Is BBA from VIPS good

Vacuum insulation in the form of VIPs and VIGs has excellent thermal insulation properties with low thickness, but is also a lot more expensive than solutions with conventional insulation materials. Nevertheless, it is worth using it wherever there is little space available, for example for energy-efficient renovation.

Text: Ulrich Heinemann

We all know from thermos flasks that a vacuum is excellent for thermal insulation. These have been manufactured for the storage and transport of hot and cold drinks since 1904. The insulating effect is essentially based on the suppression of gas thermal conductivity in an evacuated space. Cylindrical vessels are able to carry the external load pressure of the surrounding atmosphere. If you want to transfer this principle to flat elements, you have to insert a filler material between the walls of the vacuum envelope that absorbs the enormous pressure forces. After all, these correspond to a weight load of 10 t / m². Otherwise, such an element would be instantly compressed. In addition to the mechanical requirements, this filling material must be evacuable and therefore open-pored. At the same time, it should only insignificantly increase the heat transport, so that there remains a significant improvement over conventional insulation materials.

In the eighties and nineties, such flat vacuum insulation elements - vacuum insulation panels or vacuum super-insulation (VSI) - were developed and used for use in refrigerators and in the automotive sector (high-temperature batteries and latent heat storage). The application in construction goes back to a "self-experiment" by the Bavarian Center for Applied Energy Research (ZAE Bayern). There, in 1999, vacuum insulation panels were integrated into the experimental facade of a new extension to the institute building. Since then, this approach has not only been copied by other research institutions, but there are now numerous marketable products. Vacuum insulation panels for use in construction are now manufactured by five companies in Germany alone (as of early 2007). Several thousand square meters of VIP were installed in this area in 2006; the trend is increasing.

Thermal potential

While conventional insulation materials such as glass, mineral wool or polystyrene foams have thermal conductivities of around 0.035 to 0.045 W / (m K) at ambient temperature, thermal conductivities of around 0.002 to 0.008 W / (m K) can be achieved with evacuated insulation materials. Heavy gas-filled polyurethane foams have thermal conductivities of about 0.022 to 0.030 W / (m K); In the case of special, particularly finely structured materials, the thermal conductivity of dry, static, i.e. non-convecting air can still be below 0.026 W / (m K). Compared to non-evacuated insulation materials, vacuum insulation has an improvement potential of a factor of 5 to 10 (Fig. 1).

Building VIPs

The background for an increasing market development is certainly the strongly increasing requirement for the thermal insulation effect of a building envelope paired with the desire - or the necessity - to limit the wall thickness. On the technical side, the development of flat vacuum insulation elements has gone hand in hand with developments in the filling materials and in the construction of the vacuum envelope. In the area of ​​filling materials, these were particularly fine-grained SiO2 powders structured on the nanometer scale, so-called pyrogenic or highly dispersed silicas. This material, which is also known as "sand in its finest form" or "white soot", is not, however, a new product. It is used extensively both as an additive for tires, paints or varnishes and as a thickening and gelling agent for cosmetics, pharmaceuticals and food.

For this extremely finely structured filling material, the requirements for the tightness of the vacuum envelope are significantly lower than is the case, for example, for thermos flasks. Therefore, less dense high-barrier laminates, which have been specially developed for this application and consist of several layers of different functional plastic films with several vapor-deposited aluminum barrier layers a few tens of nanometers thick, come into consideration. This opens up a production technology that is significantly cheaper and more flexible than the production of casings made of glass or stainless steel. Just think of the huge amounts of different foods that are packed in barrier films today. Only the combination of the filler material with the lowest demands on the impermeability of the shell and the tightest plastic barrier films available today enables relatively inexpensive production of VIP products that also meet the enormous requirements in the building industry with regard to durability.

A VIP basically consists of a filling material and a vacuum envelope. This means that a VIP is not so much an insulation material that can be processed and cut to size as required, but rather a prefabricated, highly efficient insulation element (Fig. 2). From the point of view of insulation technology, a system change is made with the use of vacuum insulation panels. Since vacuum insulation panels are custom-made in the required size, it is advisable, for cost and logistical reasons, to adapt the application to uniform dimensions. In various respects, the planning and conditions of use of a VIP are comparable to those of a window - with corresponding consequences for use in practice. During the conception and implementation, the various connection details must therefore be given special attention, such as the dimensional accuracy of the components and the sealing of the joints.

Thermal bridges / handling on the construction site

In order not to endanger the excellent insulation function, the shell must not be damaged under any circumstances. Foil-wrapped VIPs must therefore be well protected against mechanical damage. The use of a vacuum envelope made of stainless steel is less critical. During transport and assembly, vacuum insulation panels in the facade trade are much better protected than in the shell construction. The high level of coordination required on the construction site must be taken into account, as many craftsmen do not yet have sufficient experience in dealing with VIPs and their installation.

The thermal bridges in the area of ​​the connections and fixings are a particular problem. When using VIPs, this problem is exacerbated to the same extent that the thickness of the insulation layer can be reduced with the use of this highly efficient insulation. Even still air or conventional insulation material represents a thermal bridge compared to a vacuum insulation panel. The connection details are therefore of particular importance for a thermally optimized building envelope. Since there is increased heat transmission at the edge of the elements, all butt joints should be carefully planned and their number reduced as much as possible.

Typical fields of application for VIPs in construction

Compared to conventional thermal insulation - with the same insulation effect - vacuum insulation is significantly more expensive (approx. 50 to 100 euros / m², U-value of a 2 cm thick board approx. 0.2 W / (m² K)). Added to this is the increased planning and coordination effort. The advantage of vacuum insulation panels lies in the improvement of the thermal insulation while at the same time gaining space. They are used in particular where you want to achieve a given level of insulation with a structure that is as slim as possible, or where the space available for insulation is limited or very valuable. So your specific advantages mainly come into play when

  • There is insufficient space available for conventional insulation
  • Through the use of VIPs, in particular in the case of thermal renovation, further measures can be saved, such as the offset of door and window openings or the extension of a roof overhang
  • The aim is to achieve as much usable space as possible from a given floor space (inner cities of large cities with high land prices)

Further areas of application / examples:

  • on roof terraces to avoid steps or thresholds, as is relatively widespread in Switzerland in particular. Particular attention should be paid to dry conditions during installation, as in this case the VIP is provided with a vapor-tight membrane from above and below. Trapped moisture sooner or later reduces its effect.
  • for small extensions with a particularly unfavorable surface-to-volume ratio, such as dormer windows
  • for facades in element and mullion-transom constructions. Integrating the VIPs in the factory into prefabricated facade panels for mullion and transom constructions is currently the most common way of using vacuum insulation panels in Germany. In a uniform system, transparent and opaque, highly insulating elements can be combined in an attractive, slim design. The design options in terms of color and / or choice of material for the facade elements remain unaffected. The critical point in these constructions are the thermal bridges in the edge area: If the frame construction is not designed to have a particularly low thermal bridge, the increased heat transfer in this area can completely cover the excellent insulating effect in the center of the panel. In favor of this application, however, is the fact that the risk of mechanical damage to the VIPs is reduced to a minimum due to the factory prefabrication under controlled conditions by trained personnel and the protective second shell. The »second shell«, if, as in the case of an insulating glass pane, has a low vapor diffusion design, reduces the climatic loads, especially for VIPs wrapped in film.

Structure of VIGs

Even in well-insulated buildings, the thermal weak points can still be found in the area of ​​the windows. While U-values ​​of 0.15 W / (m2 K) are achieved with opaque facades, even the UW-values ​​of high-quality windows (triple glazing, including frames) are only 0.8 W / (m2 K), which means they are around a factor of 5 worse. Since the heat transport is suppressed by evacuating the gas in the space between the panes, there is also considerable potential for improvement in the windows. A prototype of vacuum glazing with a U-value of 0.5 W / (m² K) was presented at glasstec 2006. It was the result of a research and development cooperation between ZAE Bayern and several research and industrial partners.

The technical challenges in the implementation of vacuum glazing consisted in developing supports that, analogous to the filling material in the VIPs, are able to absorb the external load (Fig. 12), but without making a significant contribution to the overall heat transport. In addition, they should appear as little as possible. The requirements for the quality of the vacuum and thus the tightness of the envelope, especially that of the edge seal, are much higher with the VIGs. If the gas pressure of the VIPs with nanostructured filling material can be several tens of millibars, the gas pressure of the vacuum insulating glass windows must be kept below a thousandth of a millibar (10–3 mbar) over the entire service life.

The pure Ug value of a VIG (0.5 W / (m² K)) hardly differs from that of triple glazing (Ug value> 0.5 W / (m² K)), but with a total structure of only 9 mm this is extraordinarily slim (for comparison: standard thermal insulation glass 24 mm, triple glazing 36 - 44 mm) - and above all light. They only weigh the weight of conventional double glazing and therefore do not place any increased mechanical demands on the frame or the suspension. Further research and development work is currently required in order to apply the knowledge gained so far to the requirements of industrial series production.

Conclusion

In recent years, various manufacturers have taken up the idea of ​​highly efficient vacuum insulation and developed marketable products. Against the background of increased requirements for structural thermal insulation, further market penetration is to be expected. There are still numerous approaches for improvement: concerning the structure of the elements as well as the connection and fastening details. For example, facade elements are conceivable in which the outer shell made of glass or metal also forms part of the vacuum shell. Architects and planners are certainly called upon to develop products together with industry. In addition, it should be pointed out that until the relevant general building inspectorate approvals have been issued for an application, an individual approval has to be obtained from the responsible building authority. •

Manufacturer of vacuum insulation panels (selected from Germany):

Panels with plastic high barrier films or laminated aluminum foils:

  • Porextherm, Kempten, contact person: Gregor Erbenich, www.porextherm.com
  • Vaku-Isotherm, Rossau, contact: Claudia Röttig / Rainer Baars, www.vaku-isotherm.de
  • Variotec, contact person Christof Stölzel, www.variotec.de
  • va-Q-tec, Würzburg, contact person Roland Caps, www.va-q-tec.de

Stainless steel coated panels:

lambdasave, Emden, www.lambdasave.com

Literature:

Heinemann, Ulrich, Roland Caps and Jochen Fricke: Characterization and optimization of filler materials for vacuum super insulations. Vuoto scienza e tecnologia, Vol. 18, N. 1-2 1999, pp. 43-46

Caps, Roland, inter alia: Application of Vacuum Insulation in Buildings. VIA Symposium: Progress in vacuum insulation, June 2000, Vancouver

Information platform on the subject of "Evacuated insulation in the building industry": www.vip-bau.de

Simmler, Hans, et al .: Vacuum Insulation Panels - Study on VIP-components and Panels for Service Life Prediction of VIP in Building Applications, IEA-Annex 39, 2005, www.vip-bau.de

Armin Binz, i.a .: Vacuum Insulation in the Building Sector, Systems and Applications, IEA-Annex 39, 2005, www.vip-bau.de

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