Hygro-thermal properties of sheep wool insulation

Abstract

The primary energy use for house heating has increased strongly in the second half of the previous century. The environmental and economical impact of this high energy consumption has augmented the use of affordable insulation materials which have moderate environmental impact. Sheep wool is one of these natural products which are becoming more popular in the construction industry. Because sheep wool insulation, as a product, is very new on the market and also produced at this moment on relatively small scale, the general properties of sheep wool and especially the properties regarding moisture transfer have not been examined in great detail yet. Therefore, in first instance, a broad research is done about the wool fiber as a material and about the moisture transfer properties of wool (samples) in the hygroscopic range. Traditionally, the vapor transfer inside a material is governed by sorption on one hand, and on the other hand by the resistance to vapor flow. For sheep wool, the (steady-state) sorption behavior of wool has been studied extensively. Also the vapor resistance has been studied in some detail. But, the ?-factor of two wool insulation products with identical mass density can lay far from each other, both being measured according to the German norm (DIN 52615). Because of these differences, it was decided to measure the ?-factor of sheep wool insulation in this study using the well-known cup method. In addition, since wool samples may exhibit varying properties, the sorption isotherm of the wool was measured as well. And finally, besides these two experiments, a third experiment was carried out which focused on the time and space-dependent vapor transfer in a column of wool. The data from this quite unique experiment may indicate whether the vapor transfer inside the wool can be described using a Fickian diffusion model (i.e. one effective differential equation for the vapor transfer) or that non-Fickian effects do occur. The analysis of the cup method indicated that the vapor resistance (?-factor) lies between 1,2 (at 60% RH) and 2,12 (at 95% RH). As the ?-factor is expected to be constant or decrease with increasing RH, it is speculated that the increase of ?-factor was probably due to dilution of the salt solution. Consequently, the measurement of the permeability at lowest measured RH at the start of the experiment is probably the most reliable one. Therefore, it is concluded from this experiment that the ?-factor is 1,2 ±0,2. In these measurements it was seen that the sorption occurred in two stages after a step in RH. The first stage was fast, and occurred within a couple of hours, while the second step was very slow and took days to reach thermodynamic equilibrium. The second sorption stage was hardly present in the 45-60% RH range, but it was clearly observed in the range of 75-90% RH. This slow second stage of the adsorption is thought to be due to swelling of wool with increasing relative humidity.