Long term durability of vacuum insulation panels: determination of the Sd-value of MF-2 foils

Vacuum insulation panels (VIP) combine high thermal performance with minor thickness making them fit for building retrofit. However, consisting of a fumed silica core at low air pressure (near vacuum) encapsulated by a protective aluminum containing covering, they are prone to puncturing which inadvertedly increases the internal air pressure and thus worsens the thermal performance. In addition to failure by puncturing, it remains unclear under which hygric circumstances VIPs degrade in time and how this is influenced by the Sd-value of the protective foil and thermal performance of the VIP. Therefore in this study the behavior of VIPs was monitored under various climates for measurement periods up to several years, allowing for a realistic assessment of the Sd-value of the foils and the degradation in time. Frequent gravimetric measurements were performed on 46 VIPs stored under ambient conditions and in climate chambers for periods up to 4 years. Based on EN ISO 12572 [1] the equivalent µd-values of the MF-2 foils were determined. Comparing measurement results to literature it is found that water vapor diffusion resistances µ are in the range of 2.66 – 7.03 . 10^7 for MF-2 type foils. Overall, it can be concluded that hygrothermal loading for extended periods has a limited effect on the stability of the foil in time, ensuring the applicability of VIPs in building components