A Computer Analysis of the Annual Thermal Performance of a Roof System with Slightly Wet Fibrous Glass Insulation under Transient Conditions

Abstract

Field and laboratory results from short-term summer tests have sug gested that small amounts of moisture in fibrous glass roof insulations can have a sig nificant effect on the dynamic thermal performance of roof systems. These results have been reproduced using the transient heat and mass transfer mathematical model. Subsequently, the model has been used to simulate roof performance in different seasons and in different climate regions. In all cases the effects continue to be signifi cant. Small amounts of water readily move up or down in the insulation depending on the direction of the vapor pressure gradient. If a large amount of water is present, it concentrates on the bottom during the summer and on the top during the winter. In addition, during most days there is a portion that cycles up and down This adds excess heat to the building during the warm afternoon which may increase the air conditioning load and removes nearly the same amount of extra heat during the cool evening which may increase the building's heating system load The results from the model calculations are reported in terms of increased cooling and increased heating. Combining these into a changed building energy load and discussing the impact on building performance requires additional analysis to show how these hourly changes impact the whole building's energy balance Using the available property data for other roofing insulations indicates that this latent heat effect is not as important as it is for fibrous glass insulation Additional computer simulations with the model suggest that the effect is notice able even for moisture concentrations at a level which can be picked up by insulations in outdoor storage Impermeable layers between insulation boards influence the magnitude of dynamic heat transfer but do not eliminate it. Calculations show that thermal performance for new roofs over old insulation where moisture is probably common may be particularly affected by these results.