Thermal bridges are defined as locations or areas in building construction that have local heat flows that are high and local surface temperature that are low, both relative to the surrounding construction. Thermal bridges created by the edges of VIP's are vital as they can result in the significant reduction of the thermal insulation, hence leading to the increment in heat loss via the building structure or an increment in the condensation rate at the insides of the building structure. The cold bridges formation should be hindered or an attempt should be made to at least reduce its effect as much as possible in order to facilitate the optimum efficiency of vacuum insulation panels. Thermal shunting is a primary consideration owing to the VIP structure. Therefore three varying levels of thermal bridging are explained for the VIP application:
Thermal bridging caused due to:
The high barrier of thin film surrounding the core material.
The edge spacers in the building components.
The link between the component and the load bearing structure through structures like a window frame or a post mullion system.
Three varying thermal bridging levels also exist in case of in situ application of vaccum insulation panels. These levels exist due to the
The high barrier of thin film surrounding the core material.
A small gap of air trapped between the two adjacent panels.
To constructional irregularities.
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Aging and service life
Accelerated aging tests have demonstrated no major effect on the thermal efficiency of the vacuum insulation panels, which was conducted utilising high relative humidity and indoor temperatures for time duration of thirty to sixty days. As the pressure applied of 5 bar on the VIP's was found to be higher than the pressure normally tolerated by the layered structure of the core material, therefore the results of the VIP toan overpressure was not decisive. It is suggested that further studies should be carried out to complete the understanding of the subject (NCR. 2006).
The table 36 demonstrates the data derived from the permeation characteristic panel aging experiments, perimeters and surface area, relevant to all the barrier types, investigated with multiple metalized polymer laminates. These values obtained are adjusted for temperature of 23°C, atmospheric pressure of 1 bar and 50% RH.
All the permeation values are within the comparable ranges with the exception of MF1. An increased area relevant dry gas transmission rates and water vapour id demonstrated by the MF1. An annual moisture and pressures can be estimated through table 36 at standard conditions for VIP of 2cm thickness and with applicable formats of 0.25 x 0.25 m2, 0.25 x 0.50 m2, 0.5 x 0.5 m2, 0.5 x 1.0 m2, and 1.0 x 1.0 m2 applied on respective areas of 0.0625 m2, 0.125 m2, 0.25 m2, 0.50 m2, and 1.00 m2. As clearly shown by the figure 69, a trend can be seen in the increase in the rates of high pressures in accordance with small formats because of perimeter contributions. As the surface contribution is rather dominant, a reliance of format is less significant with respect to accumulation of moisture. The moisture accumulation rate is lower than 0.2%-mass/yr for all formats with the exception of MF1. The increase in pressure rates is also approximately or in some cases lower than the 2 mbar/yr with the exception of MF1. The pressure rates are 2 mbar/yr or lower in case of small formats while in case of formats above 0.5 x 1.0 m2 the pressure rates are 1 mbar/yr or below (Annex 39).
Condensation
The water vapour transmission in the VIP can have long lasting effects. The pressure of water vapour environmentally is maintained as 10-20 mbar. A time may come when the internal water vapour pressure of the VIP may become equivalent to the environmental pressure which may result in the increase of the induced gas heat conduction in the silica based VIP. The phenomenon that results in the increase of the moisture is known as sorption isotherm (16). The thermal conductivity will experience an increase by 0.5 mW/mK for every sorption isotherm and mass percentage. The following figure will demonstrate this behaviour. For instance in a dry state condition the thermal conductivity is 4mW/mK and the equilibrium state for normal indoor conditions is obtained at 230c and a relative humidity of about 50%. The moisture content is 4% of the mass which will experience an increase of 4 x 0.5 mW/mK = 2 mW/mK which in turn will render a thermal conductivity of 4 + 2 = 6 mW/mK.
The relationship of thermal conductivity with the water content has been observed by researchers and it has demonstrated an increase in the coefficient of heat transfer with an increase in the water content through powder boards of fumed silica. The connection explained previously was however further elaborated upon by Schwab (6) who explained that the thermal increase per mass will be 0.29 mW/mK instead of 0.5 mW/mK. But besides that researchers have indicated three varying types of heat transfer owing to the moisture increase in the VIP. They are as follows:
Heat conduction through water vapour at partial pressure.
Heat conduction through absorbed water at the inner surface.
Heat transfer due to evaporation through absorbed water, diffusion and condensation.
A linear relationship was suggested between the thermal conductivity increase and per mass water content by Beck et. al. [19], Schwab et. al. [20], Coquard et. al [24], and Platzer et. al. [25]. But on the contrary the above researches also suggested that the total moisture effect on the thermal conductivity of a vacuum insulation panel is complex compared to the above mentioned linear relationship.
Building Application
An international research team teamed up to work on the vacuum insulation for buildings in the year 2002-2205. This research was conducted in IEC/ECBCS Annex 39, HiPTI(High Performance Thermal Insulation). Two teams subtask A and subtask B were involved in which subtask A was dealing with the vacuum insulation queries and subtask B was dealing with basic application factors behind the vacuum insulation of a building section. The finalized report was published in joint collaboration of Germany, Switzerland, Netherlands and Sweden, in the year 2005 (Binz, 2005). The National Research Council Canada is alo contributing in the research of developing new types of VIP.
The VIP utilization will reap far reaching benefits in the future. This will provide oppurtunities to provide effective solutions for the increasing insulation demand as a result of a successful combination of high thermal performance and a sleek structure, therefore saving internal space.
This new VIP technology has provided new methods for the insulation in constructions where a minimum fraction of the usually required thickness in insulation as compared to the thickness used in conventional thermal insulation materials. The multi-layered metalized polymer has given a foundation for the high barrier materials which will provide a long lasting building usage minimising the thermal bridge effect through the edge zones. But generally the moisture penetration is still an issue as compared to the aluminium foils used. Therefore it is recommended that wet, alkaline environment with high temperatures should be minimised in any sort of application.
The VIP's are used in the direct installation at building sites. the VIP insulation is frequently used in Switzerland for the insulation of flat roof terraces. As shown in figure 2.
The VIP usage eliminates complicated steps between the interior and the terrace. A plan should be prepared prior to the application. Conventional insulation at the edges can be reduced by making individual made-to-fit pieces. The VIP should always be covered by a protective covering during its application, to avoid it being damaged by a falling object or by someone walking on it. Onsite people involved should always be aware of the dry materials and the dry weather.
Rain drops in small quantities can also prove to be hazarduous as they can increase the permanent increase in the water vapour pressure in the entire construction site. thus harming the insulation properties of the material.
Examples
There are some examples of using VIP in buildings application:
First example
There is an example in Munich/Germany, where an apartment building facade has been installed with the VIP composite system, as explained by Prof. Armin Binz, Gregor Steinke 2005. An adhesive was used to fix the vacuum insulation panels to the concrete walls where the VIP's were fixed between the vertical laths of recycled polyurethane. These were fixed with the help of wall plugs. The polyurethane foam panels are attached to these laths securely on which the plaster is applied. As shown in figure 3.
Second example:
In Biiningen/Switzerland the VIP was installed on a five terraced house. These were one storey high (ca. 2.60 x 1.60 m2), within a wooden frame construction. The VIP besides from having a laminate surrounding, are also vacuum construction panels constituting of empty stainless steel cases having a fumed silica core within (figure 5). The panels have the facility of being manufactured in large sizes as well, with sizes of 3/8 meters, and have the quality of being robust and vacuum tight. An addition of a metal facing was done on the outside for aesthetic quality and the inside had a wooden covering. Repairing and maintenance of the VIP Panels is easily done by welding the damaged areas and re-vacuation.
The VIP manufacturing requires handling by trained personnel under highly controlled conditions in factory. Although a few fabricated products and systems are present for the building division. The development in this area of more VIP related products has pointed out towards the engaging of system and component manufacturers. Therefore in the future one will become accustomed to a wide range of VIP related products such as floor heating systems, exterior and interior wall elements, etc. The Arrhenius temperature aging in humid environments has indicated a time period of several decades for the service life in the appropriate constructions making sure that the regular surface condensation and stresses are absent. The real world and laboratory estimations should be regarded for the reliable service life prediction for the combined effect of temperature, moisture, mechanical and chemical aging mechanisms.
Summary:
As mentioned above the barrier materials should be protected from all sorts of harm such as moisture, water from the environment, temperatures beyond the approved limit, etc. If drying is made sure then occasional condensation is bearable.
Many heat transfer methods have been researched upon with the usage of powder, foam, fibre and the staggered beams in the filling of the VIP. Simplified models are used in the solid conductivity, which is related to the solid conductivity, porosity and the mechanical properties of the materials. The tortuously lengthy thermal paths have made the fibre and the staggered beams a preferable choice for the solid conduction. The gaseous conductivity at low pressure is relying on the size of the pore and the pressure of the core.
The diffusion approximation is used in various insulation materials for the radiative transfer, relying on the specific extinction coefficient, density and core temperature. The total effective conduciveness of the VIP can be estimated accurately based on the separately estimated thermal conductivities. The VIP developments are benefiting from the proposed analytical tools. The researches have further favoured the usage of fibre and staggered beams with radiation shield as the adequate materials for filling VIP's.
Still according to the NRC, there are some hindering factors in the increased usage of VIP in the construction industry. These factors are:
Costly application of VIP as compared to the conventional insulation materials.
Less confidence of the people in its long lasting efficiency and and the vacuum technology in VIP.
The lack of any application guidelines, designs, standards, specifications and installation instructions also discourages its usage.
