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THESE VACUUM INSULATION PANELS COULD BE OPAQUE, TRANSLUCENT OR EVEN TRANSPARENT

TECHNICAL & PRACTICE

Vacuum insulation systems are relatively well known, but for certain building types their use could be extended to the creation of complete cladding systems that would provide ultra-slim facades.

Most applications of vacuum insulation systems 1 are based on the substitution of conventional insulation material in standard facade construction. Here, the reverse approach has been chosen, to develop and study facades that are designed to match the specific criteria that are set by the new and highly effective insulation systems.

Vacuum Insulation Panels (VIPs) are characterised by several properties, which are:

having very high insulation capabilities at minimum thickness;

being able to withstand pressure only if applied across the surface;

being ductile to a certain extent; and

being impenetrable (ie for mounting).

This list of specific characteristics supports the conventional approach, which is to apply the new systems as insulation on the outside of load-bearing walls or as a core? inlayed within sandwich-like elements.

But there is another possibility, which is outlined here.

The first three properties in the list above suggest the material could be used in suspended facade constructions (contrasted schematically with ground-bearing facades in fig. 1 overleaf) if there is a desire to limit the number of other layers and elements needed.

This possibility of using suspended facade construction and the decision to use two layers of VIPs are the basis for the following details.

Creating a 50 per cent offset between the two layers of rectangular VIPs will provide regular point locations for penetrations that can be used not only for fixing and stabilising the VIP layers, but also for connecting the VIPs to a supporting substructure.

If the vacuum insulation systems can be exposed to the outdoor climate and the joint between them is resolved adequately, only a supporting structure would be needed to keep the layers in place. This could well be a linear structure (such as a post-and-rail system) or a planar one (a supporting-wall construction).

However, using cable structures on one or even on both sides would provide a more interesting and potentially more promising alternative construction, shown here as types 1 and 2 (figs. 3 and 4 overleaf).

If the envelope of the VIP cannot withstand outside conditions, then an additional insulation-protective element could be applied between the insulation layer and the supporting structure on either one or both sides. This is exemplified in type 2 (see 3 in fig. 4 overleaf). The alternation of high and low point joints not only optimises the cable structure but also helps to avoid up to 50 per cent of the point fixing thermal bridges if the pressure load on the high-point joints can be absorbed by the protecting elements and the VIPs.

For type 3 (fig. 5 overleaf), this idea has been further developed by replacing the cable structure on both sides (and potentially also the protective elements) with membranes. The theoretical sequence of developments is visualised in fig. 2.

OTHER ASPECTS All variants are designed for panels measuring 50 x 50 cm. Due to the lack of a specific building projects and therefore of realworld constraints, the proposals can only be examined in principle.

The unknowns include:

the overall dimensions;

exterior conditions (such as wind pressure and suction); and building constraints, particularly lateral connections.

Every proposed construction relies on a certain bowing allowance of the VIP, which is directly dependent on the amount of cable or membrane prestressing.

In all three examples given here, the VIP could be opaque, translucent or even transparent. The most intriguing options would be to use translucent or even transparent VIPs, as these would make it possible to see the structural system.

At present VIPs are not available in those forms, but they are feasible in principle and the subject of current research and development.

Although the construction methods outlined avoid linear penetration, the point fixing joints constitute significant thermal bridges that would require careful and precise detailing. One would recommend using materials with low thermal conductivity, such as fibre-reinforced pultrusion plastics.

The proposals show the ratio of thickness to thermal insulation to be very close to the technological optimum. The resulting U-values are determined predominantly by the thickness of the VIPs used. Lateral connections have a minor influence, depending on the overall dimensions.

TYPE 1 CONSTRUCTION Two layers of joint VIP (off-set by 50 per cent) are connected on either one or two sides to a cable structure that will deal predominantly with the horizontal forces. If used on both sides it might also take vertical forces. Fig. 3 above depicts a schematic horizontal section.

The envelope of the vacuum insulating system forms both the outer and the inner skin of the wall system. Therefore, it has to satisfy the resulting requirements (resisting the weather, mechanical forces, UV radiation etc. ). Currently available conventional VIPs are not suitable, but future modifications and improvements to the laminates used might result in appropriate materials.

As a matter of principle, cable constructions require very high prestressing forces, depending on the overall dimensions.

2TYPE 2 CONSTRUCTION This method differs from type 1 because it introduces protective elements that follow the geometry created by the gaps between the VIPs. The protective elements are also rectangular, but are rotated by 45° and stabilised by a pleat in one direction, thereby separating the VIPs. Elements lying on top of each other are rotated by 90°.

Their edges and the joints are continuously tilted by 45°, which helps to avoid the infiltration of water.

The protective elements could be manufactured from metal plates. Alternatively, plastics such as polyester, or even translucent or transparent materials such as polycarbonate (PC) or polymethyl methacrylate (PMMA) can be used, because there are no requirements for higher vapour tightness.

The introduction of high and low points in the cable construction significantly reduces the prestressing forces in the cables and leads to a differentiation of the joints between those subject to pressure and those receiving tension forces.

Fig. 4 shows a schematic horizontal section, Fig. 6 a computer rendered view of this type.

A speciality of this variant derives from the high pressureresistance of the VIPs and the pressure-distributing effect of the protective elements 3. Thus we can avoid penetrating the insulating layer at the pressure joints. Thereby the amount of point fixing thermal bridges could be reduced by 50 per cent.

Due to the very low thermal conductivity of the vacuum insulating systems, the outer layers will be strongly heated by solar radiation. This must not lead to a deterioration of the material and of the envelope of the VIP. Therefore a light colour and a high reectivity of surface is recommended.

Depending on circumstances, an additional thermally separating layer (eg a thin glass--bre layer) has to be inserted.

Furthermore, the consequences of thermal expansion have to be taken into account, especially in the design of the joints of the protective elements. The objective is to avoid asymmetrical deformation (bulge) of the whole construction.

The exterior and interior parts of the cable structure are exposed to a very different range of temperatures. The difference between the two sides leads to unbalanced thermal expansion of the cables and therefore to uneven prestressing forces.

However, the short circuit at the tension joints (low points) ensures that these disparate forces only cause minor deformations in the between sections.

The overall dimensions of the construction are limited by the necessary prestressing forces taken by a primary structure.

The prestressing of the cable construction can be carried out in several different ways, depending on the intended span and overall dimensions. An adequate solution could be to increase the crest of the arch in the areas of penetration, by lengthening the pressure bars (8 in Fig. 4) or alternatively by shortening the tension bars at the joints.

TYPE 3 CONSTRUCTION The stabilising of this particular variant (Fig. 5) is not effected by a cable structure, but by using buckled membranes (see p42) on both sides.

The aim here is to use an uncut membrane. The necessary crest of the arch then results from the material properties (eg stretch) that presuppose a soft but very strong membrane material - according to state-of-the-art technologies for example, a PTFE-coated PTFE-fabric would meet these criteria.

Important historical references for this approach are the early so-called 'Buckelzelte' (buckled tents) and the 'Flächen mit Hoch-und Tiefpunkten' (planes with high and low points) designed by Frei Otto.

4For the design of the high and low points the following has to be considered:

the support of the high and low points has to be 'smoothed' (eg with domed supports) to limit the forces within the membrane material; and the tension-loaded low point normally requires material reinforcement in the area of the clamp to withstand the wind loads affecting the facade.

If the linear joints of the VIPs are perforated and the space between the membrane and the VIP is linked to the outside (and via the perforations to the inside) air circulation will be unrestricted. This should avoid problems of persistent condensation on the membrane.

A translucent or even transparent realisation of this variant would be particularly interesting, as the geometrical and substantially different layers of this assembly indicate a visual appearance of high complexity.

BUILDING A DESIGN MODEL OF TYPE 2 A model of an approximately 1 x 1m section of the type 2 construction has been built in a project to allow U-value measurement at the Technical Centre of the Technische Universität München.

The VIPs measure 50 x 50cm x 15 mm and were sponsored by Porextherm, of Kempten, Germany.

Fig. 7 depicts all parts which are manufactured for the model. Fig. 9 illustrates the assembly process and shows the geometry that results from the 50 per cent off-set between the insulating panels.

The distribution of the beads of the protective elements follows the linear joints of the VIPs (-g. 10). Fig. 8 gives an impression of the -nished model.

RESULTS AND OUTLOOK We can expect to better the following U-values:

approx. 0.20-0.25 W/m 2K for a design with two 15 mm VIPs, approx. 0.10-0.12 W/m 2K for a design with two 30 mm VIPs.

Due to the slenderness of the material, the appearance is more that of a skin than a wall.

If using two layers of VIPs proves not to be economically viable, then one of the layers could be replaced with an alternative conventional insulation material (such as polyurethane foam) since the main purpose is to avoid linear thermal bridges.

When thinking about possible applications for the construction approaches described in this article, one should consider the following aspects:

due to the 50 per cent off-set, integration of apertures within the wall systems is impossible without disturbing the pattern repeat;

the design of connections at the edges is complex;

the necessary prestressing forces require an adequate primary structure; and

a horizontal application of the proposed constructions is not possible without modi-cations (mainly due to the water run-off from the facade and potential snow loads).

Therefore the greatest potential for such wall systems would be for:

an application in large uninterrupted areas;

buildings with a demand for constantly high room temperatures;

and buildings that already have an adequate primary structure.

Such conditions apply for industrial buildings with high performance requirements for the production environment. Other potential applications include swimming pools, large studios or museums.

Jan Cremers is scientific and teaching assistant at the Technische Universität München. The results presented here are part of a finished PhD thesis.

For more information, email: mail@jan-cremers. com

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