Metal, wood, PVC. Three profoundly different materials in origin, production processes, performance and design solutions, each of which has large crowds of supporters who, from time to time considering the technical, economic, ecological and aesthetic features, decide on the alleged superiority of one over another. In recent years studies, and resulting intake of alleged advantages to bring to bear commercially, they are opposed and those who were waiting for the confirmation of its thesis – or at least the denial of those of others – from the study “Sustainability assessment of windows and curtain walls” are definitely disappointed by the final report of the research: none of the analyzed materials demonstrates a clear supremacy. But this result allows us to focus on the undeniable methodological value of the study, to highlight some fundamental aspects related to the sustainability of the windows and systems with transparent casing.
Commissioned by the European Aluminium Association, the research was carried out by the german engineering company Drees & Sommer Advanced Building Technologies, in collaboration with PE-International, an organization of experts in sustainability.
The study, conducted in Germany and published in January 2015, was inspired by the observation that the studies available so far on the subject do not analyze how the type of the material that makes up the backbone of the windows affects the performance of windows and facades. Consequently, the researchers assessed the sustainability aspects of types considered “standard”, that is:
- windows with double casement shutter and just one turn and tilt windows (building for residential uses) utilizing aluminum, wood, wood-aluminum and PVC frames;
- portions of the mullions and transoms structural facade (office building) employing aluminum, wood and wood-aluminum profiles.
The first part of the study dealt with the evaluation of the environmental and economic impacts. Here the stages of the methodology used:
- calculation of the thermal comfort and energy consumption, by means of a thermal simulation software, for each of the two different climatic zones considered (Berlin and Rome);
- calculating the economic cost of the life cycle (LCC) according to ISO 15686-5;
- life cycle assessment (LCA) according to ISO 14040 and EN 15804.
In the second part of the overall performance of the sustainability of the various systems of windows and curtain walls were compared with a quantitative methodology, based on criteria of Deutesche nachhaltiges Gesellschaft für Bauen (DGNB) and the Green Building Rating Scheme (GBRS), including: environmental impact, economic quality, social quality, technical and process quality.
In fact, according to the main protocols sustainability of the built, the building envelope is a critical component in the overall environmental assessment because it contributes of 10% to the average for the sustainability of the whole building. The assessment was based on sustainability criteria advanced by EN 15643/1 (Building life cycle stages), widely used in Europe in the field of real estate. Below related steps:
- comparison with protocols environmental certification of building sustainability (only for the part relating to windows and facades);
- depth development of a set of indicators to assess the sustainability of different materials.
Since the holistic assessment identifies a broad set of indicators (including sensitivity analysis), the research has focused on the scenarios most relevant to the assessment, in order to provide reliable results. Finally, the research has been subjected to eminent experts in the field, in order to understand their opinions and criticisms.
PHASES IN DETAIL
In a nutshell, here is the feedback resulting from the analysis above and detailed in 80 pages that compose the study.
The comparison of performance in terms of comfort and thermal energy consumption required during the operating phase showed no substantial differences between the various materials. In general, the differences in performance are the result of the different dimensions of the profiles used and, in the same climate zone, the variations observed not exceed 1.5%.
According to the Life Cycle Costing, PVC windows for residential uses show a cost of investment and operating less than other materials. Aluminium is the best among the materials considered for the curtains in the wall, with investment costs and overall lower than those of wood and wood-aluminum. Overall, the changes in operating costs were not higher than 20% of total costs.
In Life Cycle Assessment evaluation, the environmental aspects of the production, use and end of life phase have returned a global warming potential (GWP) substantially similar for all materials, both in the windows for residential uses and in curtains walls for offices.
The operation phase is predominant in relation to other as regards the energy requirements: for curtains walls is about 90% of the total, while the windows reaches 98%. As to the potential environmental risks wood is the most problematic result (because of the use of biocides, paints and solvents characterized by long time emission and decay) than PVC (in this case the stabilizing substances were considered too little concentrated) and aluminum.
The situation does not change substantially when considering the stages of production and disposal. On one side, in fact, the use of raw material from certified plantations (FSC or PEFC) has a significant environmental value, however, the study notes as the wood is characterized by a reduced degree of reuse.
Also because of its low commercial value, the latter is generally used for the production of energy, if not sent to landfill. In practice, the final step is considered a weak point in the sustainability of the wood. Different is the situation of the aluminum, which is systematically recycled with collection rates of nearly 100%, due to the high value of scrap: aluminum recycled products are, in fact, placed on the market at a price ranging between 50% and 75% compared to a product in virgin aluminum.
As for the PVC, study shows the presence of some technical limitations to the total recycling: PVC reused is generally incorporated within a layer of the first production PVC. Overall, the PVC is positioned one step below aluminum and wood.
In general, aluminum has resulted the material better overall in terms of potential for innovation and architectural design, resistance to weathering and maintenance needs. In addition, the aluminum systems meet all the requirements of fire safety, as opposed to wood and PVC.
As part of the curtains wall, the use of mullion and transom systems in wood identified limitations with regard to mechanical properties and freedom of design, especially because of the greater thickness of the profiles, and it results the material more difficult acquisition, due to the longer time of delivery.
Finally, in the case of windows for residential use the methodology used showed a score of 93% in favor of the aluminum (predictable result considering the transmittance values assumed), 84% for PVC, 83% for the aluminum-wood and 82% for the wood, with “best practice” of 99% for aluminum (technique, functionality, design) and 87% for PVC (LCC).
In the case of curtains walls for offices, aluminum systems get an overall sustainability performance of 92% (LCA, technical, functional and design), compared with 80% of the wood-aluminum and 76% for wood; in this case the “best practice” get the highest score.
But rather than dwell on the fact that systems with aluminum profiles are ranked in first place in both simulations, it should be noted that the study actually shows that the different materials have analyzed each of the significant “pro” and “con”. Consequently, no material is much more sustainable than the other. The only concrete result that emerged from this research concerns the jmportance that transparent surfaces play to the energy aspects related to the use of the buildings – fact, however, down amply demonstrated in previous studies. The challenge on the material more “sustainable” is therefore again postponed to future and more extensive research.