EFFECT OF MOISTURE ON THERMAL INSULATION

Abstract

The effect of moisture on the thermal performance of fibreglass insulation 1s investigated over a range of temperatures and moisture levels. Two experimental rigs viz. the heat-flow-meter apparatus and the environmental chamber, both incorporating an automatic data acquisition system are developed for the investigation. Dry and moist insulation slabs are tested on the heat-flow-meter apparatus. The results reveal three distinct phases of moisture migration viz. quasi-steady, transient and final-steady states. In the second series of experiments, one face of the slab is exposed to a warm and humid ambient of the enviromental chamber and the other face is maintained at a law temperature. The results reveal a quasi-steady state of moisture migration which prevails as long as the supply of moisture is available from the ambient of the chamber and the slab is not saturated with water. A sixty hour wetting and subsequent drying experiment further reveals the three distinct phases of moisture migration and demonstrates the drying phenomenon. In both series of experiments, the thermal performance viz. temperature distribution, heat-flow, vapour flux and effective thermal conductivity are measured for different operating conditions, viz. mean slab temperature, temperature differential, moisture content and ambient relative humidity. Significant thermal behaviour is observed during the quasi-steady state. During this state the total heat flux qr consists of two components viz. the vapour latent heat flux, qL and the dry conduction heat flux, qc. During the quasi-steady state, the total heat is five times larger than that at final-steady state and the effective thermal conductivity varies from two to thirteen times the dry thermal conductivity. The moisture migration process is not significantly affected by the heat flow direction with respect to gravity. An increase in the mean or differential temperature of the insulation slab brings about an increase in the heat flux and effective thermal conductivity. During the quasi-steady state, the moistsure evaporation, diffusion and condensation rate is constant and is independent of the moisture content at the cold face of the insulation slab. The effect of increasing the moisture content at the cold end is to lengthen the duration of the quasi-steady state of moisture migration. Simple analytical models are developed for the quasi-steady state of the condensation process in both experiments. The predicted temperature distribution, heat flux, vapour flux and effective thermal conductivity agree well with the measured values.

This investigation confirms and extend the findings of previous researchers in the -field. It also serves as a starting point for further development of diagnostic methods for detecting the moisture migration process within insulations and the development of remedial action for reversing the moisture flow.