Building in a vacuum
Vacuum insulating systems are highly complex, in terms of manufacture and installation. Although no products from the vacuum insulation systems sector have been officially approved for use in construction, it is only a matter of time before one is. So, what are their benefits and drawbacks?
In conventional insulating material, thermal transmission is mainly made up of conduction through the enclosed gas (65?75 per cent), heat radiation (20-30 per cent) and thermal solid conduction (5-10 per cent).
Vacuum insulation systems were developed to minimise the biggest source of potential heat loss by reducing the transmission of heat through the gas. This is done by evacuating the unit - ridding the insulation of gas through which heat might travel.
To maintain the vacuum in an insulant over a long period of time, the evacuated space has to be surrounded by a gas-proof envelope, which can be made of glass, metal, polymer or polymer-composites. The enclosed space can either be empty (for use as a glazed element, for example), or filled with a core material whose cells are entirely open, so that it can be successfully evacuated.
Depending on the level of induced vacuum, the 'system' (ie the gas-proof envelope and the insulant combined) must resist high atmospheric pressure (equivalent to up to about 10.3 tonnes/m In Germany, as in Britain, a typical opaque system has micro-porous insulation cores. Most of them consist of boards of glass-fibre reinforced by silicon dioxide (fumed silica), which has a very high porosity and a pore size small enough to strongly suppress the gas conduction effect. Even at atmospheric pressure, its thermal conductivity of approximately 0.018 W/mK is significantly below that of the surrounding air of approximately 0.026 W/mK. Therefore, the thermal conductivity drops sharply, even at relatively moderate vacuums, and reaches its minimum, of approximately 0.004 W/mK, at 10 mbar.
The consequent reduction in demands on the manufacturing process also means that higher gas permeation rates in the envelope material and the sealing are possible.
This leads to a longer potential product life, determined by a maximum allowable pressure rise within the system. Other core materials, such as open-porous foam materials, are not relevant for the building sector at the moment because they require much lower gas permeation rates (ie greater integrity) of the envelope.
Two very different materials are used for the gas-proof envelope.
Evacuated insulation systems that use metallised plastic composite foils are generally called Vacuum Insulation Panels (VIPs), while those using stainless-steel plate as the outer layers will be referred to as Vacuum Insulating Sandwiches (VISs). Transparent highbarrier membranes, based entirely on polymers and coated with metal oxides are under further development and might offer the prospect of translucent or even transparent vacuum insulation systems in the future, if they are combined with aerogels (see AJ 17.6.04, page 56).
The thermal conductivity of a vacuum insulation system depends primarily on the internal gas pressure and on the moisture content of the core material. Any changes in these magnitudes can therefore, in principle, be used for quality control (if initial gas pressure and moisture are known). If internal effects are neglected, any pressure rise within the system mainly depends on the quality of the envelope material and of the edge condition. Inadequately sealed panels cannot usually be detected visually, particularly if they have already been installed, and therefore procedures for measuring the internal pressure offer the preferable way of quality control: methods that do not require penetration of the envelope and can be used even if the panel is only partly accessible.
The va-Q-check method (developed by va-Q-tec AG of Würzburg, Germany) enables the control of the whole product chain. Every panel is equipped with a thin metal plate covered by a thin layer of insulating fleece and placed directly under the foil (ie installed within the sealed VIP system). A specific checking device relays information from this insert, allowing the thermal conductivity of the fleece to be measured, and thus indirectly giving evidence of the internal pressure of the panel and hence the status of the system and its thermal conductivity as a whole. The procedure is fast, cheap, non-destructive, repeatable and fairly accurate but, it can not be used for VIS. In principle, the performance of installed vacuum insulation systems can also be qualitatively demonstrated using thermographic recording techniques, but only as long as there are sufficient temperature contrasts.
As already mentioned, the truth is that no products from the vacuum insulation systems sector have yet been officially approved for use in construction. It is also true that thermal conductivity values that apply to the undisturbed centre of the panel will never be achieved when the panel is fitted. Achievable U-values are primarily determined by the geometry, the choice of material, and the way in which the edges are formed, sealed and joined.
The total amount of material needed is relatively small, due to the system's high efficiency, and because it is constructed from just two materials separated by a vacuum, it is relatively easy for them to be separated for reuse. Silica boards and stainless steel are the major components by weight and have no known chemical or ecological risks in recycling. Similarly, the composite foil materials represent a minor proportion of the weight and can be partially recycled.
Careful handling of unprotected VIPs on building sites is critical because they can be easily damaged. However, because they are predominantly manufactured in factory conditions, these risks are minimised, although panel faces should still be protected in transit with sacrificial soft layers.
Furthermore, vacuum insulation systems have to be fitted without being subjected to tension, and the future user has to be aware that the system must not be penetrated: puncturing the seal with drills or nails will render the panel useless.
A thermal bridge always means a reduced level of general thermal protection, increased energy loss and the risk of condensation, mould and structural damage. The geometry of vacuum insulation systems itself creates thermal bridges, because of the higher conductivity of the edges.
Additionally, high-performance insulation increases the acute problem of thermal bridges due to the significantly lower insulation thickness in combination with the minimisation of thermal conductivity across the main area. The effect strongly depends on the edge solution, the construction and the formats used.
Any condensation will concentrate on the outer edges and joints because the panel systems themselves are principally vapour tight. With regard to the high conductive contrasts between insulation and construction materials, a rough estimation will inevitably tend to show a failure - that thermal bridging has not been prevented - and therefore more elaborate methods such as FEM (finite element method)-calculations are strongly recommended, especially if new details have been developed.
The preferred format for successful VIP applications is that of rectangular panels whose right-angle edges have the most accurate geometry possible. Other formats (even apertures) are possible, but lead to great effort, cost and a general reduction of product life due to their complexity and longer edges. Since the format of manufactured panels cannot be changed and strengthened by the risk of thermal bridges, the designer has to pay great attention to tolerances. The demands for accurate dimensioning is particularly high for the application of vacuum insulation systems. A further aspect is the economically driven pressure to have only a small number of formats.
Careful consideration should be given to the issue of 'buildability', that is, how the panels can be installed, accessed and replaced. For example, vacuum insulation systems can be easily fitted and retro-fitted into existing post-and-rail systems.
This means either placing the VIP within a double-glazed unit (necessitating alterations to the spacer distance between the panes of glass) or within a double-skin facade panel.
Thicknesses of approximately 32mm are enough to achieve U-values of approximately 0.16 W/m 2K in the centre of the panels. This is a reduction of about 75 per cent in comparison with the best thermally insulating multiple glazing. Here the U-value of the facade as a whole depends heavily on the size of the panels and on the relative proportions of the edges to the flat areas.
The integration of VIPs in a glazing unit ensures some mechanical protection and offers the possibility of almost entirely ruling out damage to the VIP during transport and assembly.
Another advantage is the space-weight reduction and the conventional look and handling of these integrated units. However, for VIPs enclosed between two layers of protective window glass, there are no quick and easy ways of checking the status of the VIP, the integrity of the vacuum seal and hence the thermal conductivity of the system as a whole.
At the moment, the only technology that can monitor this is using thermal imaging cameras after assembly.
An area-wide replacement of conventional insulation material by vacuum insulation systems is not going to happen soon, nor is it a reasonable or sensible thing to consider at the moment, but wherever the maximum insulating effect and minimum thickness justify the relatively high investment - there will always be scope for the growth of this sector. In many cases this will apply to localised applications only and rarely will the whole envelope be covered. That said, a specific field of interest lies in the refurbishment and thermal improvement of old buildings, where often virtually no space exists for conventional insulation material to be used.
For that reason, VIPs will be a growth area, but one that needs careful consideration and careful handling.
Jan Cremers is researching the architectural applications of vacuum insulating systems at Technische Universität München at the Chair of Professor Dr (Univ Rom) Thomas Herzog. For more information, email:
cremers@lrz. tum. de. Visit: www. vipbau. de/start_e. html