VACUUM INSULATION PANELS AND ARCHITECTURE: CRADLE-TO-CRADLE FAÇADE SYSTEMS

Martin TENPIERIK MSc Arch1

Arjan VAN TIMMEREN PhD MSc2 Wim VAN DER SPOEL PhD MSc3

Hans CAUBERG Prof MSc4

Keywords: facades, low energy design, cradle-to-cradle, vacuum insulation

Abstract After being used in refrigerators and transport containers for quite some time, vacuum insulation panels (VIPs) have recently been introduced to the building sector. A VIP is an insulation material consisting of a porous core material that is evacuated and then sealed into a barrier envelope. Mainly as a result of evacuation, the thermal conductivity of the material in the VIP’s core is a factor of 5 to 10 less than that of conventional thermal insulators. Vacuum insulation panels thus enable the development of very thin high performance architectural constructions. This reduction of thickness is among the most promising features for large-scale application of VIPs in buildings. Over the last decade, several buildings have been erected primarily in Switzerland and Germany acting as demonstration projects for the use of VIPs in buildings. However, in most projects this high performance insulation material replaces conventional thermal insulators, as a consequence of which the potential of vacuum insulation is not exploited to its full extent. The objective of this contribution therefore is to show how the properties of VIPs can be exploited in actual facade designs. Based upon a brief introduction into the properties of VIPs, requirements will be formulated for a new type of VIP integrated facade design. Based upon these requirements, a range of VIP integrated façade panels will be typologically introduced including critical thoughts regarding their fitness for the building industry and fitness for integrating VIPs. From this overview, two panel types, which are based upon C2C principles, will be discussed in more detail: the membrane panel and the membrane panel with stiffening elements. These design examples demonstrate the potential of VIPs for architectural constructions. With the proliferation and wide-spread application of VIPs in façade constructions, the energy performance of buildings in their occupational phase can be lowered considerably. In this respect, vacuum insulation panels can thus contribute to a more sustainable and healthy society. Figure 1 Typical example of a vacuum insulation panel with a core of fumes silica and metallised barrier.

1 Faculty of Architecture, Delft University of Technology, Delft, The Netherlands, m.j.tenpierik@tudelft.nl 2 Faculty of Architecture, Delft University of Technology, Delft, The Netherlands, a.vantimmeren@tudelft.nl 3 Faculty of Architecture, Delft University of Technology, Delft, The Netherlands, w.h.vanderspoel@tudelft.nl 4 Faculty of Architecture, Delft University of Technology, Delft, The Netherlands, h.cauberg@chri.nl

1. Introduction After being used in refrigerators and transport containers for quite some time, vacuum insulation panels (VIPs) have recently been introduced to the building sector due to tightened energy performance regulations. In light of sustainability, a reduction of greenhouse-gas emissions and consequently a cutback in primary energy generation with fossil fuels is needed. Besides the search for alternative energy sources, a reduction of the energy consumption of buildings is an important incentive for developing a sustainable energy provision. As a result, new thermal insulators with higher efficiency, like vacuum insulation, have been developed further for demanding applications, like buildings. A VIP is an insulation material consisting of a porous core material that is evacuated and then sealed into a thin barrier laminate or thick metal casing, as shown in Figure 1. In fact, VIPs, especially those based upon fumed silica, are quite analogous to evacuated packs of coffee: a powder evacuated and enclosed by a thin barrier. Mainly as a result of this evacuation, the thermal conductivity of the material in the VIP’s core is a factor of 5 to 10 less than that of conventional thermal insulators. This either results in reduced energy losses through or in a reduced thickness of the façade. The advantages of energy performance improvement and/or space-saving potential are among the most interesting properties for VIP application in buildings. However, because of the novelty of the material to building practitioners and owing to the care required for adequate and safe use, vacuum insulation panels have hardly been used in buildings until now. If integrated properly into buildings, their potential can be enormous, though. Simmler et al. (2005) even estimate that, if the entire existing non-insulated building stock of the European Union is insulated with 2 cm of vacuum insulation, the requirements of the Kyoto-protocol (UNFCCC, 1997) for the EU are easily met. Several researchers have already studied the use of VIPs in buildings. Binz et al (2005), for example, investigated exemplar projects in which they have been applied. These studies made clear that in practically all houses VIPs were applied in a traditional manner in which a conventional insulator was replaced by a VIP. Moreover, several manufacturers of façade systems filled the cavity of double glazing, or similar cavity panels with VIPs. However, due to large thermal edge effects caused by a spacer along the panel’s edge, the favourable thermal properties of VIPs are rendered ineffective. Besides, Hangleiter and Weismann (2006)  developed a precast concrete based façade system having as disadvantage that it is difficult to check whether the VIPs are still intact after installation. As part of his doctoral research, Cremers (2006a), finally, designed a façade system with a double layer of VIP protected by a metallic or polymeric sheet and stabilized on one or two sides by a cable or a membrane structure. Such a membrane-like wall combines the advantages of slenderness and high thermal performance. However, it does not allow for window openings, it introduces very complex details along the wall’s edge and introduces special requirements on the primary load-bearing structure owing to pre-stresses (Cremers, 2006b). This paper therefore continues the search for façade systems in which vacuum insulation panels are integrated meticulously.

2. Material properties With respect to vacuum insulation panels applied in façade systems mainly three properties are important. The first property is the material’s thermal performance. For most homogeneous materials the thermal performance is characterised by its thermal conductivity which is the amount of heat transferred through a material per unit thickness and unit temperature difference. Since vacuum insulation panels are not homogeneous but consist of a core material and an enveloping barrier laminate, it must be distinguished between the centre-of-panel or ideal behaviour and the thermal bridge resulting from this barrier. Two variables primarily determine the centre-of-panel thermal conductivity: the type of the core (average pore size and pore size distribution) and the pore gas pressure. At very low gas pressure, this thermal conductivity is approximately a factor 5 to 10 lower than at ambient pressure. As a result, the VIP’s core is evacuated until this pressure region is. The centre-of-panel (c.o.p.) thermal conductivity, however, does not characterise a VIP on its overall thermal performance due to additional heat losses resulting from the barrier envelope which continuous from one side of the panel to the other. This c.o.p.-value must therefore be increased with a value reflecting the effect of this barrier laminate. This value is the product of some thermal property, i.e. the linear thermal transmittance, the panel’s thickness and the panel’s perimeter length to surface area ratio: (1),

p

pp Sdψλλ = edgecopeql

+

in which λeq is the overall equivalent thermal conductivity, λcop is the centre-of-panel thermal conductivity, ψedge is the linear thermal transmittance among others reflecting the thermal properties of the barrier envelope, dp is the panel thickness and lp / Sp is the ratio of panel circumference to surface area. For a typical panel of 0.6x1.2x0.02 m3 with a metallized or an aluminium based barrier laminate, this results in an increase in thermal conductivity of 0.2·10-3 (+5%) and 4.0·10-3 W·m-1·K-1 (+100%) (Tenpierik and Cauberg, 2007). Regarding thermal performance of vacuum insulation panels, the following therefore needs to be kept in mind: thick metal barrier foils result in a high thermal bridge effect and large (square) panels are preferred to small panels with high aspect ratio.

Table 1 most relevant requirements on VIP integrated façade panels. requirement indicator criterion

statics / safety strength and stability integrity

sufficient strength and stability sufficient integrity safety in case of failure

statics / aesthetics displacements, δ [m] flatness, diagonally [mm/m]

< 1/200 * span (mullions) < 1/360 * span or < 8 mm (double glazing) < 5

energy Ueq [W m-2 K-1] < 0.4 (required by Dutch law) < 0.15 (PassivHaus standard)

service life service life, tSL [years] > 20 to 30 (facades)

The second property is a VIP’s service life. This property is strongly related to the thermal performance of a panel since its thermal performance is not constant. The gradual uptake of atmospheric gases and moisture by the VIP’s core deteriorates the panel’s thermal performance, as a consequence influencing its service life. This service life is generally defined as the time elapsed from the moment of production until the moment the thermal conductivity has increased to a certain limit often set at approximately twice the initial value (Simmler and Brunner, 2005). The main properties influencing this life are the properties of the barrier envelope and the size, aspect ratio and thickness of the panels. For a panel of 0.6x1.0x0.02 m3 with a fumed silica core and with a metallized or an aluminium based barrier laminate, this service life is more than 50 years and 300 years at 23oC and 50% relative humidity respectively. Again three remarks must be remembered: at high temperature or high relative humidity this service life reduces exponentially relative to ambient conditions (Simmler et al., 2005), thick metal barrier foils result in a high service life while metallised polymeric film laminates result in a short service life and the larger and more square the panels, the higher their service life. The third property relevant is the panel’s structural behaviour under a flexural load. For application of VIPs in facades especially the Young’s modulus, or modulus of elasticity, is of importance. For this modulus it is important to realize that a vacuum insulation panel cannot be considered as a single material but must be regarded as a composite system. A pressure difference over the barrier laminate causes the film to adhere to the core material; high shear stresses along the interface between barrier laminate and core are needed to overcome the resulting friction. As a consequence, the barrier envelope and core material co-operate to form a composite system that exhibits similar behaviour as a sandwich panel. This sandwich action improves the flexural stiffness of a VIP composite considerably as is observed from the fact that a vented VIP has a Young’s modulus approximately half that of an intact VIP (Simmler et al., 2005). Despite this combined behaviour of core and barrier laminate, the flexural modulus of a VIP does still not suffice for use in façade systems as a structural member unless it is used as part of a structural sandwich.

3. Legal requirements on façade panels The requirements on vacuum insulation panels applied in buildings are heavier than the requirements on traditional VIP applications, like for example appliances and mobile organ boxes. The European Construction Products Directive (European Council, 1988) states six product-related requirements on building products in general. Specifically regarding VIPs, one additional product-related requirement needs to be complemented, resulting in the following list of prerequisites5: a) structural requirements (mechanical resistance and stability) b) fire protection requirements (safety in case of fire) c) requirements regarding hygiene, health and environment d) application safety and fitness for use (safety in use) e) acoustical requirements (protection against noise) f) thermal requirements (energy economy and beat retention) g) service life requirements

5 In this list of requirements only product-related requirements and not process-related requirements are considered. Process-related requirements relate to the production process, transportation, storage, planning, installation, preparation, maintenance and demolition, at least as far as processes are involved. For an elaborate discussion on these requirements, it is referred to the work conducted for the IEA / ECBSC Annex 39 (Binz et al., 2005).

Only the structural and thermal behaviour of VIP integrated building panels and their service life6 will be dealt with in subsequent sections since these are strongly interrelated when designing façade panels. Table 1 presents an overview of the most important requirements on VIP integrated facade panels. Based upon this set of requirements, it is possible to investigate the (inter)relationships between these requirements for prefabricated building components, resulting in the scheme presented in Figure 2. As can be seen, several relationships among properties and requirements exist, two of which will be discussed in more detail. First, there exists an important relationship between the thermal performance of a VIP-incorporated building component on the one hand and its structural performance on the other hand. Whether this relationship is mainly determined by the properties or behaviour of the VIP itself, or by the some profile along the edge of the component, primarily depends on the mechanical behaviour of the component. Is such a profile required for structural action, then both the top facing and the edge spacer need to be dimensioned accordingly, in most cases resulting in additional thermal losses at the panel’s edge. Proper detailing and choosing the right materials can on the one hand minimize the influence of this thermal bridge significantly, but also influence the structural performance of the component as a whole. If this edge profile is dimensioned as not to partake in structural action, then the face sheets need to be structurally adhered to the VIP core to obtain a sandwich component. This creates the possibility of reducing thermal bridging due to this edge spacer, but on the other hand imposes additional structural requirements on the vacuum insulation panel itself. Second, the relationship between thermal performance and VIP service life is important. As discussed previously, the thermal conductivity of the core is not a constant value but is subjected to aging. Since the thermal performance of a VIP façade component or a VIP integrated building construction primarily depends on the centre-of-panel thermal conductivity combined with a geometry-based multiple of the linear thermal transmittance of the component edge, the application of an arbitrary value for the maximum allowable centre-of-panel thermal conductivity indirectly influences the service life of a VIP integrated building component. In general, however, besides this functional service life, which is based on physical processes, an economic and an aesthetic service life can be defined as well, which are related to maintenance and appearance respectively. These examples show how important it is to make an integral design for building components with incorporated vacuum insulation panels. All aspects have to be taken into account from the early stages of the design process until the demolition of the component, from the cradle to the grave.

Figure 2 (Inter)relationships among different requirements for VIP incorporated building panels.

Continuous and broken lines in these cases represent physical performances and customer’s desires respectively.  

6 With respect to fire safety of vacuum insulation panels, it is important to mention that the core material fumed silica is inherently fire safe. Due to the barrier envelope containing polymers, however, the fire behaviour of a VIP is negatively influenced. Fire spread along the surface of the high barrier laminate is quite rapid, while at the same time a loss of vacuum due to damage of this barrier laminate reduces the Young’s modulus of the core material with approximately a factor 2, resulting in increased deflections if the VIP is subjected to a flexural load, like in sandwich panels. The exact fire behaviour however strongly depends on the context in which the building panel is applied and will therefore not be elaborated upon in this report. Moreover, the exact behaviour of vacuum insulation panels under fire is still unknown. Extensive testing is required to determine this behaviour and to classify VIPs according to their behaviour.

Figure 3 Typological classification of traditional panels according to number of layers and structural action.

4. Existing VIP integrated façade panels: the edge-spacer type When thinking of prefabricated VIP integrated panels in general, four types are distinguished: a panel with an outside load bearing layer or an inside bearing layer, an edge spacer construction and a sandwich panel. Since VIPs need to be protected against mechanical damage, like puncture, the edge spacer and the sandwich type inherently have advantages over the other constructions. The difference between an edge spacer panel and a sandwich panel lies in the structural behaviour of the panel when subjected to a force perpendicular to the neutral plane of the panel. In practise, an edge spacer construction will act as a single plate supported by a frame of pin-ended columns (and a set of springs formed by the core material), while a sandwich will act as a composite system of three shear-connected plates. Due to this synergy effect, the deformation of a sandwich panel will be smaller than that of an edge spacer construction for the same load. Moreover, the edge profile required for edge spacer panels results in excessive thermal shunting hardly present in sandwich panels. To illustrate this, Ueq-value, i.e. the U-value which includes thermal bridge effects, of three types of edge spacer are plot as function of panel size. As can be seen from this figure, the Ueq-value of spacer type c, which can be applied to sandwich panels, is the lowest for all spacer types and panel sizes, thus reducing the amount of heat lost through the building panel as a whole. Figure 4 Three edge spacers: A) aluminium double-glazing edge (left); B) thermoplastic spacer (middle);

C) non-metallic tape (left).  Figure 5 Ueq-value of edge spacers A, B and C as function of panel length for square panels (30 mm thick

VIP with metallised barrier; 1 mm thick stainless steel face sheets).

Until now, several manufacturers have integrated VIPs into existing façade panels. In general, these façade panels are of the edge spacer type, thus consisting of a VIP core, a face sheet on both sides of the panel and a spacer along the panel’s perimeter. As explained from both a thermal and a structural perspective, however, this type of panel is disadvantageous to sandwich constructions.

5. Advanced VIP integrated facade panel systems Based upon an analysis of the thermal, hygrothermal and structural performance of VIPs and their usability, three types of VIP integrated façade panels have been principally designed and researched: a structural sandwich panel (S-panel), a panel with stainless steel encasing (Es-panel) and a panel using membrane action (M and MS-panel). In figure 6, these façade panel types are categorised according to their thermal performance, their fragility and complexity and their usability for practise. Although the membrane panel (M-panel) has the best overall thermal performance, its usability for practise is limited to highly specialised applications owing to its high complexity and fragility. In contrast, a cavity filled double-glazing panel is based on existing facade technology and therefore very practical for application in buildings although thermal concerns might render it unsuitable. The panel types in between (S-panel and Es-panel) combine the best of both worlds. In Table 2 some properties and images of all three types of façade panels are presented. They are separately discussed below. Because of the structural and thermal advantages of sandwich panels over edge spacer panels, first a VIP integrated sandwich panel and related façade system were designed. The system is primarily based on opaque sandwich panels made of a core of 40 mm VIP (fumed silica with metallized envelope) with a 1 and a 1.5 mm thick powder coated steel facing structurally adhered onto its core and placed in a grid of 1200x1200 mm2. Panels of insulated glass can be easily integrated into the system allowing visual contact with the outer environment. With this façade a very high thermal performance is possible, resulting in Ueq = 0.16 W·m-2·K-1 if all panels are of the opaque type. Although this very high thermal performance could be achieved with a panel thickness of in total only 43 mm, a good structural bond between stainless steel face sheets and VIP barrier laminate is required for sandwich behaviour. Peel strength tests with several adhesives resulted in peel strength values around 0.4 to 0.5 N·mm-1, which are significantly lower than the desired value of more than 1.0 N·mm-1. This low peel strength is caused by delaminating of the polymer-aluminium interface within the barrier film. More samples were also tested on tensile strength in their flatwise plane. During these measurements the vacuum insulation panels stayed intact, while the adhesive between facing and VIP failed at normal stresses far below the required values. Until now it has thus not been possible to obtain a good bond between both elements. Moreover, gluing inhibits the possibility of easily recycling materials. Therefore a new type of façade panel was designed.

Table 2 Overview of properties and images of the three types of VIP integrated façade panel. SANDWICH PANEL (S-PANEL) PANEL WITH STEEL ENCASING(ES) MEMBRANE PANEL (MS-PANEL) size 1200x1200 mm2 1200x3600 mm2 1200x3600 mm2 VIP thickness 40 mm 40 mm (and 10 mm PCM) 40 mm Panel thickness 42.5 mm 49.5 mm (53.5 mm with PCM) From 62.8 to 72.8 mm Up (initial) 0.16 W·m-2·K-1 0.15 W·m-2·K-1 0.14 W·m-2·K-1 Up (end of life) 0.24 W·m-2·K-1 0.23 W·m-2·K-1 0.23 W·m-2·K-1 Rendering of connection between adjoining panels

Rendering of stiffening elements

Temperature profile through a junction or stiffener

Figure 6 Categorisation of VIP integrated façade panels on their thermal performance, their complexity/

fragility and their usability for practise (S-panel is sandwich panel; Es-panel is VIP with stainless steel encasing; MS-panel is membrane panel with additional stiffening elements; M-panel is pure membrane panel) (black lines denote initial performance, grey lines performance at end of life).

This new type of panel is a membrane-based panel (M and MS-panel). Since we observed that a structural bond is very difficult to obtain, the edge spacer panel type stood as a model for this façade panel. However, in order to obtain a thermal performance fulfilling PassivHaus standard, thermal bridging by the spacer had to be minimised, also because the maximum size of a VIP is currently limited to 1000x1200 mm2. As a consequence, the contact between face sheets on both sides of the panel had to be limited solely to very thin material configurations with low thermal conductivity. Some years ago, Cremers (2006a) developed shading devices that use a membrane for creating stiff elements in which contact between two polymer encasings on both sides of a VIP were thermally separated. Using similar ideas, a façade system was first designed in which a pre-stress within a membrane helped in creating sufficient stiffness for a façade panel. This system, though, only worked for very small panels of about 900x900 mm2 or smaller. For larger façade panels, additional stiffeners needed to be added, which finally resulted in the design of which some images are presented in Table 2. Since from a perspective of sustainability it is desirable to be able to recycle or preferably up-cycle materials after being disposed of, cradle-to-cradle design principles might help create a sustainable façade system (McDonough and Braungart, 2002). During the design of the façade system presented here, especially this recycling aspect has been considered as additional guideline. The façade system is therefore made of materials that can be recycled or use renewable sources. The encasings for example are made of flax fibre (Duralin®) reinforced biopolymer, which has relatively high Young’s modulus and low thermal conductivity related to non-reinforced polymers. With a frame and 3 stiffeners of this material, an overall U-value of 0.14 W·m-2·K-1 has now been achieved for a panel size of 1200x3600 mm2. Since however this façade system is very fragile and prone to damage, a third system was designed combining the advantages of both the sandwich and membrane panels. This third façade system consists of a VIP on both sides encapsulated by a stainless steel encasing connected to each other using mechanical fasteners. Bending stiffness is created by elements within these encasings acting as stiffeners. The cavity at the warm side of the panel can be filled with a phase change material to increase the active thermal mass of the space immediately behind the façade. With this façade system an overall U-value of 0.15 W·m-2·K-1 can be achieved for a component size of 1200x3600 mm2, as can be seen from Figure 7. Images of this system are also presented in Table 2. The advantages of this system over existing high performance façade panels and the previously discussed systems are its slimness, very high performance and robustness. Moreover, since the appearance of these components does not deviate from current façade systems, acceptance by the building industry will be high. As a consequence, wide-spread application is possible resulting in a considerable improvement of current curtain-wall facades on their energy performance.

6. Conclusions As we have seen in this paper, the potential of applying vacuum insulation in façades of buildings can be very high. With a thickness of the insulation layer of only about 4 cm, PassivHaus standard for the insulation quality of the façade can be achieved. However, actual application does involve solving several technical hurdles. In the case of structural sandwiches, a good and strong bond between face sheets and VIP barrier laminate needs to be obtained. In the case of edge spacer panels, the spacer needs to be thermally and structurally optimised. With the presented façade systems, we hope to have developed systems that combine high thermal performance, structural soundness, limited thickness and aesthetical satisfaction. With the proliferation and wide-spread application of VIPs in façade constructions, the energy performance of buildings in their occupational phase can then be lowered considerably. In this respect, vacuum insulation panels can thus contribute to a more sustainable and healthy society.

Figure 7 Ueq-value of a VIP integrated Es façade panel as function of panel length for a panel with a width

of 1200 mm (40 mm thick VIP with metallised barrier; 1.5 and 2 mm thick stainless steel face sheets). Due to size limitations of VIPs (in this case 1000x1200 mm2), jumps occur in the graph at specific points. At such points a VIP is added to the system as a result of which also a stiffening element is added.

References Binz, A., A. Moosmann, G. Steinke, U. Schonhardt, F. Fregnan, H. Simmler, S. Brunner, K. Ghazi, R. Bundi, U. Heinemann, H. Schwab, J.J.M. Cauberg, M.J. Tenpierik, G.A. Johannesson, T.I. Thorsell, M. Erb and B. Nussbaumer 2005, Vacuum Insulation in the Building Sector. Systems and Applications. Subtask B, IEA ECBCS Annex 39 HiPTI, 111 pages. Cremers, J. 2006a, Einsatzmöglichkeiten von Vakuum-Dämmsystemen im Bereich der Gebäudehülle, Ph.D.-thesis, Technische Universität München, München, 248 pages. Cremers, J. 2006b, Schlank und hochgedämmt – Vakuum-Dämmsystem in nichttragenden Aussenwandsystemen, Fassade 2006 (2), pp. 19-23. European Council 1988, The Construction Products Directive, Council Directive 89/106/EC, Strasbourg, December 21, 1988. Granta Design 2008, CES Edupack 2008, Database. Hangleiter, M. and S. Weismann 2006, Systematisiertes Bauen mit vakuumgedämmten Betonfertigteilen, Bauphysik 28 (3), pp. 192-197. McDonough, W. and M. Braungart 2002, Cradle to Cradle. Remaking the Way We Make Things, North Point Press, New York, 193 pages.

Simmler, H. and S. Brunner 2005, Vacuum insulation panels for building application. Basic properties, aging mechanisms and service life, Energy and Buildings 37 (11), pp. 1122-1131. Simmler, H., S. Brunner, U. Heinemann, H. Schwab, K. Kumaran, Ph. Mukhopadhyaya, D. Quénard, H. Sallée, K. Noller, E. Kücükpinar-Niarchos, C. Stramm, M.J. Tenpierik, J.J.M. Cauberg and M. Erb 2005, Vacuum Insulation Panels. Study on VIP-components and Panels for Service Life Prediction of VIP in Building Applications. Subtask A, IEA ECBCS Annex 39 HiPTI, 153 pages. Tenpierik, M.J. and J.J.M. Cauberg 2007, Analytical Models for Calculating Thermal Bridge Effects Caused by Thin High Barrier Envelopes around Vacuum Insulation Panels, Journal of Building Physics 30 (3), pp. 185-215. United Nations Framework Convention on Climate Change 1997, Kyoto Protocol, Kyoto, 23 pages. Willems, W.M. 2003, Zur Dauerhaftigkeit ausgewählter Vakuum-Dämmsysteme, In: Proceedings of the first professional conference VakuumIsolationsPaneele – Evakuierte Dämmungen im Bauwesen, ZAE-Bayern / Projektträger Jülich / Universität Rostock / Hochschule Wismar, Rostock-Warnemünde July 10–11, 2003, pp. P1-P14.