL EQUIVALENT One of the possible purposes of Life Cycle Assessment is to evaluate the environmental impacts of alternative solutions, guiding the choice of a building product or of a building s project solution. In an LCA-based comparison, it is necessary to define the reference flow on the basis of the functional equivalent, ie the quantified functional requirements and/or technical requirements for a building or an assembled system (part of works) for use as a basis for comparison. [1] The functional unit, or functional equivalent, refers to the challenge of ensuring that two or more products (or buildings) provide the same level of service. Among all the different parameters of an LCA, the functional unit is often considered of prime importance. [2, 3] As first quantitative datum in an LCA calculation, its uncertainty can be more critical of the uncertainty of the following data. A relative uncertainty of 2 percent in each product flow yields, after 7 processes, an overall uncertainty (propagated uncertainty plus uncertainty in each flow) of about 15% [4]. 1.1 The functional equivalent in a LCA-based comparison of products A possible approach is to identify the functional equivalent starting from the knowledge of the properties and the performances of products, having identified the most representative 81

2 products on the market and, above all, having defined what is the performance more relevant in the comparison [5]. But all products are designed to perform one or more functions, providing one or more services, satisfying one or more requirements. For example, the function of a building component can be the heat insulation, but also the sound insulation, the durability, the fire resistance, the structural stability, the flexibility of use, etc. Compare alternatives with the same performance is not simple, since benefits vary from material to material and from product to product. In addition, performances are often in contrast each other (eg, thermal insulation and thermal inertia, breathability and water resistance), making it difficult to identify the best product in an absolute sense. The identification of the best product from an environmental perspective is a problem, because the comparison should be conducted with the same performance, but this is never possible. It is important that the functional unit is based on the requirements of the building in which the product will be placed. Effectively, products are chosen according to project requirements and taking into account the interrelationships that the product will have with other products within the building envelope. It is therefore incorrect to define "ecological" a product regardless of the knowledge of its location in a building, which requires certain benefits from that product. There aren't eco-friendly products in an absolute sense, but a product can be defined ecological only in relation with the performance required in a specific building. This approach allows to extend comparisons, defining more alternatives: it is possible to make a comparison between products of similar materials, but alternative (for example, two types of brick block); or between products that perform similar functions but are made of different materials (for example, two types of insulating material); or between different solutions to perform the same function (for example, changing the air through a window or through a mechanical ventilation system). Especially the last example shows a great role of the project in the identification of possible alternatives to be compared, highlighting the importance of defining the project requirements (the function that is requested to the building or to its parts) and the role of designer to identify unusual and new responses characterized by low-impact (for example through a reduction of the use of materials). For example, the requirement of avoid overheating during the summer of the transparent surfaces can be reached with two different project solutions: through the use of external solar shading or through the use of screen printing directly applied on glass surface. The second approach reduces drastically the amount of material required to meet the performance and therefore in the assessment surely will be the most environmentally (and economically) advantageous. At the same time, through this example emerges as a very distant solutions may have characteristics, behavior, performance very different from the starting function. For example, screen printing is obviously less flexible than a outer swivel shield; so, the risk is to reduce the solar gains in the winter earned through transparent surfaces and also the entrance of natural light. Moreover, the screen printing, as permanent feature, reduces the visibility from internal to external and imposes the inhabitants to a filtered vision of the exterior spaces. So a solution is choosen on the base of a primary function, or more than one, but the issues to be considered are multifaceted, and often very articulate, and the environmental impact is just one among many factors to be weighed in the choice process. The role of performance in use in the identification of the preferred solution is fundamental and it is difficult to identify a functional unit capable of expressing all the aspects involved in the path of choice. 82

3 1.2 The functional equivalent in a LCA-based comparison of buildings In the case of a comparison of buildings, the question is even more complex. It is really difficult taken into account all the requirements typical of a building into the definition of a functional equivalent. The functional equivalent of a building or an assembled system (part of works) shall include, but is not limited to, information on the following aspects: building type (e.g. office, factory etc.); relevant technical and functional requirements (e.g. the regulatory and client s specific requirements etc.); pattern of use (e.g. occupancy); required service life. The requirements become fixed when they are prescribed in the client's brief or in the project specification. [6] But in concrete, it is quite impossible to define two alternative buildings with the same characteristics and performances. It is more reasonable to use the reference unit. The assessment results of the buildings that have different functional equivalents can also be compared based on a common unit of reference. A reference unit may be dimensionless or qualified with a dimension (e.g. per m², per year, per employee, per room, ). [6] 2. THE ROLE OF PERFORMANCE IN COMPARATIVE LCA OF PRODUCTS Functionality and performances have an essential role to develope eco-products; the multifunctionality of products causes problems in the definition of the functional unit [7]. Suppose that the objective of a LCA is to compare the environmental profile of different types of thermal insulation materials for the construction of a vertical closure. In order to identify the reference flow (amount of material to meet the performance), the performance of thermal insulation (ie a functional unit as thermal resistence) will be selected as most important. Consequently, the most important properties of the material, considered in the comparison, will be the thermal conductivity and the density, since a lower conductivity allows to reach performance with less material and the density affects the amount of material in terms of weight (in LCA's studies, generally, the greater weight is related to greater impacts). The amount of material of the different types of insulating materials will be assessed through the use of LCA software or database, to find the material insulation with less impact. Table 1: Example of comparative assessment of products related to the functional unit of thermal insulation (in all tables above, the data are related to specific EPD of products). requirement: thermal insulation functional unit: thermal transmittance of 0.34 W/m 2 K (limit value according to italian Legislative Decree no. 311/06 climatic zone E) of 1 m 2 of opaque vertical closure expanded polystyrene (EPS) wood wool thermal conductivity λ (W/mK) 0,035 0,049 specific heat (J/kgK) thickness (m) 0,10 0,15 thermal transmittance 0,33 0,31 periodic thermal trasmittance 0,33 0,12 volume (m 3 ) 0,10 0,15 density ρ (kg/m 3 ) weight (kg) 1,9 37,5 embodied energy of the material (MJ/kg) embodied energy in 1 m 2 (MJ) For example, in the comparison between an expanded polystyrene (EPS) insulating and a wood fiber insulating, since the EPS is characterized by better performance of thermal 83

4 conductivity (requiring less material) and low density (being composed mainly of air bubbles), the EPS will be the material with a lower environmental impact. But this conclusion may be overturned by a different setting of the functional unit. If we change the functional unit, including other services, the result of the LCA will change. For example, if we consider, in addition to thermal insulation, also the thermal inertia, it will change the amount of material (reference flow) required to meet the functional unit. In particular, the EPS is a material with a very low thermal inertia, so, to ensure this performance it is necessary to combine it with another material capable to reach that benefit (for example a brick wall). Instead, the wood fiber has a thermal capacity that can satisfy the requirement of thermal inertia. So, in the second case one material, the wood fiber, is capable to satisfy two performances with the same amount of material, while in the first case, the EPS requires the use of another material (then the reference flow will be of two materials instead of one). The solution with the lowest impact becomes the one with the wood fiber. Table 2: Example of comparative assessment of products related to the functional unit of thermal insulation and thermal inertia. requirement: thermal insulation + thermal inertia functional unit: thermal transmittance of 0,34 W/m 2 K and periodic thermal trasmittance of 0,12 (limit value according to italian Legislative Decree no. 311/06) of 1 m 2 of opaque vertical closure EPS + brick blocks wood wool thermal conductivity λ (W/mK) 0,035 / 0,93 0,049 specific heat (J/kgK) 1450 / thickness (m) 0,10 + 0,25 0,15 thermal transmittance 0,30 0,31 periodic thermal trasmittance 0,12 0,12 volume (m 3 ) 0,10 + 0,25 0,15 density ρ (kg/m 3 ) 19 / weight (kg) 1, ,5 embodied energy of the material (MJ/kg) 130 / 3 20 embodied energy in 1 m 2 (MJ) = At this point should be defined the support of the components. In the case of the combination of EPS and brick, the brick masonry also performs the role of supporting himself and the other layers, so it is not necessary to provide for additional elements. In the case of wood fiber, to support the insulation and any additional layers, there must be a frame. Table 3: Example of comparative assessment of products related to the functional unit of thermal insulation, thermal inertia and mechanical resistance. requirement: thermal insulation + thermal inertia + mechanical resistance functional unit: thermal transmittance of 0,34 W/m 2 K and periodic thermal transmittance of 0,12 of 1 m 2 of opaque vertical closure self-sustaining EPS + brick bloc wood wool + steel frame (0,0008x0,25x1 m x 2 elements) volume (m 3 ) 0,10 + 0,25 0,15 + 0,0004 density ρ (kg/m 3 ) 19 / / 7800 weight (kg) 1, ,5 + 3,12 embodied energy of the material (MJ/kg) 130 / 3 20 / 35 embodied energy in 1 m 2 (MJ) = =

5 In this case, the incidence of the new frame makes this solution of higher impact than the brick with EPS. And again, the closure must ensure stability of performance over time and avoid interstitial condensation. Polystyrene is characterized by a high resistance to water vapor diffusion, not requiring the placement of a protective vapor barrier. Instead, the wood fiber has a high vapor permeability (μ = 5-10) and requires protective treatments or the inclusion of a vapor barrier, further raising the value of environmental impact. Table 4: Example of comparative assessment of products related to the functional unit of thermal insulation, thermal inertia, mechanical resistance and vapor impermeability. requirement: thermal insulation + thermal inertia + mechanical resistance + vapor barrier functional unit : thermal transmittance of 0,34 W/m 2 K and periodic thermal transmittance of 0,12 of 1 m 2 of opaque vertical closure self-supporting and vapor impermeable EPS + brick blocks wood wool + steel frame + vapor barrier in polyethylene volume (m 3 ) 0,10 + 0,25 0,15 + 0, ,0008 density ρ (kg/m 3 ) 19 / / 7800 / 920 weight (kg) 1, ,5 + 3,12 + 0,74 embodied energy of the material (MJ/kg) 130 / 3 20 / 35 / 90 embodied energy in 1 m 2 (MJ) = = 926 Finally, a closure must be protected by an inner and an outer coating. The types of coating that can be chosen in the two cases stem from the choices above: in the case of the traditional solution, you can refer to plaster, in the case of the lightweight solution it is necessary to provide prefabricated panels, usually in plasterboard for the interior and fiber cement to the outside. Generally prefabricated elements have a higher impact and consequently the lightweight solution turns out to be slightly more critical than the traditional solution. Table 5: Example of comparative assessment of products related to the functional unit of thermal insulation, thermal inertia, mechanical resistance, vapor impermeability and coatings. requirement: therma insulation + thermal inertia + mechanical resistance + vapor barrier + coating functional unit: thermal transmittance of 0,34 W/m 2 K and periodic thermal transmittance of 0,12 of 1 m 2 of opaque vertical closure self-sustaining, vapor impermeable and protected by internal external coatings external plaster + EPS + brick bloc + internal plaster external fibercement + wood wool + steel frame + vapor barrier in polyethylene + internal plasterboard volume (m 3 ) 0,10 + 0,25 + 0,015 0, ,15 + 0, , ,030 density ρ (kg/m 3 ) 19 / 720 / / 250 / 7800 / 920 / 900 weight (kg) 1, ,5 + 37,5 + 3,12 + 0, embodied energy of the material (MJ/kg) 130 / 3 /1,8 10 / 20 / 35 / 90 / 7 embodied energy in 1 m 2 (MJ) = = Obviously, the considerations developed here are related to the specific assumptions: if we had chosen different material, probably we would have been obtained different results. The example demonstrates how the functional unit is changed by moving from the product to the wall system, in relation to the requirements that the closure has to meet. In general, it should be noted that the increasing of the requirements increases the environmental impact because they increase the materials necessary to meet the performance. 85

6 Table 6: Definition of the building elements and materials needed to reach the performances in the two cases. opaque vertical closure requirement material of solution 1 material of solution 2 thermal insulation EPS wood wool thermal inertia + brick mechanical resistance + steel frame vapor impermeable + vapor barrier (polyethylene) coating + internal & external plaster + fibercement + plasterboard A building is the result of different products assembled into systems and of the interrelationships of the performance of products. The comparative assessment of products developed outside of the context of use and outside of the overall performance of the building is misleading. In this sense there are no ecological products: the environmental profile of the product depends on the application and use. The technical and functional requirements may include, for example, requirements for structural strength, fire resistance, indoor air quality, safety, adaptability, efficiency, accessibility, de-constructability, recyclability, maintainability, durability. These requirements define the peculiarity of the whole building, and, consequently, of the individual parts of the building works and of individual building elements. Not always the final performance is the result of the sum of the performances, but it s necessary to know the performances (and reliability) of individual building products in order to determine the overall performances. 3. THE REFERENCE UNIT OF THE RESULTS OF LCA OF BUILDINGS The results of an LCA of a building are not significant if it isn t possible to make them comparable to something. If the impact values are expressed in absolute terms (for example stating that a building has an embodied energy of 3,900 GJ) it becomes difficult to express an opinion on the fact that this value is high or low. So, it is important to refer the impact values to the size of the building. For this reason, it is appropriate to "normalize" the absolute values on the basis of the internal surface area of the building (for example stating that the building has an embodied energy of 2.6 GJ/m 2 ). Some protocols for the environmental assessment of buildings (such as the italian Protocollo Itaca, or SBTool) require this kind of standardization, in order to define benchmarking and thresholds. This type of normalization has the advantage of allowing comparisons with the results obtained with other assessment tools. For example, the energy certification uses as measure of the energy consumption the kwh/m 2 a, ie the kwh consumed during the building use "normalized" on the basis of the internal surface area (heated space). But the normalization of the LCA results based on the size of the building opens to a number of issues. First, it is not easy and obvious the decision of which is the reference unit of the building size: it can be m 2 or m 3. In the italian energy regulations, the limit values for energy consumption of buildings is expressed according to m 2 for the residence and according to m 3 for all other uses. This is because in the case of the residence the internal height is regulated (at least 2.70 m) and in general the construction companies choose to be close to the limit (to optimize the area to sold); whereby the measure normalized in m 2 is easily convertible in a measure in m 3. In the other cases (schools, offices, museums, libraries, etc..), the internal height can be highly variable and so the energy normative refers the consumptions to the m 3. 86

7 Another critical issue is that, while in the energy certification the energy consumption is clearly divided by the internal surface area or volume heated, in the case of environmental assessment, since the evaluation consider all parts of the building (for examples garage), it is more problematic the definition of the reference area and therefore it is subject to different interpretation. This opens the question of the role of accessory spaces (for example, how to consider the not heating spaces in the count) and of the intensity of use of space. A topic that should be clarified and standardized is how to quantify the intensity of use of building space in the LCA. This is especially critical for the "normalization" of the results, first of all to define the reference unit. For example, a particular problem is the role of underground parts of the building, which, moreover, generally determines the largest share of environmental impact: the "service" spaces (garages, basements, plant rooms etc..) are in relationship to the residential spaces, but cannot be counted as floor space with the same value of the living space. At the same time, the choice to omit their development (counting only the living area or volume of the building) is criticisable, because it is space available for inhabitants. A possible road, travelled by some authors [8], is the definition of a reduction factor, for example related to the commercial value of such spaces (considering 1/3 of their surface). But the commercial value does not have a firm foundation: in general the underground spaces used as integrative spaces of the apartment are sold at 50% compared to the cost of sales of living spaces; garages and basements can be sold at 10-20% compared to the cost of sales of living spaces. At this point, however, it should also be counted other ancillary spaces that have a commercial value: for example, loggias and balconies are usually sold at 50% of sales compared to the cost of sales of living spaces. Whereas accessories spaces (such as stairwells, entrance hall, and plant rooms) don't have commercial value. The issue becomes even more complicated if we consider some very innovative contemporary residential buildings, which provide community areas for cohousing. In these cases, many spaces in the building are accessories, used collectively by the inhabitants (laundry room, meeting or parties room, gym, kindergarten, etc.). These spaces have an effect on the commercial value, raising the housing quality at the time of the sale (the price of housing can rise up to 20%), but there isn't a close relationship between the amount of extra space and the increase of the value of the apartments. It is just a greater willingness to pay by the buyer according to the enhancement of quality of life in a building with more services. So, if the normalization is based on the commercial value, it tends to swing on the base of the assumptions on the quantification of ancillary spaces. One possible alternative is to find some normative reference (town planning laws). But in Italy, it is difficult to find useful information, because the ancillary spaces (stairs, lobbies, and retail spaces) and basements (cellars, garage, local plant) are not counted in the buildable volumes and thus their quantity is defined freely from the builder, according to the needs of the project and the cost containment. In this sense, therefore, it seems not correctly to take in count the auxiliary surfaces and it seems more correct to consider only the habitable surfaces or volumes. From one side, this assumption would lead to properly penalize those who build unnecessary ancillary spaces, encouraging instead the containment and the optimization of space. At the same time, the risk is to penalize those who create accessories spaces for the raising of the quality of housing, as in cases of cohousing. In addition to all these aspects, another critical element concerne how to treat the spaces under-used, especially in the case of buildings with special destinations (such as trade shows). 87

8 In these buildings, the use of spaces is reduced to a few scheduled events (which may be approximately 50 days per year). This leads to an underutilization of space that is not consider in the environmental assessment of the building, but is a negative element. The underspending is also harmful to the economic management, so much that the exhibition centres are trying to accommodate alternate functions (concerts, parades, etc..). Being able to consider the intensity of use of spaces (maybe related to the flexible management of spaces) is not simple, but it should be a matter to be investigated. 4. CONCLUSIONS Comparative LCA between different products are particularly critical if the assumptions to define the functional unit are imposed out of the context of use, taking into account the many kind of performances that contribute to the choice of a technical solution and the interrelationships with other elements of construction. Comparisons must be made in a building systems context, rather than on simple product-to-product bases. There aren't eco-friendly products in an absolute sense, but the environmental performance of products can be defined only in relation with the performance required to a product in a specific location within a specific building. Equally critical is the definition of the functional unit of buildings, necessary for comparative purposes. It must be able to consider various aspects such as intensity of use, durability, technical, functional and social performance, fundamental to the quality of life, but generally the functional unit (too complex to define at building level) is substitute by a reference unit, more simple but really difficult to define univocally. In perspective, it is necessary that the normalisation approach is clearly defined into standardisation documents, to allow comparable LCA results and to identify comparable benchmarking or limits in the certification or regulation processes. REFERENCES [1] Hischier, R., Reichart, I., Multifunctional Electronic Media Traditional Media. The Problem of an Adequate Functional Unit: A case study of a printed newspaper, an internet newspaper and a TV broadcast, Int J LCA 8 (4) (2003) [2] Cooper, J.S., Specifying Functional Units and Reference Flows for Comparable Alternatives, Int J LCA 8 (6) (2003) [3] Ciroth, A., Srocka, M., How to Obtain a Precise and Representative Estimate for Parameters in LCA. A case study for the functional unit, Int J LCA 13 (3) (2008) [4] ISO :2010. Sustainability in building construction. Framework for methods of assessment of the environmental performance of construction works. Part 1:Buildings [5] Weidema, B., Wenzel, H., Petersen, C., Hansen, K., The Product, Functional Unit and Reference Flows in LCA, Environmental News 70 (2004), Danish Ministry of Environment. [6] FprEN 15978:2011. Sustainability of construction works. Assessment of environmental performance of buildings. Calculation method [7] Lagerstedt, J., Luttropp, C., Lindfors, L.-G., Functional Priorities in LCA and Design for Environment, Int J LCA 8 (3) (2003) [8] Blengini, G.A., Di Carlo, T., Energy saving policies and low energy residential buildings: a LCA case study to support decision-makers in Piedmont (Italy), Int J LCA 15 (6) (2010). 88