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In the process of assessing design values for thermal conductivity of insulating materials, knowledge of the time-dependent deterioration of the insulation performance is important. Extruded polystyrene is affected by changes in the composition of the gas mixture in the cells (normally referred to as ageing) and in some applications by moisture accumulation, both mechanisms resulting in a higher thermal conductivity.
In the Swedish quality control system the determination of thermal conductivity is carried out 91 ± 7 days after manufacture. Part of the ageing has by then already taken place, and the rest of the ageing during the life span of a building is taken care of by a specified addition (Δλa) to the laboratory value in the assessment of the design value. The more ageing that takes place before the determination of the thermal conductivity, the more accurate the design value will be. Different ways of accelerating the ageing have been discussed. This paper describes a method of acceleration by cutting the specimens into thin slices. It is shown that, by making use of this method, no extra addition allowing for the ageing under service conditions is needed (i.e., Δλa = 0).
The paper also deals with moisture accumulation in soil insulation and inverted roofs. The moisture corrections (Δλm) in the classification system are given. A method for acceleration of the moisture pickup and correlation to in situ measurements as well as a theoretical model for predicting moisture accumulation are described. Finally, effects of moisture on thermal conductivity are discussed.
thermal conductivity, ageing, moisture, extruded polystyrene, classification, quality control
Head, Swedish National Testing Institute, Borås,