Influence of water on the total heat transfer in 'evacuated' insulations

The impact of water vapor on the thermal conductivity of vacuum insulation panels (VIPs) filled with fumed silica kernels was investigated and found to be surprisingly large. Besides thermal transport by water vapor within the pore space, the question was whether water in the adsorbed phase also contributed significantly to the total heat transfer. In order to quantify these effects, several series of thermal conductivity measurements were performed in an evacuable guarded-hot-plate. The fumed silica specimens were first investigated in the dry state. Then water vapor was added into the vacuum system in different quantities (valves closed and no pumping), yielding water content in the VIP-kernel of up to 4% by mass. Most of the water is adsorbed in the specimens; smaller amounts are to be found in the gas phase. The parameter varied was the temperature; accordingly, the partial pressure of water vapor changed. An empirical equation is presented that describes the influence of water vapor and adsorbed water on the total heat transfer.