Prediction of water vapor permeation and desiccant saturation in vacuum insulation panels with a glass fiber core

Abstract

The permeation of dry air in vacuum insulation panels (VIPs) with a glass fiber core has been often prevented using laminated films with metallized ethylene vinyl alcohol copolymer (EVOH) layers; meanwhile, desiccants are incorporated to control the increasing pressure due to the permeation of water vapor. Previously proposed models can predict the changes in long-term performance caused by dry-air permeation, but a method that accounts for water-vapor permeation and desiccant saturation has not been reported before. Predicting the long-term performance of VIPs when exposed to outdoor temperature and humidity is essential for using it as a building insulation material. In this study, we investigated the dependence of the water-vapor transmission rate (WVTR) of metallized EVOH film on temperature and relative humidity, and the effect of ambient water-vapor pressure on the WVTR of metallized EVOH film. To reduce the measurement time of WVTR, we developed a new measurement method using a small sensor device and confirmed that its accuracy is equivalent to that of the conventional method. We measured the water-vapor transmission coefficient of glass fiber core VIPs under various temperature and humidity conditions using this measurement method. For the same water-vapor pressure, the amount of transmission significantly depended on the temperature and humidity conditions. Therefore, the WVTR of metallized EVOH films should be more concerned with the effect of relative humidity than that of the water vapor pressure of the ambient air. Based on these results, a new model that accounts for desiccant saturation was proposed and validated based on the conventional model to predict the long-term performance under permeation of mixed gases. The new model accurately evaluated the long-term performance of VIPs with confirmed desiccant saturation, supporting the long-term performance of glass fiber core VIPs.